Deformation evolution during initiation of transgranular stress corrosion cracking

Abstract

Face centered cubic metals and alloys have multiple slip systems and are characterized by high dislocation velocities. Nevertheless, these materials suffer from transgranular stress corrosion cracking (T-SCC), that occurs by environmentally-induced cleavage. Since plasticity precedes fracture in all T-SCC phenomena, the evolution of deformation patterning during T-SCC is an important element of the local microfracture mode. Experimental observations show that the presence of the SCC-causing environment during straining is promoting localized plastic deformation at the near-surface region and producing an entirely different deformation pattern compared with that developing in laboratory air. The deformation evolving in the presence of the SCC electrolyte is highly localized, exhibiting closely spaced, coarse slip bands. The amount of localized strain developing at the near-surface region prior to nucleation of stress corrosion cracks is equivalent to the strain required for ductile fracture of the material in air, suggesting the existence of a fundamental fracture criterion. The above phenomenology of the deformation evolution is considered in relation to T-SCC initiation and propagation. The T-SCC is suggested to be a macroscopically brittle but microscopically ductile fracture occurring by localized plastic flow. An environment-induced deformation localization mechanism is described where the role of the environment involves generation of vacancies and subsequent dislocation nucleation from the near-surface region at loads well below those required for normal yielding. The evolution of the localized deformation pattern during T-SCC is suggested to be an outcome of nonuniformity and periodicity in the dissolution process.