| Gamma titanium aluminide (γ-TiAl) has attracted many scientific research interests as a kind of high temperature structural material, due to its low density, high specific strength, high specific stiffness and good creep resistance at high temperatures. The ductility and toughness of TiAl-based alloy at room temperature have been improved greatly during last one decade and a half, but its insufficient oxidation resistance above approximately 750℃is still one of the main barriers to its wider applications at elevated temperatures.The work of our research group shows that a composite coating can be formed by pack siliconizing with a mixture powder of silicon and alumina. The coating is composed of a thin Al2O3-dominated outer layer and a thick Ti5Si3-dominated inner layer, which bonds well with substrate. Neither cracks nor spallations occurred during the long-term cyclic oxidation at 900℃. The specimen siliconized at 1250℃for 2 h exhibits the best oxidation resistance in the siliconized specimens. This paper investigates the oxidation kinetics and the oxidation mechanisum of the siliconized TiAl-based alloy above 900℃and makes a comparation with that at 900℃, to supply certain theory and practical basis for its application at elevated temperatures.The specimens were pack sliconized with a mixture powder of silicon and alumina with the propertion of 15% to 85%, the siliconized specimens and the bare one were cyclically oxidated at 900℃, 1000℃, 1100℃, 1200℃. The experimental data were fitted by the least square method to study the oxidation kinetics that follows different laws at different oxidation stages above 900℃. The results are as follows:For the bare specimens oxided at 900℃for 4 h, a little spallation occurred and the weight gains are as high as 0.688mg/cm2, which is three time more than 0.1875 mg/cm2 that of siliconized specimens oxided for 118 h at the same temeperature. The spallation took place severely and the surfaces appereared yellow to diffferent degree for all of bare specimens oxided at 1000℃, 1100℃, 1200℃for 2 h. But oxidation resistance at 1100℃and 1200℃of the specimens siliconized at 1250℃for 2 h were improved greatly, and at 900℃and 1000℃, it behaved much better oxidation resistance. After oxidized at 900℃for 118 h, the surface and the cross-section showed no limbus changes, with the coating and the substrate bonded well with each other. The slight spal lation occurred for the siliconized specimen oxided at 1000℃for 118 h. And l l00C for 16 h, and 1200℃for 4 h different degree spallation occurred.Three kinds of oxidation kinetics behavior, parabolic, linear and quadratic, were revealed by analyzing the experimental data of 900℃, 1000℃, 1100℃and 1200℃with the least square method. At 900℃, only parabolic oxidation behavior appears during the whole cyclic oxidation test of 118 h. At 1000℃, the oxidation behavior is parabolic at the initial stage up to 50 h, and becomes linear from 50 to 118 h. At 1100℃, the oxidation behavior is parabolic at the initial stage up to 4 h, and is linear from 4 h to 8 h, and then becomes quadratic from 8 until spallation at 16 h. At 1200℃, only quadratic oxidation behavior appears until spallation at 4 h. The corresponding oxidation constants obtained by fittingthe curves are Kp≈4.924×10-5mg2cm-4h-1 at 900℃, Kp≈7.778×10-5mg2cm-4h at 1000℃, Kp≈9.392×10-2mg2cm-4h-1 at 1100℃. That is to say, when the is to say, when the temeperature rises from 900℃to 1000℃, although Kp value increaces to a certain degree, it keeps in the same order. When the temeperature rises from 1000℃to 1100℃, Kp value increaces more than 3 orders of magnitude. This steep change of Kp values when temperature increases from 1000℃to 1100℃was associated with the phase transformation of TiAl-based alloy fromα2＋γtoα＋γ, and the inverse process occurred during cyclic oxidation. This transformation maybe occur at a lower temperature in the siliconized. TiAl-based alloy with Cr and V addition than the eutectoid temperature 1125℃in binary TiAl-based alloys.
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