Aluminum comes mainly from bauxite, the ore that contains aluminum oxide minerals along with water and impurities. In industry, bauxite is refined to produce alumina (Al2O3) through the Bayer process, and then alumina is reduced to aluminum metal in the Hall-Héroult process. The other minerals listed are iron oxides (hematite and magnetite) and a lead sulfide (galena), which are ores for iron and lead, not aluminum.

Grain boundaries carry higher energy than the crystal interior, so the system tends to minimize this interfacial energy. When grains grow, many small grains disappear and merge into larger ones, which reduces the total grain boundary area. Since the grain boundary energy contributes to the overall surface energy of the material, lowering the boundary area decreases the surface energy of the system. In short, grain growth acts as a thermodynamic drive to reduce surface energy by reducing the total grain boundary area.

Substitutional solid solutions form when atoms of the solute replace atoms of the solvent in the crystal lattice, typically when they are similar in size and have the same crystal structure. Copper in silver fits this pattern well: both metals crystallize in the face-centered cubic lattice, and their atomic sizes are close enough that copper atoms can substitute for silver atoms in the lattice without causing significant distortion. This creates a solid solution with a wide range of copper–silver compositions. The other examples are not substitutional for these reasons. Oxygen in nickel is interstitial because oxygen is much smaller than nickel atoms and occupies the gaps between them rather than substituting nickel in the lattice. Hydrogen in palladium behaves similarly as an interstitial species, moving into interstitial sites. Carbon in iron is also interstitial, sitting in the spaces between iron atoms rather than replacing iron atoms in the lattice.

Oxide thickness tracks how fast oxidation proceeds under the given history. Magnesium-containing alloys oxidize more rapidly because Mg readily forms oxides (like MgO) that are less protective and allow faster diffusion of ions through the scale. This means the oxide layer can keep growing during the same heat exposure, producing a thicker film. In contrast, a Mg-free alloy, especially one with aluminum, tends to form a denser, more protective Al2O3 scale that slows further oxidation and keeps the oxide thickness thinner. So, the key idea is that Mg-containing alloys have a higher oxidation rate, leading to thicker oxide layers under the same thermal and mechanical history.