THE GRAIN-SIZE DEPENDENCE OF DUCTILE FRACTURE-TOUGHNESS OF POLYCRYSTALLINE METALS AND ALLOYS

A systematic assessment of the grain size dependence of ductile fractu re toughness has been made using available experimental data from vari ous metals and alloys. Based on this assessment the following semiempi rical equation has been proposed: K-IC=K(IC)degrees+k(F)d(-1) where K( IC)degrees and k(F) are experimental constants and d is the average gr ain diameter. The above equation has been rationalized by dislocation theory. It is proposed that after deformation at large plastic strain a polycrystalline material can be treated as a composite consisting of the grain interior and grain boundary zone. The fracture toughness K- IC of such a composite can be expressed as K-IC=K-IC(GI)+2t(K-IC(GBZ)- K-IC(GI))d(-1) where K-IC(GBZ) and K-IC(GI) are the fracture toughness of the grain boundary zone and the grain interior, and t is the grain boundary zone thickness. The grain size dependence of ductile fractur e toughness has also been discussed in terms of the influence of yield strength and strain to fracture as well as of the effect of deformati on homogeneity across the grain. It is found that the effect of grain boundaries on K-IC is complex; they can either toughen the polycrystal by enhancing the grain boundary deformation or cause embrittlement by promoting microvoid nucleation in the grain boundary zone.