Phase transformation and second phases in ternary Ni–Ti–Ta shape memory alloys

The phase transformation and microstructure in solution-treated Ni50Ti47Ta3, Ni50Ti45Ta5, Ni49Ti46Ta5 shape memory alloys (SMAs) have been investigated. Two different second-phase particles, (Ti,Ta)2Ni and a tantalum-rich solid solution (β-Ta) mostly located at grain boundaries are observed. These Ni–Ti–Ta alloys undergo a one stage B2 ↔ B19′transformation. Experimental results indicate that the Ta atoms are suggested to replace Ti atoms, instead of Ni atoms in these alloys. The transformation temperatures depression of Ni–Ti–Ta alloy is more than that of NiTi alloy under the same number of thermal cycles due to the presence of more second-phase particles. The above 4% deformation might induce plastic strain for the β-Ta. The Ni–Ti–Ta alloys exhibit a wide transformation hysteresis compared to NiTi binary alloys.