THE REACTION PROCESSING OF TITANIUM CARBIDE FROM TITANIUM METAL AND TITANIUM CARBIDE

摘要

The solid state reaction between titanium metal and titanium carbide to form substoichiometric carbide was studied by means of annealing single crystal diffusion couples at temperatures in the range 1350 to 1525(DEGREES)C. The thickness of the epitaxially grown carbide layer was almost an order of magnitude greater than that predicted from published values for chemical diffusion of carbon through titanium carbide. A value of 424exp{-88kcal/mole/RT}cm('2)sec is calculated for the chemical diffusivity of carbon in titanium carbide from the diffusion couple data of the present study. This anomalously rapid diffusion of carbon through titanium carbide is associated with short circuit diffusion along platelets of Ti(,2)C which develop parallel to {111} planes in the TiC during the reaction.;The processing of substoichiometric titanium carbide by the reactive hot pressing and liquid phase reactive sintering of fine grained mechanical mixtures of titanium metal and titanium carbide was investigated. The effects of sintering time, sintering temperature, composition, and applied pressure on Mohs hardness, final porosity, final grain size, and phases present were determined. The grain growth and densification are described by semiempirical equations adapted from the literature.;The activation energy observed for densification during sintering agrees well with the activation energy for short circuit grain boundary diffusion of carbon determined in the study of single crystal diffusion couples. This observation, combined with microstructural and crystallographic evidence of Ti(,2)C platelets, indicates that the mechanism of reaction is the diffusion of carbon along Ti(,2)C-TiC grain boundaries. The low sintered densities which result from liquid phase reactive sintering are attributed to the rapid formation of a titanium carbide skeleton.;Reactive hot pressing of titanium carbide with titanium results in high density substoichiometric titanium carbide at temperatures of 1200 to 1600(DEGREES)C and pressures of 7 to 35MPa (1000 to 5000psi). The activation energy for densification during hot pressing agrees well with the activation energies for creep and self diffusion of titanium metal. This correspondence of activation energy, and the high densities which result from reactive hot pressing, indicate that the mechanism responsible for densification during hot pressing is the deformation of titanium metal.