Hydrogen and interfacial cohesion

Hydrogen effects on interfacial cohesion are discussed in relation to models for atomistically brittle cracking. An analysis is presented of the condition for stability against dislocation blunting of a critically stressed, atomistically sharp interfacial crack tip configuration, the satisfaction of which is viewed as a pre-condition for the occurrence of brittle interfacial separation. Results are applied to estimate the necessary effect of dissolved H on cohesive energies, so as to allow brittle grain boundary cracking of fcc metals, in which grain interfaces are not normally capable of sustaining such cracks. In addition, a new approach is presented to the thermodynamics of interfacial separation in presence of a segregated mobile species such as H, and some generalizations of the Gibbs adsorption relation are derived expressing the dependence of critical fracture parameters, namely the interfacial separation energy and cohesive strength, on the equilibrating potential and surface concentration of the segregant.