MOLECULAR DYNAMIC STUDY OF GRAIN-BOUNDARY EMBRITTLEMENT FOR [101] TILT COPPER BICRYSTALS INDUCED BY BISMUTH SEGREGATION

The selective bismuth segregation and the microprocess of fracture for the three [101] tilt copper bicrystals SIGMA-9(212BARBAR) 38-94-degrees, SIGMA-11(323BARBAR) 50.48-degrees, and SIGMA-33(545BARBAR) 58.99-degrees have been studied by a molecular dynamics technique. The results show that the Bi segregation and the fracture behavior of the Cu-Bi bicrystals are strongly dependent on the grain boundary (GB) structure. The Bi segregation is strongly related to the polyhedra constructing the GB cores and the stress fields of the GB dislocations (GBDs), and the GB embrittlement of copper induced by the Bi segregation is determined by the segregated concentration and the distribution of Bi atoms. With the increase of the relative number of pentagonal bipyramids and the localization of the stress fields of the GBDs in the GBs, the bicrystals SIGMA-9, SIGMA-11, and SIGMA-33 show a decreasing propensity for the Bi segregation and subsequent different fracture behaviors. The severe intergranular brittle fracture that happens in the SIGMA-9 bicrystal is mainly caused by the breaking of weakened Cu-Cu bonds, which is related to the highly concentrated Bi segregation at the GB core. In the case of the SIGMA-11 bicrystal, the segregation of Bi atoms at the GB shows an inhomogeneous distribution characteristic, so that the fracture is intergranular but with a large amount of shear-deformation. The transgranular fraction that appears in the SIGMA-33 bicrystal is related to the low concentration of the Bi segregation and the dispersive distribution of the Bi atoms along the GB and in the grains.