Boron carbide is a strategic material, finding applications in nuclear industry, armour for personnel and vehicle safety, rocket propellant, etc. Its high hardness makes it suitable for grinding and cutting tools, ceramic bearing, wire drawing dies, etc. Boron carbide is commercially produced either by carbothermic reduction of boric acid in electric furnaces or by magnesiothermy in presence of carbon. Since many specialty applications of boron carbide require dense bodies, its densification is of great importance. Hot pressing and hot isostatic pressing are the main processes employed for densification. In the recent past, various researchers have made attempts to improve the existing methods and also invent new processes for synthesis and consolidation of boron carbide. All the techniques on synthesis and consolidation of boron carbide are discussed in detail and critically reviewed.

Synthesis and consolidation of boron carbide: a review

One of the most intriguing protein materials found in nature is bone, a material composed of assemblies of tropocollagen molecules and tiny hydroxyapatite mineral crystals that form an extremely tough, yet lightweight, adaptive and multifunctional material. Bone has evolved to provide structural support to organisms, and therefore its mechanical properties are of great physiological relevance. In this article, we review the structure and properties of bone, focusing on mechanical deformation and fracture behavior from the perspective of the multidimensional hierarchical nature of its structure. In fact, bone derives its resistance to fracture with a multitude of deformation and toughening mechanisms at many size scales ranging from the nanoscale structure of its protein molecules to the macroscopic physiological scale.

On the Mechanistic Origins of Toughness in Bone

Magnesium is a light metal, with a density two-thirds that of aluminium, is abundant on Earth and is biocompatible; it thus has the potential to improve energy efficiency and system performance in aerospace, automobile, defence, mobile electronics and biomedical applications. However, conventional synthesis and processing methods (alloying and thermomechanical processing) have reached certain limits in further improving the properties of magnesium and other metals. Ceramic particles have been introduced into metal matrices to improve the strength of the metals, but unfortunately, ceramic microparticles severely degrade the plasticity and machinability of metals, and nanoparticles, although they have the potential to improve strength while maintaining or even improving the plasticity of metals, are difficult to disperse uniformly in metal matrices. Here we show that a dense uniform dispersion of silicon carbide nanoparticles (14 per cent by volume) in magnesium can be achieved through a nanoparticle self-stabilization mechanism in molten metal. An enhancement of strength, stiffness, plasticity and high-temperature stability is simultaneously achieved, delivering a higher specific yield strength and higher specific modulus than almost all structural metals.

Processing and properties of magnesium containing a dense uniform dispersion of nanoparticles

The mechanical properties of ex-situ and in-situ metallic glass matrix composites (MGMCs) have proven to be both scientifically unique and of potentially important for practical applications. However, the underlying deformation mechanisms remain to be studied. In this article, we review the development, fabrication, microstructures, and properties of MGMCs, including the room-temperature, cryogenic-temperature, and high-temperature mechanical properties upon quasi-static and dynamic loadings. In parallel, the deformation mechanisms are experimentally and theoretically explored. Moreover, the fatigue, corrosion, and wear behaviors of MGMCs are discussed. Finally, the potential applications and important unresolved issues are identified and discussed.

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Metallic glass matrix composites

High-entropy ceramics (HECs) are solid solutions of inorganic compounds with one or more Wyckoff sites shared by equal or near-equal atomic ratios of multi-principal elements. Although in the infant stage, the emerging of this new family of materials has brought new opportunities for material design and property tailoring. Distinct from metals, the diversity in crystal structure and electronic structure of ceramics provides huge space for properties tuning through band structure engineering and phonon engineering. Aside from strengthening, hardening, and low thermal conductivity that have already been found in high-entropy alloys, new properties like colossal dielectric constant, super ionic conductivity, severe anisotropic thermal expansion coefficient, strong electromagnetic wave absorption, etc., have been discovered in HECs. As a response to the rapid development in this nascent field, this article gives a comprehensive review on the structure features, theoretical methods for stability and property prediction, processing routes, novel properties, and prospective applications of HECs. The challenges on processing, characterization, and property predictions are also emphasized. Finally, future directions for new material exploration, novel processing, fundamental understanding, in-depth characterization, and database assessments are given.

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High-entropy ceramics: Present status, challenges, and a look forward

Wetting of TiC by molten Al at 1123–1323 K

Wetting of polycrystalline SiC by molten Al and Al−Si alloys

Porous laminated SiC scaffolds with initial solid loadings of 20–40 vol% were prepared by freeze casting and then sintered at 1200 °C for 2 h in air to produce a SiO2 layer. Subsequently, the preforms were pressureless-infiltrated by an Al–12 wt% Si–10 wt% Mg alloy in a N2 atmosphere, forming a lamellar-interpenetrated structure in the composites. The resultant composites were investigated by compression tests and ultrasonic pulse echo measurements in the longitudinal and transverse directions, exhibiting weak orthotropic properties. The highest compressive strength (722±35 MPa) was achieved in the samples with 30 vol% SiC in a longitudinal direction and the largest elastic modulus (163 GPa) was exhibited in the 40 vol% SiC composites.

Lamellar-interpenetrated Al–Si–Mg/SiC composites fabricated by freeze casting and pressureless infiltration

Effect of Cr on the wetting in Cu/graphite system

The wetting and evaporation behaviors of water–ethanol mixtures on polished poly(tetrafluoroethylene) (PTFE) surfaces were studied with an emphasis on the influence of concentration. Three representative stages: (i) constant contact diameter with decreasing contact angle and drop height; (ii) increase in contact angle and drop height with decreasing contact diameter; and (iii) simultaneous decrease in contact angle, drop height, and contact diameter, were identified in the wetting process under evaporation. In the initial stage, with increasing ethanol concentration, the wetting and evaporating behaviors of the mixture drops are close to those of pure ethanol, while in the final stage they are close to those of pure water, regardless of the concentration. The most significant difference between the pure substance and mixture drops is in the intermediate stage due to the different controlling mechanisms behind the de-pinning phenomenon. Moreover, the concrete wetting behavior is dependent on the composition of the mixture drops. For an evaporating sessile drop, the wetting should be evaluated using the combined parameters of contact angle, drop height, and contact diameter. Six representative modes are proposed to describe the wetting behaviors of the water–ethanol sessile drops on the PTFE surfaces under evaporation. Copyright © 2009 John Wiley & Sons, Ltd.

Ping Shen

Papers

Wetting of TiC by molten Al at 1123–1323 K

Wetting of polycrystalline SiC by molten Al and Al−Si alloys

Lamellar-interpenetrated Al–Si–Mg/SiC composites fabricated by freeze casting and pressureless infiltration

Effect of Cr on the wetting in Cu/graphite system

Wetting and evaporation behaviors of water-ethanol sessile drops on PTFE surfaces