Welding of high entropy alloys-A review
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paper_title: Fundamental deformation behavior in high-entropy alloys: An overview

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Abstract High-entropy alloys (HEAs), as a new class of materials, are nearly equiatomic and multi-element systems, which can crystallize as a single phase or multi-phases. Most of the HEAs described in the literature contain multiple phases (secondary phases, nanoparticles, and so on), rather than a single solid-solution phase. Thus, it is essential to review the typical mechanical properties of both single-phase and multiphase HEAs thoroughly, with emphases on (1) the fundamental physical mechanisms and (2) the difference from conventional alloys. In this paper, mainly based on different mechanical properties, HEAs are classified into four types for the first time, i.e., (a) HEA alloy systems of 3d-transition metals only (Type 1), (b) HEA alloy systems of transition metals with larger atomic-radius elements (Type 2), (c) HEA alloy systems of refractory metals (Type 3), and (4) others (Type 4). Then a number of aspects of mechanical behavior are reviewed and discussed, including the elastic anisotropy, yield strength, high-temperature performance, serration behavior, fracture toughness, and fatigue responses, which may serve as a demonstrative summary for the current progress in the scientific research of HEAs. Several mechanisms that quantitatively explain the mechanical properties of single-phase and multiphase HEAs in terms of basic defects (dislocations, twinning, precipitates, etc.) are discussed. A number of future research activities are suggested, based on the emphasis on developing high-performance structural materials. The review concludes with a brief summary of major mechanical properties and insights into the deformation behavior of single-phase and multiphase HEAs. The comparison and contrast between HEAs and conventional alloys remain the most compelling motivation for future studies. With the integrated experimental and simulation investigations, a clearer picture of the fundamental deformation behavior of single-phase and multiphase HEAs could be explored.
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paper_title: Numerical study of strengths of spot-welded joints of steel

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Abstract A resistance spot-welding (RSW) joint consists of several material zones with different microstructure and properties as a result of the thermal, metallurgical and mechanical deformation process. Detailed material properties are essential to accurately simulate the mechanical behavior of a joint and its dependency on some key structural parameters (e.g. nugget size, sheet thickness etc.). The work presented in this paper utilises an inverse modelling methodology combining numerical modelling and indentation tests with a standard hardness test to characterise the detailed properties of different weld zones of spot-welded joints. The yield and strain hardening parameters of the three zones (nugget zone, HAZ: heat-affected zone and base zone) were determined and the predicted stress–strain curves for base zone were compared with standard tensile tests results. A 3-D finite element model based on the predicted constitutive material laws for different zones coupled with a fracture model was developed to predict the deformation of spot-welded joints beyond the onset of initial yield under tensile-shear loading. The deformation mode and force–displacement result showed good agreements with experimental data. The effect of nugget size and sheet thickness on the tensile-shear strength of welded joints was further systematically studied using a high performance computing system.
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paper_title: Processing, alloy composition and phase transition effect on the mechanical and corrosion properties of high entropy alloys: a review

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Abstract This paper reviews from the corpus of literatures on high-entropy alloys (HEAs), their mechanical and corrosion behavior as affected by metallurgical factors such as processing technique, composition, phase formation and transition. HEAs are a promising class of alloys which are designed based on the use of multiple component alloying elements in equimolar or near equimolar ratio. There has been surging interest in this class of alloys on account of their unique property range. Their unique metallurgical characteristics, structures, mechanical and corrosion properties, current and potential areas of applications, and suggestions for future research are discussed in this paper.
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paper_title: High-entropy alloy: challenges and prospects

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High-entropy alloys (HEAs) are presently of great research interest in materials science and engineering. Unlike conventional alloys, which contain one and rarely two base elements, HEAs comprise multiple principal elements, with the possible number of HEA compositions extending considerably more than conventional alloys. With the advent of HEAs, fundamental issues that challenge the proposed theories, models, and methods for conventional alloys also emerge. Here, we provide a critical review of the recent studies aiming to address the fundamental issues related to phase formation in HEAs. In addition, novel properties of HEAs are also discussed, such as their excellent specific strength, superior mechanical performance at high temperatures, exceptional ductility and fracture toughness at cryogenic temperatures, superparamagnetism, and superconductivity. Due to their considerable structural and functional potential as well as richness of design, HEAs are promising candidates for new applications, which warrants further studies.
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paper_title: A Fracture-Resistant High-Entropy Alloy for Cryogenic Applications.

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A CrMnFeCoNi alloy is prepared by arc melting the elements and drop casting into copper molds, followed by cold forging and cross rolling at room temperature into sheets roughly 10 mm thick.
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paper_title: The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy

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Abstract An equiatomic CoCrFeMnNi high-entropy alloy, which crystallizes in the face-centered cubic (fcc) crystal structure, was produced by arc melting and drop casting. The drop-cast ingots were homogenized, cold rolled and recrystallized to obtain single-phase microstructures with three different grain sizes in the range 4–160 μm. Quasi-static tensile tests at an engineering strain rate of 10 −3 s −1 were then performed at temperatures between 77 and 1073 K. Yield strength, ultimate tensile strength and elongation to fracture all increased with decreasing temperature. During the initial stages of plasticity (up to ∼2% strain), deformation occurs by planar dislocation glide on the normal fcc slip system, {1 1 1}〈1 1 0〉, at all the temperatures and grain sizes investigated. Undissociated 1/2〈1 1 0〉 dislocations were observed, as were numerous stacking faults, which imply the dissociation of several of these dislocations into 1/6〈1 1 2〉 Shockley partials. At later stages (∼20% strain), nanoscale deformation twins were observed after interrupted tests at 77 K, but not in specimens tested at room temperature, where plasticity occurred exclusively by the aforementioned dislocations which organized into cells. Deformation twinning, by continually introducing new interfaces and decreasing the mean free path of dislocations during tensile testing (“dynamic Hall–Petch”), produces a high degree of work hardening and a significant increase in the ultimate tensile strength. This increased work hardening prevents the early onset of necking instability and is a reason for the enhanced ductility observed at 77 K. A second reason is that twinning can provide an additional deformation mode to accommodate plasticity. However, twinning cannot explain the increase in yield strength with decreasing temperature in our high-entropy alloy since it was not observed in the early stages of plastic deformation. Since strong temperature dependencies of yield strength are also seen in binary fcc solid solution alloys, it may be an inherent solute effect, which needs further study.
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paper_title: Microstructure and Mechanical Properties of High-Entropy Alloys CoCrFeNiAl by Welding

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Five common elements Co, Cr, Fe, Ni and Al were selected, and CoCrFeNiAl was prepared by arc-melting. The microstructure and mechanical properties after spot welding were studied. The results show that the cast microstructure of high entropy alloy CoCrFeNiAl is relatively uniform with a dendritic morphology. The heat is inversely proportional with the alloy grain sizes. The greater the heat is, the smaller the grain size is, which leads to the higher hardness and more uniform tissue. But there is a critical value of the heat(Hcrit) in spot welding. When Hactu(actual heat) exceeds Hcrit, it will adversely affect the performance, resulting in crack, splash and other defects.
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paper_title: Combinatorial high throughput synthesis of high entropy alloys

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This PhD thesis is a part of the Accelerated Metallurgy (AccMet) project funded under the Seventh Framework Programme. AccMet’s aims consists on the delivery of an integrated pilot-scale facility for the combinatorial synthesis and testing of those unexplored material. ::: The contribution of this thesis to AccMet has been expanded in 3 years while focused in the understanding and development of a methodology suitable for the combinatorial synthesis of novel materials, and particularly of High Entropy Alloys (HEAs). These novel materials are composed of multiple elements at near equiatomic levels with the capacity of forming simple ::: crystalline phases such as bcc and fcc instead of the expected intermetallic compounds as well as their excellent combination of structural and functional properties compared to the traditional materials. ::: A mathematical technique known as Principal Component Analysis has been used here to identify patterns within a set of metallic systems forming a wide range of crystalline structures. This technique would not only speed up the compositional design stage but also contribute to the development of a virtual library containing all the explored systems. ::: Mercury Centre has been an important key during the synthesis of HEAs where Spark Plasma Sintering (SPS) and Electron Beam Melting (EBM) have been successfully applied for the development of the thesis. ::: The final combination of the design stage, production and characterisation of HEAs developed in this thesis would result in an advances technique suitable not only for the synthesis of novel HEAs, but also for the discovery of other unexplored systems.
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paper_title: Microstructures and properties of high-entropy alloys

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Abstract This paper reviews the recent research and development of high-entropy alloys (HEAs). HEAs are loosely defined as solid solution alloys that contain more than five principal elements in equal or near equal atomic percent (at.%). The concept of high entropy introduces a new path of developing advanced materials with unique properties, which cannot be achieved by the conventional micro-alloying approach based on only one dominant element. Up to date, many HEAs with promising properties have been reported, e.g., high wear-resistant HEAs, Co 1.5 CrFeNi 1.5 Ti and Al 0.2 Co 1.5 CrFeNi 1.5 Ti alloys; high-strength body-centered-cubic (BCC) AlCoCrFeNi HEAs at room temperature, and NbMoTaV HEA at elevated temperatures. Furthermore, the general corrosion resistance of the Cu 0.5 NiAlCoCrFeSi HEA is much better than that of the conventional 304-stainless steel. This paper first reviews HEA formation in relation to thermodynamics, kinetics, and processing. Physical, magnetic, chemical, and mechanical properties are then discussed. Great details are provided on the plastic deformation, fracture, and magnetization from the perspectives of crackling noise and Barkhausen noise measurements, and the analysis of serrations on stress–strain curves at specific strain rates or testing temperatures, as well as the serrations of the magnetization hysteresis loops. The comparison between conventional and high-entropy bulk metallic glasses is analyzed from the viewpoints of eutectic composition, dense atomic packing, and entropy of mixing. Glass forming ability and plastic properties of high-entropy bulk metallic glasses are also discussed. Modeling techniques applicable to HEAs are introduced and discussed, such as ab initio molecular dynamics simulations and CALPHAD modeling. Finally, future developments and potential new research directions for HEAs are proposed.
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paper_title: Microstructure and corrosion behavior of the heat affected zone of a stainless steel 308L-316L weld joint

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Abstract Microstructure of the heat affected zone (HAZ) of a 308L-316L stainless steel (SS) weld joint and its corrosion behavior in high temperature water were studied. Peak of the residual strain was observed to approach to the fusion boundary in the HAZ while the strain increased from the top to root areas of the HAZ. The root area of the HAZ shows a lower corrosion resistance in high temperature water than the top and middle areas of the HAZ. This is attributed to a higher level of residual strain in association with a higher density of tangled dislocations in the top area of the HAZ. The results suggest that the residual strain in the HAZ could also promote the SCC through its effect on corrosion, in addition to that on the local microstructure and mechanical property of the steel.
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paper_title: Welding Metallurgy and Weldability of Nickel-Base Alloys

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Preface. 1. Introduction. 1.1 Ni-base Alloy Classification. 1.2 History of Nickel and Ni-base Alloys. 1.3 Corrosion Resistance. 1.4 Nickel Alloy Production. 2. Alloying Additions, Phase Diagrams, and Phase Stability. 2.1 Introduction. 2.2 General Influence of Alloying Additions. 2.3 Phase Diagrams for Solid-Solution Alloys. 2.4 Phase Diagrams for Precipitation Hardened Alloys--gamma' Formers. 2.5 Phase Diagrams for Precipitation-Hardened Alloys--gamma" Formers. 2.6 Calculated Phase Stability Diagrams. 2.7 PHACOMP Phase Stability Calculations. 3. Solid-Solution Strengthened Ni-base Alloys. 3.1 Standard Alloys and Consumables. 3.2 Physical Metallurgy and Mechanical Properties. 3.3 Welding Metallurgy. 3.4 Mechanical Properties of Weldments. 3.5 Weldability. 3.6 Corrosion Resistance. 3.7 Case Studies. 4. Precipitation Strengthened Ni-base Alloys. 4.1 Standard Alloys and Consumables. 4.2 Physical Metallurgy and Mechanical Properties. 4.3 Welding Metallurgy. 4.4 Mechanical Properties of Weldments. 4.5 Weldability. 5. Oxide Dispersion Strengthened Alloys and Nickel Aluminides. 5.1 Oxide Dispersion Strengthened Alloys. 5.2 Nickel Aluminide Alloys. 6. Repair Welding of Ni-base Alloys. 6.1 Solid-Solution Strengthened Alloys. 6.2 Precipitation Strengthened Alloys. 6.3 Single Crystal Superalloys. 7. Dissimilar Welding. 7.1 Application of Dissimilar Welds. 7.2 Influence of Process Parameters on Fusion Zone Composition. 7.3 Carbon, Low Alloys and Stainless Steels. 7.4 Postweld Heat Treatment Cracking in Stainless Steels Welded with Ni-base Filler Metals. 7.5 Super Austenitic Stainless Steels. 7.6 Dissimilar Welds in Ni-base Alloys - Effect on Corrosion Resistance. 7.7 9%Ni Steels. 7.8 Super Duplex Stainless Steels. 7.9 Case Studies. 8. Weldability Testing. 8.1 Introduction. 8.2 The Varestraint Test. 8.3 Modified Cast Pin Tear Test. 8.4 The Sigmajig Test. 8.5 The Hot Ductility Test. 8.6 The Strain-to-Fracture Test. 8.7 Other Weldability Tests. Appendix A Composition of Wrought and Cast Nickel-Base Alloys. Appendix B Composition of Nickel and Nickel Alloy Consumables. Appendix C Corrosion Acceptance Testing Methods. Appendix D Etching Techniques for Ni-base Alloys and Welds. Author Index. Subject Index.
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paper_title: Microstructures and mechanical properties of a welded CoCrFeMnNi high-entropy alloy

