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 °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 (∼80 MPa) remains essentially unchanged during aging at 400–450 °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, L1₂ (Al,Si)₃(Zr,Sc,Er) nanoprecipitates, which nucleate in <1 h at 400–450 °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 α-Al(Mn,Mo)Si precipitates (which form between 1 and 11 days at 400 °C), which counterbalances the loss of Mn solid-solution strengthening. Finally, the α-Al(Mn,Mo)Si precipitates scavenge Si from the matrix, which is then not available to accelerate the coarsening of the L1₂ precipitates, thereby improving the over-aging resistance of the alloy. Iron additions (0.015 at.%), expected to replace some Mn in the α-phase Al(Fe,Mn,Mo)Si, does not affect the aging behavior.