Microstructure and Residual Stress of Alumina Scale Formed on Ti2AlC at High Temperature in Air
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Ti 2AlC ternary carbide is being explored for various high temperature applications due to its strength at high temperatures, excellent thermal-shock resistance, and high electrical conductivity. A potential advantage of Ti 2AlC over conventional Al 2O 3-forming materials is the near-identical coefficient of thermal expansion (CTE) of Ti 2AlC and α-Al 2O 3, which could result in superior spallation resistance and make Ti 2AlC a promising option for applications ranging from bondcoats for thermal barrier coatings to furnace heating elements. In this study, isothermal and cyclic oxidation were performed in air to examine the oxidation behavior of Ti 2AlC. Isothermal oxidation was performed at 1000, 1200 and 1400 °C for up to 25 h and cyclic oxidation consisted of 1,000 1-hour cycles at 1200 °C. Characteristics of the oxide scale developed in air, including mass change, residual stress in the α-Al 2O 3 scale, phase constituents and microstructure, were examined as functions of time and temperature by thermogravimetry, photostimulated luminescence, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy via focused ion beam in situ lift-out. Above a continuous and adherent α-Al 2O 3 layer, a discontinuous-transient rutile-TiO 2 scale was identified in the oxide scale developed at 1000 and 1200 °C, while a discontinuous-transient Al 2TiO 5 scale was identified at 1400 °C. The continuous α-Al 2O 3scale thickened to more than 15 μm after 25 h of isothermal oxidation at 1400 °C, and after 1,000 1-hour cycles at 1200 °C, yet remained adherent and protective. The compressive residual stress determined by photoluminescence for the α-Al 2O 3 scale remained under 0.65 GPa for the specimens oxidized up to 1400°C for 25 hours. The small magnitude of the compressive residual stress may be responsible the high spallation-resistance of the protective α-Al 2O 3 scale developed on Ti 2AlC, despite the absence of reactive element additions. © Springer Science+Business Media, LLC 2007.
author list (cited authors)
Byeon, J. W., Liu, J., Hopkins, M., Fischer, W., Garimella, N., Park, K. B., ... Sohn, Y. H.