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ABSTRACTThe response of the CoCrFeMnNi high-entropy alloy to weld thermal cycles was investigated to determine its applicability as an engineering structural material. Two processes were used: high-energy-density, low-heat-input electron beam (EB) welding and low-energy-density, high-heat-input gas tungsten arc (GTA) welding. Weldability was determined through comprehensive microstructural and mechanical property characterisation of the welds. The welds did not develop solidification cracking or heat-affected zone cracks. The microstructures in weld fusion zones are similar to that in the as-cast materials, consisting of large columnar grains with dendrite. The dendrite arm spacing and the extent of elemental segregation were less in the welds than in the cast ingot, and also were less pronounced in the EB weld than in the GTA weld. Compositional microsegregation between dendritic cores and interdendritic regions of the welds was insignificant. Both welds exhibited slightly higher yield strengths than the...
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paper_title: Development of in situ observation technique using scanning ion microscopy and demonstration of Mn depletion effect on intragranular ferrite transformation in low-alloy steel

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An in situ observation technique using scanning ion microscopy (SIM) to investigate the behaviour of phase transformation from austenite to ferrite in low-alloy steel is proposed. The most important advantage of the technique is the combined observation by SIM and fabrication by focused ion beam (FIB) at a wide range of temperature, from room temperature to 1673 K. Using this advantage, we have developed a new technique for observing intragranular ferrite (IGF) transformation in low-alloy steel, which is the phase transformation occurring at an inclusion existing inside a grain of austenite phase. We successfully observed the IGF transformation at 873 K by examining an inclusion exposed to the specimen surface by FIB fabrication at 1073 K during cooling from 1673 K. From the results of the in situ observation, we determined the role of Mn depletion due to precipitation of MnS for IGF transformation in steel.
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paper_title: Enhanced Relative Slip Distance in Gas-Tungsten-Arc-Welded Al0.5CoCrFeNi High-Entropy Alloy

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Gas-tungsten-arc-welded (GTAW) Al0.5CoCrFeNi high-entropy alloy (HEA) was analyzed using scanning electron microscopy (SEM), microhardness, and tensile testing. The weld metal having refined equiaxed and elongated columnar dendritic microstructure experienced 6.38 pct reduction in strength and marginally reduced hardness compared to the base metal (BM). Lower work hardening with enhanced relative slip distance, which was observed through the Kocks–Mecking plot and slip distance–true strain plots, was attributed to the reduced bcc fraction in the weld.
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paper_title: A critical review of high entropy alloys and related concepts

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Abstract : High entropy alloys (HEAs) are barely 12 years old. The field has stimulated new ideas and has inspired the exploration of the vast composition space offered by multi-principal element alloys (MPEAs). Here we present a critical review of this field, with the intent of summarizing key findings, uncovering major trends and providing guidance for future efforts. Major themes in this assessment include definition of terms; thermodynamic analysis of complex, concentrated alloys (CCAs); taxonomy of current alloy families; microstructures; mechanical properties; potential applications; and future efforts. Based on detailed analyses, the following major results emerge. Although classical thermodynamic concepts are unchanged, trends in MPEAs can be different than in simpler alloys. Common thermodynamic perceptions can be misleading and new trends are described. From a strong focus on 3d transition metal alloys, there are now seven distinct CCA families. A new theme of designing alloy families by selecting elements to achieve a specific, intended purpose is starting to emerge. A comprehensive microstructural assessment is performed using three datasets: experimental data drawn from 408 different alloys and two computational datasets generated using the CALculated PHAse Diagram (CALPHAD) method. Each dataset emphasizes different elements and shows different microstructural trends. Trends in these three datasets are all predicted by a structure in - structure out (SISO) analysis developed here that uses the weighted fractions of the constituent element crystal structures in each dataset. A total of 13 distinct multi-principal element single-phase fields are found in this microstructural assessment. Relationships between composition, microstructure and properties are established for 3d transition metal MPEAs, including the roles of Al, Cr and Cu. Critical evaluation shows that commercial austenitic stainless steels and nickel alloys with 3 or more principal elements are MPEAs.
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paper_title: Tensile deformation behavior and deformation twinning of an equimolar CoCrFeMnNi high-entropy alloy

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Abstract The tensile deformation and strain hardening behaviors of an equimolar CoCrFeMnNi high-entropy alloy (HEA) were investigated and compared with low and medium entropy equiatomic alloys (LEA and MEA). The HEA had a lower yield strength than the MEA because the addition of Mn weakens solid solution hardening in the HEA. However, deformation twinning induced the multiple stage strain hardening behavior of the HEA and enhanced strength and elongation. Using tensile-interrupted electron backscatter diffraction analysis, geometrically necessary dislocations were observed as plume-shaped features in grain interior, and a considerable texture was characterized, which is typical of face centered cubic metals. Moreover, the relationship between favorably oriented grains and twinning in the HEA bore a clear resemblance to the same tendency in TWIP steels. The thickness of the twin bundles was less than 100 nm. A high density of stacking defects was found in the nanotwins. Nano twinning and stacking faults were found to contribute to the remarkable mechanical properties. Deformation induced twinning not only demonstrated the dynamic Hall-Petch effect but also changed dislocation cell substructures into microband structures.
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paper_title: Welding Metallurgy and Weldability of Nickel-Base Alloys

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Preface. 1. Introduction. 1.1 Ni-base Alloy Classification. 1.2 History of Nickel and Ni-base Alloys. 1.3 Corrosion Resistance. 1.4 Nickel Alloy Production. 2. Alloying Additions, Phase Diagrams, and Phase Stability. 2.1 Introduction. 2.2 General Influence of Alloying Additions. 2.3 Phase Diagrams for Solid-Solution Alloys. 2.4 Phase Diagrams for Precipitation Hardened Alloys--gamma' Formers. 2.5 Phase Diagrams for Precipitation-Hardened Alloys--gamma" Formers. 2.6 Calculated Phase Stability Diagrams. 2.7 PHACOMP Phase Stability Calculations. 3. Solid-Solution Strengthened Ni-base Alloys. 3.1 Standard Alloys and Consumables. 3.2 Physical Metallurgy and Mechanical Properties. 3.3 Welding Metallurgy. 3.4 Mechanical Properties of Weldments. 3.5 Weldability. 3.6 Corrosion Resistance. 3.7 Case Studies. 4. Precipitation Strengthened Ni-base Alloys. 4.1 Standard Alloys and Consumables. 4.2 Physical Metallurgy and Mechanical Properties. 4.3 Welding Metallurgy. 4.4 Mechanical Properties of Weldments. 4.5 Weldability. 5. Oxide Dispersion Strengthened Alloys and Nickel Aluminides. 5.1 Oxide Dispersion Strengthened Alloys. 5.2 Nickel Aluminide Alloys. 6. Repair Welding of Ni-base Alloys. 6.1 Solid-Solution Strengthened Alloys. 6.2 Precipitation Strengthened Alloys. 6.3 Single Crystal Superalloys. 7. Dissimilar Welding. 7.1 Application of Dissimilar Welds. 7.2 Influence of Process Parameters on Fusion Zone Composition. 7.3 Carbon, Low Alloys and Stainless Steels. 7.4 Postweld Heat Treatment Cracking in Stainless Steels Welded with Ni-base Filler Metals. 7.5 Super Austenitic Stainless Steels. 7.6 Dissimilar Welds in Ni-base Alloys - Effect on Corrosion Resistance. 7.7 9%Ni Steels. 7.8 Super Duplex Stainless Steels. 7.9 Case Studies. 8. Weldability Testing. 8.1 Introduction. 8.2 The Varestraint Test. 8.3 Modified Cast Pin Tear Test. 8.4 The Sigmajig Test. 8.5 The Hot Ductility Test. 8.6 The Strain-to-Fracture Test. 8.7 Other Weldability Tests. Appendix A Composition of Wrought and Cast Nickel-Base Alloys. Appendix B Composition of Nickel and Nickel Alloy Consumables. Appendix C Corrosion Acceptance Testing Methods. Appendix D Etching Techniques for Ni-base Alloys and Welds. Author Index. Subject Index.
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paper_title: Microstructural evolution of a CoCrFeMnNi high-entropy alloy after swaging and annealing

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Abstract The processing parameters which govern the evolution of microstructure and texture during rotary swaging and subsequent heat treatments were studied in an equiatomic single-phase CoCrFeMnNi high-entropy alloy. After vacuum induction melting and casting, the diameter of the 40 mm cast ingot was reduced at room temperature to a final diameter of 16.5 mm by rotary swaging (diameter reduction of 60%/area reduction of 80%) and the alloy was then annealed at different temperatures for 1 h. The resulting microstructures were analyzed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscatter diffraction and correlated with results of microhardness measurements. It was found that the microhardness first increases slightly upon annealing below the recrystallization temperature but then drops steeply at higher annealing temperatures due to the onset of recrystallization. Special emphasis was placed on how the microstructure evolves with respect to the radial and longitudinal position in the rod. Finally, a combination of swaging and heat treatment parameters were identified that can produce CoCrFeMnNi high-entropy alloys with a homogeneous composition and grain size and almost no texture.
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paper_title: Numerical study of strengths of spot-welded joints of steel

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Abstract A resistance spot-welding (RSW) joint consists of several material zones with different microstructure and properties as a result of the thermal, metallurgical and mechanical deformation process. Detailed material properties are essential to accurately simulate the mechanical behavior of a joint and its dependency on some key structural parameters (e.g. nugget size, sheet thickness etc.). The work presented in this paper utilises an inverse modelling methodology combining numerical modelling and indentation tests with a standard hardness test to characterise the detailed properties of different weld zones of spot-welded joints. The yield and strain hardening parameters of the three zones (nugget zone, HAZ: heat-affected zone and base zone) were determined and the predicted stress–strain curves for base zone were compared with standard tensile tests results. A 3-D finite element model based on the predicted constitutive material laws for different zones coupled with a fracture model was developed to predict the deformation of spot-welded joints beyond the onset of initial yield under tensile-shear loading. The deformation mode and force–displacement result showed good agreements with experimental data. The effect of nugget size and sheet thickness on the tensile-shear strength of welded joints was further systematically studied using a high performance computing system.
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paper_title: Laser weldability of cast and rolled high-entropy alloys for cryogenic applications

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Abstract Laser similar welding of cast and rolled high-entropy alloys (HEAs) was performed using the cantor system (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2). As the welding velocity was increased from 6 to 10 m min−1, the shrinkage voids, primary dendrite arm spacing, and dendrite packet size decreased, thus improving the mechanical properties of the cast and rolled HEA welds. The cast HEA welds showed tensile properties comparable to those of the base metal (BM). In all the specimens fracture occurred near the heat-affected zone and BM at 298 K. However, the rolled HEA welds showed lower tensile strength than the BM, and fracture occurred in the weld metal (WM). This can be attributed to the larger dendrite packet size of the WM than the grain size of the BM. In addition, the tensile properties of the specimens at the cryogenic temperature were superior to those observed at 298 K, regardless of the cast and rolled HEA welds. This is because the formation of deformation twins and dislocations was predominant at 77 K. Therefore, the laser similar welds of cast and rolled HEAs are suitable for cryogenic applications.
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paper_title: Enhanced Relative Slip Distance in Gas-Tungsten-Arc-Welded Al0.5CoCrFeNi High-Entropy Alloy

