Comparative Study of Ni-Rich Nitinol Alloys with Macro-Alloyed Pd or Hf Additions

Ni-rich NiTi alloys display relatively stable shape memory and superelastic behavior. However, their potential applications are severely limited because of low transformation temperatures. Recent efforts have focused on using ternary additions to raise the transformation temperatures, while simultaneously improving their dimensional stability, through nanoscale precipitation. In the present work, we have investigated the effect of aging on the structure and shape memory behaviour of extruded Ni-48Ti-25Pd (at.%) and Ni-29.7Ti-20Hf (at.%) alloys. These alloys were aged at 450 and 550 deg. C, respectively. For the Pd-containing alloys, the precipitates had the same structure and similar chemistry, determined by electron diffraction and atom probe tomography respectively, as the P-phase in the NiTiPt alloy. These precipitates exhibited a cubioal morphology. In contrast, the Hf-containing precipitates were plate-like in shape. Load-bias thermomechanical testing was conducted and the results are compared to the various microstructures.