Oxidation of mechanically alloyed Al-rich Al-Ti powders

The oxidation behavior of Al-rich, metastable mechanically alloyed powders in the Al-Ti binary system has been examined in the context of their potential application in high-energy-density materials. Scanning calorimetry and thermogravimetric analysis in an oxygen atmosphere up to 1500°C have been performed on powders, synthesized with compositions ranging from Al _0.95Ti_0.05 to Al_0.75Ti_0.25. Oxidation proceeds in three distinguishable steps, similar to the oxidation steps observed for pure aluminum. The steps become less pronounced with increasing Ti concentration. For both the first and second oxidation steps, the apparent activation energies are close to 400 kJ/mol. Partially oxidized material was recovered from intermediate temperatures for quantitative phase analysis by X-ray diffraction, Al₂O₃ and TiO₂ are the main oxidation products. Similarly to pure aluminum, metastable γ-alumina is present at temperatures below 1000°C, suggesting that the stepwise oxidation is related to phase transitions of the alumina in the oxide scale. At temperatures above 1300°C, oxidation follows melt formation in the binary Al-Ti system, as the aluminum component oxidizes selectively, leaving a titanium-rich metallic residue. No correlation was observed between the oxidation and melting of aluminum.