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Chemically complex multicomponent alloys have garnered widespread interest owing to their exceptional properties coming from a sheer inexhaustible compositional space. The complexity poses severe challenges for atomistic modelling and interatomic potential development. Here, we explore the limits of two complementary state-of-the-art machine-learned potentials—the moment tensor potential (MTP) and the Gaussian moment neural network (GM-NN)—in simultaneously describing both the configurational and vibrational degrees of freedom in the prototype Ta-V-Cr-W alloy family. Both models are equally accurate with exceptional performance in comparison to classical potentials. A single potential is able to achieve root-mean-square-errors of 1.37-4.35 meV/atom and 0.023-0.057 eV/Å in 0 K energies and forces, respectively, across all subsystems of the alloy family, and 0.156-0.179 eV/Å in high-temperature molecular dynamics forces for the disordered quaternary. MTPs achieve faster convergence with the training size than the GM-NNs, whereas GM-NNs are faster in execution. Active learning is partially beneficial and has to be complemented with a conventional human-based training set generation.

Using a comprehensive set of recently published experimental results for training and validation, we have developed computational models appropriate for simulations of aqueous solutions of poly(ethylene oxide) alkyl ethers, an important class of micelle-forming nonionic surfactants, usually denoted CnEm. These models are suitable for use in simulations that employ a moderate amount of coarse graining and especially for dissipative particle dynamics (DPD), which we adopt in this work. The experimental data used for training and validation were reported earlier and produced in our laboratory using dynamic light scattering (DLS) measurements performed on 12 members of the CnEm compound family yielding micelle size distribution functions and mass-weighted mean aggregation numbers at each of several surfactant concentrations. The range of compounds and quality of the experimental results were designed to support the development of computational models. An essential feature of this work is that all simulation results were analyzed in a way that is consistent with the experimental data. Proper account is taken of the fact that a broad distribution of micelle sizes exists, so mass-weighted averages (rather than number-weighted averages) over this distribution are required for the proper comparison of simulation and experimental results. The resulting DPD force field reproduces several important trends seen in the experimental critical micelle concentrations and mass-averaged mean aggregation numbers with respect to surfactant characteristics and concentration. We feel it can be used to investigate a number of open questions regarding micelle sizes and shapes and their dependence on surfactant concentration for this important class of nonionic surfactants.

The need to reduce the energy consumed and the carbon footprint generated by firing ceramics has stimulated research to develop consolidation techniques operating at lower temperatures, ideally not...

Cold sintering is an innovative low-temperature processing technique which allows consolidation of several ceramics. Despite recent research activities on the cold sintering of functional and struc...

Using a comprehensive set of recently published experimental results for training and validation, we have developed computational models appropriate for simulations of aqueous solutions of poly(ethylene oxide) alkyl ethers, an important class of micelle- forming nonionic surfactants, usually denoted CnEm. These models are suitable for use in simulations that employ a moderate amount of coarse graining and especially for dissipative particle dynamics (DPD), which we adopt in this work.The experimental data used for training and validation were reported earlier and produced in our laboratory using dynamic light scattering (DLS) measurements per- formed on twelve members of the CnEm compound family yielding micelle size dis- tribution functions and mass weighted mean aggregation numbers at each of several surfactant concentrations. The range of compounds and quality of the experimental results were designed to support the development of computational models. An es- sential feature of this work is that all simulation results were analysed in a way that is consistent with the experimental data. Proper account is taken of the fact that a broad distribution of micelle sizes exists, so mass weighted averages (rather than num- ber weighted averages) over this distribution are required for the proper comparison of simulation and experimental results.The resulting DPD force field reproduces several important trends seen in the exper- imental critical micelle concentrations and mass averaged mean aggregation numbers with respect to surfactant characteristics and concentration. We feel it can be used to investigate a number of open questions regarding micelle sizes and shapes and their dependence on surfactant concentration for this important class of nonionic surfactants.

Polycrystalline Fe2AlB2 bulk including minor Al2O3 is synthesized by reactive hot pressing from Fe, Al, and B powders at 1200 °C and 30 MPa for 30 min, with a relative density of 96%. The present approach enables a markedly reduced holding time compared with previous studies. The derived Fe2AlB2 shows an electrical resistivity of 2.27±0.01 μΩ·m, Vickers hardness of 10.2±0.2 GPa, flexural strength of 232±25 MPa, compressive strength of 2101±202 MPa, fracture toughness of 5.4±0.2 MPa·m1/2 and work of fracture of 117±12 J/m2. No dominant indentation cracks are observed, indicating that Fe2AlB2 may be quite damage tolerant. Interestingly, a non-catastrophic failure is present in the SENB test, with a high work of fracture. The energy-absorbing mechanisms in inhibiting crack formation are delamination and pullout of Fe2AlB2 grains. This article is protected by copyright. All rights reserved.

