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

•Ultrasonic surface rolling process improved yield strength and fatigue properties of additively manufactured medium-entropy alloys.•Dislocation type and multiplication at low and high stress level was semi-quantitative analyzed by neutron diffraction and EBSD.•Gradient nanotwinned layer and the reduction in irregular defects on and beneath the surface suppressed fatigue crack initiation.•Gradient nanotwinned layer provided hetero-deformation induced stress at high stress level decreasing dislocation multiplication.