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 tran sformation observed in NiTi alloys. It is characterized by the appeara nce of several transformation steps during the transformation of the R phase into martensite. The present study shows that this effect can b e observed after the sample has been solution treated at 1173K and fur ther aged at 793K and is accompanied by a special microstructure which consists in a well recrystallized matrix containing small coherent pr ecipitates of the Ni4Ti3 type. These precipitates have been observed t o be the source of a local stress field which influences both the nucl eation and the growth of the R and martensitic phases. Based on the pr esent observations, a model has been developed which explains the basi c of the mechanisms involved in this phenomenon. According to this mod el, the additional transformation step can be considered to be a local ly stress-induced martensitic transformation. This model is extended t o interpret some results found in the literature.