Mechanochemical reaction between ilmenite (FeTiO₃) and aluminium

A natural ilmenite (FeTiO₃) and aluminium powder have been mechanically milled together for 100 h in a laboratory ball mill. The as-milled powder and an unmilled powder of identical composition were annealed at up to 1200°C and examined by X-ray diffraction and differential thermal analysis (DTA). The unmilled sample showed aluminium melted prior to an exothermic reaction starting at ∼850°C. The milled powder showed no thermal activity, other than a reversible phase transition at 1067 ± 4°C, indicating that reaction occurred within the mill. The products of both powders were the same, TiAl₃, Fe₄Al₁₃ and Al₂O₃, although in the milled powder these phases were nanocrystalline until annealing caused crystallite growth. The thermal reaction seemed to occur in two stages, formation of TiAl₃, Al₂O₃ and elemental iron followed by a slower, diffusion controlled reaction between the elemental iron and residual aluminium to form Fe₄Al₁₃. The reaction during milling was attributed to increased intermixing between the ilmenite and aluminium causing a change in the rate determining step from solid-state diffusion to another, unknown, controlling mechanism.