Effects of Mo and Mn microadditions on strengthening and over-aging resistance of nanoprecipitation-strengthened Al-Zr-Sc-Er-Si alloys

Combined microadditions of 0.09 at.% Mo and 0.4 at.% Mn to a dilute Al-0.10Zr-0.01Sc-0.007Er-0.10Si (at.%) alloy lead to increases in strength upon peak-aging, and improved over-aging resistance at 400, 425 and 450 degrees C for at least 6 months. These improvements are related to four cumulative effects. Firstly, Mn and Mo provide, in the as-cast state, a solid-solution-strengthening contribution of -90 MPa. The solid-solution contribution from Mo (similar to 80 MPa) remains essentially unchanged during aging at 400 similar to 450 degrees C, due to its extremely small diffusivity and precipitation. Secondly, Mn and Mo partition to the cores and shells, respectively, of the nano-size coherent, L1(2) (Al,Si)(3)(Zr,Sc,Er) nanoprecipitates, which nucleate in <1 hat 400-450 degrees C. This is associated with an increase in their number density and a reduction in their growth and coarsening kinetics, thus delaying the loss of strength upon over-aging. Third, Mn and Mo provide precipitation strengthening via submicron alpha-Al(Mn,Mo)Si precipitates (which form between 1 and 11 days at 400 degrees C), which counterbalances the loss of Mn solid-solution strengthening. Finally, the alpha-Al(Mn,Mo)Si precipitates scavenge Si from the matrix, which is then not available to accelerate the coarsening of the L1(2) precipitates, thereby improving the over-aging resistance of the alloy. Iron additions (0.015 at.%), expected to replace some Mn in the alpha-phase Al(Fe,Mn,Mo)Si, does not affect the aging behavior. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.