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

The substructures developed during the high temperature deformation of Al-5 .2Mg are observed by optical, transmission electron and scanning electron m icroscopies in samples tested over a wide range of strains (up to epsilon a pproximate to 7.2) in the spectrum 335 to 545 degrees 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 d islocation glide throughout the range, subgrain boundaries form and serve a s obstacles to dislocation motion in similarity to pure Al, albeit with a s econdary importance. The final subgrain structure does not form until about twice the strain for the flow curve to decline from the initial peak to st eady state. Although the solute raises the stress and lowers the cell size, Al-5.2Mg has the same subgrain size as Al at the same stress. Serrations o f the grain boundary form with large amplitude and lead to geometric dynami c recrystallizaton by 1) pinching off at the base to form discrete crystall ites or 2) combining from opposite sides of grains when they become very th in and thus reducing them to groups of crystallites with irregular shape. T he crystallites, with perimeters that are 30 to 100% high angle arising fro m the initial boundaries, do not grow and their development causes no chang e in the flow stress.