| Hexagonal tungsten carbide (WC) is the only stable phase among carbides of tungsten atroom temperature, and others can be formed only by high temperature quenching.Significantly, WC is of practical interest for engineering applications as a high-meltingrefractory material (T=3070K) with high hardness and mechanical stability, as cementedcarbide in combination with a cobalt binder or other carbides (e.g., TiC and Cr2C3). Thus, itis widely used for wear parts and cutting tools. Extensive efforts have been made to improvethe properties and reduce the production costs of cemented carbide by tailoring thecomposition and microstructure. Because tungsten is a strategic metal, any replacement bymore abundant and cheaper metals will be of great interest. Recently, a special class of newcarbide (W1-xAlx)Czwith the hexagonal WC structure has been synthesized by mechanicalalloying and sintering techniques, where some W4+ions are replaced by Al3+ions. Althoughthe commercial price of Al is much cheaper than that of W, this carbide has excellentmechanical properties. For instance, the hardness of the (W1/2Al1/2)Czsystems increases upto a maximum2659kg/mm2at a carbon vacancy concentration of about35%, and thedensity of (W1/2Al1/2)Czis far lower than that of WC. It should be good alternative materialfor these carbides with unusual solid solution structure and mechanical properties, thefundamental understandings of their atomic structure are still hazy, and the physicalbackground in electronic levels remains unknown.On the other hand, in the search for new hard or high incompressibility materials, therehave been many attempts to make three-dimensional network structures out ofsp2-hybridized bonds. The motivation for these attempts lies in the belief that theincompressibility of a structure is related to the bond length found in the system, e.g., theshorter the bonds, the harder the solid.. In fact, there have been some works that indicatedthe monolayer graphene derived from bulk graphite exhibited ultrahigh Young’s modulus of1.0TPa. This fact can enhance the belief of these attempts. However, on the other hand,graphite is intrinsically soft because of its planar nature. Hence, structures have beenproposed where carbon atoms form a three-dimensional network of sp2-hybridized bondsconfiguration. Some of these attempts, like the H-6and bct-4structures, revealed bulkmodulus around370GPa that are among the highest known but still lower than the value for diamond.As well known, high pressure serves as a clean and powerful tuning parameter which hasbeen used to investigate a number of carbon allotropes including graphite, C60and itsderivatives, and carbon nanotubes. The particular pressure-temperature pathway plays asignificant role in determining the final products as well as their properties, leading tocontroversial results reported in the literature. From a fundamental point of view, amorphouscarbon materials are also interesting since they are complex and largely varied. The interplaybetween sp2and sp3hybrid bonding contributes mostly to this variation and complexity.Hereinto, Glassy carbon is an amorphous carbon allotrope containing nearly100%sp2bonding at ambient conditions. Its structure includes fragments of curved graphenelikesheets, which randomly distribute throughout the network. Recently, two investigated teamsexhibited the different high pressure results by compressing glass carbon. The main divergewas focused on the conversion from sp2to sp3-hybridized bonds, which could be attributedto the different pressure condition (hydrostatic and non-hydrostatic pressure). Thus, thecarbon allotrope derived from the compressed glassy carbon by the pressure-temperaturetreatment is of interest for investigating.Computer simulations can simulate not only experiments with shorter time scales but alsoexperiments that are difficult to realize. Based on above considerations, using computersimulations, we investigated the structural characterization and mechanical properties of the(W1/2Al1/2)CZphase and high density sp2-rich amorphous carbon phase. The main results aredivided into three parts as following:Firstly, we choose the (W1/2Al1/2)CZphase as examples to study shear modulus G, elasticconstants cijand Poisson’s ratio ν as a function of Z using first-principles calculation wherethe related electronic structures are explored to find the electronic origin of the effect of Z.The results provide a theoretical support for the effect of Z on G of (W1/2Al1/2)CZ. It is foundthat the maximal G value and the lowest Poisson ratio ν value are reached at Z=3/4due toatomic configuration changes. The fact is consistent with the effect of Z on the experimentalhardness. In addition, G(Z) function and low ν values are illustrated and expected to providetheoretical guides for the alloying and solid solution treatment for these carbides.Secondly, two candidate structural types of (W1/2Al1/2)C phase are chosen, namely,WC-type and NiAs-type. Their thermodynamic and lattice dynamic stabilities, mechanicaland electronic properties are determined by using first-principles calculation. The resultsshow that the NiAs-type is more favorable than WC-type. Moreover, it is validated that the pressure will thermodynamically stabilize the (W1/2Al1/2)C phase. The energy barrier ofstructural transition from WC-type to NiAs-type is also calculated by the transition statesearch method to understand the kinetic stability of (W1/2Al1/2)C phase.Thirdly, we construct a hypothetical high density sp2-rich amorphous carbon (sp2-HDAC)from a stable glassy carbon precursor phase with compatible rich sp2-hybridized bonds andhigh volume density based on molecular dynamics (MD) simulation. The sp2-HDAC isrevealed to exhibit high resistance to compression and shear deformation in comparison withthe related H6carbon allotrope. The results are attributed to the high volume density andthe strong in-planar sp2-hybridized bonds, and provide a proof for advanced mechanicalproperties of the carbon allotrope with complete sp2-hybridized bonds. |
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