Electron microscopic study ofθ-phase martensite in Ni-Mn alloys

Abstract

Electron microscopic observations have been made onθ-phase Ni-Mn martensites which form in nearly stoichiometric Ni-Mn alloys. Three fundamental variant-variant pairs, the “A/B”, “A/C”, and “A/D” types, have been identified as well as other combinations of variants with the aid of crystallographic information given by the phenomenological theory of martensitic transformations. The complex mixing of variants, the abundant presence of dislocations, and the large twinning shear have been pointed out as main crystallographic factors hindering the shape-memory-type deformation in this alloy system, in addition to weak grain boundaries. A new grouping of martensite variants is suggested which as a group cancels the shape strain as well as that found in the normal fundamental plate groups. The new grouping also gives the proper combination of variants for which the intersection becomes a single crystal.