Investigation of the role of Sn on negative effect in two-step aging in an Al-Mg-Si alloy by XAFS analysis

Abstract

This study investigated the effect of Sn addition on clustering behavior of natural aging (NA) in Al-0.95mass%Si-0.56mass%Mg-0.04mass%Sn alloys by XAFS measurements, first-principles calculation and Vickers hardness measurements. XAFS measurements at the Sn-K edges were carried out at the BL14B2 beamline, at the Si-K and Mg-K edges were carried out at the BL27SU beamline at SPring-8. It was found that Sn addition brought about the retardation of cluster formations. This retardation was ascribed to the suppression of formations of Mg-vacancy (Va) pairs. By Sn addition, the ratio of Sn-Va pairs was increased from as-quench (AQ) to NA for 36 ks, however Sn atoms were not remarkably included in the clusters. The formation energy of the Sn-Va pair is the smallest in Al matrix, and Sn atoms preferentially bind to vacancies. Since the formation energy of Mg-Va pair is higher than that of the Si-Va pair, the formation of Mg-Va pairs is preferentially delayed. Based on the result of Vickers hardness measurements, negative effect of two-step aging was caused by the participation of Mg atoms in the clusters, not that of Si atoms. It was clarified that Mg atoms have a greater influence on negative effect in two-step aging than Si atoms have.