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We report for the first time the synthesis of a core‐shell structured composite powder with a core of Zr(Ti,Ta)C and a shell of Ti,Ta(Zr)C at 1700°C and investigate the formation mechanism for the core‐shell structure. The medium‐entropy (Ti,Zr,Ta)C ceramics with fine grains (1.1 ± 0.4 μm) and relative density of 94.8% was prepared by hot‐pressing at 2100°C. The flexural strength of (Ti,Zr,Ta)C at 1000°C (493 ± 21 MPa) was close to the room temperature (511 ± 52 MPa). As the temperature increased from 1600°C to 1800°C, the flexural strength was increased significantly, with an ultra‐high flexural strength of 725 ± 32 MPa at 1800°C. The existence of the core‐shell structure in the powder suppressed the grain growth due to the sluggish diffusion effect. The ultra‐high strength of (Ti,Zr,Ta)C ceramics was attributed to its fine microstructures, high fracture toughness, and the reinforced the grain boundary strength. [ABSTRACT FROM AUTHOR]

Medium‐entropy (Ti,Zr,Hf)C ceramics were prepared by hot pressing a dual‐phase medium‐entropy carbide powder with low oxygen content (0.45 wt%). The results demonstrate that the medium‐entropy (Ti,Zr,Hf)C ceramics sintered at 2100°C had a relative density of 99.2% and an average grain size of 1.9 ± 0.6 μm. The flexural strength of (Ti,Zr,Hf)C carbide ceramics at room temperature was 579 ± 62 MPa. With an increase in temperature to 1600°C, the flexural strength showed an increase up to 619 ± 57 MPa, and had no significant degradation even up to 1800°C. The high‐temperature flexural strengths of (Ti,Zr,Hf)C were obviously higher than those of the monocarbide ceramics (TiC, ZrC, and HfC). The primary strengthening mechanism in (Ti,Zr,Hf)C could be attributed to the high lattice parameter mismatch effects between TiC and ZrC, which not only inhibited the fast grain coarsening of (Ti,Zr,Hf)C ceramics, but also increased the grain‐boundary strength of the obtained ceramics. [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]

ZrB2–20vol% SiC (ZS) ceramics based on ZrB2 starting powders obtained by different boro/carbo-thermal reductions involving ZrO2 +B4C, ZrO2 +B4C+C, and ZrO2 +B, were fully densified by hot pressing at 1900–2000°C. The flexural strength of these ZS ceramics was measured from room temperature up to 1600°C. At 1600°C, the flexural strength of the ceramics is 460±31, 471±32 and 345±11MPa, respectively. The evolution of the strength as function of temperature is explained in terms of the differences in oxygen content, nature of fracture, grain sizes, grain boundary phases and microstructural defects. [ABSTRACT FROM AUTHOR]

SiC and HfC were used to tailor microstructures and improve properties of hot pressed HfB 2 -based ceramics. SiC and HfC particle, as the second phase, could hinder grain growth of HfB 2 . Compared with HfC, SiC grains grew slower, which was more effective to hinder grain growth of HfB 2 . The ternary ceramics HfB 2 –20vol%HfC–20vol%SiC had the finest microstructure, indicating the effect of SiC and HfC on grain growth. Several SiC clusters were found on related fracture surfaces. Large SiC clusters can cause high levels of residual stress and stress concentration, which are harmful to mechanical properties of boride-SiC ceramics. The predominant fracture mode of HfB 2 –20vol%SiC was transgranular, however, in HfB 2 –10vol%HfC–10vol%SiC and HfB 2 –20vol%HfC–20vol%SiC ceramics, the dominating fracture mode shift from transgranular to intergranular. With the incorporation of HfC, the fracture toughness of HH10S10 and HH20S20 increased to 4.34 MPa m 1/2 and 5.05 MPa m 1/2 , which were 17.3% and 26.7% higher than HS20 (3.72 MPa m 1/2 ), respectively. The flexural strengths of HfB 2 –HfC–SiC ceramics were improved to ~750 MPa, which were about 1.3 times higher than that of HfB 2 –20SiC. [ABSTRACT FROM AUTHOR]

