X-ray diffraction pattern analysis of an aged Al-Cu-Mg-Ag alloy

An approach to X-ray diffraction (XRD) pattern analysis for distinguishing the {111}_Al plates, i.e., the Ω-phase in the Al-Cu-Mg-Ag alloys, from the structurally similar equilibrium θ-phase (Al₂Cu) in the Al-Cu alloys has been developed. Three variants of the {111}_Al plates (namely the Ω-phase) characterized in the Al-Cu-Mg-Ag alloys using transmission electron microscopy (TEM) were applied to construct and refine their structures by density functional theory (DFT). The XRD patterns were simulated using DFT supercells and a multi-purpose pattern-fitting system, RIETAN-FP. The {111}_Al Ω plates are associated with the presence of the “strongest” peaks - 110_θ, (or 220_θ), 310_θ and 202_θ, if the precipitate is considered to be similar to the equilibrium θ-phase with the Vaughan II θ/Al orientation relationship. Other XRD peaks, e.g., 200_θ, 002_θ, 112_θ, 222_θ, 420_θ, and 402_θ, have reduced integral intensities or even disappear in the experimental patterns taken from the aged Al-Cu-Mg-Ag alloy. The increase of the integral intensity of the {310}_θ and {202}_θ relative to the {110}_θ (or {220_θ}) plane(s) on the XRD patterns occurs in the alloy after over-aging compared to peak-aging state. This was associated with the improvement of periodicity/ordering level within the Ω-phase structure during its plate thickening. The DFT model, composed of the hybrid precipitate including the orthorhombic θ- and hexagonal η-type phase fragments, fits better with the experimental pattern taken from the alloy after peak-aging, where these ∼1.6 nm thick {111}_Al plates are actually predominant.