Microstructural Evolution of Ultra-Fine Grained (UFGs) Aluminum in Tribological Contacts

Grain refinement is a promising strategy for improving wear resistance of metals, but in Al, previous studies have often shown undesirable wear-induced grain growth. Here, we investigated tribological properties and microstructural evolution during sliding of ultra-fine grain (UFG) aluminum prepared by accumulative rolling bonding and by physical vapor deposition (PVD). The wear-induced grain refinement was observed for the first time in UFG Al and we have identified a transition from grain growth to grain refinement as a function of the mean contact stress. According to the observations from transmission electron microscopy, the mechanism underlying the microstructural evolution is dynamic recrystallization. We have also found that the PVD-grown UFG Al was the most wear-resistant among all the samples, even though it was not the hardest one. This finding means that hardness and grain size alone are not sufficient to predict trends in wear resistance.