Significant improvement in the thermal cycling stability of Ni44.8Ti45.8Hf5Cu5 shape memory alloy by high pressure torsion and post-deformation annealing

Abstract

A newly developed quaternary Ni44.8Cu5Ti45.2Hf5 shape memory alloy (SMA) was processed by high-pressure torsion (HPT) for five turns under a pressure of 6 GPa at room temperature and then subjected to post deformation annealing (PDA) at 450 °C for 10 min. Martensitic phase transformation behavior, thermal cycling stability of the transformation, and shape memory response of the alloy were investigated after HPT and subsequent annealing and compared with the initial as-cast material. The results showed that HPT followed by annealing decreased forward (Ms and Mf) and reverse (As and Af) transformation temperatures as compared to the as-cast state, meanwhile, significantly improved thermal cycling stability of the transformation. After 50 differential scanning calorimetry (DSC) cycles, the shift in the peak temperature (Mp) of the forward B2 (austenite) → B19′ (martensite) transformation for the as-cast alloy was measured to be 28 °C, while that for the sample processed by HPT and PDA was almost null. In addition to the substantial thermal cycling stability improvement, thermal cycling experiments under a constant stress (σ = 200 MPa) revealed that processing by HPT and subsequent annealing improved shape memory behavior of the alloy, decreased the width of thermal hysteresis from 79 °C to 53 °C, and completely removed irrecoverable strain.

Keywords

• Shape memory alloys
• High pressure torsion
• Post deformation annealing
• Shape memory strain
• Thermal cycling stability