Directional Anisotropy of Crack Propagation Along $\Sigma$3 Grain Boundary in BCC Fe

Crack growth behaviour along the coherent twin boundary (CTB), i.e., $\Sigma$3{112} of BCC Fe is investigated using molecular dynamics (MD) simulations. The growth of an atomistically sharp crack with {112}$<$110$>$ orientation has been examined along the two opposite $<$111$>$ directions of CTB under mode-I loading at a constant strain rate. Separate MD simulations were carried out with crack inserted in the left side, right side and middle of the specimen model system. The results indicate that the crack grows differently along the two opposite $<$111$>$ directions. In case of a crack inserted in the left side, the crack grows in ductile manner, while it propagates in semi-brittle manner in the case of crack inserted in the right side. The directional dependence of crack growth along the CTB is also confirmed by the stress-strain behaviour. This anisotropy in crack growth behaviour has been attributed to the twinning-antitwinning asymmetry of 1/6$<$111$>$ partial dislocations on {112} planes.