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The composition complexity of Ti(C, N)-based cermets poses a great challenge to the controllability of their sintering process. The effects of both refractory TaC addition and original particle size of starting materials on densification behavior and microstructural evolution of Ti(C, N)-based cermets were investigated during liquid phase sintering. The particle rearrangement only plays a very limited role in promoting densification of Ti(C, N)-based cermets at the initial stage of liquid phase sintering, whereas the pore-filling in the presence of liquid phase becomes a dominant densification mechanism in the middle stage or even earlier. Enhanced densification of ultrafine Ti(C, N)-based cermets is realized by introducing refractory TaC additive based on the grain growth-induced liquid phase pore-filling mechanism, which is also verified by an extended study using other refractory carbide additives such as NbC, ZrC and HfC. This work provides a novel idea for the development of fully densified ultrafine Ti(C, N)-based cermets with outstanding mechanical properties, which are highlighted in machining applications of difficult-to-machine materials as well as other wear-resistant parts operating in high-temperature and high-stress environments. [Display omitted] • The effects of both TaC addition and the particle size of starting materials on the densification behavior of Ti(C, N)-based cermets are clarified. • Enhanced densification of ultrafine Ti(C, N)-based cermets is realized by introducing refractory TaC additive. • This work provides a novel idea for the development of fully densified ultrafine Ti(C, N)-based cermets. [ABSTRACT FROM AUTHOR]

Ultrafine Ti(C, N)-based cermets are vacuum-sintered by refining the particle size of raw materials and tailoring the WC/TaC ratio to achieve the improvements in comprehensive mechanical properties and cutting performance. The hardness, flexural strength and tool lifespan of ultrafine Ti(C, N)-based cermets are substantially higher than those of submicron Ti(C, N)-based cermets with the same composition because of the refined grains and the improved distributions of both metal phases and residual stress. The hardness attenuation behavior and strengthening mechanism in Ti(C, N)-based cermets at different test temperatures have been proposed. At 400 ℃, the hardness attenuation of Ti(C, N)-based cermets depends mainly upon the softening of metal phases. However, at 800 ℃, the hardness attenuation of Ti(C, N)-based cermets is mainly related to the reduced bearing capacity of hard ceramic phases and interfacial sliding. The addition of refractory TaC is beneficial for the improvement of high-temperature mechanical properties and the tool lifespan, attributed to the strengthening of the ceramic framework by tantalum atoms. • The strategy on synergistic refinement of raw powders with various constituents in ultrafine Ti(C, N)-based cermets is proposed for the first time. • The comprehensive properties of ultrafine Ti(C, N)-based cermets are substantially higher than those of submicron Ti(C, N)-based cermets. • The hardness attenuation behavior and strengthening mechanism in Ti(C, N)-based cermets at different test temperatures are proposed. [ABSTRACT FROM AUTHOR]

The single-phase (Ti 0.2 V 0.2 Nb 0.2 Ta 0.2 Mo 0.2)(C 0.9 N 0.1) high-entropy carbonitride powder with an average particle size of 203 nm and a low oxygen content of 0.2915 wt% was synthesized by the carbothermal reduction-nitridation method at 1600 °C for 4 h. Subsequently, (Ti,V,Nb,Ta,Mo)(C,N) high-entropy carbonitride ceramics were prepared by hot pressing at temperatures of 1600 °C to 1900 °C. Increasing the sintering temperature is conducive to the uniform distribution of metallic elements, however, the hardness of high-entropy carbonitride ceramics is also reduced. (Ti,V,Nb,Ta,Mo)(C,N) ceramic hot-pressed at 1600 °C for 1 h has a relative density of 99.8%, an average grain size of 0.37 ± 0.08 µm, nanoindentation hardness of 32.4 ± 0.9 GPa, Vickers' hardness of 24.0 ± 0.7 GPa at 9.8 N and fracture toughness of 4.87 ± 0.25 MPa·m1/2, which is comparable to those of high-entropy carbides and nitrides. In this study, a novel and convenient method for the preparation of (Ti,V,Nb,Ta,Mo)(C,N) high-entropy carbonitride ceramics was established. • High-entropy carbonitride powders with fine particle size can be synthesized by carbothermal reduction-nitridation method. • The carbon content in the raw powder mixture will affect the single-phase formation ability. • High-performance high-entropy carbonitride ceramics can be prepared from high-entropy carbonitride powders. [ABSTRACT FROM AUTHOR]

