Probing the local atomic structure of Sr-modified Al–Si alloys

Abstract

Extended X-ray absorption fine structure (EXAFS) spectroscopy and atom probe tomography (APT) measurements were jointly used for the first time to probe the local structure around Sr atoms in Al–10%Sr master alloy and in Al–3%Si–0.04%Sr and Al–12.5%Si–0.04%Sr eutectic alloys in order to study the impact of trace levels of Sr on the morphological changes occurring in Al–Si binary eutectic alloy. EXAFS analysis shows the oxidation states of Sr is close to Sr0 in all the alloys studied. In the Al–10%Sr master alloy, Sr atoms tend to form the intermetallic compound Al4Sr. On the other hand, in Al–Si alloys, Sr atoms segregate towards Si-rich regions, preferentially bonding to Si atoms to form Al2Si2Sr-like clusters with a coordination environment consistent with bulk Al2Si2Sr. APT study reveals the presence of nanometer-sized Al-rich and Si-rich regions and support the EXAFS results. We speculate that addition of trace levels of Sr to Al–Si binary alloy system results in the initial formation of Al2Si2Sr-like clusters and the presence of such intermetallic clusters could play an important role such as poisoning of nucleation sites, thereby delaying the nucleation of eutectic phases and causing the morphological changes of the eutectic phases in the Al–Si alloys.