The nature of special-mode softening and the mechanism of martensitic phase transition in β-phase alloys

Abstract

Abstract The nature of the special mode in the β1 phases of NiAl and Au—Ag—Cd alloys is studied in detail from the viewpoint of the dynamic mechanism of the martensitic transition. The special mode becomes soft; the softening is responsible far the(112)[111] shear and hence for the formation of close-packed layers in the martensite. The origin of special-mode softening is an increased mode-mode correlation between the C′ and C 44 modes, i.e. an enhanced anharmonicity of the β1 lattice. Based on the results obtained, we propose a model of the transition processes of the β-phase alloys. We also discuss in detail the transition mechanism and show that the special-mode softening is an instability characteristic of the D1 lattice with a two-dimensional property induced by the C′ softening.