Solidification behaviour and the effects of homogenisation on the structure of an Al–Cu–Mg–Ag–Sc alloy

The solidification behaviour and structural evolution during homogenisation annealing of a 0.17 mass% Sc and 0.1 mass% Ge modified Al–Cu–Mg–Ag alloy was examined. The formation of the primary Mg2Ge phase and the Sc-enriched θ-phase (Al2Cu) was found to occur under solidification; no evidence for the formation of the ternary W-phase (Al8−xCu4+xSc) was observed. Approximately 70% of the overall Sc content is fixed in the supersaturated solid solution. However, only two thirds of this Sc is consumed by the formation of a dispersion of nanoscale Al3Sc particles within the Al matrix under homogenisation annealing. A third of the Sc is consumed by the formation of the W-phase because of Sc diffusion into the primary θ-phase, which leads to its transformation into the W-phase. Finally, the W-phase consumes ∼50% of the overall Sc content. No effects of homogenisation annealing on the volume fraction of Mg2Ge were observed. The optimal chemical composition of alloys in the Al–Cu–Mg–Ag–Sc system is discussed.

► The structural evolution during homogenisation of a modified aluminium alloy was examined. ► Sc and Ge additions lead to extremely diverse solidification behaviours of this alloy. ► The formation of a Mg2Ge phase occurs under solidification. ► The concentration of the Mg2Ge-phase remains unchanged after the homogenisation. ► The fraction of the Sc-bearing phase increases under homogenisation annealing.