Effects of ternary additions on the deformation behavior of single crystals of MoSi 2
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Effects of ternary additions on the deformation behavior of single crystals of MoSi2 Haruyuki Inui, Koji Ishikawa and Masaharu Yamaguchi Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ABSTRACT Effects of ternary additions on the deformation behavior of single crystals of MoSi2 with the hard [001] and soft [0 15 1] orientations have been investigated in compression and compression creep. The alloying elements studied include V, Cr, Nb and Al that form a C40 disilicide with Si and W and Re that form a C11b disilicide with Si. The addition of Al is found to decrease the yield strength of MoSi2 at all temperatures while the additions of V, Cr and Nb are found to decrease the yield strength at low temperatures and to increase the yield strength at high temperatures. In contrast, the additions of W and Re are found to increase the yield strength at all temperatures. The creep strain rate for the [001] orientation is significantly lower than that for the [0 15 1] orientation. The creep strain rate for both orientations is significantly improved by alloying with ternary elements such as Re and Nb. INTRODUCTION MoSi2 with the C11b structure has received a great deal of attention as a candidate for structural materials to be used in oxidizing environments at temperatures higher than the upper limit for Ni-base superalloys because of its high melting temperature, relatively low density, good oxidation resistance and high thermal conductivity [1-3]. However, monolithic MoSi2 exhibits only a modest value of fracture toughness at low temperatures and inadequate strength at high temperatures. Thus, many of recent studies on the development of MoSi2-based alloys have focused on improving these poor mechanical properties through forming composites with ceramics [1,4]. However, the volume fraction of Si3N4 and SiC ceramic reinforcements in these MoSi2-composites generally exceeds 50 % [5,6]. Further improvements in mechanical properties of these composites will be achieved if those of the MoSi2 matrix phase are improved. The present study was undertaken to achieve this by alloying additions to MoSi2. Transition-metal atoms that form disilicides with tetragonal C11b and hexagonal C40 structures are considered as alloying elements to MoSi2. The two structures commonly possess (pseudo-) hexagonally arranged TMSi2 layers and differ from each other only in the stacking sequence of these TMSi2 layers; the C11b and C40 structures are based on the AB and ABC stacking of these layers, respectively. W and Re have been known to form a C11b disilicide with Si and they are believed to form a complete C11b solid-solution with MoSi2. Large amounts of alloying additions are possible for these alloying elements, and hence high-temperature strength is expected to be improved through a solid-solution hardening mechanism. V, Cr, Nb and Ta have been known to form a C40 disilicide with Si. Al is also known to transform MoSi2 from the C11b to the C40 structures by substituting it for Si. 1/2 dislocations
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