In Situ Atomic‐Scale Evidence of Unconventional Plastic Behavior at The Crack Tip in AuCu Nanocrystals

Abstract Understanding the plastic behavior of crack tips is crucial for improving the fracture toughness of nanometals. Although many studies are carried out, most previous studies focus on pure metals, and how the crack tip accommodates the plastic deformation of highly concentrated solid‐solution alloys is unclear owing to a lack of direct atomic‐scale evidence. In this study, the atomic‐scale plastic behavior of the crack tip in face‐centered cubic (FCC) AuCu alloy nanocrystals is observed in situ, which provides direct evidence that plastic deformation is governed by the generation of deformation twins and hexagonal close‐packed (HCP) 2H and 4H phases, recurrence of reversible FCC‐HCP phase transitions, and detwinning, which are rarely observed in pure metals. This unusual behavior originates from the inherent chemical inhomogeneity of the AuCu alloy, which inhibits twin thickening via partial dislocations on the adjacent plane, instead of random generation of deformation twins, phase transitions, and reversible processes. This naturally implies a similar behavior at the crack tip in other highly concentrated solid‐solution alloys, including high‐medium‐entropy alloys, providing important insights that greatly improve the understanding of the fracture toughness of metallic materials.