Reactive processing of titanium carbide with titanium - Part 1 Liquid-phase sintering

The reactive sintering of titanium carbide with titanium metal was studied using mechanical mixtures of fine-grained powders heated in vacuum above the TiC-Ti eutectic temperature. Mixtures with bulk compositions of TiC_0.94 to TiC_0.63 yielded nonstoichiometric carbide with less than 0.5 wt% residual titanium metal after sintering, while residual metal was observed at higher titanium concentrations. The effects of time, temperature, and composition on Mohs hardness, final porosity and final grain-size were determined using a Box-Wilson experimental design. The experimental ranges studied were sintering times of 10 to 100 min, sintering temperatures of 1650 to 1850° C, and compositions from TiC_0.94 to TiC_0.58. Over these experimental ranges, the effects of time and temperature were small compared with those of composition. The Mohs hardness increased approximately linearly from two to nine with increasing percentage of titanium metal in the starting powder. The average grain size ranged from 15 to 70μm, increasing with increasing time and temperature. For bulk compositions TiC_0.94 to TiC_0.70 grain growth was largely due to the conversion of titanium to substoichiometric carbide which grows epitaxially on the carbide grains. Substantial grain growth occurred for higher metal concentrations. The open porosity decreased from 28% to 16% as the amount of titanium metal in the starting powders was increased. Both the grain growth and the densification during reactive sintering of titanium-titanium-carbide mixtures were analysed in terms of a sintering model adapted from Kuczynski. A factor which empirically describes the behaviour of the system over a range of compositions was incorporated into the equations proposed by Kuczynski. Microstructural evidence and the activation energies for grain growth and densification all indicate that the rapid reaction between titanium metal and titanium carbide to form substoichiometric carbide occurs via short-circuit diffusion of carbon out of the carbide grains along Ti₂C platelets. Low sintered densities are attributed to the rapid formation of a solid titanium-carbide skeleton which prevents significant particle rearrangement in the eutectic liquid. Solution-precipitation processes do not appear to contribute significantly to the densification in this system.