Microstructural changes induced by Er and Zr additions to A356 alloy investigated by thermal analyses and STEM observations

This work is aimed at characterizing the changes induced in the microstructure of an Al-7Si-0.4Mg (A356) alloy when 0.3 wt% Er (alloy E3) and 0.3 wt% Er plus approximately 0.5 wt% Zr (alloy EZ35) are added. The addition of Er and Zr caused the precipitation of Al3Er and Al-3(Er, Zr) precipitates during solution heat treatment, as it was shown by isothermal DSC tests performed at the solution treatment temperature, microhardness measurements on samples subjected to different solution treatment time and STEM micrographs. The age hardening response of Er- and Zr-containing A356 is improved, thanks to the formation of further Al-3 Er and Al-3 (Er, Zr) precipitates during aging. Er- and Zr-containing precipitates were also identified inside eutectic Si and were identified as Al2Si2Er and Al2Si2 (Er, Zr) for E3 and EZ35, respectively. Finally, an Er-rich layer was identified at the interface between eutectic Si and Al after thermal treatment, possibly formed by Er atoms diffusing along the Si twins and accumulating at the eutectic Si interface with the Al matrix.