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Silica is one of the most widely used ceramics due to its excellent chemical stability and dielectric property. However, its destructive brittle nature inhabits it from wider application as a functional ceramic. An improvement in toughness is a challenging topic for silica ceramic, as well as other ceramics. In the paper, silica ceramic with different types of boron nitride powders and alumina platelets was fabricated by hot-pressing. Introduction of the additives had great influence on the composites' mechanical properties and microstructure. The silica matrix composite containing micro-sized boron nitride powders possessed the best mechanical properties, including the bending strength (134.5 MPa) and the fracture toughness (1.85 Mpa·m1/2). Meanwhile, the introduction of alumina platelets combined with boron nitride nanosheets achieved an effective enhancement of fracture toughness while maintaining the bending strength. Compared with the monolithic silica, the composite with simultaneous addition of alumina platelets and boron nitride nanosheets had a fracture toughness of 2.23 Mpa·m1/2, increased by approximately 27% (1.75 Mpa·m1/2). The crack deflection and platelet pullout were contributing to enhancement of the fracture toughness. The improved mechanical properties, combined with the intrinsic excellent dielectric and chemical properties, make the silica matrix composites promising wave transparent and thermal protection materials. [ABSTRACT FROM AUTHOR]

Alumina is one of the most commonly used and researched structural ceramic because of its excellent properties. However, its intrinsic brittleness is the fatal drawback, which hinders it from wider applications. How to improve its fracture toughness as well as the bending strength is always challenging for material researchers. In this paper, alumina matrix composites were fabricated by hot-pressing, in which some additives, including zirconia, alumina platelets, and MXene, were incorporated. The influence of the introduced additives on their microstructure and mechanical properties was investigated. Compare with the monolithic alumina, both bending strength and fracture toughness of all samples were improved greatly. Incorporation of zirconia was beneficial to the mechanical properties due to the phase-transformation strengthening and toughening mechanism. While alumina platelets resulted in high fracture toughness because of the self-toughening of elongated grains. The synergistic effect of alumina platelets and MXene enormously improved the fracture toughness from 2.9 ± 0.3 MPa·m1/2 for monolithic alumina to 7.5 ± 0.4 MPa·m1/2 for the composite, which was increased by 159%. This work will provide useful references for the fabrication of high-strength and high-toughness alumina ceramics by introducing additives properly. [ABSTRACT FROM AUTHOR]

High-entropy RE 3 MO 7 -type ceramics with multiple cations at RE and M sites have been successfully fabricated via solid-state reaction method. The microstructure and thermal properties of two compositions, namely (Y 1/3 Yb 1/3 Er 1/3) 3 (Nb 1/2 Ta 1/2)O 7 and (Y 1/3 Yb 1/3 Er 1/3) 3 (Nb 1/3 Ta 1/3 Mo 1/3)O 7 , are studied. The XRD Rietveld refinement, transmission electron microscopy and scanning electron microscopy results demonstrate that the samples have pure disordered defective fluorite structure with highly homogeneous composition. These high entropy ceramics exhibit good thermal stability, high thermal expansion coefficients (11.1–11.5 × 10−6 K−1, 1000 °C), and low thermal conductivities (0.91–1.16 W m−1 K−1, 30–1000 °C), which make them promising for applications as thermal barrier coating materials. [ABSTRACT FROM AUTHOR]

A new eutectic high-entropy alloy (EHEA) of Cr 41 CoFeNi was prepared by arc melting technique. The microstructure and corrosion resistance of the Cr 41 CoFeNi EHEA were systematically investigated by EBSD, TEM, EIS and XPS analyses. The results revealed that the Cr 41 CoFeNi EHEA showed a dual-phase structure consisting of BCC phase and FCC phase, and the morphologies displayed the lamellar eutectic and network-shaped eutectic. Compared with 316 L stainless steel, the Cr 41 CoFeNi EHEA exhibited a better anti-corrosion property in 0.5 M H 2 SO 4 solution. This was mainly attributed to the formation of a dense and stable Cr-oxide layer on the surface of Cr 41 CoFeNi EHEA, which retarded further dissolution of the matrix. Moreover, bound water could adsorb metal ions to form a new protective film, which played an important role in improving the corrosion resistance of the Cr 41 CoFeNi EHEA. However, the elemental segregation and chemical potential difference between phases increased the defects in the passive film and accelerated its pitting corrosion of low potential phase. Moreover, the (Co, Fe, and Ni)-enriched FCC structured phase was preferentially eroded. • A novel eutectic high-entropy alloy of Cr 41 CoFeNi(EHEA) was designed and fabricated • The corrosion mechanisms of EHEA and 316 L stainless steel(316 L SS) were clarified • The passive film thickness of EHEA was about 5.3 times greater than that of 316 L SS • The main constituents of passive film on EHEA were Cr-oxide and bound water [ABSTRACT FROM AUTHOR]

