TEM studies of in situ martensitic transformation in NiTi thin films

Abstract
Amorphous sputter-deposited Ti-rich NiTi thin films were annealed at different temperatures between 600°C and 900°C for 10 min to 3 hours inducing Ti₂Ni precipitation. Differential scanning calorimetry (DSC) was performed on these specimens and their microstructure was investigated by transmission electron microscopy (TEM). In samples annealed at 600°C and 700°C for a short time, the DSC curves show, in addition to the R transformation, a multiple phase transformation with broad peaks. TEM observations reveal the presence of a great number of spherical Ti₂Ni precipitates inside the grains and some larger precipitates at the grain boundaries. Annealing at higher temperature results in clear defined DSC peaks. The density of the precipitates inside the grains decreases while the number and the size of the precipitates at the grain boundaries increase. TEM studies of in-situ martensitic transformation showed that the distribution of the Ti₂Ni precipitates strongly influences the martensitic transformation behaviour while the austenite/R-phase transformation is not affected. In samples with a large amount of precipitates inside the grains, a two-step transformation was observed : the martensite appears first along all the grain boundaries and then in the centre of the grains. This unusual feature of the martensitic transformation is considered to be responsible for the multiple transformation observed in the DSC curves.