The chemical composition influence on the microstructure and superplasticity of the Al-Mg-Si-Zr-Sc-based alloys

The present research focuses on the influence of soluble Si, Mg, and precipitation-forming Sc and Zr alloying elements on the precipitate parameters, grain structure, superplastic behavior, and room temperature mechanical properties of Al-(1.2-2) Mg-(0.1-1.1)Si-1.3Cu-0.9Fe-0.9Ni-(0.2-0.3)Zr-(0.05-0.2) Sc (wt%) alloys. The (Al), Mg2Si, Al9FeNi, AlSi2Sc2 (at high Sc and Si content), Al3(Sc,Zr) (at high Zr content) phases of solidification origin and dispersoids of the L12 Al3(Sc,Zr) secondary phase were observed after homogenization annealing of as-cast alloys. The alloys composition insignificantly influenced on the mean size of the L12 precipitates,which was 12-13 nm. The fraction of dispersoids increased with increasing Sc from 0.05 to 0.1-0.2 % and it did not depend much on the Si content within a range (0.1-0.8)%. The mean size of coarse precipitates of 1-2 mu m and a total fraction of -5-7% were observed after thermomechanical treatment. Due to Zener pinning and particlestimulated nucleation phenomena and an increase solute Mg at the superplastic deformation temperature, the alloys exhibited a strain rate sensitivity coefficient of 0.4-0.5, an elongation to failure of up to -600 % at a constant strain rate of -10- 2 s- 1 and a temperature of 480 degrees C. An increase in Sc from 0.05 % to 0.1 % resulted in a significant grain refinement and a concomitant improvement in superplastic properties. However, further increases in the Sc to 0.2 % had insignificant effect. A reduction in Si increased the Mg solute content, improved grain size stability and significantly improved superplasticity, but reduced strength properties at room temperature. The alloying elements content was optimized to achieve a balance between the superplastic and room temperature mechanical properties.