Role of silicon in the precipitation kinetics of dilute Al-Sc-Er-Zr alloys

The precipitate nanostructure and the strength of an Al-0.055Sc-0.005Er-0.02Zr at% alloy with Si additions, in the range 0–0.18 at%, were investigated utilizing micro-hardness, electrical conductivity, scanning electron microscopy and atom-probe tomography techniques. Si-containing alloys are cost-effective due to the existence of Si in commercial purity Al. In all studied alloys, homogenization for at least 0.5 h at 640 °C is needed to eliminate Al₃Er primary precipitates. Alloys containing the higher Si concentrations achieve higher microhardness by increasing the heterogeneous nucleation current of (Al, Si)₃ (Sc, Zr, Er) precipitates. The alloy containing 0.18 at% Si achieves an 60% improvement in peak-microhardness compared to the Si-free alloy, during isothermal aging at 400 °C. Silicon additions reduce the peak-aging time in the temperature range 300–400 °C, indicating that the Er and Sc diffusion kinetics are accelerated. Silicon also enhance the Zr diffusion kinetics, accelerating precipitate growth during aging at 300 °C and precipitate coarsening at 400 °C. Addition of Si modifies the concentration profiles within the nanoprecipitates, enhancing the chemical homogeneity of Sc and Er in their cores, rather than forming Er-enriched-cores/Sc-enriched-shells that we have observed in prior research. Finally, the microhardness of the alloys, containing 0.12 and 0.18 at% Si, only diminishes slightly from the peak values after isothermal aging at 375 °C for about 2000 h, suggesting that the studied alloys can be practically utilized at this operating temperature.