Correlation of Electron Configuration, Electronegativity, and Ionization Energy of Strontium on the Modification of Eutectic Si in High-Pressure Die Cast A380 Al–Si Alloys

Abstract

Strontium (Sr) is commonly used to modify the eutectic Si in hypoeutectic Al–Si alloys to improve mechanical properties. The present study is focused on the effect of electron configuration and electronegativity of alkaline earth metal Sr on the relatively more electronegative Group IV elements Si and Pb. The element Pb occurs as an impurity during the refining of secondary Al. The experiment was conducted on high-pressure die casting (HPDC) of commercial grade A380 aluminum alloy with Si level 7.5–9.5% modified with Al-9.7% Sr master alloy. Samples were cast by inserting a test bar cavity in the runner system of an experimental die. Thin foil electron microscopy (TEM) along with energy-dispersive spectroscopy and high angle annular dark field imaging was used to determine the atomic percent of Si, Sr, Fe, Cu, Mn, Pb, Sn, and Ti embedded in the Al–Si eutectic matrix. Results showed that Sr was primarily segregated in eutectic Si with trace amounts in solid solution in Al. Additionally, a trace amount of Sr was also detected in the impurity element Pb. This preferential distribution is attributed to the difference in electronegativity between Sr (Group II element) and the Si and Pb (Group IV element). Sr with two valence electrons (5s¹p¹) and an electronegativity of 1.0 preferentially segregates on eutectic Si, which has four valence electrons (3s²p²) and a relatively stronger electronegative force of 1. 8. Furthermore, a higher (e/a) valence electron-to-atom ratio of Si lowers the stacking fault energy which accelerates precipitation of Sr in the wider stacking faults and inhibits the growth of eutectic Si in an expected manner. Transmission electron microscopy (TEM) of the HPDC A380 alloy, in contrast, did not reveal any presence of Sr in intermetallic compounds formed by transition elements Fe and Cu. The TEM results indicate a strong correlation between the electron configuration and electronegativity of elements in the formation of stable intermetallic compounds.