Overview of the high temperature substructure development in Al-Mg alloys

Abstract

The substructures developed during the high temperature deformation of Al-5.2Mg are observed by optical, transmission electron and scanning electron microscopes in samples tested over a wide range of strains (up to ε ≈ 7.2) in the spectrum 335 to 545°C, 0.0014 to 2 s-1, embracing both the region with power law stress exponent n = 3 and n > 5. Although solute atmosphere interaction with dislocations strongly affects dislocation glide throughout the range, subgrain boundaries form and serve as obstacles to dislocation motion in similarity to pure Al, albeit with a secondary importance. The final subgrain structure does not form until about twice the strain for the flow curve to decline from the initial peak to steady state. Although the solute raises the stress and lowers the cell size, Al-5.2 Mg has the same subgrain size as Al at the same stress. Serrations of the grain boundary form with large amplitude and lead to geometric dynamic recrystallization by 1) pinching off at the base to form discrete crystallites or 2) combining from opposite sides of grains when they become very thin and thus reducing them to groups of crystallites with irregular shape. The crystallites, with perimeters that are 30 to 100% high angle arising from the initial boundaries, do not grow and their development causes no change in the flow stress.