Coarsening resistance at 400 °c of precipitation-strengthened Al-Zr-Sc-Er alloys

The effect of substituting 0.01 or 0.02 at.% Er for Sc in an Al-0.06 Zr-0.06 Sc at.% alloy was studied to develop cost-effective high-temperature aluminum alloys for aerospace and automotive applications. Spheroidal, coherent, L1 ₂-ordered Al ₃(Sc, Zr, Er) precipitates with a structure consisting of an Er-enriched core surrounded by a Sc-enriched inner shell and a Zr-enriched outer shell (core/double-shell structure) were formed after aging at 400 °C. This core/double-shell structure strengthens the alloy, and renders it coarsening resistant for at least 64 days at 400 °C. This structure is formed due to sequential precipitation of solute elements according to their diffusivities, D, where D _Er > D _Sc > D _Zr at 400 °C. Zr and Er are effective replacements for Sc, accounting for 33 ± 1% of the total precipitate solute content in an Al-0.06 Zr-0.04 Sc-0.02 Er at.% alloy aged at 400 °C for 64 days. Er accelerates precipitation kinetics at 400 °C, resulting in: (i) strengthening due to the elimination of lobed-cuboidal precipitates in favor of spheroidal precipitates; and (ii) a decrease in the incubation time for nucleation because D _Er > D _Sc. Finally, a two-stage aging treatment (24 h at 300 °C + 8 h at 400 °C) provides peak microhardness due to optimization of the nanostructure.