Effect of heat treatment conditions on multistage R and martensitic transformations in aged Ni-rich Ti-Ni alloys

It has been demonstrated with systematic experiments that the appearance and disappearance of multistage martensitic transformation (MMT) in aged Ni-rich Ti-Ni alloys depend on the heat treatment atmosphere. No multistage transformation occurs when the evaporation of Ti and/or Ni and the preferential oxidation of Ti are prevented and the purification of heat treatment atmosphere in an evacuated quartz tube is achieved. The heterogeneity in precipitation morphology of the Ti₃Ni₄ phase, which is responsible for the multistage transformation, can be suppressed with the regulation of heat treatment atmosphere as mentioned above. We have concluded that the multistage martensitic transformation in aged Ni-rich Ti-Ni alloys is extrinsic, and is an artifact during the heat treatment, rather than intrinsic in nature. Effect of heat treatment condition on the multistage R-phase transformation (MRT) is also discussed.