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 theta-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 theta-phase, which leads to its transformation into the W-phase. Finally, the W-phase consumes similar to 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. (C) 2011 Elsevier B.V. All rights reserved.