Impurity promoted intergranular fracture

Abstract

Abstract Although ductile transgranular fracture is common in polycrystalline metals, intergranular failure by brittle fracture can be promoted by grain boundary segregation of impurity atoms. The balance between ductile and brittle fracture is altered in favour of brittle fracture by a reduction in cohesive strength in the neighbourhood of the crack tip (Edwards and Eyre 1980). The perturbation in the local density of electron states in the neighbourhood of an impurity site is calculated using a tight-binding model. A reduction in the binding of a metal atom to its nearest neighbours is obtained when electron hopping between the atom and the impurity is taken into account, this change in cohesion therefore acting to promote brittle fracture. Charge neutrality of the impurity is assumed in contrast to the model of Troiano (1960) which relates the loss of cohesion to an assumed transfer of impurity electrons into the transitionmetal d band.