Microstructure sensitivity of the low cycle fatigue crack initiation mechanisms for the Al0.3CoCrFeNi high entropy alloy: in-situ SEM study and crystal plasticity simulation

Abstract

In this study, in-situ fatigue tests under scanning electron microscopy (SEM) were performed to reveal the relationship between grain slip deformation and fatigue crack initiation of Al0.3CoCrFeNi high entropy alloy (HEA). It was found that the fine grains (FG) exhibited a higher fatigue crack initiation life than that of the coarse grains (CG), which is related to the inhibition effects of twins and grain boundaries on fatigue crack initiation. The initiation of fatigue cracks in both the CG and FG specimens occurred in the area of high density of slip lines. The fatigue cracks were observed to deviate from the single slip line, which is related to the activation of multiple slip systems inside the grain, leading to the alternation of fatigue crack propagation path. In addition, the intersection of different slip planes can be another preferential crack initiation site if two or more slip planes are activated inside one grain. By combining EBSD analysis and crystal plasticity finite element modeling (CPFEM), the influence of microstructures, such as grain orientation and grain boundaries, on the fatigue crack initiation mechanism was further analyzed. The CPFEM results confirmed that the CG specimen resulted in a higher cumulative shear strain (CSS) than that of the FG specimen under the same stress level, which led to higher fatigue damage.