The sequence of precipitation in 339 aluminum castings

The precipitation sequences in direct-quenched from the die (DQD) and solution-treated (SOL) 339 aluminum have been determined by a combination of differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). DSC scans for the alloy in both conditions exhibit two distinct exothermic peaks, each associated with a unique precipitate. The peak temperatures for precipitation in the DQD and SOL alloys differ by only a few degrees. TEM of samples heated to the lower temperature peak shows that the first precipitate to form in the DQD alloy is S′ (Al2CuMg), whereas in the SOL alloy it is β′ (Mg2Si). The principal precipitate associated with the higher temperature peak in both DQD and SOL alloys is Si. The DSC peak temperature identifies the specific precipitate in 339 Al, but the peak area is not a reliable measure of precipitate density. Nano-indentation of the dendrites shows that the strength provided by the precipitates increases in the sequence Si < S′ < β′. However, their thermal stability increases in the reverse order.