Inter-granular and Intra-granular Crack Behavior in Mg Bicrystal of [] Symmetric Tilt Grain Boundary: LEFM Prediction and Atomic Simulation

The grain boundary (GB) influences crack propagation, and the intrinsic brittleness/ductility of GB may be related to the interfacial structures. This work examined the effects of [\(1\overline{2 }10\)] symmetrical tilting grain boundary (STGB) on the crack behaviors in bicrystal Mg to investigate the fracture mechanisms of hexagonal close-packed Mg. The [\(1\overline{2}10\)] STGB bicrystal Mg models were constructed via the molecular dynamics method, and the cleavage/emission crack-tip behaviors of pre-existing inter-granular and intra-granular cracks under Mode I crack-tip stress intensity loading at T = 0 K were first studied using anisotropic linear elastic fracture mechanics and atomic simulations. For inter-granular cracks, the crack-tip behaviors depend on the intrinsic brittleness/ductility of the [\(1\overline{2 }10\)] STGB. The crack at the brittle GB and the nucleated voids around the crack tip cause crack propagation via void growth and coalescence. The crack tip at the ductile GB was blunted by dislocation emission around the crack tip. The results show that by introducing ductile [\(1\overline{2 }10\)] STGB, the crack-tip can be blunted, and the GB fracture can be prevented in Mg. For intra-granular cracks, the brittle crack tip cleaved along its cleavage plane, and the crack tip showed cleavage/emission competitive behaviors due to the influences of GB stress. The stress field of the crack-tip induced the migration of [\(1\overline{2 }10\)] STGB. Both the cleavage and emission behaviors were observed at the crack tip with very close critical stress intensity for Griffith cleavage (K_Ic) and dislocation emission (K_Ie).

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