The CoNiCr x (x = 1, 1.4, 1.8, and 2) medium-entropy alloys were synthesized by vacuum arc-melting method to systematically investigate the roles of Cr on microstructures and mechanical properties. The results of microstructural characteristics indicated that both CoNiCr and CoNiCr 1.4 alloys have a single face-centered cubic (FCC) structure, while CoNiCr 1.8 and CoNiCr 2 alloys consist of FCC plus body-centered cubic (BCC) hypoeutectic and eutectic. The formation of FCC and BCC phases could be probably evaluated using the VEC criteria. The tested mechanical properties demonstrated that CoNiCr 1.8 alloy exhibited the best comprehensive properties at room and liquid nitrogen temperatures (RT and LNT), whereas the yield strength and hardness of CoNiCr 2 alloy were higher than those of other alloys, which may be mainly attributed to the second-phase strengthening and the fine and lamellar microstructure. The yield strength of CoNiCr 2 alloy at RT and LNT was 3.8 and 1.9 times (514 MPa and 825 MPa) that of CoNiCr alloy. In addition, the precipitation of dense and ordered FCC (L1 2) phase could primarily contribute to the enhance of yield strength of CoNiCr 1.4 alloy. • Cr addition facilitates the formation of L1 2 nano-particles and BCC/B2 phase. • The CoNiCr 1.8 hypoeutectic medium-entropy demonstrates an excellent strength and ductility synergy at liquid nitrogen temperature. • A CoNiCr 2 lamellar eutectic medium-entropy with FCC/L1 2 and BCC/B2 is designed. • L-C lockers dominate the deformation of lamellar eutectic MEAs under cryogenic condition. [ABSTRACT FROM AUTHOR]

Those peritectic alloys with large positive mixing enthalpy usually display metastable liquid phase separation. Here, the solidification kinetics for a model Fe 50 Cu 50 alloy in broad cooling rate R c range was predicted by phase-field simulations and confirmed by drop tube experiments. Near-equilibrium peritectic solidification occurs in small R c regime below 5.62×10 2 K s −1 . As R c rises beyond 7.76×10 2 K s −1 , metastable phase separation takes place and results in the formation of a dispersed structure. Afterwards, multilayer core-shell structures appear, and the layer-number varies from two-layer to six-layer with further increasing R c . Once R c exceeds 3.64×10 5 K s −1 , a dispersed structure once again forms because the core-shell dynamic condition is not satisfied. Non-equilibrium peritectic solidification is induced if R c is greater than 5.85×10 5 K s −1 . [ABSTRACT FROM AUTHOR]

In this study, the nacre-like alumina ceramics, with introduction of zirconia, are fabricated by freeze casting followed by hot pressing. The experimental results show that zirconia particles are uniformly distributed between the layered alumina 'brick' in the form of 'mortar'. Due to the incorporation of zirconia with proper particle size, both flexural strength and fracture toughness of the nacre-like alumina are greatly improved, much stronger and tougher than pure alumina. In regard to the sample containing 13.3 wt% zirconia, the flexural strength reaches 522 MPa, ∼47% increase in comparison with the monolith. Particularly, when zirconia content is 10 wt%, the fracture toughness is increased by ∼66%, from 6.1 MPa m1/2 to 10.1 MPa m1/2. Microstructure observation and fracture behavior analysis reveal that the coupling effects of bioinspired nacre-like structure and stress-induced transformation toughening are responsible for the enhanced mechanical properties. The results show that it is an effective method to increase the mechanical properties of layered alumina ceramics by introducing proper interlayer phase. [ABSTRACT FROM AUTHOR]

