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The microprocess of deformation and fracture for pure and Bi‐segregated Σ3(1¯1¯1)/ 70.53° and Σ33(5¯4¯5)/ 58.99° tilt bicrystals of metal copper has been studied by the molecular dynamics method. It has been found that deformation and fracture are dependent on the grain boundary (GB) structure and bismuth segregation. For pure Σ33 bicrystal, the deformation is mainly due to the glide of partial dislocations generated from the GB structural units where the GB dislocations exist. The ductile fracture is attributed to the dislocation emission, which leads to vacancy generation and void coalescence. The bismuth segregation weakens the atomic bonds between copper atoms in the vicinity of GB. Under the action of the external load, the weakened bonds break and lead to formation of microcracks. Finally, the brittle fracture takes place along the binding weakening region. For Σ3 bicrystal, the ductile fracture is related to the void coalescence generated not by dislocation emission but by lattice distortion, and the brittle fracture induced by bismuth segregation is also caused by the breaking of weakened Cu—Cu bonds.