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

An AA 7075 aluminium alloy with an ultra-fine grained structure was prepared through equal channel angular pressing (ECAP) at pressing temperatures T-ECAP of 120, 170, and 220 degrees C. A decrease in T-ECAP from 220 to 120 degrees 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 T-ECAP 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 T-ECAP exhibited only enhanced ductility of about 200%, however this behaviour was observed at temperatures as low as 200 degrees 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.