Grain boundary characterisation in superplastic deformation of Al-Li alloy using electron backscatter diffraction

The superplastic deformation and microstructural evolution of an as processed 8090 Al-Li alloy has been investigated with particular attention to the dynamic grain size refinement and the formation of high angle grain boundaries during concurrent straining and annealing. Tensile tests were conducted at temperatures in the range 470-560°C and initial strain rates of 10^-2-10^-4 s^-1, and the starting and deformed samples were characterised using optical microscopy, scanning electron microscopy, transmission electron microscopy, and electron backscatter diffraction. The material showed superplasticity with a maximum elongation to failure of 660% at 530°C and a strain rate of 10^-3 s ^-1; the stress-strain curves exhibited a steady state flow following a rapid drop in flow stress, which corresponds to a microstructural transformation from a coarse grained to a uniform fine grained microstructure through dynamic recrystallisation (DRX). Despite the initial bimodal grain structure, the DRX process resulted in a gradual increase in average boundary misorientation angles. The development of these high angle boundaries was a result of the absorption of dislocations into sub-boundaries and grain boundary sliding induced subgrain rotation.