Chemical hardness is considered to be a useful theoretical descriptor in many experimental and theoretical studies and this concept has several important applications in chemistry. In this study, an ...equation for atomic hardness is proposed. Following the equation for atomic hardness which is based on charges, ionisation energies and electron affinities of atoms, a new equation is obtained for the calculation of the chemical hardness of groups and molecules using the atomic hardness equation and the principle of chemical hardness equalisation. Molecular chemical hardness may be calculated using α and β parameters of atoms in a molecule or group through the use of the obtained molecular hardness equation. Furthermore, the advantage of this equation over other equations in the literature is that using this equation the chemical hardness of the charged groups and molecules can also be calculated. Besides, the relationship between molecular hardness and charge of molecule is examined using the results obtained from the new molecular hardness equation, and the partial charges of atoms in molecules are calculated with the help of derived equations.
An as-received ultrafine-grained Cu powder and four nanostructured Cu-(2.5-10)vol%Al.sub.2O.sub.3 composite powders produced by high-energy mechanical milling of mixtures of the Cu powder and an ...Al.sub.2O.sub.3 nanopowder were consolidated using warm powder compaction followed by open die powder compact forging. The circular discs produced in the experiments achieved full densification. Tensile testing of the specimens cut from the forged discs showed that the Cu-forged disc had a fairly high yield strength of 330 MPa, UTS of 340 MPa and a plastic strain to fracture of 15%, but the Cu-Al.sub.2O.sub.3 composite-forged discs did not show any macroscopic plastic yielding. The fracture strength of the composite-forged discs decreased almost linearly with the increase of the volume fraction of Al.sub.2O.sub.3 nanoparticles. This study shows that a high level of consolidation of the ultrafine-grained Cu powder and the nanostructured Cu-2.5 vol%Al.sub.2O.sub.3 composite powder has been achieved by warm powder compacting at 350°C and powder compact forging at 500 and 700° C. However, this is not true for the nanostructured Cu-(5, 7.5 and 10) vol%Al.sub.2O.sub.3 composite powders, possibly due to their higher powder particle hardness at elevated temperatures in the range of 350-800°C.
Tungsten tetraboride (WBâ) is an interesting candidate as a less expensive member of the growing group of superhard transition metal borides. WBâ was successfully synthesized by arc melting from ...the elements. Characterization using powder X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) indicates that the as-synthesized material is phase pure. The zero-pressure bulk modulus, as measured by high-pressure X-ray diffraction for WBâ, is 339 GPa. Mechanical testing using microindentation gives a Vickers hardness of 43.3 ± 2.9 GPa under an applied load of 0.49 N. Various ratios of rhenium were added to WBâ in an attempt to increase hardness. With the addition of 1 at.% Re, the Vickers hardness increased to approximately 50 GPa at 0.49 N. Powders of tungsten tetraboride with and without 1 at.% Re addition are thermally stable up to approximately 400 °C in air as measured by thermal gravimetric analysis.
High-pressure torsion (HPT) processing was applied to cast pure magnesium, and the effects of the deformation on the microstructure, hardness, tensile properties and corrosion resistance were ...evaluated. The microstructures of the processed samples were examined by electron backscatter diffraction, and the mechanical properties were determined by Vickers hardness and tensile testing. The corrosion resistance was studied using electrochemical impedance spectroscopy in a 3.5% NaCl solution. The results show that HPT processing effectively refines the grain size of Mg from millimeters in the cast structure to a few micrometers after processing and also creates a basal texture on the surface. It was found that one or five turns of HPT produced no significant difference in the grain size of the processed Mg and the hardness was a maximum after one turn due to recovery in some grains. Measurements showed that the yield strength of the cast Mg increased by about seven times whereas the corrosion resistance was not significantly affected by the HPT processing.
