The grain size dependence of ductile fracture toughness of polycrystalline metals and alloys

A systematic assessment of the grain size dependence of ductile fracture toughness has been made using available experimental data from various metals and alloys. Based on this assessment the following semiempirical equation has been proposed: KIC=KIC°+kFd−1 where KICo and kF are experimental constants and d is the average grain 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 KIC of such a composite can be expressed as KIC=KICGI + 2t (KCICGBZ−KICGI)d−1 where KICGBZ and KICGI 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 fracture toughness has also been discussed in terms of the influence of yield strength and strain to fracture as well as of the effect of deformation homogeneity across the grain. It is found that the effect of grain boundaries on KIC 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.