Reaction Syntheses with Carbon Materials Chemistry: Intragranular Nanocomposites and 'Carbon Copies'

Intragranular Nanocomposites Reaction processing techniques are often employed to fabricate refractory metals, carbides and their composites into complex shaped parts that would otherwise be inaccessible by traditional powder processing and consolidation routes. Recent work by the present authors demonstrates a volume-expansive reactive infiltration technique, Displacive Compensation of Porosity, that can produce dense, near net-shape and net-dimension metal/ceramic composites (e.g., W + ZrC) whose microstructural features are inherited from a porous reactant preform. The displacement reaction utilized to produce W + ZrC composites can be written as WC + Zr(liq.) → W + ZrC (i.e., free carbon produced by the dissociation of WC diffuses outward to react with the Zr-bearing melt to form a ZrC cladding, leaving behind a carbon-depleted W layer around a shrinking WC core). Upon infiltration and complete reaction, porous WC preforms can thus produce composites containing up to 38 vol% W + 62 vol% ZrC. The final ceramic content can be further increased by utilizing preforms initially containing mixtures of ZrC and WC. However, WC displays an extensive range of solid solubility in ZrC, forming the mixed carbide (ZrxW1-x)C. The reactivity of this solid solution under melt infiltration conditions was assessed by reacting (ZrxW1-x)C powder compacts with Zr-bearing liquids, where it was discovered that mixed carbide particles may undergo an internal displacement reaction resulting in a distinct product morphology (see Figure 1) consisting of metallic nanoprecipitates embedded in a single crystal ceramic matrix. Selected thermodynamic, kinetic, and crystallographic aspects of this reaction processing technique will be discussed.