Density functional theory is used to provide theoretical insights into the ternary nanolaminated and layered transition metal boride (MAB phase) of MoAlB, with calculations of crystal structure, electronic structure, lattice dynamics and elastic properties, including a corresponding hypothetical MAX phase compound Mo2AlC for comparison. The calculated atomic configuration matches well with experiment. The metal-like electronic structure contributes to the physical origin of the high electrical conductivity of MoAlB. Strong covalent bonding is present between the B atoms, as well as between the Mo and B atoms, and significantly the much weaker Al Al bonds are consistent with the high fracture toughness and damage tolerance seen in MoAlB. With increasing pressure, the shrinkage is highest along the b axis, and lowest along the c axis. From the calculated second-order elastic constants, the bulk moduli B, shear moduli G, Young's moduli E and Poisson ratio μ are 207 GPa, 137 GPa, 336 GPa and 0.23, respectively. The G/B ratio of 0.66—similar in magnitude to values in MAX phases –demonstrate similarities in properties between MAB and MAX phases. Lattice dynamics are examined in detail, with 9 Raman-active modes and 6 infrared-active modes identified and analyzed in terms of their atomic motion and wavenumbers.

The thermal shock behavior of Ti2AlC synthesized by means of self-propagating high temperature combustion synthesis with pseudo hot isostatic pressing is investigated, with a focus on the effect of the quenching temperature and quenching times. In general, Ti2AlC exhibits a better thermal shock resistance than typical brittle ceramics like Al2O3. Although the flexural strength decreases quickly in the temperature range of 300-500 °C, no discontinuous decrease in the retained strength is observed in Ti2AlC which, as with other MAX phases, differs from the behavior of typical brittle ceramics. Overall, the initial strength (grain size) plays a determining role in the thermal shock behavior of Ti2AlC and other MAX phases. On increasing quench times to 5 cycles, the retained flexural strength decreases further, however with a lower rate of decrease compared to the first quench. Quenching at 300 °C and above, voids after the pullout of grains and cracks are present, which however are absent in the un-quenched samples, indicating the weakening of bonding among grains and the induced damage around the grain boundary during the thermal shock. This article is protected by copyright. All rights reserved.

We present an approach that enables an efficient and accurate study of dynamically unstable crystals over the full temperature range. The approach is based on an interatomic potential fitted to ab initio molecular dynamics energies for both the high- and low-temperature stable phases. We verify by comparison to explicit ab initio simulations that such a bespoke potential, for which we use here the functional form of the embedded atom method, provides accurate transformation temperatures and atomistic features of the transformation. The accuracy of the potential makes it an ideal tool to study the important impact of finite size and finite time effects. We apply our approach to the dynamically unstable $\ensuremath{\beta}$ (bcc) titanium phase and study in detail the transformation to the low-temperature stable hexagonal $\ensuremath{\omega}$ phase. We find a large set of previously unreported linear-chain disordered (LCD) structures made up of three types of ${[111]}_{\ensuremath{\beta}}$ linear-chain defects that exhibit randomly disordered arrangements in the ${(111)}_{\ensuremath{\beta}}$ plane.

We wished to compile a data set of results from the experimental literature to support the development and validation of accurate computational models (force fields) for an important class of micelle-forming nonionic surfactant compounds, the poly(ethylene oxide) alkyl ethers, usually denoted C

We have simulated the ultrahigh-temperature ceramic zirconium carbide (ZrC) in order to predict electron and\ud phonon scattering properties, including lifetimes and transport. Our predictions of heat and charge conductivity, which extend to 3000 K, are relevant to extreme-temperature applications of ZrC. Mechanisms are identified on a first-principles basis that considerably enhance or suppress heat transport at high temperature, including strain, anharmonic phonon renormalization, and four-phonon scattering. The extent to which boundary confinement and isotope scattering effects lower thermal conductivity is predicted.

We propose an approach to calculate the anharmonic part of the volumetric-strain and temperature-dependent free energy of a crystal. The method strikes an effective balance between accuracy and computational efficiency, showing a $\ifmmode\times\else\texttimes\fi{}10$ speedup on comparable free energy approaches at the level of density functional theory, with average errors less than 1 meV/atom. As a demonstration we make predictions on the thermodynamics of substoichiometric ${\mathrm{ZrC}}_{x}$, including vacancy concentration and heat capacity.

