Quantitative phase analysis in microstructures which display multiple step martensitic transformations in Ni-rich NiTi shape memory alloys

In the present study, we investigate a microstructure which shows a three-step martensitic transformation on cooling from the high temperature regime and two DSC peaks on heating. Using in situ neutron diffraction phase analysis based on Rietveld full profile fitting it is shown how the volume fractions of B2, R and B19′ phases evolve during the three step transformation on cooling and the reverse transformation on heating. We used TEM as a complementary method to support our neutron diffraction findings. The first DSC peak on cooling is related to a complete transformation of the B2 matrix to R phase and the second and third peak on cooling are associated with the formation of B19′ in two stages. On heating also, the B19′ → R transformation is accumulating in two stages, with the second stage overlapping with the R → B2 transition.