Eighteen different 5-component equimolar rare earth zirconates were successfully synthesized by solid state reaction method. Among them, 5 of them exhibit a high-entropy fluorite structure, 8 have a high-entropy pyrochlore structure, and the remaining 5 have dual-phase structures of both pyrochlore and fluorite. For high-entropy rare earth zirconates with single-phase structure, the lattice parameter is almost linearly correlated with the average cation radius ratio. In combination with a large number of previous reports, a summary is made to study the synergistic effects of the average cation radius ratio, configuration entropy and size disorder on the single-phase formation ability of multi-component rare earth zirconates. The conclusion verifies that the size disorder of about 5% is the boundary for the multi-component rare earth zirconates to form a single-phase or dual-phase region, while the average cation radius ratio of 1.467 is the demarcation between a pyrochlore and a defective fluorite structure in the single-phase zone, and the configuration entropy had less effect. Excessive size disorder leads to the enrichment or barrenness of certain components, which is the main reason for the formation of dual phase ceramics. • 18 Different equimolar 5-component rare earth zirconates have been prepared. • The size disorder (δ *) can predict the single-phase formation ability of zirconates. • δ * = 5% is the boundary between the single- and dual-phase rare earth zirconates. • The single-phase forming ability of rare-earth zirconates is independent of entropy. [ABSTRACT FROM AUTHOR]

• Five kinds of pyrochlore high-entropy rare-earth zirconates have been prepared. • High-entropy strategy improves the electrical conductivity of rare-earth zirconate. • (Nd 0.2 Eu 0.2 Sm 0.2 Gd 0.2 Dy 0.2) 2 Zr 2 O 7 has a conductivity of 7.1 × 10–3 S·cm–1 at 1073 K. • Higher mobile vacancy concentrations, lower activation energy and the preferential O-ion migration pathway result the enhanced ion conductivity. In the present work, a series of new high-entropy rare-earth zirconates were prepared by a solid state reaction method. The crystal structure, microstructure and electrical properties of high-entropy rare-earth zirconates were characterized by means of X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy and electrochemical impedance spectroscopy. All the high-entropy rare-earth zirconates have a single-phase pyrochlore structure with a relative density of more than 95.7%. The grain sizes of high-entropy rare-earth zirconates are 1.3–2.4 µm, which are finer than low-entropy rare-earth zirconates prepared by the same process. (Nd 0.2 Sm 0.2 Eu 0.2 Gd 0.2 Dy 0.2) 2 Zr 2 O 7 has the excellent electrical conductivity of 7.1 × 10–3 S·cm–1 at 1073 K. [ABSTRACT FROM AUTHOR]

(Nd 1– x Y x) 2 Zr 2 O 7 solid solutions have been prepared by solid phase method. The influence of yttrium content on the structure and electrical performance has been explored. The crystal structure is characterized by X-ray diffraction and Raman spectroscopy. The morphology is investigated by scanning electron microscopy. The electrical property is analyzed by electrochemical impedance spectroscopy measurement. (Nd 1– x Y x) 2 Zr 2 O 7 solid solutions exhibit a single-phase structure except that NdYZr 2 O 7 shows a biphasic structure. As yttrium content x raises from 0 to 1, the order degree of (Nd 1– x Y x) 2 Zr 2 O 7 solid solutions decreases gradually, and the structures of solid solutions transform from ordered pyrochlore-type structure to disordered defect fluorite-type structure. (Nd 1– x Y x) 2 Zr 2 O 7 solid solutions are dense, and the relative densities are over 94%. The total conductivity of Nd 2 Zr 2 O 7 is greatly enhanced by doping yttrium at the Nd-site in 823–1073 K. The total conductivity achieves 5.2 × 10−3 S cm−1 at 1073 K for (Nd 0.6 Y 0.4) 2 Zr 2 O 7 , which is the highest valve of (Nd 1– x Y x) 2 Zr 2 O 7 solid solutions. • (Nd 1– x Y x) 2 Zr 2 O 7 show a single-phase structure except for NdYZr 2 O 7. • Order-disorder transition takes place in (Nd 1– x Y x) 2 Zr 2 O 7 solid solutions. • The total conductivity of Nd 2 Zr 2 O 7 can be improved by doping Y 2 O 3. • (Nd 0.6 Y 0.4) 2 Zr 2 O 7 has the highest conductivity at 1073 K in (Nd 1– x Y x) 2 Zr 2 O 7 system. [ABSTRACT FROM AUTHOR]