Microstructure and high temperature deformation of an ultra-fine grained ECAP AA7075 aluminium alloy

Abstract

AbstractAn AA 7075 aluminium alloy with an ultra-fine grained structure was prepared through equal channel angular pressing (ECAP) at pressing temperatures TECAP of 120, 170, and 220 °C. A decrease in TECAP from 220 to 120 °C was found to lead to a more pronounced refinement of the microstructure and to worse stability of the microstructure – the onset of grain coarsening was displaced to lower temperatures. The material pressed with the highest TECAP exhibited superplastic behaviour at temperatures close to 400 °C and grain boundary sliding was identified as the dominant operating deformation mechanism. The materials prepared with both of the lower TECAP exhibited only enhanced ductility of about 200 %, however this behaviour was observed at temperatures as low as 200 °C. It was found that this “low temperature superplasticity” resulted from a combined operation of grain boundary sliding at selected grain boundaries and glide of lattice dislocations.