The toughening effect of twins on fracture in nanotwinned Cu during cyclic loading

Extensive studies have been focused on improving the fatigue performance of materials, as fatigue fracture is a primary mode of failure in material applications for structural components. The introduction of high-density twins emerges as a novel strategy for enhancing fracture toughness; however, the mechanisms driving this enhancement during fatigue remain elusive. In this work, the impact of twins on fracture toughness in nanotwinned Cu was investigated with correlated transmission electron microscopy under cyclic loading. The results revealed three toughening mechanisms, i.e., bridging, bending of twin lamellae and ductile cracking along twin boundaries. The occurrence of the three toughening mechanisms strongly depends on twin thickness and cracking orientations, as different dislocation modes are activated in varying scenarios. Specifically, bridging and ductile cracking along twin boundaries are facilitated by the pile-up and continuous emission of hard mode I dislocations, respectively, while the bending of twin lamellae is governed by the accumulation of same-signed soft mode dislocations triggered by crack-tip stress. These findings unveil the underlying toughening mechanisms of twins in nanotwinned metals during cyclic loading and offer valuable guidance for the design of high-toughness nanolayered materials.