Evolution of fine grained microstructure and superplasticity in warm-worked 7075 aluminum alloy

Hot deformation of warm pre-worked 7075 aluminum alloy was studied by means of 2-step tensile testing at 798 K and at various strain rates from around 10(-5) to 10(-1) s(-1), and metallographic observations. The samples contained unrecrystallized matrices of high density dislocations tangled at fine precipitates, which existed quite stably at 798 K. Rapid flow softening after a stress peak can result from fine grained structure evolved due to continuous dynamic recrystallization and subsequent gradual how hardening from grain coarsening occurring at high strains. The average size of new grains can be expressed by a unique function of flow stress during deformation. Strain rate dependence of flow stresses as well as flow curves changes clearly in the three regions of strain rate, epsilon over dot; i.e. region I (lower epsilon over dot), II (medium epsilon over dot) and III (higher epsilon over dot). Typical superplasticity taking place in region IT leads to the total elongation to fracture of over 750% and the stress exponent of 1.7. It is concluded that hot deformation of 7075 aluminum alloy, affected sensitively by the change of microstructures, can be controlled mainly by grain boundary sliding in region II.