Soft X-ray XAFS analysis of cluster formation behavior during natural aging on an Al-Mg-Si alloy

Cluster formation in the early stages of natural aging (NA) in Al-1.04mass%Si-0.55mass%Mg alloys was studied via X-ray absorption fine structure (XAFS) measurements using soft X-rays and via first-principles calculations. Soft X-ray XAFS measurements were taken at Spring-8 BL27SU beamline. The XAFS spectra were recorded at the Mg-K and Si-K edges in fluorescence mode. Density functional theory (DFT) calculations were used to estimate the cluster structure changes on basis of the changes in the XANES spectra. The results, the absorption edge energies shifted towards the high or low energy side through four stages as aging proceeded. According to simulating the Mg-K and Si-K edge spectra for some cluster models, it was demonstrated that Si-vacancy pairs, Mg-vacancy pairs, and a combination of both were formed in the early stages of NA. Furthermore, while vacancies were released from the clusters formed, clusters grew as Mg-vacancy pairs were mainly included in the clusters. These results indicate that soft-X-ray XAFS, which is capable of identifying individual elements, can be used to characterize cluster structures in details.