Phase fractions of B2, B19$'$, R-phase and Ni$\mathsf{_4}$Ti$\mathsf{_3}$ in NiTi alloys during two-step phase transformations

We investigated the phase composition of (i) solution annealed and subsequently aged Ti-50.7 at.-%Ni alloy and (ii) solution annealed Ti-50.1 at.-%Ni alloy cycled 30 times between 100 °C and-196 °C by x-ray diffraction through the martensitic phase transformation. Both samples show “two-step" transformation behaviour with two exothermic differential scanning calorimetry (DSC) peaks on cooling and one endothermic DSC peak on heating. For the aged Ni-rich material the x-ray data, which reflect the conditions at the sample surface only, indicate that in the presence of the Ni$_4$Ti$_3$ precipitates the B2 phase transforms to the R-phase, which then transforms to B19$'$ phase in the second step on cooling. The transition to B19$'$ starts well above the second peak. In the cycled, precipitate-free material, the R-phase coexists with B2, and/or B19$'$ throughout its limited temperature range of stability