SOLIDIFICATION PATHS AND CARBIDE MORPHOLOGIES IN MELT-PROCESSED MOSI2-SIC IN-SITU COMPOSITES

The present investigation was undertaken to elucidate the micro struct ural evolution of MoSi2-SiC in situ composites produced by melt proces sing. An assessment of the existing liquidus projection was performed by a combination of thermodynamic modeling, analysis of solidification microstructures, and measurements of the thermal history during solid ification. Results show that the quasibinary MoSi2-SiC eutectic occurs at similar to 2 at. pct C and 2283 K, rather than 8 at. pet C and 217 3 K, as previously reported. The ensuing L + MoSi2 + SiC monovariant l ine runs almost parallel to the SiMoSi2 binary and terminates at a ter nary L <-> Si + MoSi2 + SiC eutectic calculated at 1.5Mo 0.84C (at. pc t) and similar to 1670 K. The maximum amount of SiC that may be produc ed by solidification along the quasibinary isopleth is similar to 37 v ol pct, of which similar to 35 vol pct grows as primary. Analysis of s olidification microstructures shows SiC grows with the cubic beta poly type structure (B3), while MoSi2 grows with the tetragonal C11(b) stru cture. Primary SiC may grow as equiaxed particles, platelets, and hopp er crystals. Coupled growth with MoSi2 leads to SiC in the shape Of th in ribbons, sheets, and needles. The facets of the SiC crystals were i dentified to be-of the {111} and {002} type, in agreement with the per iodic bond chain analysis. The predominant platelike morphology was sh own to develop due to a re-entrant twin mechanism,similar to that obse rved in Si and Ge.