Experimental Observation of Cubic C_3N_4 Compound in Carbon Nitride Thin Films

Cubic C3N4 compound in the C-N thin films on Si and NaCl substrates was prepared by ion beam sputtering of a pure graphite target with discharge gas of pure N2. X-ray photoelectron spectroscopy indicated that nitrogen atoms combined with sp2- and sp3- coordinated C atoms in the film, respectively. X-ray diffraction, selected area electron diffraction and high-resolution electron microscopy were used to identify the cubic C3N4 phase. The results reconfirm the ab initio calculations on metastable structure in C-N compounds

Discovery of superhard materials via CALYPSO methodology

The study of superhard materials plays a critical role in modern industrial applications due to their widespread applications as cutting tools, abrasives, exploitation drills, and coatings. The search for new superhard materials with superior performance remains a hot topic and is mainly considered as two classes of materials:(i) the light-element compounds in the B-C-N-O(-Si) system with strong and short covalent bonds, and(ii) the transition-element light-element compounds with strong covalent bonds frameworks and high valence electron density. In this paper, we review the recent achievements in the prediction of superhard materials mostly using the advanced CALYPSO methodology. A number of novel, superhard crystals of light-element compounds and transition-metal borides, carbides, and nitrides have been theoretically identified and some of them account well for the experimentally mysterious phases. To design superhard materials via CALYPSO methodology is independent of any known structural and experimental data, resulting in many remarkable structures accelerating the development of new superhard materials.

Fibre Metallurgy Characteristic of Short Cast Iron Fibre and Its Applications for Diamond Wheel

A kind of diamond grinding wheel bonded by short cast iron fibres has been developed. ln comparison with high quality bronze bonded diamond grinding wheel. the new wheel was more suitable to grind hard and brittle materials like ceramics. For Si3N4, the grinding efficiency has been raised two times and the grinding ratio five times

Enhanced thermoelectric performance of Sb-doped Mg_(2)Si_(0.4)Sn_(0.6)via doping,alloying and nanoprecipitation

With the advantages of eco-friendliness,low cost,and low density,Mg_(2)(Si,Sn)solid solutions are promising candidates for thermoelectric applications.In this work,Sb-doped Mg_(2)Si_(0.4)Sn_(0.6)bulks were prepared with a combined method of solid-state reaction and high pressure synthesis,followed by spark plasma sintering.Our investigations show that Sb doping optimizes the carrier concentration,while Si/Sn alloying effectively suppresses the lattice thermal conductivity and induces a convergence of the two lowest-lying conduction bands.Additionally,numerous coherent Sn-rich nanoprecipitates are formed within micron-sized grains.All these factors contribute synergistically to improving the thermoelectric properties of Mg_(2)Si_(0.4)Sn_(0.6).The optimal Mg_(2)(Si_(0.4)Sn_(0.6))_(0.985)Sb_(0.015)exhibits a power factor higher than 4000 mW·m^(-1)·K^(-2)and a lattice thermal conductivity less than 0.8 W·m^(-1)·K^(-1)at temperatures higher than 600 K,leading to the highest ZT of 1.61 at 823 K.Current work demonstrates an effective approach to enhancing the thermoelectric performance of n-type Mg_(2)X solid solutions through doping,alloying,and microstructure modification.

Tempering mechanism of lath martensite induced in IF steel under high pressure

In this paper,low-and high-strength lath martensite(350 and 640 HV)was fabricated in an IF steel via high pressure martensitic transformation.The microstructure and the softening during their tempering from 200°C to 800°C for 1 h were systematically investigated.A carbon-irrelevant tempering process was proposed,exhibiting a three-stage structural evolution pattern depending upon the tempering de-gree(1-(HV-HV FP)/(HV NP-HV FP),where the HV is the instant hardness,HV NP is the non-tempered hard-ness and HV FP is the fully tempered hardness):(1)low tempered(<10%),removing the loose dislocations and dislocation boundaries within martensitic variants;(2)medium tempered(10%-50%),eliminating the martensitic variant laths via the migration of their terminal tips;(3)highly tempered(>50%),clearing up the remained variant laths via the migration of the triple junctions.Martensite-type microstructure is tailored by low-index lamellar variant boundaries and is thus intrinsically thermally stable,whereas the mobile terminal tips decrease the tempering resistance.The underlying mechanism for such carbon-irrelevant process was discussed and the potential effect on the tempering behavior of carbon-contained martensite was highlighted.

{112}<111>Twins or Twinned Variants Induced by Martensitic Transformation?

Twinned substructure in lath martensite was induced in the interstitial free(IF)steel via a high pressure thermal cycle(heating up to 1100℃and holding for 30 min,cooling at 10℃/s to room temperature under a pressure of 4 GPa).Experimental observations and theoretical simulation confrm that the twinned substructure has the origin related to the twinned variants rather than the bcc{112}<111>twins,while extra difraction spots were caused by crystal overlapping rather than any extra phase.The diferences in crystallography and electron difraction behavior between twinned variants and{112}<111>twins were discussed in detail.

