Preparation and High-temperature Oxidation of MoSi<SUB>2</SUB>/Mo/Nb/<I>γ</I>-TiAl Functionally Graded Materials
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概要
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Mo/Nb/<I>γ</I>-TiAl functionally graded materials (FGMs) were prepared using a spark plasma sintering (SPS) method, and the MoSi<SUB>2</SUB>/Mo/Nb/<I>γ</I>-TiAl FGMs were then siliconized in molten salts. The MoSi<SUB>2</SUB>/Mo/Nb/<I>γ</I>-TiAl FGMs before and after oxidation at 1323 K in air were evaluated using optical microcopy, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The Mo/Nb/<I>γ</I>-TiAl FGMs were obtained by joining at 1373 K for 5 min using SPS. The Nb foil was firmly joined to the <I>γ</I>-TiAl surface and the Mo foil was firmly joined to the Nb foil. The Δ<I>α</I><SUB>Nb-Mo</SUB>Δ<I>T</I> and Δ<I>αγ</I><SUB>-TiAl-Mo</SUB>Δ<I>T</I> values were consistent with the criterion for stable FGMs without interlayer cracking of Δ<I>α</I>Δ<I>T</I> less than 4.3×10<SUP>-3</SUP>, where Δ<I>α</I><SUB>Nb-Mo</SUB> and Δ<I>αγ</I><SUB>-TiAl-Mo</SUB> are the differences between the coefficients of thermal expansions of Nb and Mo, and between those of <I>γ</I>-TiAl and Nb, and Δ<I>T</I> is the difference between the SPS temperature and room temperature. The Mo/Nb/<I>γ</I>-TiAl FGMs were siliconized by dipping in a molten-salts mixture before heating in a mullite crucible at 1173 K for 40 h. The MoSi<SUB>2</SUB>Mo/Nb/<I>γ</I>-TiAl FGMs were dense and had neither cracks nor voids. The thickness of the MoSi<SUB>2</SUB> layer was approximately 50 <I>μ</I>m. The thickness loss of the MoSi<SUB>2</SUB>/Mo/Nb/<I>γ</I>-TiAl FGMs was 11 <I>μ</I>m after the 200 h exposure to air at 1323 K. The thickness loss of the MoSi<SUB>2</SUB>/Mo/Nb/<I>γ</I>-TiAl FGMs was approximately 20 % less than that of NbSi<SUB>2</SUB>/Nb/<I>γ</I>-TiAl FGMs.
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