Training effect on the two-way shape memory behavior of nitihf high temperature shape memory alloy

Shape Memory Alloys (SMAs) with transformation temperatures (TTs) higher than 100°C are generally utilized for high-temperature actuation applications in the aerospace industry and are called as High-Temperature Shape Memory Alloys (HTSMAs). SMAs have the ability to remember their deformed and undeformed shapes via heating and cooling against applied load. While the One-Way Shape Memory Effect (OWSME) refers to remembering the undeformed shape of the material after deformation via heating, Two-Way Shape Memory Effect (TWSME) is the phenomenon corresponding to the remembering of both the deformed and undeformed shapes via heating and cooling, respectively. To realize the TWSME in the SMAs, specific thermo-mechanical training procedures should be conducted. This training procedure either includes loading and unloading cycles at a specific temperature (i.e., superelastic training) or heating-cooling cycles under applied constant stress (i.e., isobaric training). In this study, TWMSE of Ni50Ti25Hf25 (at%) was maintained via following the isobaric training cycles. Ni50Ti25Hf25 (at%) HTSMA was produced using high purity Ni, Ti, and Hf elements via vacuum induction melting. Then it was placed in a mild steel can for applying hot extrusion at 900°C with a 4:1 area reduction, and the material was received as in the extruded condition. While one part of the batch was solution heat-treated (i.e., solutioning, homogenized) at 1050°C for 2 hours, the other part was kept in as extruded condition. Both solutionized and extruded samples were cut into dog bone-shaped tensile test samples using Wire Electron Discharge Machine (WEDM) and mechanically ground to remove the WEDM and oxidation residues from the surfaces. Both homogenized and extruded samples were first thermally cycled without applying load to understand the effect of extrusion and homogenization heat treatment on the TWSM behavior of the alloy. The degradation of TWSME with the annihilation of dislocations, which were induced by extrusion, was observed. Then, the extruded and the homogenized samples were isobarically trained under 300 MPa constant stress to achieve TWSME and to maintain the stability of this effect for long thermal cycles. After 100 training cycles, the samples were thermally cycled without applying load to characterize TWSME. Finally, TWSM strain (εTWSM) values were determined from the strain vs. temperatures curves obtained from the stress-free thermal cycles, and it was found that εTWSM values of the extruded sample decreased from 0.4% down to 0.35% and εTWSM values of the homogenized sample increased from 0.18% to 0.35%. All the rationale behind these observations was discussed in this study.