Exceptional Fatigue Strength of a Microstructurally Stable Bulk Nanocrystalline Alloy

The long-term safety of structural elements in many advanced applications, such as power generation and transportation infrastructures, is paramount, especially under cyclic damage conditions. In most coarse-grained (CG) materials (with an average grain size > 1000 nm), the damage tends to localize under cyclic loading conditions, resulting in low fatigue strength. On the other hand, earlier studies on nanocrystalline (NC) alloys (average grain size below 100 nm) show an improved fatigue limit compared to the CG counterparts. However, cyclic deformation mechanisms in such materials remain elusive. Here, we use a microstructurally stable NC Cu3at.%Ta alloy as a model material to demonstrate exceptional fatigue strength compared to other NC materials in literature and an endurance limit on par with tool steels. Through postmortem characterization combined with atomistic simulations, the underlying mechanism was found to be related to diffuse damage accumulation. During cyclic loading, many grains participate in the plasticity processes (such as the nucleation of dislocations at grain boundaries), resulting in homogenous rather than localized damage accumulation typically observed in CG materials. Atomistic simulations reveal a greatly suppressed damage accumulation in the Cu-3at.%Ta alloy and its more uniform distribution across the microstructure compared with NC Cu. The paper highlights a design strategy to develop advanced structural alloys with exceptional fatigue resistance.