Influence Of Heat Treatment Temperature On Tension, Compression And Bending Behavior Of Nanostructured NiTi Wires

Most medical applications involving superelastic Ni-Ti shape memory alloys are based on thin wires or tubes. These wires are commonly manufactured through a sequence of hot and cold working. The severe plastic deformation during cold working induces a grain size reduction that may lead to a partial amorphization. The mechanical behavior of these cold worked wires is strongly modified by heat treatments performed after the last cold working.

In this paper, Ni-Ti wire behaviors were experimentally characterized using uniaxial tensile tests and pure bending tests. The tests were performed on a NiTi cold worked wire submitted to heat treatments (HT) at temperature ranging from 300°C to 500°C. Strain fields were measured during tensile tests using Digital Image Correlation (DIC). Pure bending experiments were performed using a home-made device. The local curvatures of the samples along their curvilinear abscissa were also identified during the pure bending experiments. Uniform or localized behaviors were observed in tension and bending depending on the heat treatment temperatures.

The experimental results are analyzed to deduce compressive behaviors. For non localized tests, tension compression asymmetry is measured as function of strain. To model localized pure bending tests, tension and compression constitutive equations were proposed.