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The (TiZrHfNbTaMo)C‐15 wt.% Co ceramics with excellent comprehensive properties were successfully prepared by vacuum liquid phase sintering. The effects of WC content on the grain size, microstructure, and mechanical properties were investigated. The results showed that WC was dissolved into (TiZrHfNbTaMo)C phase to form a seven‐component solid solution, and the independent WC phase disappeared. The addition of WC improved the wettability between the ceramic phase and the metal phase and promoted the densification of the ceramics. In addition, WC played a role in refining grains and inhibiting the grains growth to a certain extent. The comprehensive properties of ceramics were improved by solid solution strengthening and high entropy effect. When the WC content was 5 wt.%, the toughness reached the maximum of 9.41 ± 0.15 MPa·m1/2, and the hardness of the ceramic with 10 wt.% WC achieved the highest value of 18.18 ± 0.20 GPa. [ABSTRACT FROM AUTHOR]

In this work, high-performance 93W–4.9Ni–2.1Fe–1La2O3 (wt pct) alloy was fabricated by a two-stage sintering method and subsequent vacuum heat-treatment, which had a high densification degree (99.4 pct) and fine grain sizes of W (only 9.76 μm). The utilization of ultrafine W–Ni–Fe composite powder, addition of La2O3 particles and optimized sintering technique effectively resulted in structure refinement. Moreover, the second-phase particles were mainly dispersed at grain boundaries (GBs), besides a small amount of them in the interior of W grains. The as-prepared alloys after sintering first at 1400 °C for 2 hour and then at 1500 °C 30 minutes exhibited enhanced mechanical properties, with a maximum tensile strength of ~ 1024 MPa, elongation of ~ 22.7 pct and hardness of ~ 457 HV1. The excellent strength could be attributed to various mechanisms, namely fine-grained strengthening and oxide dispersion strengthening. Meanwhile, the ductilization is predominantly benefitted from a synergistic effect of low angle grain boundaries, intragranular oxide particles and vacuum heat-treatment process. This work not only promotes the development of oxide-reinforced WHAs for challenging engineering applications but also provides a new perspective for the design of refractory composite materials. [ABSTRACT FROM AUTHOR]

Herein, CoCrxCuFeMnNi (x = 0, 0.5, 1.0, 1.5, and 2.0, in molar ratio) high‐entropy alloys (HEAs) are prepared through vacuum hot‐pressing sintering. The influences of Cr content on the microstructure and the corrosion behavior in 3.5% NaCl solution are studied. The alloys are mainly composed of two face‐centered cubic (FCC) phases, namely, Cr–Co–Fe–Mn–Ni (or Co–Fe–Mn–Ni)‐rich FCC1 phase and Cu–Mn–Ni‐rich FCC2 phase. With the increase of Cr content, The FCC1‐phase area expands and the FCC2 phase area shrinks. In 3.5% NaCl solution, the corrosion current density (icorr) and average corrosion rate (Vcorr) of the HEAs decrease with the increase in Cr content. The icorr and Vcorr of Cr2.0 alloy are lower at 1.87 × 10−6A cm−2 and 0.02 mm year−1, respectively. The increase of transfer resistance (Rp) and decrease of relative thickness (CPE1−1) of the passivation film indicate that the more difficult it is for ions in the corrosive medium to pass through the capacitor layer, the better the corrosion resistance of the alloy. Improvement of the corrosion resistance is mainly due to the formation of denser Cr2O3 passivation film and reduction of the segregation degree of Cu in the FCC2 phase. [ABSTRACT FROM AUTHOR]

Despite of good oxidation resistance and ablation resistance, challenge to densify and tendency to loss high temperature (H.T.) strength have limited its potential applications in aerospace for HfB 2 –SiC ceramic. In this work, dense HfB 2 –SiC ceramic with improved H.T. flexural strength was prepared using WC as sintering aid. Pure HfB 2 –SiC ceramic exhibited creep deformation at 1600 °C with measured strength value of 389 ± 82 MPa, whereas WC doped sample showed much higher flexural strength of 658 ± 69 MPa with linear elastic behavior prior to fracture. In addition to clean grain boundary, the addition of 5 vol.%WC into HfB 2 –SiC ceramic promoted the formation of fine (Hf,W)B 2 , (Hf,W)C and WB precipitates. This combinative effect is responsible for its excellent H.T. mechanical property of WC doped HfB 2 –SiC ceramic. [ABSTRACT FROM AUTHOR]

