Experimental investigation to thermal-mechanical behavior of Ni47Ti44Nb9 SMA under pure tension and pure torsion

The thermal-mechanical properties of shape memory alloy Ni47Ti44Nb9 under pure tension and pure torsion up to an equivalent strain of 16% at Ms + 30 °C and other temperatures, and the subsequent heating-induced recovery were investigated experimentally with an MTS 858 tensile-torsional testing system. It was found that the equivalent stress-strain curves under pure tension are distinctly different from that under pure torsion, the stress drop and the following plateau in the σ-ε curves as well as the macroscopic martensite band on the surface of the specimens under pure tension were not found in the results under pure torsion, where the shear stress develops monotonically with the increase of shear strain. In both pure tension and pure torsion, the equivalent stress for forward transformation increases with the increase of temperature. During heating-induced recovery, inverse transformation occurs at the onset of heating at a small rate but develops intensively at As. The equivalent critical transformation stresses and the inverse transformation temperatures in both pure tension and pure torsion are similar, which may be beneficial for the analysis of the thermal-mechanical response of the material subjected to complex loading histories. As the temperature is sufficiently high so that the applied stress would not be able to induce martensitic transformation and the following reorientation, plastic deformation may take place as the applied stress reaches the yield stress. The comparison between the thermal-mechanical behavior under pure tension and that under pure torsion implies the rationality of the application of von-Mises equivalent rule in describing the constitutive behavior of the material.