Nature of Oxides in Al-Mg Alloys

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: gamma Al2O3 (< 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.