Temperature effect on fine-grained structure formation in high-strength Al alloy 7475 during hot severe deformation

The effect of deformation temperature on grain refinement was studied in a high-strength 7475 Al alloy during multi-directional forging at temperatures from 573 to 763 K. Fine-grained structure is developed mainly by formation of microshear bands (MSBs) followed by increase in their number and boundary misorientation. With increase in processing temperature, the mechanism of plastic flow is gradually changed from that controlled by dislocation motion to that controlled mainly by grain boundary sliding (GBS). In the intermediate temperature range 573– 673 K, ultra-fine-grained (UFG) structure is developed in colony only near original grain boundaries in early deformation. At larger strains, GBS operating locally in these UFGed regions evolved may enhance non-homogeneous deformation, followed by MSBs crossing in original grain interiors, leading to acceleration of the kinetics of grain refinement. The main factors promoting grain refinement at elevated temperatures and the role of the operating deformation mechanisms in new grain formation are analyzed and discussed in detail.