Mn and Mo additions to a dilute Al-Zr-Sc-Er-Si-based alloy to improve creep resistance through solid-solution- and precipitation-strengthening

Compressive creep experiments were utilized to investigate the influence of small additions of 0.25 at.% Mn and 0.10 at.% Mo on the creep resistance of a cast Al-0.08Zr-0.02Sc-0.01Er-0.10Si at.% alloy. The Mn- and Mo-modified alloy displays significantly enhanced creep resistance at 300 and 400 degrees C, due to solid-solution strengthening and the formation of two types of precipitates: Al-3(Zr,Sc,Er)(L1(2))-nanoprecipitates and alpha-Al(Mn,Mo)Si submicron platelets or cuboidal-shaped precipitates. The creep threshold stresses at 300 and 400 degrees C are 37 and 24 MPa, respectively, versus 19 and 15 MPa for the unmodified alloy. At 300 degrees C, the creep exponent n is found to change from 4.4 in the base alloy, to 3 in the modified alloy, consistent with a change from climb- to glide-controlled dislocation creep. The Mn- and Mo-modified alloy exhibits an as-cast grain-structure, which is finer (similar to 0.35 mm versus 0.6 mm) and more equiaxed grains than the unmodified alloy, which is anticipated to enhance deformation by diffusional-creep. Nevertheless, diffusional-creep resistance at 400 degrees C remains high for the modified alloy, due to precipitation of submicron alpha-Al(Mn,Mo)Si-precipitates at grain boundaries (GBs). At 400 degrees C, the diffusional creep threshold-stress is similar to 14 MPa, three times that of the unmodified alloy, which also display fewer and coarser Al-3(Zr,Sc,Er)(D0(23)) precipitates at GBs. Creep resistance in the modified alloy does not deteriorate after 16 days of stress testing at 400 degrees C, highlighting the excellent coarsening resistance of the L1(2)- and alpha-precipitates. This new castable, heat-treatable aluminum alloy therefore represents an important technological advance for utilization at higher temperatures under stress. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd.