Molecular dynamics simulation of microstructure evolution during the fracture of nano-twinned

In this paper, the evolution of microstructure and the role of twin boundary during the fracture process of nano-twinned Ag are studied through molecular dynamics simulation. The fracture process of nano-twinned Ag with multiple twin boundaries is compared with both the fracture process of single crystal Ag and nano-twinned Ag with single twin boundary. Results demonstrate that multiple twin boundaries can significantly improve the strength of material by influence the propagation of crack and dislocation as observed by experiments. By comparing the stress-strain relationship and evolution of microstructure, we found that the elastoplastic transition in single crystal Ag is characterized by dislocation emission from the crack tip, while the elastoplastic transition in nano-twinned Ag is characterized by dislocation crossing the twin boundary. According to crack-resistance curves and observation results of microstructure evolution, multiple twin boundaries can significantly improve the fracture toughness of nano twinned Ag compared to single twin boundary. The role of twin boundary in the fracture process is mainly to influence the propagation of crack and dislocation. The crack tip in single crystal Ag gradually gets blunt with the nucleation and emission of dislocations, while the crack in nano-twinned Ag propagates by cleavage. To explain this phenomenon, the generalized stacking fault energies and surface energies of both nano-twinned Ag and single crystal Ag are calculated and compared. It is shown that twin boundary can influence the energy barrier for atomic slip while have no influence on the generation of fracture surface. Twin boundary can also change the propagation direction of dislocations and stacking faults. Stacking faults propagate along (1 1 1) planes in matrix grain and propagate along (1 1 1) planes in twin grain, showing a tortuous shape. In this process, dislocations accumulate on the twin boundary, leading to high local dislocation density and high strength of nano-twinned material, which is agreement with experiments and simulations. Our study is helpful for understanding the relationship between microscopic deformation and macroscopic properties of nano-twinned crystal material.