An examination of cyclic grain boundary migration and cavitation in an Al-3% Mg solid solution alloy

Abstract

Experiments were conducted on an Al-3% Mg solid solution alloy in the temperature range from 530 to 700 K using reverse bending fatigue at low frequencies. Metallographie observations after testing revealed the occurrence of extensive grain boundary migration during the cyclic deformation. Using scanning electron microscopy, it is demonstrated that, contrary to observations on high purity aluminum tested under similar conditions, numerous microcavities are formed at the various migratory positions of the boundaries in the Al-3% Mg alloy. These microcavities are formed throughout the cyclic deformation, and the migrating boundaries break away to leave rows of microcavities within the grains. At large numbers of testing cycles, the migration leads to the development of the standard orthogonal grain configuration. The activation energy for grain boundary migration in Al-3% Mg is estimated as 73.6 ± 5.0 kJ mol −1, and this is very close to the value for migration in pure aluminum. By prenucleating some larger cavities prior to testing, it is shown that the boundaries may be locally anchored at large cavities but ultimately the boundaries are able to break away under the action of the applied cyclic stress.