Highlights: [•] Fine-grained Mo–12Si–8.5B alloys were prepared by hot pressing sintering. [•] The microstructure control is achieved by mechanical alloying process. [•] The yield stress and especial the flexure strength of alloys are superior. [ABSTRACT FROM AUTHOR]

Abstract: Binderless WC ceramics were prepared by reactive spark plasma sintering, using tungsten trioxide, tungsten and carbon black as the starting materials. Phase assemblages and microstructure of the as-sintered ceramics were investigated. It was found that graphite existed as an impurity phase due to the volatilization of WO3, and W could compensate for the WO3 loss to form WC with a single phase. Benefiting from the enhanced sinterability, WC ceramics with high relative density and good hardness could be obtained at temperature as low as 1500°C. [Copyright &y& Elsevier]

Dense, highly textured Zr B2 and Zr B2- Mo Si2 ceramics were fabricated via a strong magnetic field alignment method followed by spark plasma sintering. Unlike with the previous studies, which only focused on the alignment of single-phase particles, both Zr B2 and Mo Si2, which exhibited a magnetic anisotropy, have been aligned in this study. The alignment of Mo Si2 in the same direction of Zr B2 enhanced the degree of orientation of Zr B2, decreased the grain size, but increased the aspect ratio of the platelet Zr B2 grains. The microstructure and anisotropic mechanical properties as well as the oxidation resistance in different directions were discussed. [ABSTRACT FROM AUTHOR]

ZrB2–SiC–ZrN composites were fabricated by reactive hot pressing using Zr, Si3N4, and B4C powders as starting materials. Sintering was conducted at temperatures of 1800 to 2000°C under a load of 20MPa in Ar atmosphere. The composite was densified at 2000°C. The in situ formed BN flakes which distributed uniformly at the grain boundaries were identified by X-ray diffraction and scanning electron microscopy. The formation of h-BN phase and its effect on the mechanical properties of the composite are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Zirconium carbide and hafnium carbide ceramics were prepared by reactive spark plasma sintering, using oxides and carbon black as the raw materials. The phase and microstructure evolution during the sintering process was investigated. It was found that ZrC and HfC powder intermediates were formed at 1750 and 1800°C, respectively. Due to the high sinterability of the in situ formed carbide particles, samples with a fine microstructure could be densified at a relatively low temperature. [Copyright &y& Elsevier]

HfC-30 vol% SiC ceramics with a relative density of 99.7% was obtained by pressureless sintering at 2300°C for 0.5 h. The resultant ceramics showed fine microstructure with HfC grain size around 1 μm. The hardness (20.5 ± 0.2 GPa), bending strength (396 ± 56 MPa), and fracture toughness (2.81 ± 0.18 MPa·m1/2) of HfC-30 vol% SiC ceramics were at least 20% higher than those of monolithic HfC ceramics. The influences of SiC particle size, volume fraction, and the oxide impurity on the microstructure evolution of HfC-based ceramics were examined. The results indicate that SiC addition and the oxygen impurity introduced by ball milling play opposite roles in the HfC grain growth during sintering. The oxide impurity introduced by ball milling caused the HfC grain coarsening, whereas SiC particles inhibited the grain growth of HfC significantly. [ABSTRACT FROM AUTHOR]

Influence of boron on densification and microstructure of zirconium diboride ( ZrB2) ceramics using pressureless sintering has been studied. The ZrB2 powders were synthesized by borothermal reduction of ZrO2 with three different ZrO2/ B ratios to produce powders nominally phase pure or that contained excess boron. The as-synthesized powders were then densified by heating to temperatures from 1800°C to 2200°C. The incorporation of 1.2 wt% excess boron increased relative density from ~54.3% to ~96.1% by reacting with and removing oxide impurities ( ZrO2 and B2 O3) after sintering at 2000°C, whereas it also led to an exaggerated grain growth from 1-3 to 20-80 μm presumably due to high grain-boundary mobility. The faster grain growth resulted in the formation of intragranular pores and prevented the ZrB2 ceramics from reaching full density. By the incorporation of 3 wt% excess boron, a fully dense ZrB2 ceramics with relative density of ~99.7% and grain size of 5-10 μm was obtained at 2000°C. The full densification was attributed to the synergistic effects of boron through oxide impurities removal and grain- growth inhibition. [ABSTRACT FROM AUTHOR]

Binderless tungsten carbide ceramics were produced by reactive spark plasma sintering, using tungsten nitride and carbon black as starting materials. The phase and microstructure evolution during the sintering process was investigated. It was found that the tungsten nitride decomposed into nano tungsten with a particle size of about 25 nm firstly, and then reacted with carbon black to form carbides. Due to the high activity of the in situ formed carbides, samples could be densified at a temperature as low as 1350°C. The bulk carbides possessed an average grain size of 270 nm and Vickers' hardness of 25.4 GPa. [ABSTRACT FROM AUTHOR]