Recent studies have shown the benefits of utilizing ceramic particles as reinforcement in metal alloys; nevertheless, certain drawbacks, including loss of ductility, embrittlement, and decreases in ...toughness, have been noted. For the objective of obtaining balanced performance, experts have suggested the addition of metal particles as supplement to the ceramic reinforcement. Consequently, high-performance metal hybrid composites have been developed. However, achieving the optimal mix for the reinforcement combination with regards to the optimal performance of developed composite remains a challenge. This research aimed to determine the optimal mixture of Alsub.50Cusub.10Snsub.5Mgsub.20Znsub.10Tisub.5 lightweight high-entropy alloy (LHEA), B4C, and ZrOsub.2 for the fabrication of trihybrid titanium composites via direct laser deposition. A mixture design was involved in the experimental design, and experimental data were modeled and optimized to achieve the optimal performance of the trihybrid composite. The ANOVA, response surface plots, and ternary maps analyses of the experimental results revealed that various combinations of reinforcement particles displayed a variety of response trends. Moreover, the analysis showed that these reinforcements significantly contributed to the magnitudes and trends of the responses. The generated models were competent for predicting response, and the best formulation consisted of 8.4% LHEA, 1.2% B4C, and 2.4% ZrOsub.2.
We study the nanohardness and Young’s modulus of randomly oriented CdSiPsub.2 (CSP) and ZnGePsub.2 (ZGP) single crystals, grown via the horizontal Bridgman method. CSP and ZGP are the only two ...pnictide chalcopyrites widely used as nonlinear optical crystals in the mid-IR part of the spectrum. Nanoindentation is employed in the continuous stiffness mode (45 Hz, 2 nm) using a Berkovich tip. Nanohardness values of 9.9 ± 0.2 GPa (Knoop hardness of 905 kg/mmsup.2) for CSP and 11.5 ± 0.1 GPa (993 kg/mmsup.2) for ZGP are derived. For Young’s modulus, we obtain 136 ± 2 GPa (CSP) and 150 ± 2 GPa (ZGP). The trend of increasing hardness with bandgap and melting point of the isostructural CSP and ZGP, as deduced from previous measurements, is not confirmed. The results for ZGP are compared to 2GaP, its binary isoelectronic analog, and the values obtained, 11.0 ± 0.3 GPa for the nanohardness and 154 ± 2 GPa for Young’s modulus, indicate good matching within the accuracy limits.
To explore the effect of different curing modes of conventional and self-adhesive dual-cure resin cements on their rates of thermal decomposition, hardness development and network integrity.
Five ...self-adhesive (PANAVIA SA, RelyX Universal Resin, RelyX Unicem 2, Bifix SE and SpeedCEM Plus) and three conventional (PANAVIA V5, Nexus Third Generation and RelyX Ultimate Universal) dual-cure resin cements were investigated. Thermal decomposition stages, initial onset temperatures, the maximum rate of mass-loss and the filler mass-fraction of each resin cement were analysed by thermogravimetric analysis (TGA). Surface hardness was measured at 1h post-cure and after 24h of dry storage at 37°C. The relative network integrities were estimated from reductions in hardness after 168h of water storage. Data were analysed via one-way ANOVA, Tukey post-hoc tests and paired/independent sample t-tests (a=0.05).
No difference was apparent between TGA data for self-cured and light-cured specimens. Numerical differentiation of mass-loss versus temperature showed either single or multiple peaks. For the set of 8 cements, the maximum rate of mass-loss (%/°C) correlated negatively with residual mass at 600°C.
All dry-stored cements increased in hardness from 1 to 24h, ranging from 20.4% to 52.6% for light-cure mode and from 41.3% to 112.6% for self-cure. After 168h water storage, the hardness of cements decreased: by 18.5%–36.2% for light-cured and by 9.8%–17.9% for self-cured. Overall, surface hardness was greater for light-cured cements.
The initial onset temperature (IOT) of thermal decomposition correlated negatively with the hardness decrease produced by water-storage: r2=0.77 for light-cure and r2=0.88 for self-cure. This provided the basis for a relative scale of composite network integrity, probably reflecting differences in cross-link density.
Light-curing, where possible, remains beneficial to the hardness and related properties of dual-cure resin cements. Combination of TG analysis and solvent softening experiments give an indication of relative network integrity – between materials – and their relative cross-link densities.
