 One such composition is approximately 77% nickel, 16% iron, 5% copper, and 2% chromium or molybdenum.More recently, mu-metal is considered to be ASTM A753 Alloy 4 and is composed of approximately 80% nickel, 5% molybdenum, small amounts of various other elements such as silicon, and the remaining 12 to 15% iron.The name came from the Greek letter mu (μ) which represents permeability in physics and engineering formulae.