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I.

INTRODUCTION

MoSi2 is a promising candidate material for high-temperature structural applications because of its high melting point and excellent oxidation resistance. However, its room-temperature toughness and high-temperature strength need to be improved before any practical structural application is considered. It is known that the mechanical properties of ordered intermetallic compounds, in some cases, can be significantly improved by macroalloying or microalloying. In general, the alloying processes function by modifying crystal structure, by changing lattice parameters or bonding nature, and by optimizing microstructure. One successful case of macroalloying is that in which the crystal structure of Co3V was changed by adding Fe and Ni from hexagonal to cubic. Consequently, the alloy (Fe,Co,Ni)3V with a cubic LI2 ordered structure exhibited more than 40 pct tensile ductility at room temperature.[1,2] As for microalloying, a well-known example is that in which the ductility of Ni3Al was dramatically increased by the microalloying of boron.[3] Encouraged by past successful cases, alloying is also being investigated as a possible method of toughening MoSi2. The idea is that the mechanical properties of MoSi2 might be improved either by crystal structure modification or by microstructural manipulation through alloying with some transition elements. It is known from past research work that the alloying potential of MoSi2 is limited.[4–9] MoSi2 with C11b structure is found to form a continuous solid solution only with a few isostructural compounds, such as WSi2, ReSi2, and MoGe2. The fact that MoSi2 has a c/a ratio of 2.452 (very close to =6) implies a nearly perfect hexagonal arrangement of atoms on the {110} planes. Therefore, MoSi2 crysDANQING YI, Postgraduate Student, and CHANGHAI LI, Associate Professor, are with the Department of Engineering Metals, Chalmers University of Technology, S-412 96 Go¨teborg, Sweden. ZONGHE LAI, Senior Researcher, is with the Swedish Institute of Production Engineering Research, S-412 85 Go¨teborg, Sweden. O.M. AKSELSEN, Director, and J.H. ULVENSOEN, Group Leader, are with SINTEF Materials Technology, N-7034 Trondheim, Norway. Manuscript submitted January 29, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A

tallographically has a close relationship with the hexagonal C40 structure. The microstructure and phase composition of MoSi2 modified with Al, B, Hf, Nb, and Re have been investigated.[10,11] The B and Hf substitutions for Si and Mo, respectively, exhibited very low solubilities in MoSi2. The Al and Nb substitutions for Si and Mo, respectively, changed the crystal structure from tetragonal C11b to hexagonal C40. The Ge and Re substitutions for Si and Mo, respectively, exhibited complete solubility and maintained the tetragonal structure.[10] Mechanical testing indicated that the brittleto-ductile transition temperatures of modified MoSi2 by the preceding elements were in the range 1250 7C to 1350 7C, somewhat higher than unmodified MoSi2. Meanwhile, oxidation testing indi