Enhanced fatigue resistance and fatigue-induced substructures in an additively manufactured CoCrNi medium-entropy alloy treated by ultrasonic surface rolling process

There is a significant need to elucidate the underlying mechanisms of cyclic plastic damage mechanism for additively manufactured materials and develop effective surface modification techniques to improve their fatigue life. This study investigates the efficacy of ultrasonic surface rolling process (USRP) technology in the creation of a ∼300 μm gradient nanotwinned structure on the surface of additively manufactured CoCrNi medium-entropy alloy (AM-MEA), which results in a beneficial result that yield strength and 107-cycle fatigue endurance limit are significantly improved, achieving the increment of 192.1 MPa and ∼130 MPa, respectively. The superior fatigue property is attributed to multiple factors that suppress crack initiation from sample surfaces jointly, including the presence of a gradient nanotwinned layer and the reduction in irregular defects located both on and beneath the surface. The cyclic plastic deformation behavior of AM-MEA samples with and without USRP under both high and low stress levels was studied in-depth through multiscale characterization techniques. When exposed to cyclic loading at a low stress level of 480 MPa, the fatigue damages of both samples were dominated by accumulation of statistical stored dislocations (SSDs) and persistent Lüders bands. There is no significant difference in the increase in dislocation density between both samples. However, under cyclic loading at a high stress level (660 MPa), the fatigue damage of the AM-MEA sample primarily originated from the accumulation of deformation nanotwins, stacking faults, geometrically necessary dislocations and SSDs. Conversely, the fatigue damage observed in the AM-MEA sample with USRP at the same stress level was dominant by an increase in stacking faults and SSDs. Notably, this increase in total dislocation density was visibly lower than that observed in the AM-MEA sample, which is ascribe to the stable gradient layer providing enhanced hetero-deformation induced stress for the core region in the AM-MEA sample with USRP at high stress level.