Effect of heat treatment atmosphere on the multistage martensitic transformation in aged Ni-rich Ti–Ni alloys

We demonstrated by systematic experiments that the appearance and disappearance of a multistage martensitic transformation in aged Ni-rich Ti–Ni alloys depends 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 by the purification of the heat treatment atmosphere in an evacuated quartz tube by a Ti-getter. The heterogeneity in precipitation morphology of the Ti3Ni4 phase, which is responsible for the multistage transformation, can thus be suppressed. The influence of changes in the Ni concentration and specimen size on the multistage transformation is also discussed. The multistage transformation is not detected in the concentrated Ni alloy such as Ti–51.4 at.% Ni. All the results obtained in the present study are consistently explained by assuming the formation of zones I and II. The zone I is considered to be a contamination region with interstitial impurity elements, which is responsible for the multistage transformation. The zone II is considered to be a zone of relatively higher Ni concentration as compared to the original alloy composition at the surface.