TEM studies of the R-phase transformation in a NiTi shape memory alloy after thermo-mechanical treatment

The Ti–51 at.% Ni shape memory alloy solution treated at 1123 K for 1 h followed by 5% cold rolling and annealing at 573 K for 1 h was studied by transmission electron microscopy, including high-resolution and in situ observations at cooling. The applied treatment suppressed the B19′ martensitic transformation and thus the studied transformation was the R-phase transition. It was found that the transformation starts by the incommensurate modulations occurring along 〈1 1 0〉 directions. The incommensurability exists in a wide temperature range and it decreases with lowering the temperature. The lock-in R-phase observed at liquid nitrogen temperature occurred in the form of relatively small domains. Additionally, another transformation was also observed at low temperatures that gave extra 1/2 〈1 1 0〉 spots at the diffraction patterns.