Gas‐solid displacement reactions in the Ti–W–C system

Abstract

Displacement reactions between binary and ternary ceramics in the Ti–W–C system and reactive gaseous atmospheres are investigated in this work. Specifically, WC and 50:50 wt% TiC:WC solid solution powders were exposed to flowing hydrogen gas, or equilibrated against an excess of titanium in the presence of iodine, to form metallic tungsten and TiC solid products. In the case of pure WC reacting with hydrogen, transformation to metallic tungsten occurred as a result of removal of chemically bound carbon as gaseous hydrocarbons. In the case of pure WC reacting with titanium iodide vapors, transformation was accompanied by the appearance of TiC as a solid product formed at the gas‐solid interface. In the case of 50:50 wt% TiC:WC solid solution powders, hydrogen was generally found to be an ineffective displacing reagent, whereas reaction with titanium iodide vapors was observed to proceed virtually to completion, resulting in a two phase product mixture comprising metallic tungsten and TiC. For the latter case, a variety of microstructures could be observed within a given batch, including tungsten platelets and/or lamellae in a TiC matrix, or coarse tungsten grains interspersed with TiC grains. These morphological variations are speculated to arise from compositional variation in the starting material and the occurrence of local rapid coarsening along fast diffusion pathways within reacting agglomerates and polycrystalline primary particles. The observed reaction products and relative efficacy of gaseous reagents to promote displacement reactions in the Ti–W–C system are rationalized on the basis of thermodynamic predictions. The reaction between 50:50 wt% TiC:WC solid solution powders and titanium iodide vapors constitutes the first known report of an internal displacement reaction proceeding via gaseous intermediates in a nonoxide ceramic system.

Intragranular nanocomposite powders as building blocks for ceramic nanocomposites

“…This majority intragranular character is thought to be attributable to the formation of a cladding layer around each nanocomposite powder grain, a unique feature of the employed reaction shown in Equation (). The interested reader is directed to prior work describing the origins of this morphology associated with metallothermic displacement reactions 49,51 …”

“…The interested reader is directed to prior work describing the origins of this morphology associated with metallothermic displacement reactions. 49,51 FIB milling was employed to cross-section intragranular nanocomposite powder grains to reveal their internal structures. Figure 2 depicts an STEM-HAADF micrograph of a nanocomposite powder grain with initial composition of (Ti 0.85 W 0.15 )C heat treated at 1000 • C for 6 h (i.e., a time insufficient to attain complete batch reaction).…”

“…A more detailed description of the thermodynamics and morphological development of intragranular nanocomposite powders has been previously reported. 51 These intragranular nanocomposite powders were subsequently chemically treated to remove any nonintragranular metal and consolidated via spark plasma sintering to produce ceramic nanocomposites. This work presents materials characterization results that illustrate the evolution of microstructure at each processing step to establish processing-microstructure relationships in the production of ceramic nanocomposites from intragranular nanocomposite powder building blocks.…”

“…While recent research suggests that spontaneous formation of a wetting liquid at interparticle contacts can greatly enhance the rate of solid‐state reactions, 50 the general applicability of this approach to nanocomposite synthesis has not been explored. Alternatively, solid–solid displacement reactions can be promoted via gaseous phase intermediates 37,51,52 . The gas‐phase mediated approach to nanocomposite synthesis possesses several notable advantages over liquid– and solid–solid reaction routes, including the avoidance of deleterious solidified byproducts and of interparticle contact issues, respectively.…”

“…The underscoring notations in Equation () refer to microstructural locations in the product intragranular nanocomposite powders composed of two regions: a core consisting of metallic tungsten precipitates in a tungsten‐depleted TiC‐based matrix and an epitaxially deposited TiC cladding layer devoid of precipitates. A more detailed description of the thermodynamics and morphological development of intragranular nanocomposite powders has been previously reported 51 . These intragranular nanocomposite powders were subsequently chemically treated to remove any non‐intragranular metal and consolidated via spark plasma sintering to produce ceramic nanocomposites.…”