High Temperature Oxidation Behavior of Al Added Mo/Mo 5 SiB 2 in-situ Composites
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High Temperature Oxidation Behavior of Al Added Mo / Mo5SiB2 in-situ Composites Akira Yamauchi, Kyousuke Yoshimi, and Shuji Hanada Institute for Material Research, Tohoku University, Sendai 980-8577, Japan ABSTRACT Isothermal oxidation behavior of Mo/Mo5SiB2 in-situ composites containing small amounts of Al was investigated under an Ar-20%O2 atmosphere in the temperature range of 1073–1673 K. The Mo/Mo5SiB2 in-situ composites, (Mo-8.7mol%Si-17.4mol%B)100-xAlx (x=0, 1, 3, and 5mol%), were prepared by Ar arc-melting, and then homogenized at 2073 K for 24 h in an Ar-flow atmosphere. Without addition of Al, Mo/Mo5SiB2 in-situ composite exhibits a rapid mass loss at the initial oxidation stage, followed by passive oxidation after the substrate is sealed with borosilicate glass in the temperature range of 1173–1473 K, whereas it exhibits a rapid mass gain around 1073 K. On the other hand, small Al additions, especially of 1 mol%, significantly improve the oxidation resistance of Mo/Mo5SiB2 in-situ composites at temperatures from 1073–1573 K. The excellent oxidation resistance is considered to be due to the rapid formation of a continuous, dense scale of Al-Si-O complex oxides. The protective oxide scales contain crystalline oxides, and the amounts of the crystalline oxides obviously increase with Al concentration.
INTRODUCTION Mo-based composites are attractive materials for high-temperature structural applications to achieve high energy efficiency. Especially, Mo-Si-B alloys have high potential as heat-resistant materials at ultra-high temperature [1-5]. Akinc et al. reported that the oxidation resistance of Mo5Si3 is dramatically improved by the addition of boron at temperatures ranging from 1073 to 1773 K in air [6]. However, it would not be possible to use monolithic Mo5Si3 for structural applications because of its fatal brittleness and strong anisotropy of thermal expansion coefficient [7]. Mo5SiB2 is only one ternary compound in the Mo-Si-B system. Ito et al. reported that the single crystal of Mo5SiB2 shows excellent strength at high temperature for a few orientations [8-9]. With respect to the oxidation resistance of Mo5SiB2, Yoshimi et al. [10] studied the oxidation behavior of Mo5SiB2-based alloy at elevated temperatures, and concluded that the oxidation resistance of Mo5SiB2-based alloy is not as good as that of boron-added Mo5Si3-based alloys. Furthermore, they worked on thermal expansion, strength and oxidation resistance of Mo/Mo5SiB2 in-situ composites, and reported that the Mo/Mo5SiB2 in-situ composite having a eutectic microstructure shows superior high temperature strength even at 1773 K [11]. However, the oxidation resistance of the Mo/Mo5SiB2 in-situ composites is worse than that of Mo5SiB2 [12]. Therefore, the improvement of oxidation resistance is a key to develop Mo-Si-B alloys for ultra-high temperature applications. Yanagihara et al. reported the effect of third elements on the oxidation resistance of MoSi2 [13-14]. The third elements having a larger affinity to oxygen than Si show the
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