Toughened ZrB 2 –SiC composites have been prepared by an introduction of amorphous carbon spheres. After hot pressing, microstructure observation showed that the carbon phases were present in the form of thin layer at grain boundaries, or in the form of agglomerates at the multigrain junctions. The carbon spheres led to significant improvement of fracture toughness from 4.96 ± 0.46 MPa m 1/2 to 7.52 ± 0.19 MPa m 1/2 . The toughening mechanism was attributed to the weak interfaces, which occurred due to the presence of carbon layer formed at grain boundaries after sintering. [ABSTRACT FROM AUTHOR]

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]

A high-entropy M 2 AlC-MC (M=Ti, Zr, Hf, Nb, Ta) composite was produced by reactive hot pressing using Ti, Zr, Hf, Nb, Ta, Al, and graphite as raw materials. Successful solution of the five transition metals in both hexagonal M 2 AlC and cubic MC was confirmed by XRD, SEM, atom-resolved STEM and EDS elemental analyses. The high-entropy M 2 AlC shows larger delocalization of atoms and higher lattice strain than the high-entropy MC along with considerable amount of Al vacancies. The formation mechanism of the high-entropy composite has been discussed. The Vickers hardness of the composite is measured to be 11.1 ± 1.1 GPa. Image, graphical abstract [ABSTRACT FROM AUTHOR]

B 4 C–ZrB 2 ceramics were reactively hot pressed by three-step process using B and C with Zr (C–B–Zr mixtures) or ZrO 2 (C–B–ZrO 2 mixtures) as raw materials. Based on thermodynamic analysis and experimental results, the C–B–Zr or C–B–ZrO 2 mixtures were firstly held at 1000 °C, then held at 1650 °C, and finally held at 2000 °C under 30 MPa. B 4 C–ZrB 2 ceramics were obtained by using the three-step process and controlling the reaction sequence. The ZrB 2 grains in B 4 C–ZrB 2 ceramics from C–B–Zr mixtures had a bimodal grain size distribution including fine grains with size of 1–2 μm and large agglomeration-like grains with size of 10–30 μm, while that in the ceramics from C–B–ZrO 2 mixtures showed homogeneous distribution with small grain size of 1–2 μm. The B 4 C–ZrB 2 ceramics from C–B–ZrO 2 mixtures showed higher relative density, more homogeneous and fine-grained microstructure, higher hardness (33.0 ± 1.3 GPa), and comparable toughness (4.1 ± 0.2 MPam 1/2 ), compared with the ceramics from C–B–Zr mixtures. [ABSTRACT FROM AUTHOR]

Chemical reactivity of hot-pressed Si 3 N 4 –ZrB 2 ceramics at 1500–1700 °C under N 2 and Ar atmosphere is investigated by X-ray diffraction, thermodynamic calculation, and phase diagram. Results show that there are chemical reactions in Si 3 N 4 –ZrB 2 ceramics. The chemical reactions accelerate the decomposition and phase transformation of Si 3 N 4 . The reaction products include ZrN, BN, ZrSi 2 , SiC or Si, which strongly depend on the atmosphere or temperature. The source of N 2 for ZrN and BN products is both atmosphere and decomposition of Si 3 N 4 at 1600 °C or above, whereas that is only atmosphere at 1500 °C. There is no reaction in Si 3 N 4 –ZrB 2 ceramics at 1500 °C under Ar. The MgO–Lu 2 O 3 sintering adds lead to the significant increase of relative density of Si 3 N 4 –ZrB 2 from 48% to 94% at 1500 °C in Ar. Therefore, it is possible that dense Si 3 N 4 –ZrB 2 ceramics with no reaction could be achieved at 1500 °C or below in Ar. [ABSTRACT FROM AUTHOR]