The rise of plastic deformation in boron nitride ceramics

Ceramics are bonded by ionic or covalent bonds,with very limited slip systems for dislocation nucleation and movement[1].The poor deformability and natural brittleness are the major drawbacks of ceramics,especially when compared with metals.Under stress,ceramics tend to fracture before noticeable plastic deformation takes place.Cracks occur and propagate rapidly in ceramics subjected to stress much lower than the theoretical strength[2].As a result,ceramics can only endure very small strains(<1%),absorb limited mechanical energy,and display poor toughness[3].Moreover,microstructure imperfections in ceramics may decrease the toughness even further.Due to the lack of significant plastic deformation capacity for ceramic materials,the catastrophic failures without warning are easy to happen under stress which critically increases the unreliability of ceramics in the applications as structural materials.

Extraordinary high-temperature mechanical properties in binder-free nanopolycrystalline WC ceramic

From the perspective of high-temperature applications,materials with excellent high-temperature mechanical properties are always desirable.The present work demonstrates that the binder-free nanopolycrystalline WC ceramic with an average grain size of 103 nm obtained by high-pressure and hightemperature sintering exhibits excellent mechanical properties at both room temperature and high temperature up to 1000℃.Specifically,the binder-free nanopolycrystalline WC ceramic still maintains a considerably high Vicker hardness H_(V)of 23.4 GPa at 1000℃,which is only 22%lower than the room temperature H_(V).This outstanding thermo-mechanical stability is superior to that of typical technical ceramics,e.g.SiC,Si_(3)N_(4),Al_(2)O_(3),etc.Nanocrystalline grains with many dislocations,numerous low-energy,highly stableΣ2 grain boundaries,and a relatively low thermal expansion coefficient,are responsible for the observed outstanding high-temperature mechanical properties.

Small onion-like BN leads to ultrafine-twinned cubic BN

Nanotwinned cubic boron nitride(nt-cBN) with remarkable hardness, toughness, and stability has attracted widespread attention due to its distinct scientific and industrial importance. The key for nt-cBN synthesis is to adopt an onion-like BN(oBN) nano-precursor and induce phase transition under high pressure. Here, we found that the size change of oBN used greatly affected the mechanical performance of products. With the precursor size decreasing from^320 to 90 nm, the Vickers hardness of nanostructured products improved from 61 to 108 GPa, due to the fact that large oBN nanoparticles possessed more flattened, orderly and graphite-like shell layers, in sharp contrast to the highly wrinkled and imperfect layers in small-diameter nanoparticles, thus resulting in the apparent reduction of ultrafinetwin substructure in the synthetic products. This study reveals that only small oBN precursor could produce complete ultrafine nt-cBN with outstanding performance. A practical route was proposed to further improve the performance of this important material.

Enhanced thermoelectric performance of Na-doped Pb Te synthesized under high pressure

Despite an effective p-type dopant for PbTe, the low solubility of Na limits the fully optimization of thermoelectric properties of Na-doped PbTe. In this work, Na-doped PbTe was synthesized under high pressure. The formation of the desired rocksalt phase with substantially increased Na content leads to a high carrier concentration of 3.2×10^20 cm^-3 for Na0.03Pb0.97Te. Moreover, dense in-grain dislocations are identified from the microstructure analysis. Benefited from the improved power factor and greatly suppressed lattice thermal conductivity, the maximal ZT of 1.7 is achieved in the optimal Na0.03Pb0.97Te. Current work thus designates the advantage of high pressure in synthesizing PbTe-based thermoelectric materials.

Heat-treated glassy carbon under pressure exhibiting superior hardness,strength and elasticity

Glassy carbon(GC)is a type of non-graphitizing disordered carbon material at ambient pressure and high temperatures,which has been widely used due to its excellent mechanical properties.Here we report the changes in the microstructure and mechanical properties of GC treated at high pressures(up to 5 GPa)and high temperatures.The formation of intermediate sp2-sp3 phases is identified at moderate treatment temperatures before the complete graphitization of GC,by analyzing synchrotron X-ray diffraction,Raman spectra,and transmission electron microscopy images.The intermediate metastable carbon materials exhibit superior mechanical properties with hardness reaching up to 10 GPa and compressive strength reaching as high as 2.5 GPa,nearly doubling those of raw GC,and improving elasticity and thermal stability.The synthesis pressure used in this study can be achieved in the industry on a commercial scale,enabling the scalable synthesis of this type of strong,hard,and elastic carbon materials.

Novel superhard boron-rich nitrides under pressure

Boron and its compounds have attracted much attention due to their interesting and complex structures[1-4].In particular,boron-rich compounds containing icosahedral structures have excellent properties,such as low density,high hardness,high melting point and low wear coffi-cient.

Twinned Martensitic Substructure in a Water Quenched Fe–1.0 wt%C Alloy

A Fe–1.0 wt%C alloy was quenched into water from 1100 ℃,leading to lath martensite and plate martensite of body-centered tetragonal structure.Both these two martensites have the twinned substructure that generates mirror symmetric diff raction patterns with extra diff raction spots around n/3(112).The twinned substructure has the origin from twinned martensitic variants,namely twin-related crystals separated by{110},rather than{112}<111>deformation twins.Tetragonality eff ect on the electron double diff raction of twinned variants was discussed.