Shape memory effect of NiTi alloy processed by equal-channel angular pressing followed by post deformation annealing

Abstract

Processing by Equal-Channel Angular Pressing (ECAP) is generally considered superior to most other SPD techniques because it uses relatively large bulk samples. However, due to their low deformability it has proven almost impossible to successfully process NiTi alloys by ECAP at room temperature and therefore the processing is conducted at elevated temperatures. Recently, a new billet design was introduced and it was used to achieve the successful processing of NiTi shape memory alloys by ECAP. In this procedure, a NiTi alloy was inserted as a core within an Fe sheath to give a core-sheath billet. In this research, a NiTi was processed by one pass ECAP with this new billet design at room temperature. The structural evolution during annealing was investigated by X-ray diffraction (XRD) and microhardness measurements. Post deformation annealing (PDA) was carried out at 400°C for 5 to 300 min and the results indicate that the shape memory effect improves by PDA after ECAP.