Dynamic evolution of the metastable structure and nano-precipitation of 7055 aluminum alloy under thermal deformation

Abstract

This paper is intended to examine the dynamic evolution of the metastable structure and nano precipitation of AA7055 under thermal deformation. Results indicate the second-phase particles produced will break up and spheroidize at low temperature. When the strain reached 0.4, precipitated η (MgZn₂) was detected to occur and gradually coarsen. Under high temperature and high strain rate, not many second phase particles were left in the alloy and the particles were nearly spherical. Coarse rod-like T (Al₂Mg₃Zn₃) particles appeared during thermal deformation, which would gradually coarsen and tend toward uniform orientation. There were also long rod-like S (Al₂CuMg) particles and some nanoscale coarse particles. During thermal deformation, dislocations quickly proliferated, entangling into dislocation cells, and then formed subgrains through slipping and climbing. When the strain reached its maximum, large-angle grain boundaries were detected below 450°C, suggesting that dynamic recrystallizion had taken place. Below 300°C, however, only dynamic recovery took place. As the strain increased, the dislocation density reduced. Subgrains developed quite completely. The intergranular misorientation was modest. The subgrains were 0.2∼ 0.6 μm in size with quite straight boundaries, but these subgrains were not stable enough. The boundary angle also displayed a tendency of developing toward the 120°. According to the diffraction pattern of zonal axis [⥘11]_Al, the orientation relationship of the η′ and η (MgZn₂) particles to the aluminum matrix was (0001)_η//(111)_Al. Under high temperature (450°C), when the strain was 0.4, subgrains with relatively clear boundaries were observed. Under low temperature, at the same strain, subgrains in the alloy were still entangling dislocaion cell grains with high intragranular and boundary dislocation densities. A few subgrain boundaries were becoming clear. The subgrain boundaries were heavily curved. Quite a lot of the subgrain boundaries were still unclear. When the strain was 0.6, under high temperature, the subgrain boundaries began to transform toward 120° stable state. Subgrains began to grow. Under low temperature, in the same state, many dynamically recrystallized grains formed in the alloy structure. The grains were small in size with clear boundaries. Some of the subgrains were still in the nucleation stage of recrystallization nuclei. When the strain was 0.8, under deformation temperature 450°C, the dislocation density in the structure reduced significantly. Equiaxial or sub-equiaxial grains more than 0.5 μm in size were observed. Under low temperature, subgrains were fairly completely developed, though the boundaries were still unstable and tended to transform toward the 120° stable state, but the recrystallized grains were fine and sized 0.2∼0.6 μm. The dislocation density in the structure reduced. Yet low-density dislocation walls not having evolved into subgrain boundaries were still observed on the boundaries of a few grains.