The role of Grain-Boundary Migration on Irradiation-Fatigue

Nanocrystalline metals and alloys have been shown to undergo fatigue-induced grain growth. Under high-cycle fatigue loading, the grain growth process precedes crack initiation and is thought to be a necessary precursor when the initial grain sizes cannot support persistent slip-based nucleation.<br/>Using a newly developed radiation-fatigue ion accelerator endstation [Briggs et al., Nucl Inst. Methods B, 2021], we have evaluated the superimposed effect of He+ irradiation on high-cycle fatigue response of nanocrystalline Ni-Fe. Surprisingly, the Ni-Fe alloy exhibited extended high-cycle fatigue lives under He+ irradiation compared to unirradiated conditions. We attribute this improved fatigue performance to the role of He+ irradiation in pinning grain boundaries. The new endstation enables future combined environment studies with ion beam modification using a 6 MV Tandem accelerator, heating up to 1200C, uniaxial loading up to 4.5 kN, or cyclic loading at rates of up to 50 Hz.<br/>To complement this capability, we employ in-situ SEM and in-situ TEM capabilities for high-cycle fatigue and irradiation-fatigue experiments. In-situ TEM high-cycle fatigue experiments on electron transparent thin films of nanocrystalline Ni and Cu have revealed not only microstructural-sensitive crack propagation, but also unexpected microstructural-scale crack healing. Based on the experimental observations, atomistic modeling, and continuum-scale microstructural modeling, the mechanism appears to be crack flank cold welding facilitated by local compressive microstructural stresses and/or grain boundary migration. While these observations are specific to pure nanocrystalline metal thin films under a high-vacuum environment, there are potentially much broader ramifications. The existing observations can be used to help rationalize suppressed fatigue crack propagation rates in vacuum, subsurface, or under contact-inducing mixed-mode stresses; and even the precipitous decline in propagation rates near the fatigue threshold.<br/>In addition to these novel experimental capabilities, we employ atomistic and hybrid atomistic kinetic monte carlo models to understand the mechanisms of microstructural evolution under fatigue and irradiation conditions. Simulated polycrystals under cyclic loading are shown to exhibit fatigue-induced grain coarsening analogous to that observed experimentally. Irradiation can also induce grain boundary migration, through a mechanism that involves both short-range processes (e.g. thermal spikes and atomic shuffling) and long-range interactions between grain boundary defects (e.g. facet junctions).<br/>Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.