A pressureless sintering process with two pre-sintering heat treatments was developed for the densification of HfB2. The use of two isothermal holdings, at 1650 and 2000°C, prior to the final sintering temperature (2200°C) proved to be more effective at removing oxygen impurities and rearranging the initial powder compact microstructure. The mechanical properties of HfB2 with 2wt.%B4C, including hardness (19.5GPa), E modulus (529GPa) and strength (469MPa), were comparable to hot-pressed samples or higher than the values using other sintering aids. [Copyright &y& Elsevier]

Four kinds of composites, ZrB2–SiC, ZrB2–SiC–ZrC, ZrB2–SiC–ZrN and ZrB2–SiC–AlN, were synthesized in situ via reactive hot pressing (RHP) and reactive spark plasma sintering (R-SPS), using Zr, Si, B4C, BN and Al as raw materials. The synthesis process plays a critical role in the microstructural features of the composites obtained. The R-SPS process can lead to a more homogeneous and finer microstructure due to its high heating rates and short holding time, while the RHP process is likely to result in coarse microstructures due to a long enough holding time for grains growth. [Copyright &y& Elsevier]

zirconium oxycarbide (ZrC x O y ) ceramics were hot pressed using ZrC-5ZrO 2 and ZrC-5ZrO 2 -5Zr (number in mol%) mixtures as starting powders, and the solid solution and densification processes were investigated. When 5 mol% ZrO 2 were added to ZrC matrix, ZrC x O y and CO were formed during the reaction between ZrC and ZrO 2 occurred above 1600 °C. And the sintered ZrC x O y ceramic had numerous pores. However, the co-doping of ZrO 2 and Zr could reduce the reaction temperatures and CO amounts in ZrC-5ZrO 2 -5Zr system, resulting in a near fully dense ZrC x O y ceramic. The chemical composition of obtained ZrC x O y ceramics were also calculated according to oxygen content and reaction processes. [ABSTRACT FROM AUTHOR]

Two different modes of placing powders in a graphite die were used in a spark plasma sintering system: loading a conductive green body (zirconium nitride, ZrN, or titanium nitride, TiN) into a non-conductive bed (boron nitride) or loading the ZrN or TiN powders into the die directly. The ceramics obtained, which had completely different morphologies, have been characterized in this work. Through experimental and theoretical analysis, possible formation mechanisms of microstructural differences have been proposed. [Copyright &y& Elsevier]

Abstract: This work compared phase composition, microstructures and mechanical properties of ZrB2—20vol.% SiC—5vol.%Yb2O3 prepared by hot pressing (HP) and spark plasma sintering (SPS) in vacuum. Despite the same raw material composition, the two densification techniques led to the appearance of different secondary phases. The HP ceramics had coarsened microstructure, predominantly intergranular fracture characteristic and high fracture toughness. The SPS ceramics exhibited refined microstructure, transgranular fracture model and high bending strength. [Copyright &y& Elsevier]

The influence of the spark plasma sintering (SPS) heating rate on densification, microstructure and strength of ZrB2–20vol.% SiC composites with or without 5vol.% Yb2O3 addition was assessed. A low heating rate promoted densification, induced grain growth and slightly increased the strength in the Yb2O3-free ceramics, whereas the low heating rate inhibited the densification, maintained the grain size and greatly decreased the strength in the Yb2O3-doped composites. [Copyright &y& Elsevier]

A pressureless densification mechanism and microstructure development in ZrB2–20vol.% SiC ceramics with boron additive were investigated. Results showed that the boron addition removed oxygen impurity and improved the densification process. By adding 1wt.% boron, fully dense ZrB2–SiC ceramics with a fine-grained microstructure was obtained. However, a higher boron addition would lead to fast grain growth of both SiC and ZrB2 grains and a deteriorated densification behavior by forming a liquid phase. The mechanical properties of the sintered ceramics with 1wt.% boron addition were also evaluated. [Copyright &y& Elsevier]

Abstract Gas burst has been commonly observed during reactive sintering, especially when combustion was initialized therein. In this study, a model system (Zr + 2B = ZrB 2) was chosen to investigate this phenomenon by a combination of thermo-analysis, mass spectrum investigation and electron microscopy studies. Results showed the combustion lead to a formation of various gaseous byproducts such as CO, CO 2 , N 2 , BO, BO 2 and B 2 O 3 etc. Especially for BO(g), its generation consumed boron in the raw materials, resulting in the residual Zr-containing phase that accelerated the formation of interlocked ZrB 2 grains in the sintered ceramics, which show relatively high strength (516 ± 42 MPa). Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

