Grain Structure, Texture Evolution and Deformation Mechanism during Low Temperature Superplasticity in 5083 Al-Mg Alloy

Low temperature superplasticity (LTSP) was observed in the 5083 Al-Mg base alloy after thermomechanical treatments (TMT). The maximum LTSP elongation was 400%, occurring at 250°C and 1 × 10^-3 s^-1. The subgrain structures formed during TMT transformed to better defined subgrains to ∼0.5 μm upon heating to 250°C. Further static annealing or superplastic straining at 250°C would produce well defined fine grains to 1.5-2.5 μm, dependent on annealing time or strain level. Superplastic loading would accelerate grain evolution rate. The near brass {110}〈112〉 and S {123}〈634〉 texture components in the as-TMT specimens evolved into a random orientation distribution after LTSP loading to 100% at 250°C. Static annealing at 250°C itself could not alter the existing texture. Under the optimum LTSP condition, the m-value was ∼0.5, compared with 0.2 for the as-received coarse-grained 5083 alloy. The activation energy Q-value was around 50-90 kJ/mol for LTSP over 200-300°C and around 145 kJ/mol for HTSP over 400-550°C. It is postulated that the rate controlling deformation mechanism in the TMT processed specimens was grain boundary sliding during the optimum LTSP condition and solute drag creep during HTSP deformation.