PREMARTENSITIC-MARTENSITE TRANSITIONS RELATED TO SHAPE MEMORY EFFECT.

Abstract

A large elastic anisotropy and a low value of the elastic modulus one-half (C//1//1-C//1//2) 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 B//2 structure, with respect to the left brace 110 right brace 110 direction 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 new results of frozen-in lattice instability and omega type premartensitic distortion in a number of splat quenched B//2 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.