Crystallography of stress-induced B 2 - ζ₂^′ (trigonal) martensitic transformation in Au-49.5 at.%Cd alloy

The crystallography of B 2 - ζ₂^′ (trigonal) stress-induced transformation in Au-Cd has been controversial. The present study was undertaken to clarify this issue by obtaining reliable experimental data, which enabled a critical test for the phenomenological crystallographic theory of martensitic transformation. The results revealed that the lattice invariant shear (LIS) was {1 0 0}〈0 1 1〉_C twinning in contrast to {0 1 1}〈1 0 0〉_C twinning, which was believed to be the case by all the previous investigators. The theoretical calculations using {1 0 0}〈0 1 1〉_C twinning agreed well with the observations in all respects. Therefore, the phenomenological crystallographic theory is shown to be well applicable to the present transformation. In addition to the stress-induced transformation, the morphologies of thermally induced martensites were examined, and two types of morphologies were observed: "roof-type" with LIS, and "herringbone-type" without LIS. Therefore, it is shown that transformations both with and without LIS are possible in the present transformation, and the previous discrepancy in LIS is rationalized.