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Gas-tungsten-arc-welded (GTAW) Al0.5CoCrFeNi high-entropy alloy (HEA) was analyzed using scanning electron microscopy (SEM), microhardness, and tensile testing. The weld metal having refined equiaxed and elongated columnar dendritic microstructure experienced 6.38 pct reduction in strength and marginally reduced hardness compared to the base metal (BM). Lower work hardening with enhanced relative slip distance, which was observed through the Kocks–Mecking plot and slip distance–true strain plots, was attributed to the reduced bcc fraction in the weld.
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paper_title: Laser beam welding of a CoCrFeNiMn-type high entropy alloy produced by self-propagating high-temperature synthesis

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Abstract Fiber laser beam welding of a CoCrFeNiMn-type high entropy alloy (HEA) produced by self-propagating high-temperature synthesis (SHS) was reported in this work. The SHS-fabricated alloy was characterized by both ∼2 times reduced Mn content in comparison with that of the other principal components and the presence of impurities including Al, C, S, and Si. The as-fabricated alloy was composed of columnar fcc grains with coarse precipitates of MnS and fine Cr-rich M23C6 carbides. Successful defect-free butt joint of the alloy was obtained using a laser power of 2 kW and a welding speed of 5 m/min. Welding resulted in changes in texture and structure of the fcc matrix. In addition, precipitation of nanoscale B2 phase particles in the weld zone was observed. A pronounced increase in microhardness from (153 ± 3) HV 0.5 (base material) to (208 ± 6) HV 0.5 (fusion zone) was observed. The B2 phase precipitation after welding was found to be in a reasonable agreement with the ThermoCalc predictions. Quantitative analysis demonstrated that the increase in hardness can be associated with the B2 phase precipitation. Possibilities of the development of HEAs with intrinsic hardening ability after laser processing are discussed.
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paper_title: Microstructure and mechanical properties of friction stir welded and laser welded high entropy alloy CrMnFeCoNi

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The high entropy alloy CrMnFeCoNi has been shown to have promising structural properties. For a new alloy to be used in a structural application it should be weldable. In the present study, friction stir welding (FSW) and laser welding (LW) techniques were used to butt weld thin plates of CrMnFeCoNi. The microstructure, chemical homogeneity and mechanical behavior of the welds were characterized and compared with the base metal. The tensile stress-strain behavior of the welded specimens were reasonable when compared with that of the base metal. FSW refined the grain size in the weld region by a factor of ∼14 when compared with the base metal. High-angle annular dark field transmission electron microscopy in combination with energy dispersive X-ray spectroscopy showed chemical inhomogeneity between dendritic and interdendritic regions in the fusion zone of LW. Large fluctuations in composition (up to 15 at%) did not change the crystal structure in the fusion zone. Hardness measurements were carried out in the weld cross section and discussed in view of the grain size, low angle grain boundaries and twin boundaries in FSW specimens and the dendritic microstructure in LW specimens.
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paper_title: Effect of post weld heat treatment on weldability of high entropy alloy welds

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ABSTRACTThe effect of post weld heat treatment (PWHT) temperature on laser beam welds in high-entropy alloys (HEAs) using a cold-rolled cantor system (CoCrFeMnNi) was investigated. Laser welding of low heat input was applied to reduce thermal distortion. The cold-rolled HEA welds indicated larger grain size and inferior tensile/hardness properties as compared to the base metal (BM). By applying PWHT, the welds showed superior hardness to the BM with no variation in the face-centred cubic phase and a decrease in the size and fraction of CrMn oxide inclusions. As the PWHT temperature increased (800–1000°C), the variation in the grain size decreased between the weld metal and heat-affected zone, thus resulting in approximately the same tensile strength and elongation of the transverse welds as compared to the BM.
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paper_title: Characterization of Laser Beam Welded Al0.5CoCrFeNi High-Entropy Alloy

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High-entropy alloys (HEA), a new generation alloy system offer superior mechanical properties with solid solution strengthening. AlxCoCrFeNi-HEA is one such system being received more attention because of its specific yield strength and ductility. In the present work, Al0.5CoCrFeNi-HEA was prepared by vacuum arc melting. The laser beam welding (LBW) was carried out on 1mm thick forged and homogenized HEA, with a beam power of 1.5 kW and at a traverse speed of 600 mm/min. The microstructural features of different regions of the weld were studied using scanning electron microscopy. The homogenized Al0.5CoCrFeNi-HEA have shown equiaxed grains of average size 60 μm. The weld metal showed a typical weld fusion zone microstructure with dendritic structure with a reduction in BCC phase due to minimal Al and Ni segregation ratio at interdendrites. Micro-chemical analysis with energy dispersive spectroscopy confirmed that there was no major segregation of elements in the weld fusion zone. The microhardness survey performed across the weld evidenced a reduction in hardness, as a consequence of significant reduction in Al-Ni rich hardening factor.
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paper_title: AlxCrFeCoNi High-Entropy Alloys: Surface Modification by Electron Beam Bead-on-Plate Melting

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Two five-component Al x CrFeCoNi high-entropy alloys, x-0.6 and 0.8, were prepared by vacuum arc-melting. Electron beam surface re-melting was employed to modify the surface properties of the two high-entropy alloys. The effects of electron beam surface re-melting on the structure and mechanical properties of the alloys were investigated using scanning electron microscopy and the Vickers hardness test. Regions of Al0.6CrFeCoNi (P3 alloy) subjected to multi-pass electron beam surface re-melting showed an average value of 374 HV, an increase of 28% when compared to base metal values, while Al0.8CrFeCoNi (P2 alloy) welds exhibited a much higher increase to 530 HV, corresponding to a 34% increase relative to the values obtained with the base metals. In the P2 alloy subjected to multi-pass surface re-melting, significant temper softening caused by overlapping tracks was detected, in contrast to the hardening of the surface induced by a single-pass. It is noteworthy that the significant increase in hardness realized in the P2 multi-pass fusion zones resulted in a microstructure with a high tendency to crack. The increased hardness of that region subjected to electron beam surface re-melting for both alloys is likely associated with a superposition of several factors related to the high cooling rates of solidification in the electron beam re-melted areas.
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paper_title: Microstructures and mechanical properties of a welded CoCrFeMnNi high-entropy alloy

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ABSTRACTThe response of the CoCrFeMnNi high-entropy alloy to weld thermal cycles was investigated to determine its applicability as an engineering structural material. Two processes were used: high-energy-density, low-heat-input electron beam (EB) welding and low-energy-density, high-heat-input gas tungsten arc (GTA) welding. Weldability was determined through comprehensive microstructural and mechanical property characterisation of the welds. The welds did not develop solidification cracking or heat-affected zone cracks. The microstructures in weld fusion zones are similar to that in the as-cast materials, consisting of large columnar grains with dendrite. The dendrite arm spacing and the extent of elemental segregation were less in the welds than in the cast ingot, and also were less pronounced in the EB weld than in the GTA weld. Compositional microsegregation between dendritic cores and interdendritic regions of the welds was insignificant. Both welds exhibited slightly higher yield strengths than the...
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paper_title: Weldability of a high entropy CrMnFeCoNi alloy

paper_content:

Abstract High-entropy alloys are unique alloys in which five or more elements are presented all in high concentrations. To determine its potential as a structural alloy, a model face-centered-cubic CrMnFeCoNi alloy was selected to investigate its weldability. Welds produced by electron beam welding show no cracking. The grain structures within the fusion zone (FZ) are controlled by the solidification behavior of the weld pool. The weldment possesses mechanical properties comparable to those of the base metal (BM) at both room and cryogenic temperatures. Compared with the BM, deformation twinning was more pronounced in the FZ of the tested alloy.
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paper_title: Effects of (W, Cr) carbide on grain refinement and mechanical properties for CoCrFeMnNi high entropy alloys

paper_content:

Abstract In this study, we investigated the influence of (W, Cr) carbide on the hardness and tensile properties of friction stir welds (FSWs) for CoCrFeMnNi high entropy alloys. FSW without cracks or voids were achieved for rotational speeds of 400–1000 rpm. As the rotational speed increased, the thinning of the weld thickness compared to the thickness of the base metal was increased. Specifically, an abnormal tornado-shaped region was observed in the stir zone at rotation speeds of 600 rpm and above. The tensile strength increased as the rotation speed increased from 400 to 800 rpm and decreased at a rotation speed of 1000 rpm. The abnormal region with a fine (W, Cr) carbide of 0.5 μm or less was dispersed in the grain boundaries. At a rotation speed of 800 rpm, the dispersion of fine (W, Cr) carbide was optimized to produce grain refinement and maximum tensile strength. However, at 1000 rpm, (W, Cr) carbide coarsened due to high heat input, and the number of carbide particles per unit area decreased, thereby decreasing the hardness and tensile strength.
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paper_title: Friction stir welding of a CoCrFeNiAl0.3 high entropy alloy

paper_content:

Abstract A typical face-centered cubic CoCrFeNiAl 0.3 high entropy alloy was welded by friction stir welding. Sound joints without welding defects were obtained. Four typical regions, the stir zone, thermomechanically affected zone, heat affected zone and base metal could be detected. The stir zone showed a refined equiaxed microstructure due to recrystallization and it exhibited the highest hardness with grain refinement. The grain size decreased slightly with increasing the welding speed. The thermomechanically affected zone exhibited a mixed microstructure comprising coarse and fine grains due to partial recryatllization. The present result shows a potential capability for joining high entropy alloys through friction stir welding and may enhance their engineering applications.
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paper_title: Numerical analysis and analytical modeling of the spatial distribution of heat flux during friction stir welding

paper_content:

Abstract The quantitative characterization of the in-process energy input during friction stir welding (FSW) is crucial for the prediction and the control of microstructure and properties in the welds. In this study, a novel CFD model is established to quantitatively investigate the spatial distribution of frictional heat flux and plastic deformation heat flux during the FSW of AA2024-T4, in which an advanced frictional boundary condition is used to capture the complex interplay between the spatial distribution of heat flux and the contact states at the welding tool/workpiece interface. It is found that the heat flux distribution is closely related to the contact states at the interface. An analytical model of the heat flux distribution is proposed based on the simulation results. The distribution of frictional heat flux at the welding tool/workpiece interface could be represented by a modified Gaussian heat source model and the distribution of plastic deformation heat flux at proximity of the welding tool could be represented by the Two - phase Hill function. The analytical model in this study provides a more realistic mathematical description of the heat flux distribution during FSW, which provides further support for the process optimization and thermal analysis of FSW.
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paper_title: Friction stir welding of a сarbon-doped CoCrFeNiMn high-entropy alloy

paper_content:

Abstract Butt-joint seam of a high entropy alloy (HEA) of the CoCrFeNiMn system was successfully obtained by friction stir welding (FSW). The HEA was produced by self-propagating high-temperature synthesis. Along with the principal elements, a small amount (0.9 at.%) of C was added to the alloy. The as-cast alloy was cold rolled and annealed at 900 °C to produce refined microstructure. The structure of the annealed alloy consisted of a recrystallized face-centered cubic matrix with a grain size of 9.2 μm and fine Cr-rich M23C6 carbides. FSW of the HEA resulted in microstructure refinement to d = 4.6 μm in the stir zone. A noticeable rise in strength and some decrease in ductility of the processed alloy in comparison with the initial condition can be associated with the microstructure refinement and some increase in the volume fraction of M23C6 carbides.
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paper_title: Enhanced strength and ductility in a friction stir processing engineered dual phase high entropy alloy

paper_content:

The potential of high-entropy alloys (HEAs) to exhibit an extraordinary combination of properties by shifting the compositional regime from the corners towards the centers of phase diagrams has led to worldwide attention by material scientists. Here we present a strong and ductile non-equiatomic HEA obtained after friction stir processing (FSP). A transformation-induced plasticity (TRIP) assisted HEA with composition Fe50Mn30Co10Cr10 (at.%) was severely deformed by FSP and evaluated for its microstructure-mechanical property relationship. The FSP-engineered microstructure of the TRIP HEA exhibited a substantially smaller grain size, and optimized fractions of face-centered cubic (f.c.c., γ) and hexagonal close-packed (h.c.p., ε) phases, as compared to the as-homogenized reference material. This results in synergistic strengthening via TRIP, grain boundary strengthening, and effective strain partitioning between the γ and ε phases during deformation, thus leading to enhanced strength and ductility of the TRIP-assisted dual-phase HEA engineered via FSP.
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paper_title: Friction-stir welding of a ductile high entropy alloy: microstructural evolution and weld strength

paper_content:

