Synthesis of Ti matrix composites reinforced with TiC particles: in situ synchrotron X-ray diffraction and modeling

The reaction tending toward thermodynamic equilibrium during the synthesis of Ti/TiC MMC prepared by the powder metallurgy route was studied by in situ synchrotron X-ray diffraction. The carbide composition was found to change rapidly from its initial stoichiometric value TiC_0.96 toward a substoichiometric value (TiC_0.57) corresponding to thermodynamic equilibrium with the C-saturated Ti matrix. The reaction rate is very fast, and the solid-state reaction is almost complete after only a few minutes at 1073 K (800 °C) for the smallest particles, whereas the rate-limiting step remains the particle size. In addition, modeling of the diffusion processes in MMCs, i.e., initial dissolution of particles and their trend toward equilibrium composition, was performed using three particles size classes and the calculations were performed using the ThermoCalc and Dictra package. First, dissolution of the smallest particles (10% of the initial TiC_0.96 particles) is expected to be achieved after only 1 s at 800 °C. Second, the change in TiC composition leads to an increase in the total amount of carbide in the composite from 16 to 19 mass%. The consequences on the industrial process of Ti/TiC MMC synthesis have also been considered. A typical industrial heat treatment of a MMC billet, 1 h at 900 °C, was modeled, and the results showing an increase in the total amount of carbide in the composite from 16 to 22 mass% are in rather good agreement with the experimental value (21 mass%). This highlights the potential of thermodynamic and kinetic modeling to help understand and optimize industrial processes for MMC synthesis.