The role of grain boundary sliding in microstructural evolution during superplastic deformation of a 7055 aluminum alloy

The microstructure evolution in a 7055 aluminum alloy subjected to thermomechanical processing (TMP) was studied at 450degreesC and (epsilon) over dot = 1.7 x 10(-3) s(-1) at which the material exhibits super-plastic behavior with a total elongation of 720% and the coefficient m = 0.58. Partially recrystallized initial structure of the as-processed 7055 Al consisted of bands of recrystallized grains with a mean size of 11 mum alternating with bands of recovered subgrains with a mean size of 2 mum. The true stress-true strain curve exhibits a well-defined peak stress, followed by gradual strain softening. The coefficient of strain rate sensitivity, In, remains unchanged at epsilon less than or equal to 1 and tends to decrease with strain at epsilon > 1. The initial microstructure persists near the peak strain. Following strain leads to evolution of initial partially recrystallized structure into uniform fully recrystallized structure due to occurrence of continuous dynamic reactions, i.e. continuous dynamic recrystallization (CDRX). The data of microstructural observation and misorientation analysis show that low-angle boundaries (LAB) gradually convert to high-angle boundaries (HAB) resulting in an extensive flow softening. It was shown that grain boundary sliding (GBS) provides superplastic flow at all strains. Concurrently, GBS plays an important role in the dynamic evolution of new grains facilitating conversion of LABs to HABs.