Effect of Hf addition on the high-temperature deformation behavior of Ni-Ti-Al alloys

The microstructure and high temperature deformation behavior of Ni₅₀Ti_46-xAl₄Hf_x (x=0-6, at%) alloys were investigated by compressive tests at elevated temperatures. Results show that the Ni-Ti-Al-Hf alloys consist of NiTi and Ti₂Ni phases. Hf exists as the solid solution element in both of NiTi and Ti₂Ni, and promote the precipitation of Ti₂Ni, thus greatly improving the yield stress of the alloys at high temperatures. The alloys possess good compressive property. The flow stress generally decreases with the increasing deformation temperature or the decreasing initial strain rate. The deformation feature of the Ni-Ti-Al-Hf alloys could be adequately described by standard power law. The addition of Hf increases the stress exponent (n) and activity energy (Q), resulting from its strong solid solution strengthening effect and high-temperature stability and the promotion of precipitation strengthening effect of Ti₂Ni phase.