The effect of small scandium additions to AA7050 on the as-cast and homogenized microstructure

The effect of scandium additions, in the range 0-0.5wt%, on the microstructure of an as-cast and homogenized 7050 alloy have been investigated. Scandium in excess of a critical concentration leads to the precipitation of primary Al3(Sc,Zr) particles during solidification, which have a powerful grain refining effect. This grain refinement is accompanied by an increase in the concentration of zinc, magnesium and copper retained in solid solution after casting. Microprobe analysis has shown that, after solidification, scandium is distributed relatively evenly across the grains. This promotes the precipitation of a uniform dispersoid distribution during subsequent homogenization and reduces the width of the dispersoid free zones adjacent to the grain boundaries by an order of magnitude compared to those in conventional 7050 alloys.