Fatigue Crack Growth Behavior and Associated Microstructure in a Metastable High-Entropy Alloy

High-entropy alloys (HEAs) containing different kinds of high-concentration solute atoms provide new concepts for obtaining excellent balance of strength and ductility. In particular, a metastable dual-phase HEA (Fe30Mn10Cr10Co; FCC matrix and HCP second phase) shows superior ductility and strength owing to the transformation-induced plasticity effect associated with deformation-induced HCP-martensitic transformation. In this context, the fatigue properties of metastable HEAs are to be investigated towards practical applications as structure materials. In this study, the fatigue crack growth behaviors of HEA and type 316L austenitic stainless steel (FCC single phase) were comparatively examined. The crack growth rate of HEA was comparable to that of 316L. In HEA, the fatigue crack was covered by a large amount of HCP-martensite. In general, the HCP-martensite was cracked easily because of the smaller number of slip systems. However, the negative effect of HCP-martensite did not appear in the fatigue crack growth rate of HEA. By electron channeling contrast imaging, we found that the HCP-martensite beneath the fracture surface contained significant orientation gradient and high density of dislocations, indicating that HCP-martensite in the present Fe30Mn10Cr10Co HEA had high plastic deformability and associated stress accommodation capacity.