To understand the potential for use of the Hf–Al–C ternary compounds, (HfC)nAl3C2 (Hf2Al3C4 and Hf3Al3C5) and (HfC)nAl4C3 (Hf2Al4C5 and Hf3Al4C6) were investigated using density functional theory, including crystal structure, electronic structure, compressibility, and elastic properties. The theoretical density of (HfC)nAl3C2 (4.10–4.16 g/cm3) is higher than that of (HfC)nAl4C3 (3.92–3.98 g/cm3), due to the smaller number of lighter Al–C layers. With increasing numbers of Hf–C layers, the Hf–C and Al–C bond lengths remain almost unchanged. In none of the compounds is there a gap around the Fermi energy (Ef), which implies they are metal-like conductors. With increasing pressure, there is greater shrinkage along the c axis than the a axis. The bond stiffness increases with increasing pressure. In general, (HfC)nAl3C2 has higher elastic stiffness than (HfC)nAl4C3, with the moduli increasing with the number of Hf–C layers. The Hf–Al–C compounds as well as the brittle Zr–Al–C compounds all have low shear moduli/bulk moduli ratio (G/B) from 0.71 to 0.78, suggesting that the G/B ratio is not always a suitable measure of ductility.

The growth of high In content InGaN with sufficiently high crystal quality is challenging due to the differences in the GaN and InN thermodynamics. The surprisingly different thermodynamics is due to a complex competition between strain and chemistry and mediated by the different indium and gallium atomic radii as well as their different bonding enthalpies with nitrogen. In the present work, we investigate bulk and surface thermodynamics of molecular beam epitaxial (MBE) growth of InxGa1−xN for the technologically relevant (0001) and growth planes by means of density functional theory calculations. Our calculations confirm that coherent growth fully suppresses phase separation through spinodal decomposition. However, the biaxial strain is found to have a marginal effect on the critical temperatures for InxGa1−xN decomposition. Furthermore, the thermal stability of excess indium is found to be remarkably higher on N-polar surfaces than on the Ga-polar surfaces. Phase diagram of In covered (a) Ga-polar and (b) N-polar surfaces as function of In pressure.

unrelaxed structures are easily reproducible, enabling them to be used by other authors to readily test potential formalisms other than those considered in this work. Specifically, we will perform our calculations on unrelaxed bcc � -iron (ferrite) with interstitial carbon. This will enable us to make a contribution to the ongoing debate as to how the ferritic phase of steel might be best represented using a semi-empirical potential. 3–6) This is an important topic because many properties of steel originate from the interaction of carbon (and nitrogen) with point and extended defects. These interactions must be treated using a semiempirical or empirical approach, since the numbers of atoms involved in such interactions place them well out of reach of current ab-initio methods such as density functional theory (DFT). 7,8)

Thermodynamic properties of ZrC are calculated up to the melting point (${T}^{\mathrm{melt}}\ensuremath{\approx}3700\phantom{\rule{0.28em}{0ex}}\text{K}$), using density functional theory (DFT) to obtain the fully anharmonic vibrational contribution, and including electronic excitations. A significant improvement is found in comparison to results calculated within the quasiharmonic approximation. The calculated thermal expansion is in better agreement with experiment and the heat capacity reproduces rather closely a CALPHAD estimate. The calculations are presented as an application of a development of the upsampled thermodynamic integration using Langevin dynamics (UP-TILD) approach. This development, referred to here as two-stage upsampled thermodynamic integration using Langevin dynamics (TU-TILD), is the inclusion of tailored interatomic potentials to characterize an intermediate reference state of anharmonic vibrations on a two-stage path of thermodynamic integration between the original DFT quasiharmonic free energy and the fully anharmonic DFT free energy. This approach greatly accelerates the convergence of the calculation, giving a factor of improvement in efficiency of $\ensuremath{\sim}50$ in the present case compared to the original UP-TILD approach, and it can be applied to a wide range of materials.

