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 coars eningand creep-resistance up to 400 degrees 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 degrees C exhibit Mn partitioning at the Scand Er-rich precipitate cores, and Mo throughout the precipitates. Manganese-modified L12-precipitates exhibit a higher number density (-7.5 x 1022 m3 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 degrees C are observed. Isochronal aging displays, however, that a Mo addition delays the formation of Al/Si/Mn-rich alpha-precipitates from 425 degrees C to 475 degrees C. Both Mn and Mo additions improve the creep resistance of the alloys at 300 degrees 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 alpha-precipitates (<20 nm) formed during the creep experiments at 300 degrees 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 alpha-precipitates formed during creep.

Automated summary

Micro-additions of 0.25 at.% Mn and/or 0.10 at.% Mo to dilute Al-0.08Zr-0.014Sc-0.008Er-0.09Si alloys affect precipitate evolution and resulting strengths, providing solid-solution strengthening and improving ambient-temperature strength and elevated-temperature creep resistance. Mn-modified precipitates exhibit higher number density, while Mo-modified precipitates display improved coarsening-resistance. Both Mn and Mo additions enhance creep resistance at 300 degrees C, with Mn-bearing alloys showing a more significant effect.