Soft X-ray XAFS Analysis of Cluster Formation Process during 353 K Aging in Al-Mg-Si Alloys

The cluster formation process in the early stages of 353 K aging in Al–1.04 mass%Si–0.55 mass%Mg alloys was studied by X-ray absorption near edge structure (XANES) measurements and XANES calculation using first-principles calculation by density functional theory (DFT). Soft-X-ray XAFS measurements were carried out at BL27SU beamline at SPring-8 near the Si-K and Mg-K edges. To compare the spectral changes in the experimental and calculated spectra for different cluster models, difference spectra were obtained. Then, it was valued whether the difference spectra shape for the measured and calculated results were consistent, and determined which cluster models were formed majority. It was suggested that Mg atoms and Si atoms first bonded with vacancies (Va) to form Mg-Va pairs and Si-Va pairs, respectively. Subsequently, multiple Mg-Va pairs were mainly ordered along (100) to form L1₀ clusters that Mg atoms ordered along (100), on the other hand multiple Si-Va pairs formed Si-pyramid structures consisting Si atoms. Thus, multiple Mg-Va pairs and Si-Va pairs were individually united. Furthermore, as 353K aging proceeding, some of the Mg atoms formed L1₀ bonds with Si atoms, and some of the Si atoms formed Si-pyramid bonds with Mg atoms, and then monolayer and multilayer clusters developed.