Isothermal Oxidation of Ti ₃Al _0.6Ga _0.4C ₂ MAX Phase Solid Solution in Air at 1000 °C to 1300 °C

Abstract

The atomically laminated Ti ₂AlC, Ti ₃AlC ₂ and Cr ₂AlC MAX phases, with A = Al, form adherent, passivating α-alumina, Al ₂O ₃, oxide scales when heated in air. The effect of solid solutions on the A layers in affecting the oxidation kinetics remains a subject of open research. Herein we synthesize a dense bulk polycrystalline Ti ₃Al _1−xGa _xC ₂ (x ≈ 0.4) solid-solution and investigate its isothermal oxidation in ambient air, in the 1000 °C–1300 °C temperature range, for times varying between 15 and 300 h. At 1000 °C, a passivating dense Al ₂O ₃ layer ( ≈ 1–2.6 μm thick) with near cubic kinetics and an overall weight gain that is slightly less than either Ti ₃AlC ₂ or Ti ₂AlC is formed. At 1200 °C, the Al ₂O ₃ layer thickens (3.5–12 μm thick) with some scale delamination on the corners initiating at 15 h. At 1300 °C, the Al ₂O ₃ layer (7.6–20.7 μm thick) wrinkles and Al ₂TiO ₅ forms. Though the Al ₂O ₃ grains coarsen at 1200 °C and 1300 °C, the weight gain is higher than that for Ti ₃AlC ₂ or Ti ₂AlC. At around 7 at. %, this is one of the lowest, if not lowest, Al mole fraction in a Ti-based alloy/compound that forms an Al ₂O ₃ passivating layer. We further provide compelling microstructural evidence, in the form of a duplex oxide, that at 1000 °C, the outward Al flux, J _Al, and the inward O flux, J _O, are related such that 2 J _Al = 3 J _O. A fraction of these fluxes combine, at the duplex oxide interface, to nucleate small grains