(Ti,Zr)C powder was synthesized by carbothermal reduction and subsequently aged at 1150–2000 °C. The phase composition and microstructure was investigated using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and electron backscatter diffraction. It was found that the as-synthesized (Ti,Zr)C particles have a concentration gradient with a higher concentration of Ti at the surface of the particles. Furthermore, during aging the (Ti,Zr)C decomposes into Ti-rich and Zr-rich lamellae. During aging at 1400 and 1800 °C for 10 h, most Zr-rich and Ti-rich domains precipitate at grain boundaries, inheriting the crystal orientation of the parent grain behind the growth front. When the precipitate grows into another (Ti,Zr)C grain, that grain adopts the same crystal orientation as the parent grain. The crystallographic misorientation between adjacent lamellae is 0–5°. Based on these microstructural observations it is hypothesized that the mechanism of decomposition is discontinuous precipitation.

The mixed carbide Ti-Zr-C has been synthesized through carbothermal reduction of TiZrO₄ at 2200 °C, 2300 °C, and 2400 °C. As-synthesized carbide was subsequently aged at 1400 °C to study phase separation. Microstructural investigations and nanoindentation measurements were performed. It was found that the synthesis temperature is important for the homogeneity and porosity of the as-synthesized powder. The initial structure strongly influences the subsequent phase separation upon aging. The phase separation occurs via discontinuous precipitation, and high-angle boundaries are preferred. Finally, fully decomposed particles are slightly harder than the unaged carbide particles.

A nanoindentation and first-principles calculation study of a self-organizing nanostructured lamellar (Ti,Zr)C powder has been performed. The nanoindentation measurements reveal that the hardness of the carbide is comparable to the hardest transition metal carbides that have been reported previously. The origin of the super-high hardness is postulated to be due to the inherent bond strength and the large coherency strains that are generated when the carbide demixes within the miscibility gap. The high hardness is maintained at a high level even after 500 h aging treatment at 1300°C. Therefore, it is believed that the new superhard mixed carbide has a high potential in various engineering applications such as in bulk cemented carbide and cermet cutting tools, and in surface coatings.

The ternary carbides (V,Nb)C and (V,Ta)C have been synthesized by heat-treating powder mixtures of the corresponding binary carbides. The effect of different mixing and milling conditions as well as the addition of small amounts of Fe powder on the resulting carbide microstructure after synthesis was investigated. The as-synthesized carbides were aged at 900 and 1200 °C, which are inside the miscibility gap for both systems, to study the decomposition. The microstructure after aging was characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and electron backscatter diffraction. It was found that (V,Nb)C with small additions of Fe decomposed in a way that resembles discontinuous precipitation upon aging at 1200 °C. (V,Ta)C with and without additions of Fe was found to decompose upon aging at 1200 °C, but giving a different morphology compared to (V,Nb)C. The hardness of both as-synthesized and aged carbides was measured using nano-indentation and the hardness was found to be 26.5 ± 1 GPa and 30.2 ± 1.3 GPa for (V,Nb)C and (V,Ta)C respectively. The high hardness was found to be maintained in (V,Nb)C after decomposing into V-rich and Nb-rich lamellae. 

(Ti,Zr)C powder was sintered with WC-Co following an industrial process, including an isotherm at 1410 °C. A series of interrupted sintering trials was performed with the aim of studying the sintering behavior and the microstructural evolution during both solid-state and liquid-state sintering. Reference samples, using the same elemental compositions but with the starting components TiC and ZrC instead of (Ti,Zr)C, were also sintered. The microstructure was investigated using scanning electron microscopy and energy dispersive X-ray spectroscopy. It is found that the (Ti,Zr)C phase decomposes into Ti-rich and Zr-rich nano-scale lamellae before the liquid-state of the sintering initiates. The final microstructure consists of the binder and WC as well as two different γ phases, rich in either Ti (γ₁) or Zr (γ₂). The γ₂ phase grains have a core-shell structure with a (Ti,Zr)C core following the full sintering cycle. The major differences observed in (Ti,Zr)C with respect to the reference samples after the full sintering cycle were the referred core-shell structure and the carbide grain sizes; additionally, the microstructural evolution during sintering differs. The grain size of carbides (WC, γ₁, and γ₂) is about 10% smaller in WC-(Ti,Zr)C-Co than WC-TiC-ZrC-Co. The shrinkage behavior and hardness of both composites are reported and discussed.