Transition-metal carbides☆

Aluminium was incorporated into the reactant mixture with a molar ratio of Ti/C of 1.0 to study the effect of its addition on the combustion reaction between titanium and carbon to form TiC. Thermal analysis of the reactant mixture and component analysis of the reaction product suggest that the combustion reaction of the Ti-C-Al system proceeds in such a way that aluminium initially reacts with titanium to form titanium aluminide compounds of TiAl3, Ti2Al, and TiAl with heat evolution, and then the reaction between titanium and carbon and the decomposition of titanium aluminide to titanium and aluminium is subsequently followed. As the amount of aluminium incorporated was increased over the range of 0 to 40 wt%, the grain size of TiC decreased from approximately 15 μm to 0.4 μm. It was also observed that most of the aluminium in the TiC-Al composite was distributed on the surface of the spherical TiC grain.

Effect of aluminium addition on the combustion reaction of titanium and carbon to form TiC

For group VB transition metal carbides, as compared to group IVB carbides, an anomalous rise in hardness occurs for substoichiometric carbon concentrations as compared to the stoichiometric monocarbides. Here we report the computationally derived elastic constants, electronic density of states, and activation energies for carbon vacancy migration as a function of carbon content to elucidate their effect on the hardening responses. The changes in elastic properties with respect to carbon vacancy concentration show similar behaviors of elastic softening and decreasing hardness for all of the cubic carbides. The consistent trends of vacancy diffusion energy barriers between all of the group IVB and VB transition metal carbides also suggests that carbon diffusion may not be a significant contributor to the reported hardness differences. Consequently, we propose that the anomalous hardening for substoichiometric behavior is a competition between elastic constant softening and a microstructural-based effect, i.e. domain hardening, that is present in group VB carbides but not in group IVB carbides.

Influence of carbon vacancy formation on the elastic constants and hardening mechanisms in transition metal carbides

The series of books represents a thorough treatment of ultra-high temperature materials with melting (sublimation or decomposition) points around or over 2500 °C. In the third volume are included physical (structural, thermal, electromagnetic, optical, mechanical, nuclear) and chemical (more than 1100 binary, ternary and multi-component systems, including those used for materials design, data on solid-state diffusion, wettability, interaction with chemicals, gases and aqueous solutions) properties of titanium monocarbide TiC1–x, vanadium monocarbide VC1–x and related 2D-molecular (graphene-like) carbide MXenes, discovered and developed only in the last decade. TiC1–x and VC1–x are also widely applied in the general technological and engineering practice in a wide range of temperatures as parts of highly hard materials (alloys) and special steels. This book will be of interest to researchers, engineers, postgraduate, graduate and undergraduate students alike. For the named materials, readers/users are provided with the full qualitative and quantitative assessment, which is based on the latest updates in the field of fundamental physics, chemistry, nanotechnology, materials science, design and engineering.

http://link.springer.com/content/pdf/10.1007/978-94-024-2039-5.pdf

Ultra-High Temperature Materials III: Refractory Carbides II (Ti and V Carbides)

The refractory compounds TiC, ZrC, HfC, VC, NbC and TaC comprise a family of materials characterized by high melting point, great hardness and low chemical reactivity. These properties, which are exploited in tribological applications, are reviewed and discussed in terms of models taken from solid state physics. Particular attention is given to the important role of non-stoichiometry in these compounds and its influence on bonding and on transport properties. It is argued that the non-stoichiometry is a metastable condition, however, in view of the presece of ordered phases at certain values of carbon-to-metal ratio. The substantial impact of order-disorder transformations on transport properties is analyzed. The partially covalent bonding is discussed in terms of mixing of 2p and metal 3d or 4d atomic states, and the high Peierls stress resisting dislocation motion is thereby justified.

Physics of transition metal carbides

Single crystals of TiCx (x=0.84) were exposed to a carbon source at temperatures near 2000°C. The penetration of carbon into the structure, reflected in a gradient in the value of x, was evaluated with the electron microprobe analyzer. The chemical diffusivity D was then determined using the Boltzmann‐Matano method of analysis. The experimental value of D was found to increase with decreasing carbon vacancy concentration, 1−x, and to have the average value of 220 exp (−97 700/RT) cm2/sec. This value of D is in good agreement with one generated from Sarian's 14C tracer measurements on TiC crystals and DePoorter's semitheoretical model for activity. Because of the abundance of vacancies in the carbon fcc sublattice of TiCx, the experimental activation energy represents an energy of motion only. Lower experimental values for the diffusion activation energy have been obtained by other workers with the phase‐boundary advance method. The present work shows that the difference between the activation energy me...

D. L. Kohlstedt

Papers

Chemical Diffusion in Titanium Carbide Crystals