Effect of Heat Treatment Atmosphere on Multistage R-Phase Transformation in an Aged Ti–51.0 at%Ni Alloy

The present study systematically investigates the effect of heat treatment atmosphere on the multistage R-phase transformation (MRT) in an aged Ti–51.0 at%Ni alloy. No MRT occurs when the heat treatments were completed under the regulated atmosphere. On the other hand, the MRT is observed in the specimen heat treated under the unregulated atmosphere. It is apparent from transmission electron microscope observations that the first and the second transformations take place around the grain boundary and at the grain interior, respectively. We conclude that the MRT is extrinsic, and is an artifact during the heat treatment, rather than intrinsic in nature.