Microstructures and Mechanical Properties of MoSi 2 / Mo 5 Si 3 / Mo5Si3C Ternary Eutectic Composite
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Microstructures and Mechanical Properties of MoSi2 / Mo5Si3 / Mo5Si3C Ternary Eutectic Composite Hirotaka Matsunoshita1, Kosuke Fujiwara1, Yuta Sasai1, Yuichiro Kondo1, Kyosuke Kishida1, 2 and Haruyuki Inui1, 2 1Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan 2Center for Elements Strategy Initiative for Structural Materials (ESISM), Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
ABSTRACT Microstructures and mechanical properties of directionally solidified (DS) MoSi2 / Mo5Si3 / Mo5Si3C ternary eutectic composites were investigated. Ternary eutectic microstructure of a script-lamellar type that is characterized by rod-shaped Mo5Si3 and Mo5Si3C phases extending along the growth direction in the MoSi2 single crystalline matrix was developed simply by directional solidification at a growth rate of 10 mm/h. Compression tests along [1¯ 10]MoSi2 nearly parallel to the growth direction revealed that the DS ternary eutectic composites were plastically deformed above 1000 °C. Yield stresses of the DS ternary eutectic composites were much higher than those of binary composites mainly because of a smaller average thickness of MoSi2 matrix.
INTRODUCTION MoSi2 with the tetragonal C11b structure has been considered as one of the promising base materials for ultra-high temperature application because of its high melting temperature (2020 °C), excellent oxidation resistance and relatively low density (figure 1a) [1]. However, poor fracture toughness at room temperature and insufficient strength at elevated temperature are major drawbacks for its practical applications. One possible method to improve these drawbacks is to form a composite with a hard secondary phase. Among various candidates for the secondary phase, Mo5Si3 with the tetragonal D8m structure (figure 1b) is attractive because of eutectic reaction between MoSi2 and Mo5Si3 occurring at around 1900 °C. MoSi2 / Mo5Si3 eutectic composites are expected to have higher probability of improving the insufficient properties of monolithic MoSi2 through controlling their interface properties because of fine microstructure of so-called script lamellar type formed simply by directional solidification (DS) [2, 3]. Orientation relationship between MoSi2 and Mo5Si3 phases in DS eutectic composites has been reported that [1¯ 10]MoSi2 // [001]Mo5Si3, being approximately parallel to the growth direction, [110]MoSi2 // [1¯ 10]Mo5Si3 and (001)MoSi2 // (110)Mo5Si3 [2, 4]. Recently, we have systematically investigated the effects of ternary additions on microstructures and mechanical properties of MoSi2 / Mo5Si3 eutectic composites and found that about 1 at.% addition of carbon result in the formation of a MoSi2 / Mo5Si3 / Mo5Si3C ternary eutectic microstructure [5]. A so-called Nowotny phase of Mo5Si3C has the hexagonal D88 structure (figure 1c). Since further improvement in mechanical properties of MoSi2-based composites by controlling properties of newly introduced interfaces
Figure 1. Crystal structures of (a) MoSi2 (C11b), (b) Mo5Si
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