A pressureless sintering process using different amounts of boron carbide (B 4 C) as sintering aid was investigated for the densification of zirconium diboride (ZrB 2 ). The sintering mechanism was converted from solid state sintering to liquid phase sintering as sintering temperature increased from 2150 to 2250 °C. B 4 C addition improved densification by removing the oxide impurities during solid state sintering. Besides, it could restrain the growth of ZrB 2 grains. At 2250 °C, ZrB 2 and B 4 C formed liquid phase which was well wetting ZrB 2 grains during liquid phase sintering process. The different roles of B 4 C on the microstructure development, mechanical properties and oxidation resistance of the sintered ZrB 2 -based ceramics were investigated. [ABSTRACT FROM AUTHOR]

Abstract: AlB2 is used as a sintering additive to densify ZrB2 ceramics. The AlB2 phase plays a positive role in the densification process of ZrB2 ceramics by removing the oxygen impurity and liquid phase sintering. With 5vol% AlB2 addition, both mechanical properties and oxidation resistance can be improved compared with monolithic ZrB2 ceramics. [Copyright &y& Elsevier]

Abstract: A solid state reaction of ZrH2, Si3N4 and B4C has been proposed to synthesize machinable ZrB2–SiC–BN composites. Dense ZrB2–SiC–BN composites with fine grain size and homogeneous microstructure were fabricated via spark plasma sintering at 1900°C in vacuum. The kinetics of the reaction process was studied with XRD results. SEM and TEM results showed that the in-situ formed BN phase was composed of micro-sized intergranular and nano-sized intragranular h-BN particles. The influences of the BN content on the mechanical properties as well as the thermal conductivities of the composites were studied. The composites that contained >20vol% BN exhibited excellent machinability and high strength. [Copyright &y& Elsevier]

ZrB2–SiC–BN composites were fabricated from a powder mixture of ZrH2, Si3N4 and B4C via reactive hot pressing (RHP) and reactive spark plasma sintering (R-SPS). The phase composition, density, microstructure and mechanical properties of the final products were studied and compared. The sample fabricated by the R-SPS process showed higher density, flexural strength and fracture toughness and had a more homogeneous microstructure compared to the one fabricated by the RHP process. [Copyright &y& Elsevier]

Abstract: Nanosized WC powder was synthesized by solid state reaction between tungsten nitride (W2N) and carbon black. The reaction path and the phase evolution during the synthesis were studied. It was found that W2N firstly decomposed to tungsten metal in the middle stage of synthesis, and then it reacted with carbon black to form W2C and WC in sequence. Using this WC powder as the starting material, binderless WC hardmetals were prepared by hot pressing, and the microstructure and mechanical properties of the material were investigated. [Copyright &y& Elsevier]

Abstract: The influence of the pressure loading cycle on spark plasma sintered ZrB2–SiC–Yb2O3 ceramics was investigated. Spark plasma sintering consists of three stages, i.e., a heating-up, isothermal and cooling stage. Applying the load during isothermal stage allowed evaporation of volatile species. Applying the load during the heating-up and isothermal stages resulted in the entrapment of volatile species and the formation of nanometer-sized spherical SiO2 particles. The presence of SiO2 particles led to a large thermal residual stress, and improved fracture toughness in the ZrB2–SiC–Yb2O3 ceramics. [Copyright &y& Elsevier]

Textured hexagonal boron nitride ceramics were prepared by hot pressing, using boron nitride powders as starting materials. The results showed that a high sintering temperature and moderate external pressure helped to form a textured microstructure when using boron nitride powder with a low degree of order and a broad particle size distribution. The flake-like boron nitride grains tended to orient along the hot pressing direction via pressure-induced preferential grain growth. The obtained textured boron nitride ceramics demonstrated strong anisotropic mechanical and thermal properties. [Copyright &y& Elsevier]

Abstract: Vanadium-doped bismuth titanate ceramics of composition Bi4Ti3−x V x O12+x/2 (BTV, x =0.02, 0.03, 0.04, 0.06, 0.08) were prepared using the conventional solid-state reaction method. The sintering activity was significantly improved by the addition of even a small amount of vanadium. The effects of the incorporation of vanadium in the Bi4Ti3O12 lattice on the microstructure, Raman characteristics, dielectric and ferroelectric properties of the samples were investigated. [Copyright &y& Elsevier]