Relationships of fracture toughness and dislocation mobility in intermetallics

An analytical method has been developed and used to compute the Peierls-Nabarro (P-N) barrier energy, UP-N, for relevant slip systems in several intermetallics, including NiAl, FeAl, Nb-Ti-Al (B2), Ni3Al (L1(2)), TiAl (L1(0)), TiCr2, NbCr2 (C14, C15), Nb5Si3 (D8(l)), Mo5SiB2 (D8(l)), and Mo5Si3 (D8(m)). The P-N barrier energy and a generalized fault energy, gamma(F), are combined and used as a measure of dislocation mobility. Furthermore, a fracture model has been developed to describe the process of thermally activated dislocations moving away from the crack tip and to predict the corresponding fracture resistance. A ductility index defined in terms of the ratio of gamma(s)/(UP-N + gamma(F)), where gamma(s) is the surface energy, is used to correlate with the fracture toughness, K-C, of individual intermetallics. The correlation indicates that fracture toughness increases with increasing values of gamma(s)/(UP-N + gamma(F)), in accordance with the fracture model formulated based on thermally activated slip. The use of the fracture model for predicting the effects of slip behavior, temperature, and alloy additions on fracture resistance is demonstrated for selected intermetallics including NiAl, TiAl, Laves phase, and Nb5Si3.