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γ-M23C6 carbide often forms at grain boundaries in creep-resistant steels and plays a crucial role in creep resistance by blocking microstructural changes at elevated temperatures. Given that the dislocations and stacking faults (SFs) in carbides may affect their stability, the observation of SF formation in γ-M23C6 using atomic-scale microscopy has implications for Cr-rich creep-resistant steels. Our analysis of SF energies (SFEs) derived from density functional theory calculations reveals that the SFEs are high, which suggests that the SF is not induced by external shear stress deformation but by lattice misfit between conjoined subgrains nucleated from the same austenite grain.

The relationship between creep lifetime and microstructure, especially precipitate morphology, of a new 9Cr heat-resistant steel containing 0.011 wt% boron was examined. The B-added steel showed superior creep lifetime compared to conventional Gr. 91 steel due to the microstructural stability of the new steel at 873 K. The new steel also exhibited prior-austenite grain boundaries with a high coverage of precipitates, which increased from ~30% to ~40% during creep tests, whereas that of the conventional Gr. 91 steel was nearly stable, at ~25%. In addition, the B-containing M23C6 in the B-added steel had a lower coherency, i.e., a higher interface energy, between the precipitate and the matrix when compared to conventional Gr, 91 steel. These factors led to a higher pinning pressure to resist interface migration in the B-added steel. Microscopy revealed that chemical stabilization had a significant effect on the coarsening behavior of M23C6. Chemical analyses revealed that M23C6 became Cr-rich during the early stages of creep, in which rapid coarsening was observed at a rate contrast of >1.0 × 10−28 m3/s; in this state, M23C6 was approaching equilibrium Cr concentration. In later stages of creep, moderate coarsening occurred with a rate constant of ~8.1 × 10−30 m3/s; in this state, chemical stability was achieved but diffusion and dissolution of small precipitates were dominant.

In our recent work [Electrochim. Acta 240 (2017) 388–398], we examined the adsorption energy of glucose on graphene via Ni nanoparticles. In the present study, we have responded to comments to clarify the intricacies of dispersion effect and adsorption energies in our previous work. We have explored the performance and accuracy of some established and promising theoretical schemes, namely, the two-, three- (DFT-D2/3), and many-body (MBD@rSC) dispersion approaches, to account properly for the dispersive force contributions, which were not previously included in the system. Incorporating dispersive forces drastically increased the adsorption energy of glucose (Eads), where different active sites were obtained. The calculated energies were in the order Eads(DFT−D2)

Although welding results in premature failure by type IV fracture under high temperature creep conditions, the alloy design of light elements such as boron addition and nitrogen reduction enhances the creep lifetime of 9Cr heat resistant steel. In particular, the simulated heat affected zone (SHAZ) sample of new 9Cr steel (called TA steel) shows about 10 times longer creep lifetime than that of the standard Gr. 91 steel. The welded TA steel is thus expected to exhibit good creep properties because its SHAZ sample has coarser grains and suppresses type IV fracture. The preservation of base metal’s microstructure after welding results from the precipitate morphology, such as high grain boundary coverage by precipitates and low amount of MX being nucleation sites of ferrite grains during the a-g phase transformation. In addition, the increase of stability of M23C6 affects high pinning pressure toward grain boundary migration upon rapid heating during welding. First-principles calculations confirm the increased stability when boron is absorbed by M23C6. Moreover, the calculations reveals that boron decreases the coherency between matrix and M23C6, suppressing grain coarsening during creep tests in TA steel. It is concluded that the increased microstructural stability during welding and long high temperature exposure generates the elongated creep lifetime in welded TA steel including about 0.01 wt% boron and less than 0.01 wt% nitrogen.

