Abstract: Here we present new neural network potentials capable of accurately modeling the transformations between the α, β, and ω phases of titanium (Ti) and zirconium (Zr), including accurate prediction of the equilibrium phase diagram. The potentials are constructed based on the rapid artificial neural network (RANN) formalism which bases its structural fingerprint on the modified embedded atom method. This implementation allows the potential to reproduce density functional theory results including elastic and plastic properties, phonon spectra, and relative energies of each of the three phases at classical molecular dynamics (MD) speeds. Transitions between each of the phase pairs are observed in dynamic simulation and, using calculations of the Gibbs free energy, both potentials are shown to accurately predict the experimentally observed phase transformation temperatures and pressures over the entire phase diagram. The calculated triple points are 8.67 GPa and 1058 K for Ti and 5.04 GPa and 988.35 K for Zr, close to their experimentally observed values. The mechanism of transformation is also observed for each phase pair. The neural network potentials can be used to further investigate the behavior of each phase and their interaction.

Abstract: Zr alloys are commonly used materials for in-core components of pressurized water reactors. The evolution of defects in Zr under irradiation conditions will directly contribute to the degradation of reactor components. For a better understanding of the evolution process of defects in Zr, a multiscale simulation was performed to study the diffusion behavior of radiation-induced point defects. Because the existing potentials cannot accurately describe the energy differences of stable states self-interstitial atoms (SIAs) in Zr, an embedded atom method potential that can exactly reproduce the energy difference between metastable states and ground states, and the binding energy of divacancy of Zr, was first developed by fitting a potential to point defect properties calculated by density functional theory and other basic crystal properties. Based on the constructed potential, molecular dynamics (MD) and kinetic Monte Carlo simulations were conducted to investigate the migration of point defects. We determined several frequent migration paths of SIAs by kinetic study, which is rarely reported in previous MD studies. The SIA exhibits obvious anisotropic diffusion characteristics at low temperatures (<600K). The most frequent migration path for SIAs is the jump between the two nearest basal octahedral (BO) sites in the basal plane, which means that SIAs diffuse faster along the basal plane than along the c axis. It is found that diffusion within the basal plane tends to be two-dimensional (2D) diffusion rather than 1D diffusion, and there is a significant correlation effect for diffusion of SIAs along the basal plane. Additionally, the existence of trivacancy-SIA complex was found in the process of divacancy migration, which can inhibit divacancy migration. Monovacancies and divacancies exhibit anisotropic diffusion characteristics in the considered temperature range Divacancies have a much faster diffusion rate than monovacancies in the present MD simulation and can easily dissociate at high temperatures (>900 K). The rapid migration of the divacancies may contribute to the formation of vacancy dislocation loops. These results are meaningful to understand the evolution process of radiation-induced defects.

Abstract: In recent years, some 30 studies have been published on the molecular dynamics (MD) of zirconium, primarily of its twinning deformation and response to radiation damage. Its low thermal neutron absorption makes it uniquely suited for the latter application. Surprisingly, currently used interatomic potentials do not encapsulate the unique properties of Zr, namely its high stacking-fault energy, anomolous self-diffusion, melting and phase transformation under temperature and pressure (or alloying). Ab initio calculations have shown deficiencies in the description of point defects, both vacancies and interstitials, using existing interatomic potentials, deficiencies that can now be rectified by refitting. Here, we show the calculation of phase transitions self-consistently and present a potential for Zr that correctly reproduces the energetics of our extended database of ab initio configurations and high-temperature phase transitions. The potential has an analytic many-body form, making it suitable for existing large-scale MD codes. We also present a best-fit potential for the hcp structure and its defects.

Abstract: In recent years, some 30 studies have been published on the molecular dynamics (MD) of zirconium, primarily of its twinning deformation and response to radiation damage. Its low thermal neutron absorption makes it uniquely suited for the latter application. Surprisingly, currently used interatomic potentials do not encapsulate the unique properties of Zr, namely its high stacking-fault energy, anomolous self-diffusion, melting and phase transformation under temperature and pressure (or alloying). Ab initio calculations have shown deficiencies in the description of point defects, both vacancies and interstitials, using existing interatomic potentials, deficiencies that can now be rectified by refitting. Here, we show the calculation of phase transitions self-consistently and present a potential for Zr that correctly reproduces the energetics of our extended database of ab initio configurations and high-temperature phase transitions. The potential has an analytic many-body form, making it suitable for existing large-scale MD codes. We also present a best-fit potential for the hcp structure and its defects.

