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 °C, due to solid-solution strengthening and the formation of two types of precipitates: Al3(Zr,Sc,Er)(L12)-nanoprecipitates and α-Al(Mn,Mo)Si submicron platelets or cuboidal-shaped precipitates. The creep threshold stresses at 300 and 400 °C are 37 and 24 MPa, respectively, versus 19 and 15 MPa for the unmodified alloy. At 300 °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 (~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 °C remains high for the modified alloy, due to precipitation of submicron α-Al(Mn,Mo)Si-precipitates at grain boundaries (GBs). At 400 °C, the diffusional creep threshold-stress is ~14 MPa, three times that of the unmodified alloy, which also display fewer and coarser Al3(Zr,Sc,Er)(D023) precipitates at GBs. Creep resistance in the modified alloy does not deteriorate after 16 days of stress testing at 400 °C, highlighting the excellent coarsening resistance of the L12- and α-precipitates. This new castable, heat-treatable aluminum alloy therefore represents an important technological advance for utilization at higher temperatures under stress.