Conventional and in-situ transmission electron microscopy investigations into multistage martensitic transformations in Ni-rich NiTi shape memory alloys

Investigations of multi-step martensitic transformations in Ni-rich NiTi single crystals and polycrystals are reported. Attention is mainly given to processes that involve a two-step R → B19′ transition. Single crystals and polycrystals with a Ni content close to 51 at.% were subjected to standard heat-treatments during which microstructures with a homogeneous distribution of Ni4Ti3 precipitates form. Results obtained using differential scanning calorimetry and conventional and in-situ transmission electron microscopy clearly show that these microstructures transform from R phase into B19′ martensite in two steps. We suggest that the nucleation conditions for the B19′ phase must be taken into account to rationalize the obtained results.