Individual and synergistic effects of Mn and Mo micro-additions on precipitation and strengthening of a dilute Al–Zr-Sc-Er-Si alloy

Dilute Al–Er-Sc-Zr-Si alloys strengthened by coherent Al₃(Er,Sc,Zr)(L1₂) nanoprecipitates have excellent coarsening- and creep-resistance up to 400 °C. Herein, the effects of micro-additions of 0.25 at.% Mn and/or 0.10 at.% Mo to a dilute Al-0.08Zr-0.014Sc-0.008Er-0.09Si (at.%) alloy are investigated with respect to precipitate evolution and the resulting strengths after different aging treatments. Both Mn and Mo provide solid-solution strengthening, contributing to ambient-temperature strength, in addition to elevated-temperature creep resistance. L1₂-core-shell nanoprecipitates created upon aging at 400 °C exhibit Mn partitioning at the Sc- and Er-rich precipitate cores, and Mo throughout the precipitates. Manganese-modified L1₂-precipitates exhibit a higher number density (~7.5 × 10²² m⁻³ for peak-aged condition), while Mo-modified L1₂-nanoprecipitates display significantly improved coarsening-resistance. No notable synergistic effect of Mn and Mo additions strengthening upon isothermal aging at 400 °C are observed. Isochronal aging displays, however, that a Mo addition delays the formation of Al/Si/Mn-rich α-precipitates from 425 °C to 475 °C. Both Mn and Mo additions improve the creep resistance of the alloys at 300 °C. Manganese-bearing alloys exhibit a more significant effect, as it doubles the threshold stress compared to the Mn-free base alloy. This strong effect could be a result of fine α-precipitates (<20 nm) formed during the creep experiments at 300 °C for the peak-aged sample. The over-aged Mn-containing samples are less creep resistant due to coarsening of both existing L1₂-nanoprecipitates and the absence of fine α-precipitates formed during creep.