Abstract: In recent years, some 30 studies have been published on the molecular dynamics (MD) of zirconium, primarily of its twinning deformation and response to radiation damage. Its low thermal neutron absorption makes it uniquely suited for the latter application. Surprisingly, currently used interatomic potentials do not encapsulate the unique properties of Zr, namely its high stacking-fault energy, anomolous self-diffusion, melting and phase transformation under temperature and pressure (or alloying). Ab initio calculations have shown deficiencies in the description of point defects, both vacancies and interstitials, using existing interatomic potentials, deficiencies that can now be rectified by refitting. Here, we show the calculation of phase transitions self-consistently and present a potential for Zr that correctly reproduces the energetics of our extended database of ab initio configurations and high-temperature phase transitions. The potential has an analytic many-body form, making it suitable for existing large-scale MD codes. We also present a best-fit potential for the hcp structure and its defects.

Abstract: Semiempirical interatomic potentials for hcp elements, Ti and Zr, have been developed based on the MEAM (modified embedded-atom method) formalism. The new potentials do not cause the stability problem previously reported in MEAM for hcp elements, and describe wide range of physical properties (bulk properties, point defect properties, planar defect properties, and thermal properties) of pure Ti and Zr, in good agreement with experimental information. The applicability of the potentials to atomistic approaches for investigation of various materials behavior (slip, irradiation, amorphous behavior, etc.) in Ti or Zr-based alloys is demonstrated by showing that the related material properties are correctly reproduced using the present potentials and that the potentials can be easily extended to multicomponent systems.

Abstract: Recent molecular dynamics simulations of the growth of [Ni0.8Fe0.2/Au] multilayers have revealed the formation of misfit-strain-reducing dislocation structures very similar to those observed experimentally. Here we report similar simulations showing the formation of edge dislocations near the interfaces of vapor-deposited (111) [NiFe/CoFe/Cu] multilayers. Unlike misfit dislocations that accommodate lattice mismatch, the dislocation structures observed here increase the mismatch strain energy. Stop-action observations of the dynamically evolving atomic structures indicate that during deposition on the (111) surface of a fcc lattice, adatoms may occupy either fcc sites or hcp sites. This results in the random formation of fcc and hcp domains, with dislocations at the domain boundaries. These dislocations enable atoms to undergo a shift from fcc to hcp sites, or vice versa. These shifts lead to missing atoms, and therefore a later deposited layer can have missing planes compared to a previously deposited layer. This dislocation formation mechanism can create tensile stress in fcc films. The probability that such dislocations are formed was found to quickly diminish under energetic deposition conditions.

Abstract: A many-body interatomic potential has been developed for the h.c.p. metal α-zirconium using the same methodology as that used by Ackland for α-titanium. The repulsive pair part of the potential has been constructed so that the model can be employed for simulating atomic collisions. The favoured self-interstitial configurations are the 〈1120〉 crowdion and split defects, and they are highly mobile in the basal plane. The energy of surfaces is not strongly dependent on the crystallographic orientation, and the I2 stacking fault on the basal plane is not stable. The displacement threshold energy in a crystal at 0 K exhibits a similar orientation dependence to that computed recently for α-titanium by Bacon et al. and has the same minimum of 27·5 eV along the 〈1120〉 directions, but the mean value of 55 eV averaged over all orientations is higher than that of 30 eV in titanium.

Abstract: Recent molecular dynamics simulations of the growth of [Ni0.8Fe0.2/Au] multilayers have revealed the formation of misfit-strain-reducing dislocation structures very similar to those observed experimentally. Here we report similar simulations showing the formation of edge dislocations near the interfaces of vapor-deposited (111) [NiFe/CoFe/Cu] multilayers. Unlike misfit dislocations that accommodate lattice mismatch, the dislocation structures observed here increase the mismatch strain energy. Stop-action observations of the dynamically evolving atomic structures indicate that during deposition on the (111) surface of a fcc lattice, adatoms may occupy either fcc sites or hcp sites. This results in the random formation of fcc and hcp domains, with dislocations at the domain boundaries. These dislocations enable atoms to undergo a shift from fcc to hcp sites, or vice versa. These shifts lead to missing atoms, and therefore a later deposited layer can have missing planes compared to a previously deposited layer. This dislocation formation mechanism can create tensile stress in fcc films. The probability that such dislocations are formed was found to quickly diminish under energetic deposition conditions.

