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 $\left(111\right)$ planes in matrix grain and propagate along $\left(11\overline{1}\right)$ 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.

本文通过分子动力学模拟研究了纳米孪晶Ag断裂过程中微观结构的演变以及孪晶界的作用。将具有多个孪晶边界的纳米孪晶Ag的断裂过程与单晶Ag和具有单个孪晶边界的纳米孪晶Ag的断裂过程进行了比较。结果表明，多个孪晶边界可以通过影响裂纹扩展和位错，从而显着提高材料强度，如实验观察到的那样。通过比较应力-应变关系和微观结构的演变，我们发现单晶Ag的弹塑性转变的特征是裂纹尖端的位错发射，而纳米孪晶Ag的弹塑性转变的特征是位错越过孪晶边界。根据抗裂曲线和微观组织演变的观察结果，与单孪晶边界相比，多个孪晶边界可以显着提高纳米孪晶银的断裂韧性。孪晶边界在断裂过程中的作用主要是影响裂纹的扩展和位错。单晶Ag的裂纹尖端随着位错的成核和发射而逐渐变钝，而纳米孪晶Ag的裂纹则通过解理扩展。为了解释这种现象，计算并比较了纳米孪晶Ag和单晶Ag的广义堆垛层错能和表面能。结果表明，孪晶边界可以影响原子滑移的能垒，而对断裂面的生成没有影响。双晶界也可以改变位错和堆垛层错的传播方向。堆垛层错沿$（\mathrm{1个}\mathrm{1个}\mathrm{1个}）$ 平面在基体颗粒中传播 $（\mathrm{1个}\mathrm{1个}\overline{\mathrm{1个}}）$双晶的飞机，呈现出曲折的形状。在此过程中，位错在孪晶边界上积累，从而导致高位错密度和纳米孪晶材料的高强度，这与实验和模拟一致。我们的研究有助于理解纳米孪晶材料的微观形变与宏观性能之间的关系。