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

Abstract

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−xNb_x (x = 5, 10, 15) alloys and (Ni₅₀Ti₃₀Zr₁₀Hf₁₀)_100−yTa_y (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.

Cold-worked Ti–Zr–Hf–Ni–(Nb, Ta) alloys and strain–temperature curves under thermal cycling.