The propagation of transgranular stress-corrosion cracks in admiralty metal

Abstract

An investigation has been made of the propagation of transgranular stress-corrosion cracks in monocrystals and polycrystals of Admiralty Metal tested in ammoniacal solutions. Studies were made of the topography and orientation of the fracture surfaces, the acoustic emission during crack propagation, and the stress dependence of the crack velocity. Cracking was found to occur on parallel but displaced {110 facets, and the steps between the fracture facets also were crystallographic in nature, consisting of alternating segments of two {111 planes. The overall direction of the steps was found to be perpendicular to the crack front, and examination of opposing fracture surfaces indicated that the surface features are matching and interlocking. While not generally resolvable with the SEM, crack-arrest markings could be detected on the fracture faces under certain conditions, and this, together with the occurrence of discrete acoustic-emission events, indicated that propagation is discontinuous. Based on these and other observations, it is suggested that propagation occurs by discontinuous cleavage on {110 planes and that the steps are produced by shear on {111 planes. Possible mechanisms of embrittlement are discussed.