Abstract
The precipitation and coarsening characteristics of an alloy of nominal composition (wt-%) AI-4.2Cu-l.6Mg-0.2Si were investigated at 200, 265, and 305°C for times up to 650 h. All aging conditions produced a mixture of homogeneously nucleated S′ and the cubic phase σ. The quench rate did not alter appreciably the volume fraction of cubic phase. The lattice parameter of the σ phase was 0.831 ± 0.005 nm and the aspect ratio of the σ precipitates was 1.4 ± 0.1 for treatments at 200 and 265°C, increasing to 1.8 for the specimens aged at 305°C for 24 h. The precipitated σ phase exhibited better resistance to coarsening and dissolution in long term aging treatments at 265°C than the concurrently precipitated S′, although the S′ was a more effective precipitation. hardener.
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