DSC and TEM characterizations of thermal stability of an Al–Cu–Mg–Ag alloy

Abstract

Assessment of long-term stability of an aluminium alloy exposed to elevated temperatures is important in the design of lightweight aerospace structures. The manner in which differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) are used together in monitoring microstructural evolution, and thereby assess phase stabilities in an Al–5.1Cu–0.8 Mg–0.5 Ag–0.7 Mn–0.13 Zr (wt%) alloy, are described. DSC thermograms of the alloy, spanning room temperature to 400°C, revealed the presence of two endotherms and an exotherm. TEM investigation has identified these thermal events to be associated with Ω, S′, and θ′ precipitates. Quantitative TEM was used to measure diameter, thickness, number density, and volume fraction of the precipitates in the alloy exposed at 135°C for times as long as 3000 h. The quantitative TEM results are correlated with the DSC signatures relating to precipitation, dissolution, and coarsening reactions affecting the Ω, S′, and θ′ precipitates in the exposed alloy.