High entropy alloys (HEAs) are a novel subset of metallic systems with complex compositions usually yielding simple phase formation. To verify their potential engineering applications, a novel Co 16 Fe 28 Ni 28 Cr 28 HEA with a low content of expensive Co was developed and its welding characteristics through friction-stir welding (FSW) were investigated. The HEA shows a stable face-centered-cubic (FCC) structure with an excellent ductility of about 70%. The microstructural evolution during FSW was dominated by discontinuous recrystallization through grain bulging and the B/ B ¯ {112} shear texture formed in the stir zone (SZ). A white band (WB) containing W-rich and Cr-rich phases was detected in the SZ. The WB exhibited refined grains compared with the normal SZ, which may be associated with the particle-stimulated nucleation (PSN). The present understanding of the microstructural evolution during FSW of HEAs may help tailor the weld properties to pave the way for their engineering applications.
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paper_title: Microstructure and mechanical properties of friction stir welded and laser welded high entropy alloy CrMnFeCoNi

paper_content:

The high entropy alloy CrMnFeCoNi has been shown to have promising structural properties. For a new alloy to be used in a structural application it should be weldable. In the present study, friction stir welding (FSW) and laser welding (LW) techniques were used to butt weld thin plates of CrMnFeCoNi. The microstructure, chemical homogeneity and mechanical behavior of the welds were characterized and compared with the base metal. The tensile stress-strain behavior of the welded specimens were reasonable when compared with that of the base metal. FSW refined the grain size in the weld region by a factor of ∼14 when compared with the base metal. High-angle annular dark field transmission electron microscopy in combination with energy dispersive X-ray spectroscopy showed chemical inhomogeneity between dendritic and interdendritic regions in the fusion zone of LW. Large fluctuations in composition (up to 15 at%) did not change the crystal structure in the fusion zone. Hardness measurements were carried out in the weld cross section and discussed in view of the grain size, low angle grain boundaries and twin boundaries in FSW specimens and the dendritic microstructure in LW specimens.
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paper_title: Extremely high strength and work hardening ability in a metastable high entropy alloy

paper_content:

Design of multi-phase high entropy alloys uses metastability of phases to tune the strain accommodation by favoring transformation and/or twinning during deformation. Inspired by this, here we present Si containing dual phase Fe42Mn28Co10Cr15Si5 high entropy alloy (DP-5Si-HEA) exhibiting very high strength (1.15 GPa) and work hardening (WH) ability. The addition of Si in DP-5Si-HEA decreased the stability of f.c.c. (γ) matrix thereby promoting pronounced transformation induced plastic deformation in both as-cast and grain refined DP-5Si-HEAs. Higher yet sustained WH ability in fine grained DP-5Si-HEA is associated with the uniform strain partitioning among the metastable γ phase and resultant h.c.p. (ε) phase thereby resulting in total elongation of 12%. Hence, design of dual phase HEAs for improved strength and work hardenability can be attained by tuning the metastability of γ matrix through proper choice of alloy chemistry from the abundant compositional space of HEAs.
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paper_title: Friction stir welding of a CoCrFeNiAl0.3 high entropy alloy

paper_content:

Abstract A typical face-centered cubic CoCrFeNiAl 0.3 high entropy alloy was welded by friction stir welding. Sound joints without welding defects were obtained. Four typical regions, the stir zone, thermomechanically affected zone, heat affected zone and base metal could be detected. The stir zone showed a refined equiaxed microstructure due to recrystallization and it exhibited the highest hardness with grain refinement. The grain size decreased slightly with increasing the welding speed. The thermomechanically affected zone exhibited a mixed microstructure comprising coarse and fine grains due to partial recryatllization. The present result shows a potential capability for joining high entropy alloys through friction stir welding and may enhance their engineering applications.
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paper_title: Synergic strengthening by oxide and coherent precipitate dispersions in high-entropy alloy prepared by powder metallurgy

paper_content:

Abstract The present work deals with the synthesis and evaluation of the microstructural and mechanical properties of face-centred cubic Ni1.5Co1.5CrFeTi0.5 high-entropy alloy strengthened by nano-sized titanium oxide dispersion and coherent precipitates of Ni3Ti intermetallic phase produced by powder metallurgy. The alloy's properties are well tuneable by the use of proper heat treatment. The alloy exhibits an excellent combination of tensile strength and ductility, reaching an ultimate tensile strength of 1460 MPa and elongation of 14.5% in solution annealed state and 1660 MPa and 4.8% in the aged condition. Most of the strengthening contribution was attributed to Orowan-type strengthening mechanism.
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paper_title: Friction stir welding of a сarbon-doped CoCrFeNiMn high-entropy alloy

paper_content:

Abstract Butt-joint seam of a high entropy alloy (HEA) of the CoCrFeNiMn system was successfully obtained by friction stir welding (FSW). The HEA was produced by self-propagating high-temperature synthesis. Along with the principal elements, a small amount (0.9 at.%) of C was added to the alloy. The as-cast alloy was cold rolled and annealed at 900 °C to produce refined microstructure. The structure of the annealed alloy consisted of a recrystallized face-centered cubic matrix with a grain size of 9.2 μm and fine Cr-rich M23C6 carbides. FSW of the HEA resulted in microstructure refinement to d = 4.6 μm in the stir zone. A noticeable rise in strength and some decrease in ductility of the processed alloy in comparison with the initial condition can be associated with the microstructure refinement and some increase in the volume fraction of M23C6 carbides.
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paper_title: Microstructures and mechanical properties of a welded CoCrFeMnNi high-entropy alloy

paper_content:

ABSTRACTThe response of the CoCrFeMnNi high-entropy alloy to weld thermal cycles was investigated to determine its applicability as an engineering structural material. Two processes were used: high-energy-density, low-heat-input electron beam (EB) welding and low-energy-density, high-heat-input gas tungsten arc (GTA) welding. Weldability was determined through comprehensive microstructural and mechanical property characterisation of the welds. The welds did not develop solidification cracking or heat-affected zone cracks. The microstructures in weld fusion zones are similar to that in the as-cast materials, consisting of large columnar grains with dendrite. The dendrite arm spacing and the extent of elemental segregation were less in the welds than in the cast ingot, and also were less pronounced in the EB weld than in the GTA weld. Compositional microsegregation between dendritic cores and interdendritic regions of the welds was insignificant. Both welds exhibited slightly higher yield strengths than the...
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paper_title: Evolution of microstructure, hardness, and corrosion properties of high-entropy Al0.5CoCrFeNi alloy

paper_content:

Abstract In this study, we investigate the microstructure, hardness, and corrosion properties of as-cast Al 0.5 CoCrFeNi alloy as well as Al 0.5 CoCrFeNi alloys aged at temperatures of 350 °C, 500 °C, 650 °C, 800 °C, and 950 °C for 24 h. The microstructures of the various specimens are investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe X-ray microanalysis (EPMA). The results show that the microstructure of as-cast Al 0.5 CoCrFeNi comprises an FCC solid solution matrix and droplet-shaped phases (Al–Ni rich phases). At aging temperatures of between 350 and 950 °C, the alloy microstructure comprises an FCC + BCC solid solution with a matrix, droplet-shaped phases (Al–Ni rich phase), wall-shaped phases, and needle-shaped phases (Al–(Ni, Co, Cr, Fe) phase). The aging process induces a spinodal decomposition reaction which reduces the amount of the Al–Ni rich phase in the aged microstructure and increases the amount of the Al–(Ni, Co, Cr, Fe) phase. The hardness of the Al 0.5 CoCrFeNi alloy increases after aging. The optimal hardness is obtained at aging temperatures in the range 350–800 °C, and the hardening effect decreases at higher temperatures. Both the as-cast and aged specimens are considerably corroded when immersed in a 3.5% NaCl solution because of the segregation of the Al–Ni rich phase precipitate formed in the FCC matrix. Cl − ions preferentially attack the Al–Ni rich phase, which is a sensitive zone exhibiting an appreciable potential difference, with consequent galvanic action.
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paper_title: Enhanced Relative Slip Distance in Gas-Tungsten-Arc-Welded Al0.5CoCrFeNi High-Entropy Alloy

paper_content:

Gas-tungsten-arc-welded (GTAW) Al0.5CoCrFeNi high-entropy alloy (HEA) was analyzed using scanning electron microscopy (SEM), microhardness, and tensile testing. The weld metal having refined equiaxed and elongated columnar dendritic microstructure experienced 6.38 pct reduction in strength and marginally reduced hardness compared to the base metal (BM). Lower work hardening with enhanced relative slip distance, which was observed through the Kocks–Mecking plot and slip distance–true strain plots, was attributed to the reduced bcc fraction in the weld.
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paper_title: Laser beam welding of a CoCrFeNiMn-type high entropy alloy produced by self-propagating high-temperature synthesis

paper_content:

Abstract Fiber laser beam welding of a CoCrFeNiMn-type high entropy alloy (HEA) produced by self-propagating high-temperature synthesis (SHS) was reported in this work. The SHS-fabricated alloy was characterized by both ∼2 times reduced Mn content in comparison with that of the other principal components and the presence of impurities including Al, C, S, and Si. The as-fabricated alloy was composed of columnar fcc grains with coarse precipitates of MnS and fine Cr-rich M23C6 carbides. Successful defect-free butt joint of the alloy was obtained using a laser power of 2 kW and a welding speed of 5 m/min. Welding resulted in changes in texture and structure of the fcc matrix. In addition, precipitation of nanoscale B2 phase particles in the weld zone was observed. A pronounced increase in microhardness from (153 ± 3) HV 0.5 (base material) to (208 ± 6) HV 0.5 (fusion zone) was observed. The B2 phase precipitation after welding was found to be in a reasonable agreement with the ThermoCalc predictions. Quantitative analysis demonstrated that the increase in hardness can be associated with the B2 phase precipitation. Possibilities of the development of HEAs with intrinsic hardening ability after laser processing are discussed.
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paper_title: Lattice distortion in a strong and ductile refractory high-entropy alloy

paper_content:

Abstract The maximization of the mixing entropy with the optimal range of enthalpy in high-entropy alloys (HEAs) can promote the formation of a stable single solid-solution phase with the absence of competing intermetallic compounds. The resultant effects, such as lattice distortion, can contribute to excellent mechanical properties, which has motivated numerous efforts to develop and design single-phase HEAs. However, challenges still remain, particularly on quantifying the lattice distortion and relating it to materials properties. In this study, we have developed a NbTaTiV refractory HEA with a single body-centered-cubic (BCC) structure using an integrated experimental and theoretical approach. The theoretical efforts include thermodynamic modeling, i.e., CALculation of PHAse Diagram (CALPHAD). The microstructural evolutions have been investigated by systematic heat-treatment processes. The typical dendrite microstructure was observed, which is caused by the elemental segregation during the solidification in the as-cast condition. The structural inhomogeneity and chemical segregation were completely eliminated by the proper homogenization treatment at 1200 °C for 3 days. The homogeneous elemental distribution was quantitatively verified by the Atom Probe Tomography (APT) technique. Importantly, results indicate that this HEA exhibits the high yield strength and ductility at both room and high temperatures (up to 900 °C). Furthermore, the effects of the high mixing entropy on the mechanical properties are discussed and quantified in terms of lattice distortions and interatomic interactions of the NbTaTiV HEA via first-principles calculations. It is found that the local severe lattice distortions are induced, due to the atomic interactions and atomic-size mismatch in the homogenization-treated NbTaTiV refractory HEA.
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paper_title: Combinatorial high throughput synthesis of high entropy alloys

paper_content:

This PhD thesis is a part of the Accelerated Metallurgy (AccMet) project funded under the Seventh Framework Programme. AccMet’s aims consists on the delivery of an integrated pilot-scale facility for the combinatorial synthesis and testing of those unexplored material. ::: The contribution of this thesis to AccMet has been expanded in 3 years while focused in the understanding and development of a methodology suitable for the combinatorial synthesis of novel materials, and particularly of High Entropy Alloys (HEAs). These novel materials are composed of multiple elements at near equiatomic levels with the capacity of forming simple ::: crystalline phases such as bcc and fcc instead of the expected intermetallic compounds as well as their excellent combination of structural and functional properties compared to the traditional materials. ::: A mathematical technique known as Principal Component Analysis has been used here to identify patterns within a set of metallic systems forming a wide range of crystalline structures. This technique would not only speed up the compositional design stage but also contribute to the development of a virtual library containing all the explored systems. ::: Mercury Centre has been an important key during the synthesis of HEAs where Spark Plasma Sintering (SPS) and Electron Beam Melting (EBM) have been successfully applied for the development of the thesis. ::: The final combination of the design stage, production and characterisation of HEAs developed in this thesis would result in an advances technique suitable not only for the synthesis of novel HEAs, but also for the discovery of other unexplored systems.
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paper_title: Microstructures and properties of high-entropy alloys

paper_content:

