Synthesis of Ti matrix composites reinforced with TiC particles: in situ synchrotron X-ray diffraction and modeling
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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 TiC0.96 toward a substoichiometric value (TiC0.57) 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 TiC0.96 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.
This work was undertaken in the framework of the COMETTi project funded by the French national research agency (ANR) [Grant Number ANR-09-MAPR-0021]. O.D. is very grateful to Dr. S. Fries and Pr. I. Steinbach from ICAMS institute at Bochum University (Germany) for allowing DICTRA calculations on their informatics cluster. J.A. thanks ID15B beamline staff for their help during beam time and ESRF for the provision of beamtime through in-house research during his postdoctoral position. The authors thank Gilles RENOU for the STEM observation performed at the “Consortium des Moyens Technologiques Communs” (CMTC, http://cmtc.grenoble-inp.fr). Composite powders were provided by the Mecachrome company (www.mecachrome.fr).
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Conflicts of interest
The authors declare that they have no conflict of interest.
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