Crack initiation and propagation in 50.9 At. pct Ni-Ti pseudoelastic shape-memory wires in bending-rotation fatigue

The structural fatigue of pseudoelastic Ni-Ti wires (50.9 at. pct Ni) was investigated using bending-rotation fatigue (BRF) tests, where a bent and otherwise unconstrained wire was forced to rotate at different rotational speeds. The number of cycles to failure (N-f) was measured for different bending radii and wire thicknesses (1.0, 1.2, and 1.4 mm). The wires consisted of an alloy with a 50-nm grain size, no precipitates, and some TiC inclusions. In BRF tests, the surface of the wire is subjected to tension-compression cycles, and fatigue lives can be related to the maximum tension and compression strain amplitudes (epsilon(a)) in the wire surface. The resulting epsilon(a)-N-f curves can be subdivided into three regimes. At epsilon(a) > 1 pct rupture occurs early (low N-f) and the fatigue-rupture characteristics were strongly dependent on epsilon(a) and the rotational speed (regime 1). For 0.75 PCt < epsilon(a) < 1 pct, fatigue lives strongly increase and are characterized by a significant statistical scatter (regime 2). For Ea < 0.75 pct, no fatigue rupture occurs up to cycle numbers of 10(6) (regime 3). Using scanning electron microscopy (SEM), it was shown that surface cracks formed in regions with local stress raisers (such as inclusions and/or scratches). The growth of surface cracks during fatigue loading produced striations on the rupture surface; during final rupture, ductile voids form. The microstructural details of fatigue-damage accumulation during BRF testing are described and discussed.