Microstructure evolution during fracture process of gradient twinned ag using molecular dynamics

In recent years, the materials with high strength and good toughness have become a focus of intense research. In this paper, FCC Ag with three different microstructures: single crystal (SC), uniformly arranged twin boundaries (UTB), and gradient arranged twin boundaries (GTB), are designed and compared during the fracture process through molecular dynamics simulation. The result shows that both strength and toughness are the highest in GTB, followed by UTB, and lowest in SC. Differences in three microstructural evolutions are compared to explain macroscopic strength and toughness differences. For SC, brittle cleavage dominates the fracture process, accompanied by the generation of a large number of shear bands. While in UTB, brittle cleavage and crack deflection occur successively, and the propagation of numerous shear bands is segmented by twin boundaries, resulting in the improvement toughness and strength. For GTB, crack tip bluntness and dislocation transmission occur sequentially during the crack initiation stage, and the emitted dislocations discontinuously bridge the gradient arranged twin boundaries and crack tip, which causes the reduction of the local stress and strain experienced by the crack tip. In addition, the shear bands formed from the crack tip in GTB are impeded by the gradient arranged twin boundaries, effectively slowing down the propagation of the plastic zone and the crack tip. The above microstructural mechanisms work together to make GTB exhibit high strength and toughness during the fracture process. Finally, the role of dislocation types is analyzed, Hirth dislocations are activated in advance in GTB, leading to crack tip blunting. The Stair-rod dislocations near the stress peak impedes the plastic deformation in GTB, effectively improves toughness and suppresses the strength decline trend in GTB. This study helps to understand the relationship between microscopic deformation and macroscopic properties of gradient nanotwinned materials.