Nature of Oxides in Al–Mg Alloys

Abstract

Acting as substrates for heterogeneous nucleation, native oxides in Al–Mg alloys have shown their potential for grain refinement. However, the limited knowledge about the nature of the oxides in Al–Mg alloys impedes the widespread application as native grain refiners. The aim of this work is to comprehensively investigate the native oxides in Al–Mg alloys through electron microscopy. Our results show that the predominant inclusions in Al–Mg alloys are oxides in three types of oxide films at the micrometer scales: young films, old films and oxide skins. All oxide films consist of discrete oxide particles of three types in nanometer scale depending on the Mg contents: γAl2O3 (< 0.4 wt.%), MgAl2O4 (0.08–3.5 wt.%) and MgO (> 2 wt.%). Specifically, MgAl2O4 particles have sizes ranging from a few tens to a few hundreds nanometer and possess an elementary shape of octahedron faceted by {111} planes. In Al–Mg alloys, the native oxides have a lognormal size distribution, with the average mean size fluctuating in accordance with the oxide configurations as Mg content varies. The agglomerating feature causes inhomogeneous sampling, and dual-peak lognormal curves are found for low-Mg-content alloys (0.08/0.4%), which could be eliminated by increasing the Mg content (2.0/3.5%) or by using the high-shear melt conditioning (HSMC) technology. Understanding the native oxides in Al–Mg alloys shall provide instructions on their application in grain refinement.