A neutron diffraction study of the martensitic transformations in aged Ni-rich NiTi alloys

The transformation of a Ni-rich NiTi alloy of nominal composition 50.7 atomic percent nickel which was solution annealed, water quenched and aged at 500 °C for 20 hours was investigated. The material shows a two-step transition on cooling with differential scanning calorimetry peaks at 180 °C and 7 °C. On heating, a single peak occurs at 43 °C. The aging leads to the formation of Ni$_4$Ti$_3$ precipitates in the microstructure, and in the presence of the precipitates the martensitic transformation occurs from the cubic B2 phase via the R-phase to B19$'$ martensite. However, the first peak on cooling is not due to the formation of R-phase alone. Below the peak temperature, R-phase is present along with some B19$'$ and with residual B2 and the precipitates. The second peak is related to the transformation of R-phase and B2 to B19$'$. This result confirms our earlier conclusions which were obtained for the same alloy aged at 400 °C for 20 hours