Development of Ni–Ti–Zr–Hf–(Nb, Ta) Multi-Principal Element High-Temperature Shape Memory Alloys with High Cold Workability

Novel Ni–Ti–Zr–Hf–Nb and Ni–Ti–Zr–Hf–Ta high temperature shape memory alloys with multi-principal elements were developed, and differences in the effects of Nb and Ta on cold workability and shape memory properties were investigated. Constituent phases, microstructure, cold workability, transformation temperatures, shape memory properties were investigated in (Ni50Ti30Zr10Hf10)100−xNbx (x = 5, 10, 15) alloys and (Ni50Ti30Zr10Hf10)100−yTay (y = 5, 10, 15) alloys. Although both of Nb and Ta were effective to improve cold workability of Ni50Ti30Zr10Hf10 alloy by forming a ductile β phase with a disordered body-centered cubic structure, it was found that Ta was more effective than Nb in improving cold workability. The addition of Ta was also effective to suppress the formation of Ti2Ni-type intermetallic compound. Transformation temperatures were not significantly affected by the addition of Nb, while the transformation temperatures increased by the addition of Ta. According to thermal cycling tests, the (Ni50Ti30Zr10Hf10)85Nb15, (Ni50Ti30Zr10Hf10)90Ta10 and (Ni50Ti30Zr10Hf10)85Ta15 alloys exhibited almost full shape recovery under 200 MPa. These alloys are suggested as promising candidates for practical high temperature shape memory alloys that can be worked at room temperature.