Influence of Thermal Treatment on the Appearance of a Three Step Martensitic Transformation in NiTi

Wires of Ti-51.14 at.%Ni have been either solution treated and subsequently aged at 520°C or only aged. Differential thermal analysis (DTA) has been used to characterise the macroscopic behaviour of the martensitic transformation and transmission electron microscopy (TEM) to study the microstructure. It has been found that after certain heat treatments the martensitic transformation occurs in three steps. The occurrence of the R-phase is followed by a two step martensitic transformation. In these samples the recristallisation has occured and a coherent precipitation of Ni 4 Ti 3 is observed by TEM.