Microstructure and mechanical properties of cast Al-Ce-Sc-Zr-(Er) alloys strengthened by Al11Ce3 micro-platelets and L12 Al3(Sc,Zr,Er) nano-precipitates

Abstract

The microstructure and mechanical properties of two die-cast hypoeutectic Al-Ce-based alloys modified with Sc, Zr and Er micro-additions – Al-1.5Ce-0.14Sc-0.03Zr with high Sc content and Al-1.4Ce-0.02Sc-0.06Zr-0.003Er (at%) with low Sc content – are studied, with focus on the two strengthening phases: micron-scale Al11Ce3 platelets formed on solidification and L12 Al3(Sc,Zr,Er) nanoprecipitates formed during aging. The evolution of microstructure and microhardness upon isothermal aging at 350 and 400 °C, and the creep resistance at 300 °C, are studied and compared to alloys containing only one of these strengthening phases. The Al11Ce3 and L12 phases form mostly independently of each other, except in the low-Sc alloy where scavenging of Er and Si by Al11Ce3 on solidification reduces the kinetics of precipitation and number density of the L12 precipitates formed on subsequent aging. The two strengthening phases, while mostly independent of each other chemically, interact synergistically in terms of strengthening. At ambient temperature, the hardness of the Al11Ce3- and L12-strengthened alloys is higher than that of similar alloys containing only one strengthening phase. At 300 °C, the creep strain rate is reduced by as much as five orders of magnitude in the Al11Ce3- and L12-strengthened alloys compared to alloys containing only L12 precipitates. This improvement in both room and elevated-temperature mechanical properties is modeled through a combination of load transfer from the Al11Ce3 micro-platelets, and precipitation strengthening from the Al3(Sc,Zr,Er) nanoprecipitates.