Host: The Japan Society of Mechanical Engineers
Name : [in Japanese]
Date : November 02, 2019 - November 04, 2019
High-cycle fatigue crack initiation and propagation in an equiatomic CrMnFeCoNi high-entropy alloy (HEA) were investigated using smooth specimens. The microstructural deformation characteristics, i.e., planar dislocation slip, significantly affected the fatigue crack initiation and small fatigue crack propagation. Multiple crack initiation was observed. In the grain where a crack initiated, the fatigue crack plane at the initiation site was along the slip plane, but the planar dislocation array was aligned on another slip plane, which suggests slip deformation highly localized on a single {111} atomistic layer where the crack initiated. High dislocation planarity of HEA caused the localization of slip deformation, which led to multiple crack initiation. Dislocation planar array was observed near the fatigue crack of HEA. In the same line, the planar dislocation motion associated with a low SFE increases the dislocation density on specific slip planes and stress arising from the nearest dislocation to the crack tip, which strongly shields crack tip stress during loading. This effect is referred to as dislocation stress shielding. Dislocation-driven stress shielding contributed to the decrease of fatigue crack growth rate. Here, considering the crack propagation mechanism, we recognize two modes: (A) Mode I type driven by dislocation emission from a crack tip, and (B) Mode II type associated with dislocation accumulation on limited slip planes ahead of crack tip. In HEA, fatigue crack propagation along slip lines was frequency observed, which suggested the promotion of Mode II crack propagation. Mode II crack propagation was promoted because Mode I crack propagation was suppressed by dislocation-driven stress shielding and instead shear-induced damage was localized along a slip plane near the crack tip.