Abstract: In order to investigate the phase separation behavior in Cu–Zr–Ag bulk metallic glasses (BMGs) on an atomic level, a modified embedded-atom interatomic method potential for the Cu–Zr–Ag system has been newly developed. A clear tendency of phase separation of Ag-rich phases could be observed in the supercooled liquid, in reasonable agreement with experimental information. The potential can be used for atomistic investigations of the effects of alloying element Ag on a wide range of amorphous properties of Cu–Zr BMG.

Abstract: In order to investigate the phase separation behavior in Cu–Zr–Ag bulk metallic glasses (BMGs) on an atomic level, a modified embedded-atom interatomic method potential for the Cu–Zr–Ag system has been newly developed. A clear tendency of phase separation of Ag-rich phases could be observed in the supercooled liquid, in reasonable agreement with experimental information. The potential can be used for atomistic investigations of the effects of alloying element Ag on a wide range of amorphous properties of Cu–Zr BMG.

Abstract: Interatomic potentials for the Al-Ti, Al-Ta, Al-Zr, Al-Nb and Al-Hf binary systems have been developed based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. The fundamental materials properties (structural, thermodynamic and elastic behaviors of different intermetallics) could be readily described with the potentials using molecular dynamic simulation (MD), in rational agreements with experimental or first principles data. The potentials are further utilized to develop an interatomic potential for the (TiZrNbHfTa)Al₃ high entropy intermetallic compound (HEIC), which open the door to understand atomic scale behavior of HEICs.

Abstract: Interatomic potentials for the Al-Ti, Al-Ta, Al-Zr, Al-Nb and Al-Hf binary systems have been developed based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. The fundamental materials properties (structural, thermodynamic and elastic behaviors of different intermetallics) could be readily described with the potentials using molecular dynamic simulation (MD), in rational agreements with experimental or first principles data. The potentials are further utilized to develop an interatomic potential for the (TiZrNbHfTa)Al₃ high entropy intermetallic compound (HEIC), which open the door to understand atomic scale behavior of HEICs.

Abstract: We report on the first interatomic potentials for Zirconium Diboride and Hafnium Diboride. The potentials are of the Tersoff form, and are obtained by fitting to a first-principles database of basic properties of elemental Zr, Hf, B, and the compounds ZrB₂ and HfB₂. Two variants of the Zr-B potentials have been obtained, and one for Hf-B. The potentials have been tested against a variety of properties of the compound, with the conclusion that they are stable and provide a reasonable representation of the desired properties of the two diborides.

Abstract: Atomistic Green-Kubo simulations are performed to evaluate the lattice thermal conductivity for single crystals of the ultra high temperature ceramics ZrB₂ and HfB₂. Recently developed interatomic potentials are used for these simulations. Heat current correlation functions show rapid oscillations, which can be identified with mixed metal-Boron optical phonon modes. Results for temperatures from 300K to 1000K are presented.

Abstract: A combination of ab initio, atomistic, and finite element methods (FEM) was used to investigate fundamental properties of grain boundaries and grain boundary networks and their impact on lattice thermal conductivity in the ultra high-temperature ceramic ZrB₂. The structure, energetics, and lattice thermal conductance of certain low energy grain boundaries were studied. Atomic models of these boundaries were relaxed using density functional theory. Information about bonding across the interfaces was determined from the electron localization function. Interfacial thermal conductances were computed using nonequilibrium molecular dynamics. Microstructural models were used to determine the reduction in lattice thermal conductivity due grain boundary networks where FEM meshes were constructed on top of microstructural images.

