Abstract
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.
Highlights
IntroductionAs reported for Ni- and Co-based superalloys, the formation of L12ordered Al3M nanoprecipitates (where M is a transition metal, such as Sc, Zr, or Ti) improve dramatically the mechanical properties of aluminum [1À5]
As reported for Ni- and Co-based superalloys, the formation of L12ordered Al3M nanoprecipitates improve dramatically the mechanical properties of aluminum [1À5]
Further increasing the concentration of L12-forming elements in conventionally cast alloys usually leads to grain refinement caused by the precipitation of primary Al3M(L12)-intermetallic precipitates, which is detrimental for high-temperature creep
Summary
As reported for Ni- and Co-based superalloys, the formation of L12ordered Al3M nanoprecipitates (where M is a transition metal, such as Sc, Zr, or Ti) improve dramatically the mechanical properties of aluminum [1À5] These L12-strengthened Al-superalloys exhibit good coarsening resistance at 400 °C [6À9], which is ~72% of the absolute melting point of Al, and far above the maximum usage temperature of typical commercial aluminum alloys. While the base-alloy is strengthened solely by coherent (Al,Si)3(Zr,Sc,Er)(L12)-nanoprecipitates, which form upon aging and exhibit slow coarsening up to 400 °C; the modified alloy displays improved microhardness values and increased coarsening resistance at higher temperatures (400 - 450 °C).
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