Formation process of the incompatible martensite microstructure in a beta-titanium shape memory alloy

The formation modes of habit plane variant (HV) clusters (HVCs) in a Ti-23Nb-3Al (mol%) shape memory alloy were investigated by in situ optical microscopy with high-speed digital video camera. In addition to the kinematic compatibility condition, the nucleation mode and the interfacial energy between martensites were clearly shown to have critical roles. The V-I cluster, which is formed by two HVs connected by a {111} type I twin, predominated in the early stage of the martensitic transformation, and had a lower energy barrier to nucleation and growth than did the V-II cluster, which is formed by two HVs connected by a <211> type II twin. Eighty percent of the V-I clusters were observed to form by paired nucleation or branching, in which the junction plane corresponds to the nucleation site of the relevant HV(s). In contrast, V-II clusters formed as a result of collisions of HVs. Neither paired nucleation nor branching was observed for V-II. V-I nuclei rather than V-II nuclei had a strong tendency to form, and this difference was not fully explained by the kinematic compatibility (KC) conditions. The junction planes formed by the collision of growing HVs should have larger deviation from the KC condition. The compatibility of the junction plane between HVs depends on the formation modes. (c) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.