Effect of Ni/Ti Ratio and Ta Content on NiTiTa Alloys

Influences of Ta and Ni/Ti ratio on transformation temperatures, microstructures, and super-elastic properties of NiTiTa alloys were investigated. It is found that: (1) increasing Ni/Ti ratio increases the solubility limit of Ta in NiTi matrix and suppresses the formation of Ta-rich secondary phases. 4 at.% Ta can be dissolved in alloys with Ni/Ti ratio of ~ 1.1; (2) high Ni/Ti ratio promotes the formation of Ti₂Ni₃ phase, which is suppressed by increasing Ta additions; (3) phase transformation temperatures increase with Ta content at a rate of ~ 70–80 °C/at.% Ta, but rapidly decrease with increasing Ni/Ti ratio; (4) super-elastic properties of NiTiTa alloys can be optimized by adjusting residual cold work, Ta content, and Ni/Ti ratio; and finally, (5) phase transformation temperatures of NiTiTa alloys decrease with increasing valence electron concentration in general.