Aluminum(3)(scandium, zirconium) dispersoids in aluminum alloys: Coarsening and recrystallization control

With proper metallurgical techniques, the addition of scandium and/or zirconium to aluminum will form recrystallization inhibitors in wrought product called "dispersoids". Zirconium forms Al3Zr dispersoids with aluminum, which is currently the most potent dispersoid in commercial use. However, scandium forms Al3Sc dispersoids with aluminum, which have been shown to surpass the effectiveness of Al3Zr in some cases. Scandium is not currently a common addition to commercial Al alloys as little is known about its performance compared to Al-Zr. In this work, recrystallization and dispersoid coarsening are systematically studied as an effect of Sc and Zr content in Al. Comparison is made between the performance of wrought experimental Al alloys containing Al3Zr, Al3Sc, and Al3(Sc, Zr) dispersoids. Effectiveness of Al3Sc is limited to dispersoids less than 25nm radius, the point at which Al3Sc transforms from coherent to non-coherent. Alloys containing Al3(Sc, Zr) more effectively control recrystallization through combined volume fraction and thermal stability effects compared to alloys containing Al3Sc. Scandium shifts the recrystallization mechanism of A1 and Al-Zr alloys from nucleation-and-growth of new grains to boundary migration pinned by dispersoids. During annealing of cold rolled alloys, impinging boundaries dissociate coherent Al3Sc for which a disordering mechanism is proposed. As a practical measure, Sc and/or excess Zr are added to 7050 for comparison with the experimental alloys. The performance of modified 7050 alloys resembles the trends of the experimental alloys. In summary, the Al3(Sc, Zr) dispersoid is a more effective recrystallization inhibitor than any other dispersoid currently in use.