Lattice Softening, Phase Stability and Elastic Anomaly of the β-Au-Cu-Zn Alloys

Abstract

The β-Au x Cu 1- x Zn alloys transform martensitically from a Heusler-type ordered structure (b. c. c.) into an orthorhombic crystal at lower temperatures. The phase stability associated with the transformation was investigated by an examination of the elastic constants in the Heusler phase. The shear constant ( C 11 – C 12 )/2 approaches zero near the martensite temperature, and its temperature coefficient is positive in the region above martensite temperature. Thus the transformation in these alloys proved itself to be deeply connected with a lattice softening and the elastic anisotropy. The free energy difference between the Heusler and the martensite phases mainly comes from the change in vibrational entropy, which can explain the thermoelastic property of the transformation. Pseudoelastic properties including ferroelasticity and the memory effect of these alloys were also investigated from their strain-stress diagrams. The properties have a close relation to the large elastic anisotropy and the latti...