Structural and functional properties of a semi equiatomic NiTi shape memory alloy processed by multi-axial forging

The capability of multi-axial forging (MAF) as an efficient severe plastic deformation (SPD) method to improve the shape memory responses of a semi equiatomic NiTi alloy was investigated. The MAF cycles were performed at temperatures in two different ranges, where the governing deformation mechanisms were thought to be different. This was confirmed through the material corresponding true stress-true strain flow behaviors. The presence of different phases was traced by means of X-ray analysis. The martensitic transformation was hindered in the specimens deformed at 150 °C due to the presence of high dislocation density and in consequence the nano-crystalline austenite phase was the main phase after deformation. The specimens deformed at 400 °C however, experienced restoration phenomena thereby brought the martensite beside the austenite phase in the microstructure. The latter was related to shifting the transformation point to lower temperatures. The processed material at 400 °C exhibited both shape memory effect and super-elasticity due to the coexistence of restored and processed martensite and austenite phases. However the related recovery strain was reached to 3.2 pct. due to the corresponding shape memory effect. In addition, the results indicated that the multi-axial forging should be considered as a method to produce homogenous bulk NiTi alloy possessing an improved mechanical properties. To this end the microhardness, ultimate strength and elongation were improved from 220Hv, 920 MPa and 18.5% in as-hot rolled condition to ~590Hv, ~1700 MPa and 23% in optimum MAF condition scheme, respectively.