MWCNT/alumina composites were prepared by a flocculation method combined with acid treatment of the MWCNTs and spark plasma sintering. The effects of varying the processing conditions, namely the acid treatment temperature (50–80 °C), MWCNT content (0–10 vol.%) and sintering temperature (1200–1600 °C) on the composites’ microstructure, electrical conductivity and mechanical properties were investigated. This found that flocculation processing brings about a good dispersibility of the MWCNTs, especially with higher acid treatment temperature, which enhances their reinforcing and grain refining effect. When combined with a low sintering temperature, this significantly improves the mechanical properties of the composites.As a result, a composite prepared using a sintering temperature of 1200 °C and an acid treatment temperature of 80 °C exhibited a fracture toughness greater than that of a monolithic alumina, even with a MWCNT content as high as 10 vol.%. [ABSTRACT FROM AUTHOR]

Abstract Graphene oxide (GO) is one of the most important ceramic-toughening candidates due to its outstanding physical and chemical properties. In this paper, nacre-like alumina ceramics reinforced with GO have been prepared by freeze casting followed by hot pressing. The experimental results showed that a laminated structure with homogeneous distribution of GO was obtained in the composites. Microstructural analysis, relative densities, flexural strength and fracture toughness of the samples were investigated to reveal the effect of GO. Higher flexural strength and fracture toughness (~380 MPa and ~7.5 MPa m1/2, respectively) were achieved and the multiple length-scale toughening mechanisms of composites contributed to the improved mechanical properties. The results show that it is a promising method to enhance the mechanical properties of nacre-like alumina by the addition of GO. [ABSTRACT FROM AUTHOR]

High entropy alloys (HEAs) have five or more principal elements with four core effects: high entropy, sluggish diffusion, severe lattice distortion, and cocktail effects. These effects lead to some distinct properties of HEAs. Some HEAs are promising for high temperature applications and have the potential to replace Ni-base superalloys as the next generation high-temperature materials, such as high entropy superalloys (HESAs) and refractory HEAs. The microstructures of HESAs consisting of γ and γ ′ phases are similar to that of Ni-base superalloys. Refractory HEAs contain refractory elements with high melting temperatures. Thus, a number of aspects of HESAs and refractory HEAs are reviewed and discussed in the present paper, including microstructure, density, room-temperature mechanical properties, high-temperature strength, creep behavior, and oxidation resistance. Furthermore, a number of future research topics are suggested, emphasizing on developing high-performance high-temperature materials. [ABSTRACT FROM AUTHOR]

In order to overcome intrinsic brittleness and poor mechanical properties of fused silica (FS), boron nitride nanosheets (BNNSs) as a novel reinforcement were employed for fabrication of BNNSs/fused silica composites. BNNSs with micron lateral size were homogeneously dispersed with FS powder using a surfactant-free flocculation method and then consolidated by hot pressing. The flexural strength and fracture toughness of the composite with the addition of only 0.5 wt.% BNNSs increased by 53% and 32%, respectively, compared with those of pure FS. However, for higher BNNSs contents the improvement in mechanical properties was limited. Microstructural analyzes have shown that the toughening mechanisms are combinations of the pull-out, crack bridging, and crack deflection mechanisms. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties of (Hf 0.2 Nb 0.2 Ta 0.2 Ti 0.2 Zr 0.2)B 2 high-entropy boride (HEB) were first predicted by first-principles calculations combined with virtual crystal approximation (VCA). The results verified the suitability of VCA scheme in HEB studying. Besides, single-phase (Hf 0.2 Nb 0.2 Ta 0.2 Ti 0.2 Zr 0.2)B 2 ceramics were successfully fabricated using boro/carbothermal reduction (BCTR) method and subsequent spark plasma sintering (SPS); furthermore, the effects of different amounts of B 4 C on microstructure and mechanical properties were evaluated. Due to the addition of B 4 C and C, all samples formed single-phase solid solutions after SPS. When the excess amount of B 4 C increased to 5 wt%, the sample with fine grains exhibited superior comprehensive properties with the hardness of 18.1 ± 1.0 GPa, flexural strength of 376 ± 25 MPa, and fracture toughness of 4.70 ± 0.27 MPa m1/2. Nonetheless, 10 wt% excess of B 4 C coarsened the grains and decreased the strength of the ceramic. Moreover, the nanohardness (34.5–36.9 GPa) and Young's modulus (519–571 GPa) values with different B 4 C contents just showed a slight difference and were within ranges commonly observed in high-entropy diboride ceramics. [ABSTRACT FROM AUTHOR]

