Isothermal Oxidation of Ti 3 Al 0.6 Ga 0.4 C 2 MAX Phase Solid Solution in Air at 1000 °C to 1300 °C

Abstract

The atomically laminated Ti 2 AlC, Ti 3 AlC 2 and Cr 2 AlC MAX phases, with A = Al, form adherent, passivating α -alumina, Al 2 O 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 3 Al 1−x Ga x C 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 2 O 3 layer ( ≈ 1–2.6 μ m thick) with near cubic kinetics and an overall weight gain that is slightly less than either Ti 3 AlC 2 or Ti 2 AlC is formed. At 1200 °C, the Al 2 O 3 layer thickens (3.5–12 μ m thick) with some scale delamination on the corners initiating at 15 h. At 1300 °C, the Al 2 O 3 layer (7.6–20.7 μ m thick) wrinkles and Al 2 TiO 5 forms. Though the Al 2 O 3 grains coarsen at 1200 °C and 1300 °C, the weight gain is higher than that for Ti 3 AlC 2 or Ti 2 AlC. 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 2 O 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