Stability of the B2-type structure and R-phase transformation behavior of Fe or Co doped Ti–Ni alloys

We have investigated the R-phase transformation behavior in a series of Ti–(50 − x)Ni–xFe (2 ≤ x ≤ 20) and Ti–(50 − x)Ni–xCo (4 ≤ x ≤ 16) alloys. We found that the alloys with Fe/Co content of 6/14 at.% and more do not show a latent heat and surface relief although the alloys with Fe/Co content of 4/12 at.% and less show the R-phase transformation with an obvious latent heat and surface relief. However, some of the former alloys (6Fe, 8Fe, 10Fe, 14Co, 16Co) show anomalies in specific heat, such as the existence of broad peak and a low Debye temperature. Transmission electron microscope observation of the 6Fe alloy revealed the existence of an incommensurate–commensurate transition at 140 K. That is, diffuse scatterings continuously move to 1/3[1 1 0] position as temperature decreases and lock in to the exact position at 140 K. The commensurate phase inherits a nanoscale domain-like structure, which is introduced in the incommensurate phase beforehand.