Effect of initial grain structure on the microstructure evolution and superplastic behavior of Al-Mg-Sc-Zr alloy
Piaoqi Lu, Hongfeng Huang, Qianqian Ren, Jianan Yang, Lili Wei, Shuhui Liu, Xuda Xu, Hu Xie
Abstract
In this work, the effect of initial grain structure on the microstructure evolution and superplastic behavior of Al-7Mg-0.3Sc-0.2Zr (wt%) alloy were studied. The results show that the alloy suffered multi-axial forging (MAF) deformation possesses high density dislocations and low angle grain boundaries (LAGBs). During the subsequent low strain rate thermostatic uniaxial tensile testing , the long-range migration of grain boundaries was impeded by nano-scale coherent Al 3 (Sc, Zr) particles, therefore the new recrystallized grain with high angle grain boundaries (HAGBs) occurred in form of grain boundary transformation (from LAGBs to HAGBs) instead of nucleation & growth. As the results, the recrystallization mechanism was dominated by continuous dynamic recrystallization (CDRX) and the superplasticity is not obvious, exhibiting high strength but low elongation to failure. For the alloy after friction stir processing (FSP), the ultra-fine grains with HAGBs structure are dominant. Due to the deficiency of recrystallization driving force and the presence of both in-coherent & coherent Al 3 (Sc, Zr) particles, the growth of grains was not obvious and the grain boundary sliding (GBS) tended to occurr during thermostatic uniaxial tensile testing . In this situation, the FSP specimen showed the best superplasticity but low strength. By contrast, the alloy produced by rolling + annealing (R&A) shows HAGBs & LAGBs mixed grain structure, and both CDRX and GBS occurred during tensile testing, resulting higher strength than that of FSP alloy and better superplasticity than that of MAF alloy.