Formation of endogenous MgO and MgAl₂O₄ particles and their possibility of acting as substrate for heterogeneous nucleation of aluminum grains

Aluminum containing 4 wt.% magnesium was oxidized at a temperature for different oxidation times and analyzed by high-resolution electron microscopy. A thin oxidized layer of about 5 µm, which is composed of MgO, forms at short oxidation time and gradually increases. High-resolution microstructures reveal that the oxidized layers are porous regardless of oxidation time. After extended oxidation time, discrete MgAl2O4 particles formed as a result of the reaction of initially formed MgO, liquid aluminum, and oxygen introduced from air through the porous MgO. Furthermore, it is clear by high-resolution lattice images that MgAl2O4 particles are covered with thin Al2O3, whereas MgO is bonded intimately to aluminum. Therefore, MgAl2O4 particles that form naturally during oxidation are difficult to act as a direct substrate for nucleation of aluminum grains because of the coverage of Al2O3. In contrast, MgO shows the possibility of acting as a substrate for the aluminum nucleation. The formation mechanism of MgO and MgAl2O4 and their possibility of acting as substrates for nucleation of aluminum grains suggest that atomic level bonding and mismatches of nucleant/nucleus metal should be considered for correct evaluation of the possibility of heterogeneous nucleation of metallic matrix on a potent nucleant.