Precipitation in liquid-quenched Al-Mg alloys; a study using X-ray diffraction line shift and line broadening

Abstract

The precipitation process in liquid-quenched alloys of aluminium with 3.2, 10.5 and 16.7 at% Mg on ageing at 404, 450 and 492 K was investigated by X-ray diffraction analysis. It was found that only β′-phase precipitation occurred. During ageing, the average aluminium-matrix lattice parameter was determined from the line position, while a measure for the lattice spacing variations was deduced from line broadening. The β′-phase precipitation was found to occur inhomogeneously; for one alloy, splitting of diffraction lines was observed. The (asymmetrical) line-broadening changes observed during the β′-phase precipitation could consistently be conceived as dominated by the inhomogeneous nature of the β′-phase precipitation process. The (symmetrical) line broadening retained after completed precipitation was ascribed to the volume misfit between the precipitated β′-phase particles and the aluminium-matrix. Analysis of the aluminium-matrix lattice parameter changes with ageing time revealed that the β′-phase precipitation rate is governed by the magnesium volume diffusion.