Grain boundary sliding and grain refinement in as-cast 7475 aluminum alloy under hot-deformation

Hot deformation and microstructural changes in an as-cast 7475 Al alloy were studied at a temperature of 490degreesC in the range of strain rates from 10(-5) to 10(-1) s(-1). The two deformation domains can be categorized due to different mechanical and microstructural behaviors. At strain rates lower than similar to 10(-2) s(-1), the sigma vs epsilon behavior shows a significant strain softening just after yielding and a steady-state flow at higher strains. The structural changes are mainly characterized by development of deformation bands at an early stage of deformation, followed by new grain evolution during steady state flow due to operation of grain boundary sliding (GBS). Grain refinement occurs by a deformation-induced continuous reaction; that is continuous dynamic recrystallization. In the region of strain rate higher than similar to10(-2) s(-1), a steady-state flow following a yield drop takes place in a relatively low strain and no grain refinement occurs even in large strain. The critical condition and the mechanism of grain refinement in connection with GBS are discussed.