Cyclic Stability of Ni_44.8Cu₅Ti_45.2Hf₅ and Zr‐Substituted Ni_44.8Cu₅Ti_40.2Hf₅Zr₅ Medium‐Entropy Shape Memory Alloys

Abstract

An experimental investigation on the cyclic stability of solutionized Ni44.8Cu5Ti45.2Hf5 and Zr‐substituted Ni44.8Cu5Ti40.2Hf5Zr5 (atomic percent, at%) medium‐entropy shape memory alloys (MESMAs) was carried out. The shift of the peak temperature for the forward B2B19′ transformation in Ni44.8Cu5Ti45.2Hf5 alloy after 50 differential scanning calorimetry (DSC) cycles was measured to be 23 °C, substituting 5 at% Zr for Ti increased it to 41 °C. No residual strain accumulation was observed in Ni44.8Cu5Ti45.2Hf5 alloy after 10 thermal cycles under a tensile stress of 200 MPa, and meanwhile the shape memory strain increased from 5% to 6.5%. A pseudoelastic strain (εPE) of ≈2.7% was obtained in Zr‐substituted Ni44.8Cu5Ti40.2Hf5Zr5 alloy at 55 °C (Af + 10 °C) for a maximum deformation (εmax.) of 4.4%. In addition, the stability of pseudoelasticity in Ni44.8Cu5Ti40.2Hf5Zr5 alloy during 100 loading–unloading cycles was examined in the solution annealed (1000 °C for 1 h) and aged (400 °C for 1 h) conditions. The results showed that aging treatment could effectively retard the degradation of pseudoelasticity in the alloy. These preliminary findings suggest that the idea of increasing chemical complexity in NiTi‐based SMAs holds promising potential in developing novel SMAs with enhanced cyclic stability.