Al-Fe-Si Intermetallics in Sc-added Cast Al-4.5Zn-2Mg Alloy

Al-Zn-Mg based alloys without and with Sc were prepared by remelting the base alloy without and with Al-Sc master alloy in a pot furnace followed by casting in metal mould. Fe and Si often remain as impurities in Al-Zn-Mg alloys. By using optical, scanning electron and transmission electron microscopy, the presence of intermetallics and the hardening effects of micro-alloyed Sc on the microstructure of super-high-strength Al-4.5Zn-2Mg alloys were observed and structure-property relationship was studied. The microstructures of the alloys were investigated for different heat treatment conditions (natural and artificial ageing). Scandium added alloys were harder compared to base alloys in the naturally aged condition by virtue of presence of large number of fine Al3Sc precipitates generated at the time of casting and could not be dissolved during subsequent homogenization treatment. These particles are highly effective in refining the microstructures, retarding recrystallisation, pinning dislocations and low angle grain boundaries. These Al3Sc precipitate act as potent nucleating site for hard η precipitate at the time of artificial ageing and hence suppresses GP zone formation as noticed from the differential scanning calorimetry study. However, the morphology of AlFeSi intermetallics present in cast Al-Zn-Mg alloys was not altered in presence of Sc.