Effect of isothermal multidirectional compression on microstructure and mechanical properties of Al-Zn-Mg-Cu-Zr alloy with minor Sc addition

Sc microalloying and isothermal multidirectional compression (IMC) are important methods to refine grains and improve the mechanical properties of Al-Zn-Mg-Cu-Zr alloys. Herein, EBSD, SAXS and TEM have been carried out to investigate the effect of IMC and minor Sc on the microstructure, especially for grain boundary wetting, and mechanical properties of aged Al-Zn-Mg-Cu-Zr alloys by melting and hot compression process parameter optimization. The results show that the strength and plasticity can be obviously improved simultaneously by minor Sc addition and IMC. Especially, stripy GBs (one kind of typical GB complexion) have been widely observed for alloy samples after microalloying Sc and IMC, which indicates the occurrence of GB wetting for Al-Zn-Mg-Cu-Zr alloy. And the GB wetting degree gradually increases for alloys with only microalloying Sc, only IMC and (microalloying Sc + IMC). Combined with GB wetting, the formation of Al3(Sc, Zr) due to Sc addition could inhibit the motion of dislocation and grain boundary, which promotes formation and inhibit growth of recrystallized grains, leading to further grain refinement. Besides, GB wetting degree would affect the precipitate distribution, especially for grain boundary precipitates. Also, GB wetting probably promote grain bridging, which is beneficial to improve the yield strength and ductility of Al-Zn-Mg-Cu-Zr alloys. The concurrent effect of grain refinement, the precipitate including finer Al3(Sc, Zr) particles, dislocation strengthening and grain bridging contribute to the simultaneous improvement of the strength and ductility of Al-Zn-Mg-Cu-Zr alloys under IMC and Sc. This study provides a new understanding on microalloying Sc combined with IMC on the microstructure and properties of Al-Zn-Mg-Cu-Zr alloys.