Abstract This paper reviews the recent research and development of high-entropy alloys (HEAs). HEAs are loosely defined as solid solution alloys that contain more than five principal elements in equal or near equal atomic percent (at.%). The concept of high entropy introduces a new path of developing advanced materials with unique properties, which cannot be achieved by the conventional micro-alloying approach based on only one dominant element. Up to date, many HEAs with promising properties have been reported, e.g., high wear-resistant HEAs, Co 1.5 CrFeNi 1.5 Ti and Al 0.2 Co 1.5 CrFeNi 1.5 Ti alloys; high-strength body-centered-cubic (BCC) AlCoCrFeNi HEAs at room temperature, and NbMoTaV HEA at elevated temperatures. Furthermore, the general corrosion resistance of the Cu 0.5 NiAlCoCrFeSi HEA is much better than that of the conventional 304-stainless steel. This paper first reviews HEA formation in relation to thermodynamics, kinetics, and processing. Physical, magnetic, chemical, and mechanical properties are then discussed. Great details are provided on the plastic deformation, fracture, and magnetization from the perspectives of crackling noise and Barkhausen noise measurements, and the analysis of serrations on stress–strain curves at specific strain rates or testing temperatures, as well as the serrations of the magnetization hysteresis loops. The comparison between conventional and high-entropy bulk metallic glasses is analyzed from the viewpoints of eutectic composition, dense atomic packing, and entropy of mixing. Glass forming ability and plastic properties of high-entropy bulk metallic glasses are also discussed. Modeling techniques applicable to HEAs are introduced and discussed, such as ab initio molecular dynamics simulations and CALPHAD modeling. Finally, future developments and potential new research directions for HEAs are proposed.
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paper_title: Enhanced strength and ductility in a friction stir processing engineered dual phase high entropy alloy

paper_content:

The potential of high-entropy alloys (HEAs) to exhibit an extraordinary combination of properties by shifting the compositional regime from the corners towards the centers of phase diagrams has led to worldwide attention by material scientists. Here we present a strong and ductile non-equiatomic HEA obtained after friction stir processing (FSP). A transformation-induced plasticity (TRIP) assisted HEA with composition Fe50Mn30Co10Cr10 (at.%) was severely deformed by FSP and evaluated for its microstructure-mechanical property relationship. The FSP-engineered microstructure of the TRIP HEA exhibited a substantially smaller grain size, and optimized fractions of face-centered cubic (f.c.c., γ) and hexagonal close-packed (h.c.p., ε) phases, as compared to the as-homogenized reference material. This results in synergistic strengthening via TRIP, grain boundary strengthening, and effective strain partitioning between the γ and ε phases during deformation, thus leading to enhanced strength and ductility of the TRIP-assisted dual-phase HEA engineered via FSP.
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paper_title: Microstructure and corrosion behavior of the heat affected zone of a stainless steel 308L-316L weld joint

paper_content:

Abstract Microstructure of the heat affected zone (HAZ) of a 308L-316L stainless steel (SS) weld joint and its corrosion behavior in high temperature water were studied. Peak of the residual strain was observed to approach to the fusion boundary in the HAZ while the strain increased from the top to root areas of the HAZ. The root area of the HAZ shows a lower corrosion resistance in high temperature water than the top and middle areas of the HAZ. This is attributed to a higher level of residual strain in association with a higher density of tangled dislocations in the top area of the HAZ. The results suggest that the residual strain in the HAZ could also promote the SCC through its effect on corrosion, in addition to that on the local microstructure and mechanical property of the steel.
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paper_title: Mechanical properties of a new high entropy alloy with a duplex ultra-fine grained structure

paper_content:

Abstract A new approach to increase the tensile performance of high entropy alloys (HEAs) by producing a duplex ultrafine-grained (UFG) structure was reported in this work. A novel HEA based on the CoCrFeNiMn system with substantial amounts of Al and C was used for the illustration of this approach. In the as-cast condition the alloy had almost entirely a single face-centered cubic (fcc) phase structure with an insignificant amount of M23C6 carbides. After cold rolling and annealing at 800–1000 °C an increased amount of fine second phases, namely M23C6 carbides and B2 phase, effectively pinned boundaries of recrystallized fcc grains. As a result, a duplex UFG structure composed of the recrystallized fcc grains and M23C6 and B2 particles was produced. The alloy with the UFG structure demonstrated attractive mechanical properties. For example, after annealing at 900 °C the alloy had the yield strength of 785 MPa, the ultimate tensile strength of 985 MPa, and elongation to fracture of 32%. The phase composition of the alloy in different conditions was compared with the equilibrium phase diagram obtained using a Thermo-Calc software. Strengthening mechanisms were qualitatively analyzed, and some possibilities for further improvement of strength of the alloy were discussed.
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paper_title: Friction-stir welding of a ductile high entropy alloy: microstructural evolution and weld strength

paper_content:

High entropy alloys (HEAs) are a novel subset of metallic systems with complex compositions usually yielding simple phase formation. To verify their potential engineering applications, a novel Co 16 Fe 28 Ni 28 Cr 28 HEA with a low content of expensive Co was developed and its welding characteristics through friction-stir welding (FSW) were investigated. The HEA shows a stable face-centered-cubic (FCC) structure with an excellent ductility of about 70%. The microstructural evolution during FSW was dominated by discontinuous recrystallization through grain bulging and the B/ B ¯ {112} shear texture formed in the stir zone (SZ). A white band (WB) containing W-rich and Cr-rich phases was detected in the SZ. The WB exhibited refined grains compared with the normal SZ, which may be associated with the particle-stimulated nucleation (PSN). The present understanding of the microstructural evolution during FSW of HEAs may help tailor the weld properties to pave the way for their engineering applications.
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paper_title: Microstructure and mechanical properties of friction stir welded and laser welded high entropy alloy CrMnFeCoNi

paper_content:

The high entropy alloy CrMnFeCoNi has been shown to have promising structural properties. For a new alloy to be used in a structural application it should be weldable. In the present study, friction stir welding (FSW) and laser welding (LW) techniques were used to butt weld thin plates of CrMnFeCoNi. The microstructure, chemical homogeneity and mechanical behavior of the welds were characterized and compared with the base metal. The tensile stress-strain behavior of the welded specimens were reasonable when compared with that of the base metal. FSW refined the grain size in the weld region by a factor of ∼14 when compared with the base metal. High-angle annular dark field transmission electron microscopy in combination with energy dispersive X-ray spectroscopy showed chemical inhomogeneity between dendritic and interdendritic regions in the fusion zone of LW. Large fluctuations in composition (up to 15 at%) did not change the crystal structure in the fusion zone. Hardness measurements were carried out in the weld cross section and discussed in view of the grain size, low angle grain boundaries and twin boundaries in FSW specimens and the dendritic microstructure in LW specimens.
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paper_title: Tensile ductility of an AlCoCrFeNi multi-phase high-entropy alloy through hot isostatic pressing (HIP) and homogenization

paper_content:

Abstract The microstructure and phase composition of an AlCoCrFeNi high-entropy alloy (HEA) were studied in as-cast (AlCoCrFeNi-AC, AC represents as-cast) and homogenized (AlCoCrFeNi-HP, HP signifies hot isostatic pressed and homogenized) conditions. The AlCoCrFeNi-AC ally has a dendritric structure in the consisting primarily of a nano-lamellar mixture of A2 (disordered body-centered-cubic (BCC)) and B2 (ordered BCC) phases, formed by an eutectic reaction. The homogenization heat treatment, consisting of hot isostatic pressed for 1 h at 1100 °C, 207 MPa and annealing at 1150 °C for 50 h, resulted in an increase in the volume fraction of the A1 phase and formation of a Sigma (σ) phase. Tensile properties in as-cast and homogenized conditions are reported at 700 °C. The ultimate tensile strength was virtually unaffected by heat treatment, and was 396±4 MPa at 700 °C. However, homogenization produced a noticeable increase in ductility. The AlCoCrFeNi-AC alloy showed a tensile elongation of only 1.0%, while after the heat-treatment, the elongation of AlCoCrFeNi-HP was 11.7%. Thermodynamic modeling of non-equilibrium and equilibrium phase diagrams for the AlCoCrFeNi HEA gave good agreement with the experimental observations of the phase contents in the AlCoCrFeNi-AC and AlCoCrFeNi-HP. The reasons for the improvement of ductility after the heat treatment and the crack initiation subjected to tensile loading were discussed.
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paper_title: Welding Metallurgy and Weldability of Nickel-Base Alloys

paper_content:

Preface. 1. Introduction. 1.1 Ni-base Alloy Classification. 1.2 History of Nickel and Ni-base Alloys. 1.3 Corrosion Resistance. 1.4 Nickel Alloy Production. 2. Alloying Additions, Phase Diagrams, and Phase Stability. 2.1 Introduction. 2.2 General Influence of Alloying Additions. 2.3 Phase Diagrams for Solid-Solution Alloys. 2.4 Phase Diagrams for Precipitation Hardened Alloys--gamma' Formers. 2.5 Phase Diagrams for Precipitation-Hardened Alloys--gamma" Formers. 2.6 Calculated Phase Stability Diagrams. 2.7 PHACOMP Phase Stability Calculations. 3. Solid-Solution Strengthened Ni-base Alloys. 3.1 Standard Alloys and Consumables. 3.2 Physical Metallurgy and Mechanical Properties. 3.3 Welding Metallurgy. 3.4 Mechanical Properties of Weldments. 3.5 Weldability. 3.6 Corrosion Resistance. 3.7 Case Studies. 4. Precipitation Strengthened Ni-base Alloys. 4.1 Standard Alloys and Consumables. 4.2 Physical Metallurgy and Mechanical Properties. 4.3 Welding Metallurgy. 4.4 Mechanical Properties of Weldments. 4.5 Weldability. 5. Oxide Dispersion Strengthened Alloys and Nickel Aluminides. 5.1 Oxide Dispersion Strengthened Alloys. 5.2 Nickel Aluminide Alloys. 6. Repair Welding of Ni-base Alloys. 6.1 Solid-Solution Strengthened Alloys. 6.2 Precipitation Strengthened Alloys. 6.3 Single Crystal Superalloys. 7. Dissimilar Welding. 7.1 Application of Dissimilar Welds. 7.2 Influence of Process Parameters on Fusion Zone Composition. 7.3 Carbon, Low Alloys and Stainless Steels. 7.4 Postweld Heat Treatment Cracking in Stainless Steels Welded with Ni-base Filler Metals. 7.5 Super Austenitic Stainless Steels. 7.6 Dissimilar Welds in Ni-base Alloys - Effect on Corrosion Resistance. 7.7 9%Ni Steels. 7.8 Super Duplex Stainless Steels. 7.9 Case Studies. 8. Weldability Testing. 8.1 Introduction. 8.2 The Varestraint Test. 8.3 Modified Cast Pin Tear Test. 8.4 The Sigmajig Test. 8.5 The Hot Ductility Test. 8.6 The Strain-to-Fracture Test. 8.7 Other Weldability Tests. Appendix A Composition of Wrought and Cast Nickel-Base Alloys. Appendix B Composition of Nickel and Nickel Alloy Consumables. Appendix C Corrosion Acceptance Testing Methods. Appendix D Etching Techniques for Ni-base Alloys and Welds. Author Index. Subject Index.
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paper_title: Characterization of Laser Beam Welded Al0.5CoCrFeNi High-Entropy Alloy

paper_content:

High-entropy alloys (HEA), a new generation alloy system offer superior mechanical properties with solid solution strengthening. AlxCoCrFeNi-HEA is one such system being received more attention because of its specific yield strength and ductility. In the present work, Al0.5CoCrFeNi-HEA was prepared by vacuum arc melting. The laser beam welding (LBW) was carried out on 1mm thick forged and homogenized HEA, with a beam power of 1.5 kW and at a traverse speed of 600 mm/min. The microstructural features of different regions of the weld were studied using scanning electron microscopy. The homogenized Al0.5CoCrFeNi-HEA have shown equiaxed grains of average size 60 μm. The weld metal showed a typical weld fusion zone microstructure with dendritic structure with a reduction in BCC phase due to minimal Al and Ni segregation ratio at interdendrites. Micro-chemical analysis with energy dispersive spectroscopy confirmed that there was no major segregation of elements in the weld fusion zone. The microhardness survey performed across the weld evidenced a reduction in hardness, as a consequence of significant reduction in Al-Ni rich hardening factor.
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paper_title: Size effect, critical resolved shear stress, stacking fault energy, and solid solution strengthening in the CrMnFeCoNi high-entropy alloy

paper_content:

