Brittle behavior of ductile metals during stress-corrosion cracking

Abstract

The transgranular stress-corrosion cracking of ductile alloys (and at least one pure metal) leads to a fracture appearance characteristic of cleavage. The experimental and theoretical background of this form of metal failure is reviewed. The discontinuous nature of crack growth in α-brass and copper is demonstrated using a combination of acoustic and electrochemical measurements. Short-range cleavage of ductile metals is shown to be theoretically possible provided the thin surface film which is responsible for crack initiation has the appropriate properties. The important parameters determining the effectiveness of films in initiating cleavage include the film-substrate misfit, the strength of the bonding across the film-substrate interface, the film thickness and the film ductility. Suitable combinations of these parameters which can lead to micro cleavage are determined by the state of coherency of the interface and the fracture toughness of the substrate. For example, if a ductile de-alloyed layer in a brass remains coherent with the substrate and has a misfit of 0–02 (equivalent to complete de-alloying of a 70-30 brass), it must be ∼200 monolayers thick in order to initiate a microcieavage event. Crack jump distances of 1-10/an are rationalized using a model derived in part from molecular-dynamics simulations.