The intention of the study is to find the nature of interactions that exist in inclusion complexes formed between the steroids, nandrolone, and testosterone with cucurbit[n]urnils (n = 7 and 8) host, using density functional theory incorporated with empirical dispersion correction. Upon encapsulation, nandrolone caused a larger geometrical distortion in cucurbit[8]uril geometry, while testosterone inclusion complex is formed with a larger number of intermolecular hydrogen bonds. The molecular electrostatic potential examination shows that the positive potential observed on the eight-membered ring in CB7 got reduced upon encapsulation, while on the nandrolone the negative potential on carbonyl unit has increased. AIM analysis shows that in inclusion complexes, the observed electron density are higher for the interactions between the oxygen atoms of carbonyl fringe of cucurbituril molecule and the steroid molecules. The NCI isosurface of nandrolone@CB7 has green patches in between the nandrolone and cucurbituril molecule, evenly distributed. In the testosterone@CB7, along with the green patches, red patches, due to the steric crowding between the testosterone and cucurbit[7]uril, were observed. The energy decomposition analysis parameters show that Pauli’s repulsive term was highest for nandrolone@CB7. When testosterone is the guest, repulsive energy was found to be larger than nandrolone guest. From the above interference, it can be confirmed that the steric hindrance that arises during the interaction of testosterone with CB7 reduces the stability of the complex, and the nandrolone best fit inside the CB7 cavity with the combination of hydrogen bonding and weak van der Waals bonding as intermolecular interactions.

A three-dimensional (3D) and hybrid hydrogel based on gelatin methacryloyl (GelMA), Ni nanoparticles (Ni-NPs), and reduced graphene oxide (RGO) (GelMA:Ni-RGO) was synthesized for enzymeless glucose sensing. Hybrid GelMA:Ni-RGO hydrogel was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. GelMA avoided the agglomeration of Ni-RGO in aqueous solution as it provided hydrophilic functional groups on the surface of nanoparticles. As a result, an interconnected conductive network within GelMA:Ni-RGO was created for effective charge transfer in glucose sensing. GelMA:Ni-RGO demonstrated non-enzymatic responses to glucose oxidation at a low potential of 0.47 V with high sensitivity of 0.056 mAmM−1, a wide detection range from 0.15 μM to 10 mM, and a detection limit of 5 nM. Also, no significant interference was observed from potentially interference species, such as ascorbic acid, acetaminophen, and uric acid. The proposed sensor was used for determination of glucose in human blood serum. High electrochemical performance of GelMA:Ni-RGO can be attributed to large electroactive surface area, porous structure, interconnected conductive networks, and synergistic interaction between GelMA and Ni-RGO in 3D hybrid hydrogel. First-principles calculations were employed to reveal molecular interactions of glucose molecule with GelMA:Ni-RGO. The results showed hydrophobic interaction between graphene and GelMA molecule. Ni nanoparticles were adsorbed on graphene as Ni13 nanocluster with icosahedral geometry. Moreover, glucose was effectively adsorbed on the graphene sheet through Ni13 nanoclusters and the adsorption energy was large in the presence of GelMA.

In this study, we used a facile approach to decorate Ni nanoparticles (Ni-NPs) with ethylene glycol followed by their adsorption on reduced graphene oxide (rGO) nanosheets for the development of enzyme free electrochemical glucose sensor. The effect of Ni-NP content (0.14, 0.28, 0.42, and 0.56 wt.%) on electrochemical properties and the molecular interaction of glucose with Ni-NPs/rGO in glucose sensing were evaluated. Ni-NPs/rGO showed significantly higher electrochemical performance for the glucose oxidation in alkaline solution compared to rGO. Specifically, 0.42% Ni-NPs/rGO revealed excellent performance for glucose determination with a wide linear range (0.25 μM to 1200 μM), a highly reproducible response and long-term stability with the calculated detection limit of 0.01 μM. Moreover, the reliability of the Ni-NPs/rGO sensor was confirmed by evaluating glucose concentration in human blood serum. Ab initio calculations were employed to reveal molecular interactions of glucose with Ni-NPs/rGO. The results showed Ni 13 cluster with icosahedral geometry facing the graphene sheet is the most stable structure. Furthermore, the glucose molecule was able to adsorb on the graphene sheet through Ni 13 nanoclusters and the adsorption process is closely dependent on the orientation of Ni 13 nanoclusters.

We investigated the relative stability and structural properties of Al-Cu intermetallic compounds using the density-functional theory (DFT) with different approximations. We reveal the importance of the on-site Coulomb repulsion of Cu 3d electrons on the energetics and structural properties of Al-Cu compounds of free-electron nature. The finite-temperature effect was included by accounting for the vibrational free energy within the Debye model. The present study shows that θ-Al2Cu is the ground state phase, agreeing with the experimental observations in the literature. The DFT + U approach could be useful to predict accurate formation energies of other Cu-containing precipitates in high-strength Al-alloys.