High-entropy alloys (HEAs) comprise a novel class of scientifically and technologically interesting materials. Among these, equatomic CrMnFeCoNi with the face-centered cubic (FCC) structure is noteworthy because its ductility and strength increase with decreasing temperature while maintaining outstanding fracture toughness at cryogenic temperatures. Here we report for the first time by single-crystal micropillar compression that its bulk room temperature critical resolved shear stress (CRSS) is ~33-43 MPa, ~10 times higher than that of pure nickel. CRSS depends on pillar size with an inverse power-law scaling exponent of -0.63 independent of orientation. Planar ½ < 110 > {111} dislocations dissociate into Shockley partials whose separations range from ~3.5-4.5 nm near the screw orientation to ~5-8 nm near the edge, yielding a stacking fault energy of 30 ± 5 mJ/m2. Dislocations are smoothly curved without any preferred line orientation indicating no significant anisotropy in mobilities of edge and screw segments. The shear-modulus-normalized CRSS of the HEA is not exceptionally high compared to those of certain concentrated binary FCC solid solutions. Its rough magnitude calculated using the Fleischer/Labusch models corresponds to that of a hypothetical binary with the elastic constants of our HEA, solute concentrations of 20-50 at.%, and atomic size misfit of ~4%.
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paper_title: A perspective on residual stresses in welding

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AbstractWelding induced residual stress occurs due to non-uniform simultaneous heating and cooling, local variation in shrinkage because of variable cooling rates in different regions of the weld, and strains associated with metallurgical phase transformations. The residual stress in a welded joint can augment the externally applied load and cause structural failure. Prediction and mitigation of residual stresses still remain important issues in welding. The purpose of this special issue of Science and Technology of Welding and Joining is to present recent research on calculation, measurement and alleviation of residual stresses in welding.
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paper_title: Numerical study of strengths of spot-welded joints of steel

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Abstract A resistance spot-welding (RSW) joint consists of several material zones with different microstructure and properties as a result of the thermal, metallurgical and mechanical deformation process. Detailed material properties are essential to accurately simulate the mechanical behavior of a joint and its dependency on some key structural parameters (e.g. nugget size, sheet thickness etc.). The work presented in this paper utilises an inverse modelling methodology combining numerical modelling and indentation tests with a standard hardness test to characterise the detailed properties of different weld zones of spot-welded joints. The yield and strain hardening parameters of the three zones (nugget zone, HAZ: heat-affected zone and base zone) were determined and the predicted stress–strain curves for base zone were compared with standard tensile tests results. A 3-D finite element model based on the predicted constitutive material laws for different zones coupled with a fracture model was developed to predict the deformation of spot-welded joints beyond the onset of initial yield under tensile-shear loading. The deformation mode and force–displacement result showed good agreements with experimental data. The effect of nugget size and sheet thickness on the tensile-shear strength of welded joints was further systematically studied using a high performance computing system.
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paper_title: Microstructure and mechanical properties of non-equilibrium solidified CoCrFeNi high entropy alloy

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Abstract Supercooled CoCrFeNi high entropy alloy (HEA) with undercooling up to 300 K and mass above 100 g are fabricated using glass fluxing method. Evolution of non-equilibrium solidification microstructure with different undercoolings is described in detail by SEM, XRD and TEM. Results show that with the increasing undercooling, a typical grain refinement effect and existence of BCC structured Cr-rich phase are observed. Based on the bulk mass supercooled specimens, micro-hardness and compressive measurement at room temperature are carried out to investigate the influence of supercooling on the CoCrFeNi HEA. A strong enhancement of hardness as well as strength for the non-equilibrium solidified samples is evidenced. More importantly, even small undercooling (50 K in this paper) can enhance CoCrFeNi HEAs’ mechanical properties remarkably. The present study shows that using supercooling technique to make non-equilibrium solidified sample is an efficient method to improve the mechanical properties of HEAs.
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paper_title: Failure of super 13Cr stainless steel due to excessive hardness in the welded joint

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Abstract Supermartensitic stainless steels (SMSS) with 11–13%Cr were developed to show higher corrosion resistance, toughness and weldability than conventional martensitic stainless steels (MSS). The main compositional characteristics of SMSS's, in a comparison to common MSS's, are the extra low C content, Ni and Mo addition. This new sub-family of stainless steels has been used in the oil and gas offshore production, as tubulars and casings in the wheel. Despite of the extra low C (
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paper_title: Friction stir welding of a CoCrFeNiAl0.3 high entropy alloy

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Abstract A typical face-centered cubic CoCrFeNiAl 0.3 high entropy alloy was welded by friction stir welding. Sound joints without welding defects were obtained. Four typical regions, the stir zone, thermomechanically affected zone, heat affected zone and base metal could be detected. The stir zone showed a refined equiaxed microstructure due to recrystallization and it exhibited the highest hardness with grain refinement. The grain size decreased slightly with increasing the welding speed. The thermomechanically affected zone exhibited a mixed microstructure comprising coarse and fine grains due to partial recryatllization. The present result shows a potential capability for joining high entropy alloys through friction stir welding and may enhance their engineering applications.
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paper_title: Grain size dependence of strain rate sensitivity in a single phase FCC high entropy alloy Al0.3CoCrFeNi

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Abstract High-entropy alloys (HEAs) can offer exceptional strain-rate sensitivity (SRS) due to high lattice-friction and strong solid-solution strengthening, and in some cases, low stacking fault energy. In this study, SRS of a single phase FCC HEA- Al0.3CoCrFeNi was investigated at two grain sizes. Due to its outstanding Hall-Petch coefficient, grain size becomes a crucial microstructural feature in determination of strength and SRS. Change in SRS due to grain refinement was derived to be inversely proportional to associated strength gain, in coarse microstructures where grain size >> dislocation forest cell size. This correlation was proven using two microstructures with grain sizes of 12 µm and 150 µm, with yield strengths of 313 MPa and 145 MPa, and demonstrated SRS of m = 0.029 and 0.064, respectively. SRS was also derived to increase linearly with strength contribution from thermal short-range obstacles. The slope reflects the maximum upper limit on SRS possible upon elimination of all other obstacles of long-range nature. This limiting value of SRS was derived to be 0.118 for this HEA.
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paper_title: Comparative study of AISI 304L to AISI 316L stainless steels joints by TIG and Nd:YAG laser welding

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Abstract In the present study, investigation and comparison laser and tungsten inert gas (TIG) welding techniques for dissimilar butt joint of 304L and 316L stainless steel sheets was carried out. The effects of voltage and laser beam spot size during laser welding and effects of voltage, traveling speed and presence of backing gas during TIG welding on yield strength, elongation, hardness and weld width were investigated. Moreover, in order to examine the mechanical and metallurgical behavior of the weld, microstructure analysis was used. Results showed that welding depth of TIG welding was higher than laser method. It was observed that strength and toughness of samples decreased by increasing voltage in TIG and laser welding. Additionally, by increasing beam spot size in laser welding, strength, toughness and hardness increased. Also, the presence of backing gas during TIG welding resulted in the increase of strength, toughness and hardness of the samples.
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paper_title: Thermal stability and grain boundary strengthening in ultrafine-grained CoCrFeNi high entropy alloy composite

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Abstract Thermal stability of CoCrFeNi high entropy alloy in as-milled and sintered conditions was investigated using X-ray diffraction, differential scanning calorimetry, transmission electron microscopy, and atom probe tomography. Composite microstructure consists of FCC and carbide with a fine dispersion of oxide was observed in the sintered condition. Unsolicited contamination of carbon and oxygen in the as-milled powder due to the milling medium had led to the formation of composite microstructure. An exceptional thermal stability was observed upon exposure of sintered compact to higher temperatures (0.56 T m to 0.68 T m ) for the prolonged duration of 600 h. Sintered compact exposed to 700 °C (0.56 T m ) for 600 h showed negligible change in hardness and grain size. Analysis based on the modified Hall-Petch model for two phase alloy indicates the phase boundaries act as a strong obstacle while the major contribution to strengthening comes from grain boundaries.
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paper_title: Microstructure and mechanical properties of Al0.7CoCrFeNi high-entropy-alloy prepared by directional solidification

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Abstract The high-entropy-alloy Al0.7CoCrFeNi (molar ratio) was prepared by vacuum arc melting followed by directional solidification (DS) with oriented seed. The unique lamellar-dendrite microstructure was obtained over a wide cooling rate range. During solidification, Fe and Co are prone to segregate to the dendrite, while Cr and Al segregate to interdendrite. The solute pile-up of Cr and Al at the solid/liquid interface leads to the dendritic solidification. During the following cooling process, the BCC phase precipitates from the FCC dendrite to form the lamellar structure, while the ordered B2 phase precipitates from the interdendrite. Moreover, the lamellar spacing is significantly refined with increasing cooling rate, resulting in the higher hardness and compressive yield strength. Directional solidification is proved to be an efficient way to improve the mechanical properties of multi-phases high-entropy alloys.
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paper_title: Synergic strengthening by oxide and coherent precipitate dispersions in high-entropy alloy prepared by powder metallurgy

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Abstract The present work deals with the synthesis and evaluation of the microstructural and mechanical properties of face-centred cubic Ni1.5Co1.5CrFeTi0.5 high-entropy alloy strengthened by nano-sized titanium oxide dispersion and coherent precipitates of Ni3Ti intermetallic phase produced by powder metallurgy. The alloy's properties are well tuneable by the use of proper heat treatment. The alloy exhibits an excellent combination of tensile strength and ductility, reaching an ultimate tensile strength of 1460 MPa and elongation of 14.5% in solution annealed state and 1660 MPa and 4.8% in the aged condition. Most of the strengthening contribution was attributed to Orowan-type strengthening mechanism.
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paper_title: Enhanced strength and ductility of bulk CoCrFeMnNi high entropy alloy having fully recrystallized ultrafine-grained structure

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Abstract A high efficient magnetic levitation melting technique was reported for fabricating bulk equiatomic CoCrFeMnNi high-entropy alloy (HEA) ingot with a diameter of 110 mm. The bulk ingot can be either forged or rolled. In particular, fully recrystallized ultrafine-grained (UFG) HEA with a minimum grain size of 503 ± 181 nm was successfully obtained through a simple cold rolling and annealing process. The tensile properties of the HEA were studied by changing the grain size from the UFG regime to the coarse-grained regime. The UFG HEA exhibited a good balance of strength and ductility due to the low stacking fault energy. The linear Hall-Petch relationship was well fitted when the grain sizes range from 503 nm to 88.9 μm.
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paper_title: Friction stir welding of a сarbon-doped CoCrFeNiMn high-entropy alloy

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Abstract Butt-joint seam of a high entropy alloy (HEA) of the CoCrFeNiMn system was successfully obtained by friction stir welding (FSW). The HEA was produced by self-propagating high-temperature synthesis. Along with the principal elements, a small amount (0.9 at.%) of C was added to the alloy. The as-cast alloy was cold rolled and annealed at 900 °C to produce refined microstructure. The structure of the annealed alloy consisted of a recrystallized face-centered cubic matrix with a grain size of 9.2 μm and fine Cr-rich M23C6 carbides. FSW of the HEA resulted in microstructure refinement to d = 4.6 μm in the stir zone. A noticeable rise in strength and some decrease in ductility of the processed alloy in comparison with the initial condition can be associated with the microstructure refinement and some increase in the volume fraction of M23C6 carbides.
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paper_title: Microstructures and mechanical properties of a welded CoCrFeMnNi high-entropy alloy

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ABSTRACTThe response of the CoCrFeMnNi high-entropy alloy to weld thermal cycles was investigated to determine its applicability as an engineering structural material. Two processes were used: high-energy-density, low-heat-input electron beam (EB) welding and low-energy-density, high-heat-input gas tungsten arc (GTA) welding. Weldability was determined through comprehensive microstructural and mechanical property characterisation of the welds. The welds did not develop solidification cracking or heat-affected zone cracks. The microstructures in weld fusion zones are similar to that in the as-cast materials, consisting of large columnar grains with dendrite. The dendrite arm spacing and the extent of elemental segregation were less in the welds than in the cast ingot, and also were less pronounced in the EB weld than in the GTA weld. Compositional microsegregation between dendritic cores and interdendritic regions of the welds was insignificant. Both welds exhibited slightly higher yield strengths than the...
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paper_title: Development of in situ observation technique using scanning ion microscopy and demonstration of Mn depletion effect on intragranular ferrite transformation in low-alloy steel

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An in situ observation technique using scanning ion microscopy (SIM) to investigate the behaviour of phase transformation from austenite to ferrite in low-alloy steel is proposed. The most important advantage of the technique is the combined observation by SIM and fabrication by focused ion beam (FIB) at a wide range of temperature, from room temperature to 1673 K. Using this advantage, we have developed a new technique for observing intragranular ferrite (IGF) transformation in low-alloy steel, which is the phase transformation occurring at an inclusion existing inside a grain of austenite phase. We successfully observed the IGF transformation at 873 K by examining an inclusion exposed to the specimen surface by FIB fabrication at 1073 K during cooling from 1673 K. From the results of the in situ observation, we determined the role of Mn depletion due to precipitation of MnS for IGF transformation in steel.
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paper_title: Recovery, recrystallization, grain growth and phase stability of a family of FCC-structured multi-component equiatomic solid solution alloys