MEAMfit v1.02. Changes: various bug fixes; speed of single-shot energy and force calculations (not optimization) increased by × 10 ; elements up to Cn ( Z = 112 ) now correctly read from vasprun.xml files; EAM fits now produce Camelion output files; changed max number of vasprun.xml files to 10,000 (an unnecessary lower limit of 10 was allowed in the previous version). New version program summary Program title: MEAMfit Catalogue identifier: AEWY_v1_1 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEWY_v1_1.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: BSD 2-Clause License No. of lines in distributed program, including test data, etc.: 5202334 No. of bytes in distributed program, including test data, etc.: 66637698 Distribution format: tar.gz Programming language: Fortran. Computer: Linux based workstations. Operating system: Linux. RAM: 120 MB Classification: 16.1. Catalogue identifier of previous version: AEWY_v1_0 Journal reference of previous version: Comput. Phys. Comm. 196 (2015) 439 External routines: TOMS611 Unconstrained Minimization [1] included in the MEAMfit code. Does the new version supersede the previous version?: Yes Nature of problem: Fitting embedded atom method (EAM) and reference-free modified embedded atom method (RF-MEAM) potentials [2–3] to energies and forces produced by VASP [4–7]. Solution method: A computer program is presented which uses a conjugate-gradient minimizer paired with a genetic algorithm to fit EAM and RF-MEAM potentials to energies and/or atomic forces read directly from VASP output files. Potentials produced by the code are directly usable with the LAMMPS [8] or Camelion [9] molecular-dynamics packages. Reasons for new version: Bugs fixed; improvements made. Summary of revisions: Various bug fixes; speed of single-shot energy and force calculations (not optimization) increased by × 10 ; elements up to Cn ( Z = 112 ) now correctly read from vasprun.xml files; EAM fits now produce Camelion output files; changed max number of vasprun.xml files to 10,000 (an unnecessary lower limit of 10 was allowed in the previous version). Additional comments: User manual provided. !!!!! The distribution file for this program is over 65 MB and therefore is not delivered directly when download or Email is requested. Instead a html file giving details of how the program can be obtained is sent. !!!!! Running time: The run-time depends on the required level of accuracy of the final potential. For an EAM potential fit to 670 energies, a few hours on a single core is usually sufficient to produce a potential with R = 12 % – 13 % (see Equation. 9 in main-text for definition). To ensure a maximally optimized potential however ( R = 12 % ), a run-time of 24 h is recommended. To optimize a RF-MEAM potential, a further 24 h should be allowed. One will already find an improvement over the EAM using just a single core, however to ensure a maximally optimized potential, one should run several instances of MEAMfit in parallel. References: [1] J. E. Dennis, D. Gay and R. E. Welsch, ACM Trans. on Math. Soft., 7 (1981) 348–368. [2] M. I. Baskes, Materials Science and Engineering A, 261 (1999), 165. [3] M. Timonova and B. J. Thijsse, Modelling Simul. Mater. Sci. Eng., 19 (2011) 015003 [4] G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558. [5] G. Kresse, J. Hafner, Phys. Rev. B 49 (1994) 14251. [6] G. Kresse, J. Furthmuller, Comput. Mat. Sci. 6 (1996) 15. [7] G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169. [8] S. Plimpton, J. Comp. Phys. 117 (1995) [9] http//://tinyurl.com/camelion11-53

The incorporation of In into the technologically relevant $(0001)$ (Ga-polar) and $(000\overline{1})$ (N-polar) surfaces of ${\mathrm{In}}_{0.25}{\mathrm{Ga}}_{0.75}\mathrm{N}$ is investigated using density functional theory. The cases of coherent pseudomorphic growth on GaN and on lattice-matched heterointerfaces are considered. For pseudomorphic growth on GaN, In incorporation into the ${0001}$ surface layers is limited to a tiny growth window corresponding to extreme In-rich growth conditions and at the In-rich/Ga-poor region of the metal chemical potentials. Lattice-matched growth, however, allows for a wider growth window. Surface phase diagrams are constructed as a function of growth conditions and reveal similarities between the two polar growth planes. However, a strong driving force is found for segregation of In atoms to the first III-N layer for Ga-polar growth, but not for N-polar growth. The former was found to be mainly due to chemical effects (stronger Ga-N as compared to In-N bonds), absent in the case of N-polar growth. Furthermore, finite-temperature calculations show that In incorporated into the first III-N layer is stable to $\ensuremath{\approx}150$ K higher temperatures in the N-polar surface than in the Ga-polar surface, indicating that for a given level of In incorporation, higher temperatures can be used for N-polar growth as compared to Ga-polar growth.

