Influence of tantalum additions on the microstructure and shape memory response of Ti50.5Ni24.5Pd25 high-temperature shape memory alloy

A Ti50.5Ni24.5Pd25 (at%) high-temperature shape memory alloy (HTSMA) was alloyed with 1, 3, and 5 at% Ta additions in an effort to enhance the shape memory properties, particularly the dimensional stability for actuator applications. Thermomechanical characterization indicated that the dimensional stability of Ti50.5Ni24.5Pd25 improved significantly, but mostly for the highest Ta addition. This improvement was attributed in part to a particulate strengthening effect, since both Ti47.5Ni24.5Pd25Ta3 and Ti45.5Ni24.5Pd25Ta5 were found to have similar amounts of Ta, approximately 2.2 at%, in solid solution. However, by 5% Ta addition, the transformation temperatures decreased by about 65 °C and reductions in transformation strain and fracture resistance occurred. Consequently, among the alloys studied, Ti47.5Ni24.5Pd25Ta3 seemed to be the optimum composition with a transformation temperature around 150 °C, a transformation strain value close to that of the unalloyed composition, and improved dimensional stability.