Abstract: Tungsten (W) is a material of choice for the divertor material due to its high melting temperature, thermal conductivity, and sputtering threshold. However, W has a very high brittle-to-ductile transition temperature, and at fusion reactor temperatures (≥1000 K), it may undergo recrystallization and grain growth. Dispersion-strengthening W with zirconium carbide (ZrC) can improve ductility and limit grain growth, but much of the effects of the dispersoids on microstructural evolution and thermomechanical properties at high temperatures are still unknown. We present a machine learned Spectral Neighbor Analysis Potential for W-ZrC that can now be used to study these materials. In order to construct a potential suitable for large-scale atomistic simulations at fusion reactor temperatures, it is necessary to train on ab initio data generated for a diverse set of structures, chemical environments, and temperatures. Further accuracy and stability tests of the potential were achieved using objective functions for both material properties and high temperature stability. Validation of lattice parameters, surface energies, bulk moduli, and thermal expansion is confirmed on the optimized potential. Tensile tests of W/ZrC bicrystals show that although the W(110)-ZrC(111) C-terminated bicrystal has the highest ultimate tensile strength (UTS) at room temperature, observed strength decreases with increasing temperature. At 2500 K, the terminating C layer diffuses into the W, resulting in a weaker W-Zr interface. Meanwhile, the W(110)-ZrC(111) Zr-terminated bicrystal has the highest UTS at 2500 K.

Abstract: The fast increase in available computation power allowed us to decrease the cooling rate in molecular dynamics (MD) simulation of vitrification by several orders of magnitude. While the reliability of the MD simulation should obviously benefit from this increase in the computational power, in some cases, it led to unexpected results. In particular, Ryltsev et al. [J. Chem. Phys. 149, 164502 (2018)] found that the most popular potentials for the Cu-Zr and Cu-Zr-Al alloys from Mendelev et al. [Philos. Mag. 89, 967 (2009)] and Cheng et al. [Phys. Rev. Lett. 102, 245501 (2009)] do not actually describe good glass forming systems but in contradiction with experiment predict rather fast crystallization of the Cu_64.5Zr_35.5 alloy which is the well-known example of bulk metallic glasses. In this paper, we present a new Cu-Zr semiempirical potential suitable to simulate vitrification. No crystal nucleation was observed in MD simulation using this potential in the concentration range from 75% to 5% of Zr. Since the new potential leads to about the same liquid structure and viscosity as the Cu-Zr potential from Mendelev et al. [Philos. Mag. 89, 967 (2009)] which failed to describe the good glass formability, our study clearly shows that no reliable conclusions about the glass formability can be deduced based solely on the analysis of the liquid properties and a nucleation/crystal growth study should be performed to address this question.

Abstract: Dislocation nucleation from grain boundaries (GB) can control plastic deformation in nano-crystalline metals under certain conditions, but little is known about what controls dislocation nucleation, because when data from different materials are compared, the variations of many interacting properties tend to obscure the effects of any single property. In this study, we seek clarification by applying a unique capability of semi-empirical potentials in molecular dynamics simulations: the potentials can be modified such that all significant material properties but one, are kept constant. Using a set of potentials developed to isolate the effects of stacking fault energy, we show that for a given grain boundary, loading orientation and strain rate, the yield stress depends linearly on both the stable and unstable stacking fault energies. The coefficients of proportionality depend on the GB structure and the value of the yield stress is related to the density of the E structural units in the GB. While the impact of the stable stacking fault energy is easy to understand, the unstable stacking fault energy requires more elucidation and we provide a framework for understanding how it affects the nucleation and propagation process.

Abstract: We present a new semi-empirical potential suitable for molecular dynamics simulations of liquid and amorphous Cu–Zr alloys. To provide input data for developing the potential, new experimental measurements of the structure factors for amorphous Cu64.5Zr35.5 alloy were performed. In this work, we propose a new method to include diffraction data in the potential development procedure, which also includes fitting to first-principles and liquid density and enthalpy of mixing data. To refine the new potential, we used first-principles and liquid enthalpy of mixing data published earlier combined with the densities of liquid Cu64.5Zr35.5 measured over a range of temperatures. We show that the potential predicts a liquid-to-glass transition temperature that agrees reasonably well with experimental data. Finally, we compare the new potential with two previously developed semi-empirical potentials for Cu–Zr alloys and examine their comparative and contrasting descriptions of structure and properties for Cu64.5Zr35.5 liquids and glasses.

