The non-uniform characteristics of cleavage-cracking across high-angle grain boundaries were analyzed in detail. To break through a grain boundary, a cleavage front would first penetrate the boundary at its central part, with the side-sections being locally arrested. Eventually, as the persistent grain boundary areas were separated, the crack-front bypassed the grain boundary. The critical condition of unstable crack propagation was determined by both the local fracture resistance and its increased rate with respect to the expansion of the break-through window. The grain boundary toughness was dominated by the effective grain boundary ductility. The persistent grain boundary outside the break-through window caused a significant crack-trapping effect, which could result in an increase in grain boundary toughness of 5 to 6 times, compared with the local fracture toughness inside the break-through window. The overall fracture resistance offered by a high-angle grain boundary was quite insensitive to the elastic properties and the grain size. The dominant factors were the grain boundary ductility as well as the crystallographic misorientation, particularly the twist misorientation.

Nonuniform Cleavage Cracking Across Persistent Grain Boundary. W.Lu, J.Chen, X.Kong, S.S.Chakravarthula, Y.Qiao: Mechanics of Materials, 2011, 43[10], 567-73