Precipitation in 339 and 2124 aluminum: A caveat for calorimetry

The sequence of precipitation in solutionized (SOL) 2124 aluminum and direct-quenched from the die (DQD) 339 aluminum has been identified by a combination of differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Both alloys form S' (Al2CuMg) as the first precipitate after GP zone dissolution. In each alloy a second phase forms at higher temperatures—Si for DQD 339 Al, θ′ (CuAl2) for SOL 2124 Al. These results illustrate two difficulties associated with the interpretation of calorimetric observations. 1) The S′ phase precipitates at a much higher temperature in 2124 Al than in 339 Al. Calorimetric determinations of activation energies for GP zone dissolution and S' precipitation suggest that the former is the rate-determining step for the latter. Since this or similar effects can be expected to control precipitation rates in other alloys, a precipitate is not uniquely identified simply by the DSC peak temperature. Accordingly, the literature must be viewed with caution unless the precipitate assigned to a DSC peak is identified by TEM. 2) As Si forms in DQD 339 aluminum, 40% of the S' precipitate dissolves. In this circumstance, where two calorimetrically opposed processes occur simultaneously, activation energies determined by differential isothermal calorimetry are erroneous.