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Abstract The equiatomic high-entropy alloy FeNiCoCrMn is known to crystallize as a single phase with the face-centered cubic (FCC) crystal structure. To better understand this quinary solid solution alloy, we investigate various binary, ternary and quaternary alloys made from its constituent elements. Our goals are twofold: ( i ) to investigate which of these lower order systems also form solid solution alloys consisting of a single FCC phase, and ( ii ) to characterize their phase stability and recovery, recrystallization, and grain growth behaviors. X-ray diffraction (XRD) and scanning electron microscopy with backscattered electron images showed that three of the five possible quaternaries (FeNiCoCr, FeNiCoMn and NiCoCrMn), five of the ten possible ternaries (FeNiCo, FeNiCr, FeNiMn, NiCoCr, and NiCoMn), and two of the ten possible binaries (FeNi and NiCo) were single-phase FCC solid solutions in the cast and homogenized condition, whereas the others either had different crystal structures or were multi-phase. The single-phase FCC quaternary, FeNiCoCr, along with its equiatomic ternary and binary subsidiaries, were selected for further investigations of phase stability and the thermomechanical processing needed to obtain equiaxed grain structures. Only four of these subsidiary alloys—two binaries (FeNi and NiCo) and two ternaries (FeNiCo and NiCoCr)—were found to be single-phase FCC after rolling at room temperature followed by annealing for 1 h at temperatures of 300–1100 °C. Pure Ni, which is FCC and one of the constituents of the quinary high-entropy alloy (FeNiCoCrMn), was also investigated for comparison with the higher order alloys. Among the materials investigated after thermomechanical processing (FeNiCoCr, FeNiCo, NiCoCr, FeNi, NiCo, and Ni), FeNiCo and Ni showed abnormal grain growth at relatively low annealing temperatures, while the other four showed normal grain growth behavior. The grain growth exponents for all five of the equiatomic alloys were found to be ∼0.25 (compared to ∼0.5 for unalloyed Ni), suggesting that solute drag may control grain growth in the alloys. For all five alloys, as well as for pure Ni, microhardness increases as the grain size decreases in a Hall-Petch type way. The ternary alloy NiCoCr was the hardest of the alloys investigated in this study, even when compared to the quaternary FeNiCoCr alloy. This suggests that solute hardening in equiatomic alloys depends not just on the number of alloying elements but also their type.
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paper_title: Gleeble simulation of the HAZ in Inconel 617 welding

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Abstract Five kinds of thermal cycle, with different peak temperatures ranging between 1150 °C and 1350 °C were taken to simulate HAZs with same heating and cooling rate. The microstructure of different simulated HAZs was characterized by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The micro-hardness, grain size and tensile test at 750 °C of simulated HAZs were conducted. Coarsened carbides along the grain boundaries are observed in the HAZs simulated at 1200 °C and 1250 °C. For the HAZs simulated at 1300 °C and 1350 °C, the carbides inside grains vanish, and continuous lamellar structure occurs at the grain boundaries. The microstructure of HAZ simulated at 1150 °C is similar to that of base metal (BM). The micro-hardness and average grain size of simulated HAZs have no obvious variation compared to the BM. Tensile test results show that the yield stress (YS) and ultimate tensile stress (UTS) decrease as simulation peak temperature increases, which attributes to the coarsened carbides and continuous lamellar structure occurring at the grain boundaries at higher temperature.
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paper_title: Gradient hierarchical grain structures of Al0.1CoCrFeNi high-entropy alloys through dynamic torsion

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Abstract Al0.1CoCrFeNi high-entropy alloys are firstly processed by cyclic dynamic torsion (DT) using Split-Hopkinson Torsional Bar. Gradient hierarchical microstructures produced by DT are investigated using electron backscattered diffraction, transmission electron microscopy, and nanoindentation tests. Based on the gradient torsional strain, significant grain refinement, remarkable work hardening, and hierarchical deformation substructures are introduced into the current DT-HEAs.
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paper_title: Evolution of microstructure, hardness, and corrosion properties of high-entropy Al0.5CoCrFeNi alloy

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Abstract In this study, we investigate the microstructure, hardness, and corrosion properties of as-cast Al 0.5 CoCrFeNi alloy as well as Al 0.5 CoCrFeNi alloys aged at temperatures of 350 °C, 500 °C, 650 °C, 800 °C, and 950 °C for 24 h. The microstructures of the various specimens are investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe X-ray microanalysis (EPMA). The results show that the microstructure of as-cast Al 0.5 CoCrFeNi comprises an FCC solid solution matrix and droplet-shaped phases (Al–Ni rich phases). At aging temperatures of between 350 and 950 °C, the alloy microstructure comprises an FCC + BCC solid solution with a matrix, droplet-shaped phases (Al–Ni rich phase), wall-shaped phases, and needle-shaped phases (Al–(Ni, Co, Cr, Fe) phase). The aging process induces a spinodal decomposition reaction which reduces the amount of the Al–Ni rich phase in the aged microstructure and increases the amount of the Al–(Ni, Co, Cr, Fe) phase. The hardness of the Al 0.5 CoCrFeNi alloy increases after aging. The optimal hardness is obtained at aging temperatures in the range 350–800 °C, and the hardening effect decreases at higher temperatures. Both the as-cast and aged specimens are considerably corroded when immersed in a 3.5% NaCl solution because of the segregation of the Al–Ni rich phase precipitate formed in the FCC matrix. Cl − ions preferentially attack the Al–Ni rich phase, which is a sensitive zone exhibiting an appreciable potential difference, with consequent galvanic action.
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paper_title: Enhanced Relative Slip Distance in Gas-Tungsten-Arc-Welded Al0.5CoCrFeNi High-Entropy Alloy

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Gas-tungsten-arc-welded (GTAW) Al0.5CoCrFeNi high-entropy alloy (HEA) was analyzed using scanning electron microscopy (SEM), microhardness, and tensile testing. The weld metal having refined equiaxed and elongated columnar dendritic microstructure experienced 6.38 pct reduction in strength and marginally reduced hardness compared to the base metal (BM). Lower work hardening with enhanced relative slip distance, which was observed through the Kocks–Mecking plot and slip distance–true strain plots, was attributed to the reduced bcc fraction in the weld.
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paper_title: A critical review of high entropy alloys and related concepts

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Abstract : High entropy alloys (HEAs) are barely 12 years old. The field has stimulated new ideas and has inspired the exploration of the vast composition space offered by multi-principal element alloys (MPEAs). Here we present a critical review of this field, with the intent of summarizing key findings, uncovering major trends and providing guidance for future efforts. Major themes in this assessment include definition of terms; thermodynamic analysis of complex, concentrated alloys (CCAs); taxonomy of current alloy families; microstructures; mechanical properties; potential applications; and future efforts. Based on detailed analyses, the following major results emerge. Although classical thermodynamic concepts are unchanged, trends in MPEAs can be different than in simpler alloys. Common thermodynamic perceptions can be misleading and new trends are described. From a strong focus on 3d transition metal alloys, there are now seven distinct CCA families. A new theme of designing alloy families by selecting elements to achieve a specific, intended purpose is starting to emerge. A comprehensive microstructural assessment is performed using three datasets: experimental data drawn from 408 different alloys and two computational datasets generated using the CALculated PHAse Diagram (CALPHAD) method. Each dataset emphasizes different elements and shows different microstructural trends. Trends in these three datasets are all predicted by a structure in - structure out (SISO) analysis developed here that uses the weighted fractions of the constituent element crystal structures in each dataset. A total of 13 distinct multi-principal element single-phase fields are found in this microstructural assessment. Relationships between composition, microstructure and properties are established for 3d transition metal MPEAs, including the roles of Al, Cr and Cu. Critical evaluation shows that commercial austenitic stainless steels and nickel alloys with 3 or more principal elements are MPEAs.
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paper_title: The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy

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Abstract An equiatomic CoCrFeMnNi high-entropy alloy, which crystallizes in the face-centered cubic (fcc) crystal structure, was produced by arc melting and drop casting. The drop-cast ingots were homogenized, cold rolled and recrystallized to obtain single-phase microstructures with three different grain sizes in the range 4–160 μm. Quasi-static tensile tests at an engineering strain rate of 10 −3 s −1 were then performed at temperatures between 77 and 1073 K. Yield strength, ultimate tensile strength and elongation to fracture all increased with decreasing temperature. During the initial stages of plasticity (up to ∼2% strain), deformation occurs by planar dislocation glide on the normal fcc slip system, {1 1 1}〈1 1 0〉, at all the temperatures and grain sizes investigated. Undissociated 1/2〈1 1 0〉 dislocations were observed, as were numerous stacking faults, which imply the dissociation of several of these dislocations into 1/6〈1 1 2〉 Shockley partials. At later stages (∼20% strain), nanoscale deformation twins were observed after interrupted tests at 77 K, but not in specimens tested at room temperature, where plasticity occurred exclusively by the aforementioned dislocations which organized into cells. Deformation twinning, by continually introducing new interfaces and decreasing the mean free path of dislocations during tensile testing (“dynamic Hall–Petch”), produces a high degree of work hardening and a significant increase in the ultimate tensile strength. This increased work hardening prevents the early onset of necking instability and is a reason for the enhanced ductility observed at 77 K. A second reason is that twinning can provide an additional deformation mode to accommodate plasticity. However, twinning cannot explain the increase in yield strength with decreasing temperature in our high-entropy alloy since it was not observed in the early stages of plastic deformation. Since strong temperature dependencies of yield strength are also seen in binary fcc solid solution alloys, it may be an inherent solute effect, which needs further study.
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paper_title: Laser beam welding of a CoCrFeNiMn-type high entropy alloy produced by self-propagating high-temperature synthesis

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Abstract Fiber laser beam welding of a CoCrFeNiMn-type high entropy alloy (HEA) produced by self-propagating high-temperature synthesis (SHS) was reported in this work. The SHS-fabricated alloy was characterized by both ∼2 times reduced Mn content in comparison with that of the other principal components and the presence of impurities including Al, C, S, and Si. The as-fabricated alloy was composed of columnar fcc grains with coarse precipitates of MnS and fine Cr-rich M23C6 carbides. Successful defect-free butt joint of the alloy was obtained using a laser power of 2 kW and a welding speed of 5 m/min. Welding resulted in changes in texture and structure of the fcc matrix. In addition, precipitation of nanoscale B2 phase particles in the weld zone was observed. A pronounced increase in microhardness from (153 ± 3) HV 0.5 (base material) to (208 ± 6) HV 0.5 (fusion zone) was observed. The B2 phase precipitation after welding was found to be in a reasonable agreement with the ThermoCalc predictions. Quantitative analysis demonstrated that the increase in hardness can be associated with the B2 phase precipitation. Possibilities of the development of HEAs with intrinsic hardening ability after laser processing are discussed.
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paper_title: Weldability of a high entropy CrMnFeCoNi alloy

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Abstract High-entropy alloys are unique alloys in which five or more elements are presented all in high concentrations. To determine its potential as a structural alloy, a model face-centered-cubic CrMnFeCoNi alloy was selected to investigate its weldability. Welds produced by electron beam welding show no cracking. The grain structures within the fusion zone (FZ) are controlled by the solidification behavior of the weld pool. The weldment possesses mechanical properties comparable to those of the base metal (BM) at both room and cryogenic temperatures. Compared with the BM, deformation twinning was more pronounced in the FZ of the tested alloy.
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paper_title: Strong grain-size effect on deformation twinning of an Al0.1CoCrFeNi high-entropy alloy

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ABSTRACTAn Al0.1CoCrFeNi high-entropy alloy (HEA) with a face-centered cubic structure in the as-cast and the recrystallized states is investigated. The mean grain size of the as-cast HEA is 14 times larger than that of the recrystallized HEA. The tension tests reveal that deformation twinning is the main mechanism dominating the plastic deformation of two HEAs. With a decrease in the grain size, twin spacing increases, and twin thickness decreases, which results in a low twinning activity. The twinning activity of the recrystallized HEA is strongly inhibited by grain refinement, which degrades the promotion of twinning on the strain-hardening ability and the tensile ductility.
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paper_title: Computational materials design of a corrosion resistant high entropy alloy for harsh environments

