Effect of grain size on fatigue limit in CrMnFeCoNi high-entropy alloy fabricated by spark plasma sintering under four-point bending

Abstract

In order to investigate the effect of grain size on the fatigue limit of single-phase equiatomic high-entropy alloys (HEAs); CrMnFeCoNi, fabricated by spark plasma sintering, four-point bending fatigue tests were conducted at stress ratio of 0.1 under ambient condition. The surfaces of fatigued specimen were observed by in-situ optical microscopy, and fatigue crack initiation and propagation behaviors were analyzed by electron backscatter diffraction to elucidate the fatigue fracture mechanism in CrMnFeCoNi alloy with different grain sizes. The grain size of CrMnFeCoNi sintered compacts decreased with decreasing particle size of CrMnFeCoNi powders, and their fatigue limits tended to increase with decreasing grain size due to the Hall-Petch effect. Furthermore, fatigue cracks in the coarse-grained CrMnFeCoNi sintered compact were initiated at the boundary of annealing twins, whereas annealing twin boundaries were not observed near the crack initiation site of the fine-grained one. The fraction of twin boundaries in CrMnFeCoNi alloy tended to decrease with decreasing grain size; therefore, the grain refinement is an effective approach for increasing the fatigue limit of CrMnFeCoNi alloy because of both the Hall-Petch effect and the increase in the fatigue crack initiation resistance.