Formation Process of Surface Markings during Fatigue of Aluminum at Elevated Temperatures

Abstract

Observations were conducted on the specimen surface after high temperature fatigue of aluminum by means of optical and interference microscopy. The surface markings produced during high temperature fatigue were convex or concave under the action of tension-compression stress cycles, whereas they were like stairs under the action of tension-tension stress cycles. Based on the metallographic observations, a model for the formation process of surface markings was proposed. It was confirmed experimentally that the first faint groove, which appeared in a specimen subjected to a process of annealing-mechanical polishing-electrolytic polishing prior to a fatigue test, corresponds to the position of grain boundary just after annealing. However, the grain boundary position just before the fatigue test was different from that after electrical polishing. This is because the grain boundaries which intersect free surface are liable to move during the heating-up process prior to high temperature fatigue in order to decrease their interfacial energy. This explains the first large displacement of grain boundary migration. A tentative model was proposed to account for the reason why the displacement of grain boundary migration during high temperature fatigue is large notwithstanding the small unidirectional deformation.