Orientation effect on intergranular fracture behaviors along symmetrical tilt grain boundaries in bcc iron

The fracture of crystalline metals preferentially occurs in intergranular way. This work investigates orientation effect on intergranular fracture behaviors along symmetrical tilt coherent and incoherent Sigma 3 grain boundaries (GBs) in bcc iron. The analytical results obtained based on Rice concept and numerical results obtained based on molecular dynamics theory are presented. The two inconsistencies observed between the analytical and numerical results achieved for stacking fault formation on the plane coincident with crack plane and twinning formation on the plane not coincident with crack plane are fully discussed. The results in this work show that intergranular cracks on coherent Sigma 3 GB prefer to propagate in a ductile way, while those on incoherent Sigma 3 GB prefer to propagate in a brittle way. Along both symmetrical tilt coherent and incoherent Sigma 3 GBs, intergranular crack propagation depends on advance direction and usually presents directional anisotropy due to non-mirror symmetrical atomic distribution along the plane vertical to crack advance direction. Intergranular crack propagation also depends on front direction. On account of the differences in the type and nucleation ability of plastic behavior, intergranular cracks with different front directions have different ductile-brittle levels and corresponding models have different maximum tensile stresses. Investigation on orientation effect can provide a good reference to improve material reliability.