Premartensitic-Martensite Transitions Related to Shape Memory Effect

Abstract

The literature pertaining to premartensitic-martensitic transition is reviewed with special emphasis on the role of premartensitic lattice instability in shape-memory mechanism. A large elastic anisotropy and a low value of the elastic modulus 1/2 (C11-C12) as necessary criteria for the thermoelastic type martensitic transition are scrutinized in light of published and new experimental results. Progressive lattice softening and instability of B2 structure, with respect to the {ll0} shear, approaching the transition temperature are analyzed in terms of martensitic nucleation. Some new experimental results on the premartensitic instabilities in the Ti-Ni alloy, the effect of stress on the x-ray diffraction peaks in the Ti-Ni alloys and some new results of frozen-in lattice instability and omega type premartensitic distortion in a number of splat quenched B2 alloys are presented. The presence of omega type reflection, profuse streaking in electron diffraction patterns and anomalous broadening of (110) x-ray diffraction line indicate a pretransformation instability of the parent phase. These results also suggest a progressive cooperative shear and lattice distortion which produces the final martensitic structure.