Toughening of brittle materials by grain boundary engineering

Microstructural aspects of fracture processes and the origin of the brittleness in brittle materials are discussed in connection with structure-dependent intergranular fracture. The possibility of the control of “intrinsic” and “extrinsic” brittleness in polycrystalline materials by the grain boundary engineering is proposed on the basis of the optimization of the grain boundary microstructure by using newly introduced parameters, i.e., the grain boundary character distribution (GBCD) and the grain boundary connectivity. Several examples of successful control of the brittleness achieved by the present authors and coworkers are shown for intrinsic intergranular brittleness in molybdenum, and for oxidation-induced intergranular brittleness in nickel–iron alloy and SiC. The effect of GBCD on the fracture toughness may become significant in nanocrystalline materials with high density of grain boundaries.