Study of Deformation Behaviors Associated with ζ2'→ γ2' and β2→γ2' Stress-Induced Martensitic Transformations in a Au-49.5at%Cd Alloy

¹ University of Tsukuba, Institute of Materials Science, Tsukuba, Ibaraki 305, Japan
² Teikyo University, Department of Materials Science and Engineering, Toyosatodai, Utsunomiya 320, Japan

Abstract

Two types of the stress-induced martensitic transformations, i.e. the ζ₂'(trigonal)→γ₂' and the β2(B2) →γ₂' stress-induced transformations, in a Au-49.5at%Cd alloy were studied and their deformation behaviors were compared to each other. The stress-induced γ₂' martensite, both from the ζ₂' and β₂ phases, was confirmed to be orthorhombic and the lattice parameters were accurately determined by the single crystal X-ray diffraction method under stress. The observed hysteresis in the superelastic curve for the ζ₂ ' → γ₂' transformation was much larger than that for the β2→γ₂' transformation, which means that the nucleation for the former transformation is more difficult compared with that for the latter one. A considerable temperature dependence of the superelasticity was observed for the former transformation, while the latter transformation showed a less dependence. Thus, a phase diagram for a Au-49.5at%Cd alloy in the temperature-stress coordinate was proposed, since the structures of all phases and the associated deformation behaviors became all clear. It was also found that the former transformation is considerably affected by the previous ageing of the ζ₂’ martensite.