Wear-induced microstructural evolution of ultra-fine grained (UFGs) aluminum

We investigated hardness and tribological properties of ultra-fine grain (UFG) aluminum samples prepared by accumulative rolling bonding (ARB) and by physical vapor deposition (PVD). We have found wear-induced grain refinement in this material for the first time and we have identified a transition from grain growth to grain refinement as a function of the mean contact stress. This trend is the same for the PVD and ARB samples, which means it does not depend on the synthesis method. The hardness was found to increase with a decreasing grain size, but the results deviated from the Hall-Petch relation. Among ARB samples, UFG Al with the highest initial hardness was found to be the most wear-resistant. However, PVD-grown UFG Al was found to be most wear-resistant among all samples, even though it was not the hardest. The reason lies in the different dislocation contents of the samples prepared by different methods and in a subsequent wear-induced evolution of dislocation networks. The mechanisms of microstructural evolution in UFG Al were analyzed using transmission electron microscopy and the main mechanism identified is the dynamic recrystallization (DRX), including both continuous DRX and discontinuous DRX.