Interaction between microstructure and multiple-step transformation in binary NiTi alloys using in-situ transmission electron microscopy observations

The multiple-step transformation is a special case of martensitic transformation observed in NiTi alloys. It is characterized by the appearance of several transformation steps during the transformation of the R phase into martensite. The present study shows that this effect can be observed after the sample has been solution treated at 1173K and further aged at 793 K and is accompanied by a special microstructure which consists in a well recrystallized matrix containing small coherent precipitates of the Ni 4 Ti 3 type. These precipitates have been observed to be the source of a local stress field which influences both the nucleation and the growth of the R and martensitic phases. Based on the present observations, a model has been developed which explains the basic of the mechanisms involved in this phenomenon. According to this model, the additional transformation step can be considered to be a locally stress-induced martensitic transformation. This model is extended to interpret some results found in the literature.