Enhanced mass transport in titanium carbide at large departures from stoichiometry

Significantly enhanced mass-transport related effects such as sintering and grain growth have been reported at large departures from stoichiometry in titanium carbide. The diffusivities of both C and Ti in this compound are coupled and the rate of mass transport is determined by the lower mobility of Ti. Quantitative grain-growth measurements of samples with different C/Ti ratios confirmed the large mass transport effects at low carbon content. A model based on the statistical distribution of vacant carbon sites in the immediate environment of the Ti atoms was put forward in order to account for the greatly enhanced diffusivity of Ti at low values of the C/Ti ratio. The dominant determining factor at that ratio is the reduced enthalpy of formation of Ti vacancies in the absence of nearest neighbor carbon atoms. The composition dependence of the observed grain growth is in reasonable agreement with the predictions of the model.