To investigate the surface micro-hardness (VHN) and fracture toughness (KIC) of resin-composites, with and without incorporated short fibers, after solvent storage.
Three resin-composites ...incorporating fibers, additional to particle reinforcement, were examined: everX™, NovoPro Fill™ and NovoPro Flow™. Four composites were used as controls, with only particle reinforcement: Filtek bulk Fill™, Filtek bulk one™, Filtek XTE™, and Filtek Flow XTE™. For hardness measurement, materials were cured in 2mm thick molds for 20s by a LED source of average irradiance 1.2W/cm2. Specimens (n=6/group) were stored dry for 1h and then in either water or 75% ethanol/water for 1h, 1 day and 30 days at 37±1°C. Vickers hardness was measured under a load of 300g for 15s. For fracture toughness (KIC) measurements, single-edge-notched specimens (n=6/group) were prepared: (32×6×3mm) for 3-point bending and stored for 1 and 7 days in water at 37°C. Fractured surfaces of fiber-reinforced composite were examined by scanning electron microscopy (SEM). VHN data were analyzed using three-way ANOVA, one-way ANOVA and the Tukey post hoc test (p≤0.05). KIC data were analyzed by two-way ANOVA and one-way ANOVA and the Tukey post hoc test (p≤0.05). An independent t-test was used to detect differences (α=0.05) in KIC between stored groups for each material.
VHN decreased for all composites with storage time in both solvents, but more appreciably in 75% ethanol/water (an average of 20%). KIC ranged from 2.14 (everX Posterior) to 0.96 NovoPro Flow) MPa.m0.5. The longer storage period (7 days) had no significant effect on this property relative to 1 day storage.
Reinforcement with short fibers, and possibly matrix compositional differences, significantly enhanced the fracture toughness of EVX. However, for nano-fiber containing composites, there were no evident beneficial effects upon either their fracture toughness or hardness compared to a range of control composites. Water storage for 7 days of all these resin-composites produced no significant change in their KIC values, relative to 1 day storage.
First-principles evolutionary simulations are used to predict the stable compounds of TiC Formula omittedN Formula omitted. Our structural searching results reveal that TiC Formula omittedN Formula ...omitted system has a wide compositional range and several TiC Formula omittedN Formula omitted structures with new stoichiometries are found. All the predicted TiC Formula omittedN Formula omitted compounds are dynamically stable at ambient pressure. The elastic properties, hardness and fracture toughness of these TiC Formula omittedN Formula omitted compounds have been systematically investigated. New ternary compound Ti Formula omittedC Formula omittedN Formula omitted is predicted to be the hardest and has a Vickers hardness of 29 GPa and a high fracture toughness (3.38 MPa m Formula omitted). The high hardness for Ti Formula omittedC Formula omittedN Formula omitted originates from the strong interactions between Ti-3d and C-2p electrons according to the electronic structure analysis. The relationship between elastic properties and valence electron concentration (VEC) is further discussed. The results show that the hardness and shear moduli of TiC Formula omittedN Formula omitted compounds simultaneously reach their maximum when the VEC is around 8.4.
Uniaxial compressive strength (UCS), which is one of the most important engineering properties of natural building blocks, exhibits a decrease at various rates under the effect of freeze–thaw (FT) ...cycles. In this study, practical estimation of UCS loss of 21 groups of carbonate building stones exposed to 4, 7, 10, 14, 20, 28, 35, 56, 70 and 84 FT cycles is aimed by proposed estimation models. In all models, the Leeb hardness (HL
D
), which is practically determined by a dynamic surface hardness test method, was used as major input parameter. Other basic stone properties and number of FT cycles were also used as additional input parameters in developed models. The test results indicated that HL
D
and UCS values decreased up to 13 and 40%, respectively, at the end of 84 FT cycles. Proposed equations are reliable and beneficial in estimation of deteriorated UCS values of carbonate building stones in practical engineering applications and scientific studies.
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