Lattice Softening and Martensitic Transformation in Cu–Ni–Zn β Phase Alloys

Abstract

Cu_50−xNi_xZn₅₀ β phase alloys undergo a martensitic transformation from the CsCl-type ordered cubic (B2 structure) to the AuCuI-type tetragonal (L1₀ structure). The martensite L1₀ phase appears at room temperature when Ni composition x becomes larger than 32.5 at.%, i.e. when electron atom ratio (e⁄a) becomes smaller than 1.175. In order to clarify the origin of B2–L1₀ transition, composition and temperature dependence of the elastic constants in Cu_50−xNi_xZn₅₀ alloys were measured. The elastic constant C′=(C₁₁−C₁₂)⁄2 decreases with increase in x and dC′⁄dT is positive (i.e. lattice softening) in the cubic phase, which are suggested to have an important role on the martensitic transformation in these alloys.