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

Abstract

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 (ɛ _a ) in the wire surface. The resulting ɛ _a -N _f curves can be subdivided into three regimes. At ɛ _a > 1 pct rupture occurs early (low N _f ) and the fatigue-rupture characteristics were strongly dependent on ɛ _a and the rotational speed (regime 1). For 0.75 pct < ɛ _a < 1 pct, fatigue lives strongly increase and are characterized by a significant statistical scatter (regime 2). For ɛ _a < 0.75 pct, no fatigue rupture occurs up to cycle numbers of 10⁶ (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.