Abstract: A modified embedded-atom method (MEAM) interatomic potential for the Cu–Zr system has been developed based on the previously developed MEAM potentials for pure Cu and Zr. The potential describes fundamental physical properties and alloy behavior of the Cu–Zr binary system reasonably well. The applicability of the potential to atomistic investigations of mechanical and deformation behavior for the Cu–Zr binary and Cu–Zr-based multicomponent amorphous alloys is also demonstrated by showing that fully relaxed and realistic amorphous structures can be generated by molecular dynamics simulations.

Abstract: We propose a method of using atomistic computer simulations to obtain partial pair correlation functions from wide angle diffraction experiments with metallic liquids and their glasses. In this method, a model is first created using a semiempirical interatomic potential and then an additional atomic force is added to improve the agreement with experimental diffraction data. To illustrate this approach, the structure of an amorphous Cu64.5Zr35.5 alloy is highlighted, where we present the results for the semiempirical many-body potential and fitting to x-ray diffraction data. While only x-ray diffraction data were used in the present work, the method can be easily adapted to the case when there are also data from neutron diffraction or even in combination. Moreover, this method can be employed in the case of multicomponent systems when the data of several diffraction experiments can be combined.

Abstract: Glass forming ability has been found in only a small number of binary alloys, one being CuZr. In order to simulate this glass, we fitted an interatomic potential within Effective Medium Theory (EMT). For this purpose we use basic properties of the B2 crystal structure as calculated from Density Functional Theory (DFT) or obtained from experiments. We then performed Molecular Dynamics (MD) simulations of the cooling process and studied the thermodynamics and structure of CuZr glass. We find that the potential gives a good description of the CuZr glass, with a glass transition temperature and elastic constants close to the experimental values. The local atomic order, as witnessed by the radial distribution function, is also consistent with similar experimental data.

Abstract: A comparative study on hydrogen diffusion in amorphous and simple crystalline structures has been carried out using molecular dynamics simulations. The Cu-Zr bulk metallic glass (BMG) system is selected as the model material and a modified embedded-atom method (MEAM) interatomic potential for the Cu-Zr-H ternary system is developed for the atomistic simulation. It is found that the diffusivity of hydrogen in amorphous alloys is lower than that in open structured crystals but higher than that in close-packed crystals. The hydrogen diffusion in amorphous alloys is strongly hydrogen concentration dependent compared to crystals, increasing as the hydrogen content increases, and the Arrhenius plot of hydrogen diffusion in amorphous alloys shows an upward curvature. The reasons to rationalize all the findings are discussed based on the variety of energy state and migration energy barrier for interstitial sites in amorphous alloys.

Abstract: Chemical short-range order (CSRO) in high entropy alloys (HEAs) has attracted interests recently and is believed to be capable for tuning their mechanical properties. However, the characterization of CSRO in HEAs through experimental methods remains challenging. In this work, a modified embedded-atom method interatomic potential with good accuracy for studying CSRO in HfNbTaTiZr alloy system was developed. By employing the potential, molecular dynamic/Monte Carlo simulation was performed to investigate the CSRO in HfNbTaZr HEA. The results indicated that Hf-Zr and Nb-Ta atom pairs were preferred in the BCC solid solution of HfNbTaZr, and a new type of CSRO with topological B2 order was predicted, which can help to understand the mechanical properties of HfNbTaZr HEA. It was also found that forming of CSRO was an incubation process for the precipitation in HfNbTaZr, implying the significance of CSRO on the phase stability or precipitation behavior of HEAs. The findings in the present work can help in understanding CSRO and establishing its relationship with precipitates in HEAs, and more topics related to CSRO and phase stability in HfNbTaTiZr alloy system can be further investigated by atomistic simulation.

Abstract: High Entropy Alloys (HEAs) have attracted much interest over the past 20 years because of their remarkable mechanical properties. Recent works on BCC refractory HEAs have demonstrated high strength even at extreme temperatures with an unusual mix of strength and ductility. They also show excellent strain-hardening behavior. This study focuses on the VNbTaTiZr alloy, which stands out for its favorable qualities including relatively low density, impressive yield strength, and ductility at room temperature. To better understand the atomic behavior and microstructural features inherent to this alloy, a Modified Embedded Atom Method (MEAM) potential is developed, based on first-principles computations. Through accurate modeling of lattice constants, elastic constants, and formation enthalpies, a hybrid Molecular Dynamics/Monte Carlo (MD/MC) simulation of an equimolar VNbTaTiZr refractory HEA was performed to explore the role of local chemical compositions to its mechanical response. The current MEAM potential aligns closely with recent experimental work, validating its effectiveness. Adding Zr to the VNbTaTi alloy induces more lattice distortion, matching recent experimental observations. The potential also predicts that for RHEAs, deformation behavior is dominated by edge dislocations, unlike in pure BCC elements where screw dislocations prevail. Overall, this potential will be useful for unraveling the intricate atomic-level processes that give this alloy its remarkable mechanical performance.

