Characterisation of Oxide Films in Al-Mg Alloy Melts

Abstract

Oxide films in Al-0.7Mg and Al-9.4Si-2.3Cu-1.0Zn-0.49Mg (in wt%) alloy melts were characterised using advanced analytical electron microscopy. The oxides were collected by pressurised melt filtration for direct examination by SEM and TEM. The results showed that the oxide films consisted of numerous sub-micron sized oxide particles, rather than continuous solid films. The oxide particles formed in the two Mg-containing alloys were identified as MgAl2O4spinel by selected area electron diffraction (SAED) and high resolution TEM combined with EDS analysis. The low level of Mg in the melt resulted in the change of the oxide from alumina to MgAl2O4spinel. The MgAl2O4crystals were typically faceted with their {1 1 1} crystal planes and were about 0.2-1.2 μm in size. High resolution TEM examination of the MgAl2O4/ a-Al interfaces revealed that there was a cube-on-cube orientation relationship between the two crystals. The possibility of MgAl2O4particles to act as nucleation sites for α-Al grains during solidification is discussed in terms of the lattice matching at the MgAl2O4/ α-Al interfaces along the specific crystallographic orientation relationship.