Abstract
The microstructure evolution in a 7055 aluminum alloy subjected to thermomechanical processing (TMP) was studied at 450°C and \\dotε=1.7×10−3 s−1 at which the material exhibits superplastic 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 \\micron alternating with bands of recovered subgrains with a mean size of 2 \\micron. The true stress–true strain curve exhibits a well-defined peak stress, followed by gradual strain softening. The coefficient of strain rate sensitivity, m, remains unchanged at ε≤1 and tends to decrease with strain at ε>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.
