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 degrees C. This core/double-shell structure strengthens the alloy, and renders it coarsening resistant for at least 64 days at 400 degrees 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 degrees 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 degrees C for 64 days. Er accelerates precipitation kinetics at 400 degrees 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 degrees C + 8 h at 400 degrees C) provides peak microhardness due to optimization of the nanostructure. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.