Remarkable High-Cycle Fatigue Resistance of the TiZrNbHfTa High-Entropy Alloy and Associated Mechanisms

The TiZrNbHfTa alloy with a single body-centered cubic (bcc) phase is of particular interest among refractory high-entropy alloys (HEAs) because it exhibits significantly large ductility compared to other refractory HEAs. To have a convincing appraisal of its potential in structural applications, its fatigue behavior needs to be understood in depth as fatigue is one of the major failure modes in practice. The in-air, room-temperature high-cycle fatigue study of the alloy at a stress ratio of 0.1 and a frequency of 10 Hz gives a fatigue strength of 512 MPa and a fatigue ratio of 0.45 at 107 fatigue cycles. The comparison of fatigue resistance of many alloy systems at nearly equivalent conditions (stress ratio of -1) suggests that the TiZrNbHfTa alloy is superior to all other HEAs with reported high-cycle fatigue data, dilute bcc alloys, and many structural alloys in commercial applications such as steels, titanium alloys, and aluminum alloys. Through in-depth analyses of crack-propagation trajectories, fracture-surface morphologies and deformation plasticity by means of various micro-structural analysis techniques and theoretical frameworks, the remarkable fatigue resistance in the alloy is attributed to a synergy of intrinsic toughening (i.e., dislocation-mediated crack tip blunting) and extrinsic toughening (i.e., crack deflection and meandering, fracture-debris-induced crack closure, roughness-induced crack closure, and crack branching). Moreover, the stochastic nature of the fatigue data is neatly captured by the 2-parameter Weibull distribution model.