Grain boundary elimination by twinning and dislocation nucleation in front of intergranular crack tips in BCC iron

Abstract

Grain boundary structures with high resistance to intergranular fracture are the target of grain boundary design. In this work, grain boundary elimination in front of crack tip is observed in two special bcc iron bicrystals through molecular dynamics simulations under mode I loading. Grain boundary elimination depends on crack advance direction and enhances resistance to intergranular fracture. Direction-dependent elimination leads to directional anisotropy of intergranular crack propagation. By analytical analysis and molecular dynamics simulation, grain boundary elimination is found to be attributed to the activities of twinning and dislocation. All twinning bands are formed by atomic slip along ordinary twinning direction, but all dislocations are nucleated by atomic slip along anti-twinning direction. Mechanisms of twinning formation and dislocation nucleation are revealed by calculating energy barriers of atomic slip and shear stress field. According to the mechanism of grain boundary elimination, conditions for grain boundary elimination are proposed to find all special grain boundaries. Results show that twist grain boundaries cannot be eliminated under mode I loading, while tilt grain boundaries with axes of 〈110〉 can. This work provides a good reference for grain boundary design.