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

Abstract The phase transformation and microstructure in solution-treated Ni 50 Ti 47 Ta 3 , Ni 50 Ti 45 Ta 5 , Ni 49 Ti 46 Ta 5 shape memory alloys (SMAs) have been investigated. Two different second-phase particles, (Ti,Ta) 2 Ni 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.