Practical and reliable methods for theoretically determining the properties of B, C, N, and O in bcc iron have been explored by systematic DFT calculations. The energies of solution and migration, and the elastic strain fields due to the solute atom have been evaluated by supercell calculations under various conditions. By applying correction for spurious elastic interaction of the solute atom with its images in the periodic supercells, reasonable estimates of the solution energy have been obtained without employing very large supercells. The correction turned out unimportant for the migration energy, as it modifies the energies of the stable position and the saddle-point similarly. The lambda tensor, which uniquely characterizes the strain field induced by a solute atom, has been evaluated for the four species of various configurations, first through the force-dipole tensor obtained in zero-strain calculations, and second from changes in supercell dimensions in zero-stress calculations. The two procedures give similar results that typically differ by a few per cent from each other. The computed values for C and N in octahedral interstitial sites are comparable to experimental values. When experimental data become available for B and O, these evaluations will resolve the as-yet contentious location of these atomic species in bcc iron.

To realize bottom-up design of alloys based on theoretical calculations, the thermodynamic stabilities of phases in Ti binary alloys were estimated by a combination of density functional theory calculations for the internal enthalpy energy, the Bragg-Williams approximation for the mixing entropy contribution, the Debye model for the vibrational free energy, and the Sommerfeld model for the electronic excitation entropy. The special quasirandom structure (SQS) model was used to describe the disordered distribution of the alloying element in the solid solution state. We focused on Ti–Mo, Ti–Nb, Ti–Al, and Ti–Zr binary alloys, which have different phases, such as the α phase in the hexagonal close-packed (hcp) structure and the β phase in the body-centered cubic (bcc) structure, depending on the temperature and alloying element fraction. The elastic constants, bulk modulus, and Poisson's ratios were calculated using a strain energy method. Excitations from the vibrational contribution to the quasi-harmonic Debye approximation were added to the 0 K free energy originally derived from ab initio calculations. The effect of temperature up to 1000 K on phase stability was analyzed. Furthermore, to compare phase stabilities, the free energies of formation were calculated using the ground states of the constituent phases as references. The calculated elastic property indicated the mechanical instability of most bcc Ti–Al and bcc Ti–Zr alloys, hcp Ti–Mo and hcp Ti–Nb at high fraction range. The SQS supercell models showed good agreement in elastic constant, bulk modulus, and Poisson's ratio compared to the previous experimental and theoretical results. Free energy results showed that Mo and Nb are β-phase stabilizers, Al is an α-phase stabilizer, and Zr is a neutral element. As the fraction of the alloying element changed, stabilizing or destabilizing effects were observed under different temperatures. Moreover, the linear relationship between the filling of the d band and phase stability was identified in low temperature range. For the β phase, Mo had a stronger stabilizing effect than Nb; both Mo and Nb destabilized the α phase at low temperatures, whereas high temperatures increased the stability of the α phase and the temperature effect became more significant than the element effect. In the examined temperature range, the α phase Ti–Al alloys were stable at all Al fractions, where the thermal effect was negligible. All the α Ti–Zr alloys in this study had similar stabilities to their constituent phases (hcp Ti and hcp Zr) over a wide temperature range.

The effect of mixed partial occupation of metal sites on the phase stability of the γ-Cr23−xFe x C6 (x = 0–3) carbides is explored as function of composition and temperature. Ab initio calculations combined with statistical thermodynamics approaches reveal that the site occupation of the carbides may be incorrectly predicted when only the commonly used approach of full sublattice occupation is considered. We found that the γ-M23C6 structure can be understood as a familiar sodium chloride structure with positively charged rhombic dodecahedron (M(4a) M12(48h)) and negatively charged cubo-octahedron (M8(32f) C6(24e)) super-ion clusters, together with interstitial metal atoms at the 8c sites. The stability of the partially occupied phase can be easily rationalized on the basis of a super-ion analysis of the carbide phase. This new understanding of γ-M23C6 carbides may facilitate further development of high-chromium heat-resistant steels.

