Solidification paths and carbide morphologies in melt-processed MoSi2-SiC In Situ composites

The present investigation was undertaken to elucidate the microstructural evolution of MoSi₂-SiC in situ composites produced by melt processing. 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 solidification. Results show that the quasibinary MoSi₂-SiC eutectic occurs at ∼2 at. pct C and 2283 K, rather than 8 at. pct C and 2173 K, as previously reported. The ensuing L+MoSi₂+SiC monovariant line runs almost parallel to the SiMoSi₂ binary and terminates at a ternary L ↔ Si+MoSi₂+SiC eutectic calculated at 1.5Mo-0.84C (at. pct) and ∼1670 K. The maximum amount of SiC that may be produced by solidification along the quasibinary isopleth is ∼37 vol pct, of which ∼35 vol pct grows as primary. Analysis of solidification microstructures shows SiC grows with the cubic β polytype structure (B3), while MoSi₂ grows with the tetragonal C11 _b structure. Primary SiC may grow as equiaxed particles, platelets, and hopper crystals. Coupled growth with MoSi₂ leads to SiC in the shape of thin ribbons, sheets, and needles. The facets of the SiC crystals were identified to be of the {111} and {002} type, in agreement with the periodic bond chain analysis. The predominant platelike morphology was shown to develop due to a re-entrant twin mechanism similar to that observed in Si and Ge.