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

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.