To verify the assumption that the anelastic relaxation effect observed in Ni3Al is due to stress-induced reorientation of antisite Al atoms [Numakura and Nishi, Mater. Sci. Eng. A 442 (2006) 59-62], the magnitudes of the anisotropic distortion produced by the intrinsic point defects have been evaluated by ab initio calculations. The anisotropy of the λ tensor (the strain per unit concentration of a particular defect) for the two candidate defect species, namely a Ni vacancy and an antisite Al atom, has been computed by full structure optimization of a supercell containing a single point defect: the difference in the principal values is +0.46 and −1.12, respectively. The relaxation strength estimated for antisite Al atoms agrees fairly well with experiment, while that for Ni vacancies is far too small because of their much lower concentration. The relaxation is, therefore, conclusively attributed to antisite Al atoms.

An original approach accounting for multiple cavity occupancy, host lattice relaxation and the description of the quantum nature of guest behaviour has been used for the estimation of the thermodynamic properties of pure hydrogen and binary C2H6 + H2 hydrates with the possibility of multiple filling of cavities by guest molecules. It has been found that the pure hydrogen cubic structure II (CS-II) hydrate is more thermodynamically stable than the cubic structure I (CS-I) hydrate in a wide range of p–T regions. However, at low pressure, the stabilisation of the CS-I hydrate can be realised for H2–C2H6–H2O systems even with small concentrations of ethane in the gas phase. However, in this case, the amount of stored hydrogen strongly depends on the ethane concentrations in the gas phase. At low concentration of ethane, the amount of hydrogen stored, 2.5 wt%, in CS-I hydrate can be achieved at T = 250 K. We believe that the present approach can be useful for understanding the thermodynamic properties of the b...

In the present study, we report the results of a systematic investigation of cage-like water structures using the first-principles calculations. These results show that, in the case of methane hydrate, the following nucleation mechanism can be revealed. The formation of small water cavities filled with methane is the first step of the formation of methane hydrate. It is not necessary to occupy all dodecahedral cages by guest molecules. After that small cavities start to form the H-bonding network with surrounding water molecules and a small number of water molecules is enough for the formation of a stable hydrogen-bonding network. The structural information contained in such nuclei is conserved in the forming crystal. Moreover, the presence of a methane molecule between small cages is also important to prevent the adhesion of cavities. It found that the ozone molecule can also stabilize the small cage since the value of the interaction energy between the ozone guest and the water host framework is very close to that obtained for the methane case. However, ozone affects the structure of large cavities and hence, the second guest is necessary to stabilize the hydrate structure.

The thermodynamic properties of propane clathrate hydrate with cubic structure IV were studied using a method based on the solid solution theory of van der Waals and Platteeuw but allows one to take into account the influence of guest molecules on the host lattice and guest-guest interactions. The free energies, equations of state, and chemical potentials of this hydrate were estimated using this approach. The proposed theory was used for construction of “guest gas–hydrate–ice I h ” equilibrium curves of propane hydrates. It was found that the water framework of structure IV was dynamically stable and that the fully C3H8 filled structure was thermodynamically stable in the region of pressure from 43 to 50 MPa as compared with the hexagonal ice. The formation pressure of propane hydrate with structure IV is higher than that of propane hydrate with cubic structure II. However, a structural transformation from structure II to IV of propane hydrate was estimated under a pressure of 78 MPa at 290 K.

Single crystal elastic constants of bcc iron and bct FeC and FeN alloys, martensites, have been evaluated by ab initio calculations based on the density functional theory. The energy of a strained crystal has been computed using the supercell method at several values of the strain intensity, and the stiffness coefficient has been determined from the slope of the energy versus square of strain relation. Some of the third order elastic constants have also been evaluated. The absolute magnitudes of the calculated values for bcc iron are in fair agreement with experiment, including the third order constants, although the computed elastic anisotropy is much weaker than measured. The tetragonally distorted dilute FeC and FeN alloys exhibit lower stiffness than bcc iron, particularly in the tensor component C33, while the elastic anisotropy is virtually the same. Average values of elastic moduli for polycrystalline aggregates are also computed. Youngs modulus and the rigidity modulus, as well as the bulk modulus, are decreased by about 10 per cent by the addition of C or N to 3.7 atomic per cent, which agrees with the experimental data for FeC martensite., 10 pages,12 figures, and 6 tables