Anisotropic Tensile Behavior Of NiTi Tubes And Its Dependence On Temperature

NiTi thin walled tubes are highly textured structures used in the fabrication of nearly 60% of self-expanding stents. The presence of texture leads to strong anisotropic mechanical behavior, which has important consequences in the final behavior of devices manufactured from tubes. In this work the anisotropic superelastic mechanical behavior of a 50.8%atNi-Ti thin walled tube was studied through isothermal tensile tests performed in samples cut at five different orientations from the tube drawing direction. The tube was flattened by a one-step shape setting process and dogbone samples were cut by laser. Tests were performed at three temperatures above A_f (the reverse martensitic transformation finish temperature). Results show that transformation strains strongly depend on orientation but not on temperature. Transformation stresses also depend on orientation and this dependence is different for forward (A→M) and reverse (M→A) transformations. Clausius-Clapeyron coefficients also show an important orientation-dependent behavior. The observed difference between forward and reverse Clausius-Clapeyron coefficients are explained through the dependences of dissipated and stored energies with temperature, often neglected in the Clausius-Clapeyron relation.