The anisotropy of the deformation twinning behaviour of iron-nickel martensite crystals possessing a transformation twin microstructure

Abstract

Martensite crystals in metal alloys often possess a transformation microstructure which consists of an array of micro twins. The presence of this type of microstructure would be expected to influence the ease of propagation of deformation modes and therefore to be a controlling factor in the deformation behaviour of martensite crystals. The deformation twinning behaviour of internally twinned martensites in bulk specimens has been studied comprehensively for the first time, and was made possible by the use of the Kossel microdiffraction technique for the determination of the orientation of the narrow martensite crystals. The investigation of the operative deformation twinning modes in both as-transformed and deformed Fe-29-33 % Ni martensites showed that twinning occurs almost exclusively on {112} planes and that the number of variants formed and the relative predominance of the operative variants is modified by the microtwinning which accompanies the transformation. This is explained in terms of the intersection mechanisms of the deformation twins and transformation twins.