Lattice Strain and Texture Evolution Of a Martensitic Nitinol Wire During Cyclic Tension

Abstract: Although nitinol have dominated the medical applicaitons such as stents, guidewires, endovascular aneurysm repair devices, inferior vena cava filters and embolic protection devices in the past decade. The micromechanics of this alloy have not been thoroughly studied, especially for materials in the wire form. The development of internal stress strain and the evolution of texture during martensite de-twinning and stress-induced martensite phase transformation are still not fully understood to date. In this study, in-situ cyclic tension test was carried out on a martensitic nitinol wire at beam line 11-ID-C at APS, Argonne National Laboratory. By using the synchrotron X-ray diffraction, the evolution of lattice strains and texture during cyclic tension was monitored. It is found that the thermal martensitic nitinol wire had the (1-20), (120) and (102) fiber texture. Detwinning converted the (120) and (102) fibers to the (1-20) fiber, but ended up to the strong (1-30) fiber texture probably by the rigid body rotation.  Continuous deformation rotates the texture towards the (110) fiber. In addition, martensite detwinning process also changes status of the internal stress; while some grains show residual tensile stress,  others show residual compression after detwinning. The load partitioning during the plastic deformation will be discussed as well.