The deformation behavior of TiNi shape memory alloy under axial dynamic compression

The deformation behavior of Ti-50.9at%Ni shape memory alloy under axial compression dynamic loads was investigated by an MTS 858 Mini Bionix test machine. The alloy were aged at 500°C for an hour before being machined into specimens. The compression experiments were conducted at 20°C and the variety of dynamic loads were controlled by the strain rate, which was 3 mm/min, 15 mm/min, 30 mm/min and 50 mm/ min, respectively. The experimental results indicate that in the case of 3 mm/ min, stress-induced martensitic transformation occurs at about 350 MPa when loading and reverse transformation at about 200 MPa when unloading, during which the aged Ti-50.9at%Ni alloy shows the recoverable nonlinear pseudoelastic strain of 4.3% with the residual strain of 1.2% reserved. With the strain rate increasing, the area encloses by loading-curve and unloading-curve, i.e. stress (strain) hysteresis becomes smaller and smaller and the residual strain also decreases, while critical stress for inducing martensitic transformation rises. At a higher strain rate the alloy exhibits linear-like pseudoelasticity, which is up to 4.5%.