Table III and Fig. 7 should be replaced with those listed below. Text on page 6 should read starting with the last sentence of the first paragraph that ends on p. 6 : Just 2 at. % Si substitutionally dissolved in bcc Fe reduces the C diffusivity by 39% at 500 K and by 14% at 1000 K. There are two mechanisms by which Si reduces the diffusivity of C: a the first and second neighbor shells around Si are so high in energy that C is blocked from these sites, this reduces the number of positions through which C can diffuse labyrinth mechanism ; and b at slightly greater distances C gets weakly trapped around Si so that it remains immobilized for some time, depending on the temperature trapping mechanism14 . The KMC simulations allow us to precisely pinpoint the contribution of each mechanism. When the attractive Si-C interactions in the third through sixth shells are set to zero, the C diffusion in Fe with 2 at. % Si is reduced by 10% both at 500 K and at 1000 K. This indicates that at 1000 K the labyrinth mechanism is the most important. When, instead simulations are repeated whereby the repulsive Si-C interactions in the first and second shells are set to zero, diffusivity in Fe with 2 at. % Si is reduced by 35% at 500 K and by 8% at 1000 K. This indicates that at 500 K the trapping mechanism is dominant, but that at 1000 K labyrinth and trapping mechanisms contribute about equally. The last lines in the Sect. IV. SUMMARY should read: The Si-C interaction is reflected also in the influence of Si on the C diffusivity: I at high temperatures of about 1000 K, the strong repulsion between C and Si in the first and second neighbor shells plays an important role. This causes a mild reduction in the C diffusivity because there are fewer diffusion paths as C is blocked from sites very close to Si. II at lower temperatures of about 500 K, the energetically weaker attraction plays a dominant role in reducing the diffusivity of C through entrapment of C at a large number of interstitial sites in the vicinity of Si. At 500 K, 1 at. % of Si in solution may reduce the diffusivity by as much as 22% relative to Si-free bcc Fe.

The interaction of interstitial carbon with substitutional silicon and the effect of this interaction on the diffusion of carbon within body-centered-cubic iron, are computed using electronic density-functional theory. Both the activation energy for diffusion and the diffusion prefactor are predicted. Good agreement is found for those cases where a comparison with experimental data is possible.

A Hydrogen atom immersed in a finite jellium sphere is solved using variational quantum Monte Carlo (VQMC). The same system is also solved using density functional theory (DFT), in both the local spin density (LSDA) and self-interaction correction (SIC) approximations. The immersion energies calculated using these methods, as functions of the background density of the jellium, are found to lie within 1eV of each other with minima in approximately the same positions. The DFT results show overbinding relative to the VQMC result. The immersion energies also suggest an improved performance of the SIC over the LSDA relative to the VQMC results. The atom-induced density is also calculated and shows a difference between the methods, with a more extended Friedel oscillation in the case of the VQMC result.

Based on the evaluation of lattice parameter maps in aberration corrected high resolution transmission electron microscopy images, we propose a simple method that allows quantifying the composition and disorder of a semiconductor alloy at the unit cell scale with high accuracy. This is realized by considering, next to the out-of-plane, also the in-plane lattice parameter component allowing to separate the chemical composition from the strain field. Considering only the out-of-plane lattice parameter component not only yields large deviations from the true local alloy content but also carries the risk of identifying false ordering phenomena like formations of chains or platelets. Our method is demonstrated on image simulations of relaxed supercells, as well as on experimental images of an In0.20Ga0.80N quantum well. Principally, our approach is applicable to all epitaxially strained compounds in the form of quantum wells, free standing islands, quantum dots, or wires.

The unique and unanticipated properties of multiple principal component alloys have reinvigorated the field of alloy design and drawn strong interest across scientific disciplines. The vast compositional parameter space makes these alloys a unique area of exploration by means of computational design. However, as of now a method to compute efficiently, yet with high accuracy the thermodynamic properties of such alloys has been missing. One of the underlying reasons is the lack of accurate and efficient approaches to compute vibrational free energies—including anharmonicity—for these chemically complex multicomponent alloys. In this work, a density-functional-theory based approach to overcome this issue is developed based on a combination of thermodynamic integration and a machine-learning potential. We demonstrate the performance of the approach by computing the anharmonic free energy of the prototypical five-component VNbMoTaW refractory high entropy alloy.

With density functional theory we have performed molecular dynamics simulations of ZrC, which displayed spontaneous carbon Frenkel pair formation at a temperature of $3200\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, some 500\ifmmode^\circ\else\textdegree\fi{} below the melting point. To understand this behavior, rarely seen in equilibrium simulations, we quenched and examined a set of lattices containing a Frenkel pair. Five metastable structures were found, and their formation energies and electronic properties were studied. Their thermal generation was found to be facilitated by a reduction of between 0.7 and 1.5 eV in formation energy due to thermal expansion of the lattice. With input from a quasiharmonic description of the defect-free energy of formation, an ideal solution model was used to estimate lower bounds on their concentration as a function of temperature and stoichiometry. At 3000 K (0.81 of the melting temperature) their concentration was estimated to be 1.2% per mole in a stoichiometric crystal, and 0.3% per mole in a crystal with 10% per mole of constitutional vacancies. Their contribution to heat capacity, thermal expansion, and bulk modulus was estimated.