Abstract
Dynamic evolution of fine grains and superplasticity of an unrecrystallized 7075 aluminum alloy were studied by means of two-step tensile testing as well as optical, SEM and TEM metallographic observation. Typical superplasticity takes place accompanied by the evolution of new fine grains in the medium region of strain rate at 798 K. Newly evolved grain size decreases with increase in the reduction of prior cold-rolling, i.e. with decrease in the thickness of layered initial grains (DST). Strain rate dependence of flow stress and total elongation to fracture is sensitively affected by cold-rolling reduction because of changes in DST. Grain boundary sliding (GBS) frequently takes place just after yielding even in the layered pre-existing grain boundaries parallel to tensile axis. With further straining, GBS brings about subgrain rotation, followed by transformation of low angle boundaries to high angle ones. Repitition of this process can result in the evolution of new fine grains with high angle boundaries at high strains. It is concluded from the mechanical and metallographic results that GBS can play a key role in the dynamic evolution of fine grains as well as the appearance of superplasticity.
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