Incidence of Grain Boundary Cracking in Creep Fracture

Abstract

IN a recent investigation of void nucleation¹ the incidence of cavitation was determined metallographically by an examination of longitudinal sections of fractured specimens. Lines were drawn parallel and at right angles to the direction of the applied stress, and all grain boundaries intersected by these lines were separated into three groups depending on the angle made on the polished section of the specimen by the grain boundary with the tensile axis. For each angle interval (0°–30°, 30°–60°, 60°–90°), a count was made of the number of boundaries with and without cavities located on them. The work was carried out on a dilute nickel alloy (containing 0.1 atomic per cent gold) and the incidence of cavitation measured over a limited range of tensile creep stresses. Over this narrow range, the crack incidence appeared to be independent of the applied tensile stress. It was therefore decided to apply the same method of assessing crack incidence over a wider stress range.