The True Origin of Ductile Fracture in Aluminum Alloys

It has generally been assumed that metals usually fail as a result of microvoid nucleation induced by particle fracture. Here, we concentrate on high-density micropores filled with hydrogen in aluminum, existence of which has been largely overlooked until quite recently. These micropores exhibit premature growth under external loading, thereby inducing ductile fracture, whereas the particle fracture mechanism operates only incidentally. Conclusive evidence of a micropore mechanism is provided by the observation of an instantaneous release of gas at failure. We can therefore conclude that the growth of micropores dominates ductile fracture. Since the material we used has a standard pore density, we can assume that an identical fracture mechanism operates in other aluminum alloys. This finding suggests that intense heat treatment, which is generally believed to enhance the mechanical properties through homogenization, may have entirely the opposite effect. This revelation will have a major impact on the engineering design of metals. (C) The Minerals, Metals & Materials Society and ASM International 2013