The processing of NiTi shape memory alloys by equal-channel angular pressing at room temperature

The processing of NiTi alloys by severe plastic deformation (SPD) leads to significant grain refinement and improves both the mechanical properties of the alloy and also the shape memory effect and superelasticity. Nevertheless, it is generally not possible to process NiTi alloys by equal-channel angular pressing (ECAP) at room temperature because of low deformability and the development of cracking due to strain-induced austenite formation. It is shown in this investigation that near equi-atomic NiTi shape memory alloys may be successfully processed by ECAP at room temperature by containing the NiTi samples within Fe sheaths. Processing was performed successfully for up to two passes of ECAP by controlling the processing variables and the initial structure of the NiTi alloys. The results show that the austenitic transformation and cracking can be suppressed if adiabatic heating is minimized by reducing the processing ram speed and the specimen size.