How fatigue frequency and load level govern grain growth kinetics in indentation and micro-bending beam fatigue in confined volumes

Nanocrystalline metals offer exceptional strength but are prone to microstructural instability under cyclic loading. Fatigue-induced grain growth, in particular, limits their long-term reliability. This work examines how fatigue frequency, load ratio, and volume confinement govern microstructural evolution in nanocrystalline nickel across different loading regimes. Bulk indentation fatigue produces predominantly homogeneous grain coarsening confined to the plastically deformed zone, whereas micromechanical bending fatigue promotes localized grain growth along fatigue cracks and ahead of crack tips. Increasing fatigue frequency in indentation fatigue further alters the deformation mode, shifting the response from pileup-dominated plasticity toward material flow and grain alignment. These results identify fatigue frequency, load level, and confinement as key boundary conditions controlling fatigue- and strain-induced grain growth in nanocrystalline metals.