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Effects of Ternary Additions on the Microstructure of Directionally-Solidified MoSi2/Mo5Si3 Eutectic Composites Kosuke Fujiwara, Yuta Sasai, Kyosuke Kishida and Haruyuki Inui Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ABSTRACT Microstructural variations of directionally-solidified (DS) MoSi2/Mo5Si3 eutectic composites caused by ternary additions of V, Cr, Nb, Ta and W were investigated. Ternary addition of relatively large amount of V resulted in the apparent modification of the morphology as well as the macroscopic inclination angle of one type of the interfaces extending nearly along the growth direction in the DS ingots. Ternary addition of Cr was found to change the microstructures of the DS ingots from a cellular-type with coarse boundary regions to a plate-like cellular-type with increasing the amount of the ternary addition. No significant change of scriptlamellar morphology was observed for the DS ingots containing Nb, Ta and W irrespective of the amount of the ternary addition. INTRODUCTION The intermetallic compound MoSi2 with the C11b structure is one of the promising structural materials for ultra-high temperature applications due to its very high melting point (2020 °C) and excellent oxidation resistance [1-5]. However, poor fracture toughness at room temperature and insufficient high-temperature strength and creep strength have hindered the wide-spread practical use of the material. Extensive studies have been carried out to overcome such drawbacks by forming an in-situ composite with a secondary phase [4-9]. Among possible candidates of the secondary phase, the MoSi2/Mo5Si3 eutectic composites are quite attractive since they possess high eutectic temperature (1900 °C for binary alloys) and also because fine microstructures of a script-lamellar type have been reported to be formed simply by various unidirectional solidification processes (see figure 1) [5]. Only limited studies on microstructures and mechanical properties of binary MoSi2/Mo5Si3 eutectic composites have been done so far and revealed that the creep strengths of directionally-solidified (DS) MoSi2/Mo5Si3 eutectic composites are superior to those of MoSi2 single crystals [8]. Very recently, we prepared various DS ingots of binary eutectic composite with different thicknesses of both MoSi2 and Mo5Si3 phases by controlling the growth rate during DS process and revealed that high-temperature strength drastically increases with decreasing the thickness of MoSi2 phase [10]. In addition, the microstructure and mechanical properties of two-phase composites are generally influenced by interface properties, such as lattice misfits and segregation behavior of additional ternary elements. It is thus quite important to establish ways to optimize interface properties of the MoSi2/Mo5Si3 eutectic alloys. However, the effects of ternary additions on the microstructures and mechanical properties have not been studied extensively yet. In the present study, we investigate the effects of some ternary ad