Characterization of a cubic phase in an Al-Cu-Mg-Ag alloy

The overall objective of this study is to investigate the microstructural stability and mechanical properties of Al-Cu-Mg-Ag alloys subjected to elevated temperatures. The addition of Ag to Al-Cu-Mg alloys with correct Cu-to-Mg ratio has been shown to generate a precipitate phase, designated Ω, which displays superior thermal stability compared with the normally occurring S′ and θ′. Samples produced for this study contained the expected Ω, θ′ and S′. In addition a cubic phase, previously designated σ(Al5Cu6Mg2), was obtained. The σ phase was seen to be a semicoherent and coplanar phase with the Al matrix, i.e., {1 0 0}σ//{1 0 0}Al and 〈0 1 0〉σ//〈0 1 0〉Al. The coarsening rate of the σ phase was found to be much lower than the θ′ phase at 200°C. An ingot was produced of the σ phase, which was verified by X-ray diffraction. The ultrasonic technique was used to determine Young's modulus and the shear modulus. Estimates for the structural interfacial energy were determined. A hot-stage Vickers hardness measurement on the equilibrium σ phase indicates a high yield strength up to 350°C. The data from the present study indicate that an Al alloy with the σ phase may exhibit superior elevated-temperature stability.