The sequence of precipitation in 339 aluminum castings

The precipitation sequences in direct-quenched from the die (DQD) and solut ion-treated (SOL) 339 aluminum have been determined by a combination of dif ferential scanning calorimetry (DSC) and transmission electron microscopy ( TEM). DSC scans for the alloy in both conditions exhibit two distinct exoth ermic peaks, each associated with a unique precipitate. The peak temperatur es 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 al loy it is beta' (Mg2Si). The principal precipitate associated with the high er temperature peak in both DQD and SOL alloys is Si. The DSC peak temperat ure identifies the specific precipitate in 339 Al, but the peak area is not a reliable measure of precipitate density. Nano-indentation of the dendrit es shows that the strength provided by the precipitates increases in the se quence Si < S' < beta'. However, their thermal stability increases in the r everse order. (C) 2001 Kluwer Academic Publishers.