High-strain-rate superplastic behavior of equal-channel angular-pressed 5083 Al-0.2 wt pct Sc

Abstract

Tensile tests were carried out at temperatures of 673 to 773 K and strain rates of 1×10⁻³ to 1 s⁻¹ on an ultrafine-grained (UFG) 5083 Al alloy containing 0.2 wt pct Sc fabricated by equal-channel angular pressing, in order to examine its high-strain-rate superplastic characteristics. The mechanical data for the alloy at 723 and 773 K exhibited a sigmoidal behavior in a double logarithmic plot of the maximum true stress vs true strain rate. The strain-rate sensitivity was 0.25 to 0.3 in the low-($\dot \varepsilon$<5×10⁻³ s⁻¹) and high- ($\dot \varepsilon$>5×10⁻² s⁻¹) strain-rate regions, and ∼0.5 in the intermediate-strain-rate region (5×10⁻³ s⁻¹<$\dot \varepsilon$<5 × 10⁻² s⁻¹). The maximum elongation to failure of ∼740 pct was obtained at 1×10⁻² s⁻¹ and 773 K. By contrast, no sigmoidal behavior was observed at 673 K. Instead, the strain-rate sensitivity of 0.3 was measured in both intermediate-and low-strain-rate regions, but it was about 0.25 in the high-strain-rate region. High-strain-rate superplasticity (HSRS) in the intermediate-strain-rate region at 723 and 773 K was dominated by grain-boundary sliding (GBS) associated with continuous recrystallization and preservation of fine recrystallized grains by second-phase particles. However, the activation energy for HSRS of the present alloy was lower than that predicted for any standard high-temperature deformation mechanism. The low activation energy was likely the result of the not-fully equilibrated microstructure due to the prior severe plastic deformation (SPD). For 673 K, the mechanical data and the microstructural examination revealed that viscous glide was a dominant deformation mechanism in the intermediate- and low-strain-rate regions. Deformation in the high-strain-rate region at all testing temperatures was attributed to dislocation breakaway from solute atmospheres.