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The integrated computational materials engineering approach is inherently well suited to explore the vast, multi-dimensional high entropy alloy (HEA) compositional and processing space, and has been adopted in this work, coupled with empiricism, to the design of highly corrosion resistant HEAs. Using the combination of empirical and computational approaches, three non-equimolar HEA compositions were identified for their predicted ability to form a single-phase structure and to exhibit high corrosion resistance. One of them, Ni 38 Cr 21 Fe 20 Ru 13 Mo 6 W 2 , was successfully synthesized on the lab-scale and homogenized at 1250 °C for 120 h. Exceedingly high corrosion resistance of the Ni-rich HEA was demonstrated in electrochemical testing.
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paper_title: Optimizing the coupled effects of Hall-Petch and precipitation strengthening in a Al0.3CoCrFeNi high entropy alloy

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Abstract A successful demonstration of applying integrated strengthening using Hall-Petch strengthening (grains size effect) and precipitation strengthening is shown in the fcc based high entropy alloy (HEA) Al 0.3 CoCrFeNi, leading to quantitative determinations of the Hall-Petch coefficients for both hardness and tensile yield strength, as well as the enhancements in the yield strength from two distinct types of ordered precipitates, L1 2 and B2. An excellent combination of yield strength (~ 490 MPa), ultimate tensile strength (~ 850 MPa), and ductility (~ 45% elongation) was achieved by optimizing and coupling both strengthening mechanisms, resulting from a refined grain size as well as both L1 2 and B2 ordered precipitates. This opens up new avenues for the future development of HEAs, with the appropriate balance of properties required for engineering applications.
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paper_title: Corrosion characteristics of high entropy alloys

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AbstractThe present paper reports on the electrochemical properties of a wide range of high entropy alloys (HEAs) in a 0.6 M NaCl solution. A consolidated treatise of the topic has to date been lacking, and the purpose of the work herein is to present a primitive galvanic series for numerous HEAs, along with a broad survey of results typifying their electrochemical characteristics, passivity and comparative electrochemistry. The results are coupled with microstructural characterisation. The range of potentials for HEAs is comparable to or nobler than austenitic stainless steel, with a number of HEAs displaying higher pitting potential (Epit) values than stainless steels, in spite of possessing heterogeneous microstructures.
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paper_title: Tensile deformation behavior and deformation twinning of an equimolar CoCrFeMnNi high-entropy alloy

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Abstract The tensile deformation and strain hardening behaviors of an equimolar CoCrFeMnNi high-entropy alloy (HEA) were investigated and compared with low and medium entropy equiatomic alloys (LEA and MEA). The HEA had a lower yield strength than the MEA because the addition of Mn weakens solid solution hardening in the HEA. However, deformation twinning induced the multiple stage strain hardening behavior of the HEA and enhanced strength and elongation. Using tensile-interrupted electron backscatter diffraction analysis, geometrically necessary dislocations were observed as plume-shaped features in grain interior, and a considerable texture was characterized, which is typical of face centered cubic metals. Moreover, the relationship between favorably oriented grains and twinning in the HEA bore a clear resemblance to the same tendency in TWIP steels. The thickness of the twin bundles was less than 100 nm. A high density of stacking defects was found in the nanotwins. Nano twinning and stacking faults were found to contribute to the remarkable mechanical properties. Deformation induced twinning not only demonstrated the dynamic Hall-Petch effect but also changed dislocation cell substructures into microband structures.
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paper_title: Friction-stir welding of a ductile high entropy alloy: microstructural evolution and weld strength

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High entropy alloys (HEAs) are a novel subset of metallic systems with complex compositions usually yielding simple phase formation. To verify their potential engineering applications, a novel Co 16 Fe 28 Ni 28 Cr 28 HEA with a low content of expensive Co was developed and its welding characteristics through friction-stir welding (FSW) were investigated. The HEA shows a stable face-centered-cubic (FCC) structure with an excellent ductility of about 70%. The microstructural evolution during FSW was dominated by discontinuous recrystallization through grain bulging and the B/ B ¯ {112} shear texture formed in the stir zone (SZ). A white band (WB) containing W-rich and Cr-rich phases was detected in the SZ. The WB exhibited refined grains compared with the normal SZ, which may be associated with the particle-stimulated nucleation (PSN). The present understanding of the microstructural evolution during FSW of HEAs may help tailor the weld properties to pave the way for their engineering applications.
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paper_title: Tensile ductility of an AlCoCrFeNi multi-phase high-entropy alloy through hot isostatic pressing (HIP) and homogenization

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Abstract The microstructure and phase composition of an AlCoCrFeNi high-entropy alloy (HEA) were studied in as-cast (AlCoCrFeNi-AC, AC represents as-cast) and homogenized (AlCoCrFeNi-HP, HP signifies hot isostatic pressed and homogenized) conditions. The AlCoCrFeNi-AC ally has a dendritric structure in the consisting primarily of a nano-lamellar mixture of A2 (disordered body-centered-cubic (BCC)) and B2 (ordered BCC) phases, formed by an eutectic reaction. The homogenization heat treatment, consisting of hot isostatic pressed for 1 h at 1100 °C, 207 MPa and annealing at 1150 °C for 50 h, resulted in an increase in the volume fraction of the A1 phase and formation of a Sigma (σ) phase. Tensile properties in as-cast and homogenized conditions are reported at 700 °C. The ultimate tensile strength was virtually unaffected by heat treatment, and was 396±4 MPa at 700 °C. However, homogenization produced a noticeable increase in ductility. The AlCoCrFeNi-AC alloy showed a tensile elongation of only 1.0%, while after the heat-treatment, the elongation of AlCoCrFeNi-HP was 11.7%. Thermodynamic modeling of non-equilibrium and equilibrium phase diagrams for the AlCoCrFeNi HEA gave good agreement with the experimental observations of the phase contents in the AlCoCrFeNi-AC and AlCoCrFeNi-HP. The reasons for the improvement of ductility after the heat treatment and the crack initiation subjected to tensile loading were discussed.
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paper_title: Welding Metallurgy and Weldability of Nickel-Base Alloys

paper_content:

Preface. 1. Introduction. 1.1 Ni-base Alloy Classification. 1.2 History of Nickel and Ni-base Alloys. 1.3 Corrosion Resistance. 1.4 Nickel Alloy Production. 2. Alloying Additions, Phase Diagrams, and Phase Stability. 2.1 Introduction. 2.2 General Influence of Alloying Additions. 2.3 Phase Diagrams for Solid-Solution Alloys. 2.4 Phase Diagrams for Precipitation Hardened Alloys--gamma' Formers. 2.5 Phase Diagrams for Precipitation-Hardened Alloys--gamma" Formers. 2.6 Calculated Phase Stability Diagrams. 2.7 PHACOMP Phase Stability Calculations. 3. Solid-Solution Strengthened Ni-base Alloys. 3.1 Standard Alloys and Consumables. 3.2 Physical Metallurgy and Mechanical Properties. 3.3 Welding Metallurgy. 3.4 Mechanical Properties of Weldments. 3.5 Weldability. 3.6 Corrosion Resistance. 3.7 Case Studies. 4. Precipitation Strengthened Ni-base Alloys. 4.1 Standard Alloys and Consumables. 4.2 Physical Metallurgy and Mechanical Properties. 4.3 Welding Metallurgy. 4.4 Mechanical Properties of Weldments. 4.5 Weldability. 5. Oxide Dispersion Strengthened Alloys and Nickel Aluminides. 5.1 Oxide Dispersion Strengthened Alloys. 5.2 Nickel Aluminide Alloys. 6. Repair Welding of Ni-base Alloys. 6.1 Solid-Solution Strengthened Alloys. 6.2 Precipitation Strengthened Alloys. 6.3 Single Crystal Superalloys. 7. Dissimilar Welding. 7.1 Application of Dissimilar Welds. 7.2 Influence of Process Parameters on Fusion Zone Composition. 7.3 Carbon, Low Alloys and Stainless Steels. 7.4 Postweld Heat Treatment Cracking in Stainless Steels Welded with Ni-base Filler Metals. 7.5 Super Austenitic Stainless Steels. 7.6 Dissimilar Welds in Ni-base Alloys - Effect on Corrosion Resistance. 7.7 9%Ni Steels. 7.8 Super Duplex Stainless Steels. 7.9 Case Studies. 8. Weldability Testing. 8.1 Introduction. 8.2 The Varestraint Test. 8.3 Modified Cast Pin Tear Test. 8.4 The Sigmajig Test. 8.5 The Hot Ductility Test. 8.6 The Strain-to-Fracture Test. 8.7 Other Weldability Tests. Appendix A Composition of Wrought and Cast Nickel-Base Alloys. Appendix B Composition of Nickel and Nickel Alloy Consumables. Appendix C Corrosion Acceptance Testing Methods. Appendix D Etching Techniques for Ni-base Alloys and Welds. Author Index. Subject Index.
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paper_title: Characterization of Laser Beam Welded Al0.5CoCrFeNi High-Entropy Alloy

paper_content:

High-entropy alloys (HEA), a new generation alloy system offer superior mechanical properties with solid solution strengthening. AlxCoCrFeNi-HEA is one such system being received more attention because of its specific yield strength and ductility. In the present work, Al0.5CoCrFeNi-HEA was prepared by vacuum arc melting. The laser beam welding (LBW) was carried out on 1mm thick forged and homogenized HEA, with a beam power of 1.5 kW and at a traverse speed of 600 mm/min. The microstructural features of different regions of the weld were studied using scanning electron microscopy. The homogenized Al0.5CoCrFeNi-HEA have shown equiaxed grains of average size 60 μm. The weld metal showed a typical weld fusion zone microstructure with dendritic structure with a reduction in BCC phase due to minimal Al and Ni segregation ratio at interdendrites. Micro-chemical analysis with energy dispersive spectroscopy confirmed that there was no major segregation of elements in the weld fusion zone. The microhardness survey performed across the weld evidenced a reduction in hardness, as a consequence of significant reduction in Al-Ni rich hardening factor.
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paper_title: Grain growth and the Hall–Petch relationship in a high-entropy FeCrNiCoMn alloy

paper_content:

A high-entropy FeCoNiCrMn alloy with a single face-centered cubic phase was synthesized and subsequently annealed at different temperatures to systematically investigate the grain growth behavior. It was observed that the growth kinetics could be described by a power law of 3 and the activation energy for growth was about 321.7 kJ mol−1. The hardness of the alloys was measured as a function of grain size, and the result was found to follow the classical Hall–Petch strengthening, though with a relatively high hardening coefficient.
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paper_title: Microstructure and mechanical properties of equimolar FeCoCrNi high entropy alloy prepared via powder extrusion

paper_content:

Abstract An equiatomic FeCoCrNi high alloy (HEA) with both high tensile strength and ductility was produced by a powder metallurgy (P/M) method. The P/M process includes a gas atomization and a hot extrusion of pre-alloyed HEA powder. Microstructures and mechanical properties were characterized using optical microscopy (OP), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and tensile tests. The results show that the P/M FeCoCrNi HEA has a single face centered cubic (fcc) structure and an equiaxed microstructure. No obvious porosity and brittle intermetallic phase was found. The as-extruded alloy exhibits a very high tensile strength of 712.5 MPa, and still maintains an elongation as high as 56%. The improvement of the tensile properties is caused by the solid solution strengthening, grain boundary strengthening and homogenous microstructure. Therefore, the powder hot extrusion can be considered as a promising way for preparing large-sized HEAs with high mechanical properties.
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Title: Welding of High Entropy Alloys - A Review

Section 1: Introduction
Description 1: Provide an overview of the paper by emphasizing the significance of high-entropy alloys (HEAs) in comparison to classic alloys, and briefly discuss the challenges and innovations in welding HEAs.

Section 2: Arc Welding of HEAs
Description 2: Discuss the use of arc welding methods for HEAs, including different types of arc welding processes and their effects on the microstructure and mechanical properties of HEAs.

Section 3: Laser Welding/Laser Beam Welding
Description 3: Present research findings on laser beam welding (LBW) of HEAs, detailing the process, benefits, and influence on microstructure and mechanical integrity.

Section 4: Electron Beam Welding
Description 4: Examine studies on electron beam welding (EBW) of HEAs, including the conditions, results, and comparisons with other welding methods.

Section 5: Friction Stir Welding
Description 5: Explore the friction stir welding (FSW) technique for HEAs, describing the process, microstructural changes, and mechanical properties resulting from FSW.

Section 6: Grain Structure, Element Distribution and Precipitations in Welded Structures of HEAs
Description 6: Analyze the grain structure, element distribution, and precipitation phenomena in welded HEAs, and discuss their implications on the mechanical and corrosion properties.

Section 7: Discussion and Future Works
Description 7: Summarize the key findings and current challenges in the welding of HEAs. Highlight the potential areas for future research and development to improve welding techniques and the application of HEAs.

Section 8: Summary
Description 8: Provide a concise summary of the paper, reiterating the main points reviewed and conclusions drawn on the welding of HEAs and their mechanical properties.