Synthesis of in situ composites through solid-state reactions: thermodynamic, mass-balance and kinetic considerations

The applications of thermodynamics, mass balance and kinetics to the synthesis of in situ composites through solid-state reactions are discussed. The proper choice of starting materials, the principles governing the diffusion paths and the formation of desirable microstructures are considered. The use of stability diagrams for rationalizing diffusion and reactions during composite synthesis is also demonstrated. Applying these principles to the synthesis of NbSi2-SiC composites, it is shown that the proper starting materials are NbC1−x and Si. The probable microstructure is an aggregate type, composed of NbSi2 and SiC. Preliminary experimental results of the study of bulk NbC1−x-Si diffusion couples annealed at 1300 °C for 60 h revealed that the microstructure is indeed a two-phase mixture of NbSi2 and SiC. Discontinous SiC particles with an average size of 1 μm are homogeneously dispersed in the NbSi2 matrix. This microstructure is considered favorable for high-temperature structural composite application. Further characterization of mechanical and other properties for this material are currently being performed.