Transient softening in lubricated sliding contacts due to strain path changes

The present work investigates microstructure evolution in pure aluminium under lubricated contact conditions. The initial wear scars were obtained in unidirectional sliding up to a pre-defined number of cycles N. After this pre-imparted severe plastic deformation, the sliding direction was reversed and the resulting microstructure was characterized using hardness measurements and microscopy techniques. The results show that immediately after reversing the strain path, the material significantly softens, as known in uniaxial tension-compression experiments. After the transient softening stage, the new microstructure becomes established and the polycrystal work-hardens again. The results obtained proof the presence of strain path change effects in metals undergoing sliding contact and highlight the importance to take these effects into account in component design in order to avoid undesired material softening.