• A novel nanostructured Fe 39 Cr 39 Ni 14 C 8 eutectic high entropy alloy was developed. • The rapid solidification alloy exhibits a typical fully lamellar FCC1/FCC2 eutectic structure. • FCC1 phase has a unique cube-cube coherent relationship with FCC2 phase. • Its high hardness and high yield strength originate mainly from the phase interface strengthening. A novel nanostructured Fe 39 Cr 39 Ni 14 C 8 eutectic high entropy alloy (EHEA) was developed by rapid solidification. Detailed characterization reveals that the rapidly solidified Fe 39 Cr 39 Ni 14 C 8 EHEA shows a nano-lamellar structure consisting of alternative FCC1 (∼120 nm in width) and FCC2 (∼80 nm) phases with a coherent relationship and possesses a high hardness of 463HV (∼4.5 GPa) and a high yield strength of 1.45GPa. They are mainly attributed to the unique ultrafine eutectic structure. This finding would shed new light on the development of FCC1/FCC2-nanostructured EHEAs with excellent mechanical properties. [ABSTRACT FROM AUTHOR]

Alumina composites reinforced with multiwalled carbon nanotubes (MWCNTs) at up to 3.7 vol% are prepared by a precursor method followed by a spark plasma sintering. We systematically and quantitatively investigate the effects of acid-treatment time of the MWCNTs on not only bending strength and fracture toughness of the composites but also on the mechanical strength and dispersibility of the MWCNTs, the grain size of the alumina matrix, and the interfacial strength between MWCNT and alumina. The main objective of this study is to evaluate how these parameters influence the mechanical properties with the aid of multiple regression analysis. We demonstrate that the matrix grain size, the mechanical strength of the MWCNTs, and the interfacial strength have little impact on the mechanical properties for the composites prepared in this study. On the other hand, the dispersibility of MWCNTs has the significant influence on the mechanical properties. Both the dispersibility of the MWCNTs and the mechanical properties of the composites increase as the acid-treatment time increases up to 2 h at low MWCNT content (0.9 vol%). Conversely, at a higher amount of MWCNTs, the degradation in the mechanical properties is shown to be associated with the deterioration of MWCNTs' dispersibility. As MWCNT agglomerates are anticipated to act as imperfections, they may override the effects of the strength of MWCNTs, matrix grain size, and interfacial strength. By means of the multiple regression analysis, we quantitatively show that improving MWCNTs' dispersibility is one of the most important factors in enhancing the mechanical properties of MWCNT/alumina composites. [ABSTRACT FROM AUTHOR]

Microstructure–property relationships of pressureless-sintered multi-walled carbon nanotube (MWCNT)/alumina composites have been investigated using four types of MWCNTs having different mechanical characteristics as well as almost the same diameter and length; the influences of grain size, location of MWCNTs, fracture patterns, dispersion states of MWCNTs, and crack bridging characteristics of MWCNTs on the composites׳ mechanical properties have been explored. It has been found that the improvement in bending strength and fracture toughness observed for all types of the composites was primarily attributable to the grain refining effect by adding the MWCNTs and to the energy-dissipation by a MWCNT debonding and pullout, respectively. The number and size of clustered MWCNTs were increased in response to the increase in the MWCNT content, which resulted in the decrease in mechanical properties. The pressureless-sintered composites made with MWCNTs having larger load-bearing ability led to ~25% and ~45% improvement in bending strength (742.6±13.1 MPa) and fracture toughness (5.83±0.19 MPa m 1/2 ), respectively, than a MWCNT-free alumina. [ABSTRACT FROM AUTHOR]