Precipitation of phases in Al-Mg-Si-Cu alloy with Sc and Zr additions during heat treatment

In the present work the sequence of phases in Al-1%Mg-0.6%Si-1%Cu (in wt %) alloys with 0.4%Sc and 0.2%Zr additions during heat treatment has been studied. The investigated alloys were cast into copper mould to ensure high cooling rate during solidification and the retention of Sc and Zr in the supersaturated solid solution. Two-step preliminary heat treatment was applied: the annealing at temperature 300 degrees C or 350 degrees C followed by quenching from 540 degrees C to water. The precipitation of the Q' and Al3Sc phases has been observed during the first step of annealing. The annealing at 540 degrees C has led to complete dissolution of the Q' particles and growth of the spherical particles of the Al3Sc phase. The increase of hardness during subsequent ageing at 165 degrees C (typical ageing temperature of the 6xxx alloys) has been caused by the formation of needle-like Q' phase which coexist with Al3Sc precipitates.