Abstract: We present a new classical interatomic potential for a study of the binary Zr-Nb system, taking into account a wide range of the components concentrations. The potential was developed by virtue of the force-matching method that is capable of ensuring a high accuracy at the description of the complex systems containing diverse crystal phases. At simulation of pure Zr, the potential correctly describes a relative stability of Zr phases (α-Zr, β-Zr and ω-Zr) and qualitatively reproduces the right arrangement of these phases in the phase diagram. It is remarkable that β-Zr phase is found to have a dynamically unstable structure at the low temperature, in agreement with the ab initio calculations. The potential can also play a role in considering the tasks related to the crystal defects in the Zr-Nb system. In support of this statement, we show the simulation results proving adequate representation of a number of key properties of the crystal defects in Zr-Nb system. In particular, the offered potential reproduces formation/solution energies of point defects with well accuracy. To illustrate wide application possibilities for the model, we made a prediction of atomic self-diffusion and impurity diffusion in Zr and Nb. Also, the potential ensures correct description of a screw dislocation in niobium, which is a crucial point for the investigation of plasticity.

Abstract: We report a new attempt to study properties of Zr-Nb structural alloys. For this purpose we constructed an angular-dependent many-body interatomic potential. The potential functions were fitted towards the ab initio data computed for a large set of reference structures. The fitting procedure is described, and its accuracy is discussed. We show that the structure and properties of all Nb and Zr phases existing in the Zr-Nb binary system are reproduced with good accuracy. The interatomic potential is appropriate for study of the high-pressure hexagonal ω-phase of Zr. We also estimated characteristics of the point defects in α-Zr, β-Zr and Nb; results are proven to correlate with the existing experimental and theoretical data. In case of α-Zr the model reveals anisotropy of the vacancy diffusion, in agreement with previous calculations and experiments. The potential provides an opportunity for simulation of Zr-Nb alloys based on α-Zr and β-Zr. This conclusion is illustrated by the results obtained for the alloys with different niobium concentrations: up to 7% in case of hcp alloys and up to 50% for bcc alloys.

Abstract: Solid–liquid interface (SLI) properties of the Ni–Zr B33 phase were determined from molecular dynamics simulations. In order to perform these measurements, a new semi-empirical potential for Ni–Zr alloy was developed that well reproduces the material properties required to model SLIs in the Ni50.0Zr50.0 alloy. In particular, the developed potential is shown to provide that the solid phase emerging from the liquid Ni50.0Zr50.0 alloy is B33 (apart from a small fraction of point defects), in agreement with the experimental phase diagram. The SLI properties obtained using the developed potential exhibit an extraordinary degree of anisotropy. It is observed that anisotropies in both the interfacial free energy and mobility are an order of magnitude larger than those measured to date in any other metallic compound. Moreover, the [0 1 0] interface is shown to play a significant role in the observed anisotropy. Our data suggest that the [0 1 0] interface simultaneously corresponds to the lowest mobility, the lowest free energy and the highest stiffness of all inclinations in B33 Ni–Zr. This finding can be understood by taking into account a rather complicated crystal structure in this crystallographic direction.

Abstract: A new interatomic potential for the Ni–Zr system is presented. This potential was developed specifically to match experimental scattering data from Ni, Zr and NiZr2 liquids. Both ab initio and published thermodynamic data were used to optimise the potential to study the liquid and amorphous structure of the NiZr2 alloy. This potential has the C16 phase, being more stable than C11b phase in the NiZr2 alloy, consistent with experiments. The potential leads to the correct glass structure in the molecular dynamics simulation and, therefore, can be used to study the liquid–glass transformation in the NiZr2 alloy.

Abstract: Though a number of atomistic-model studies of yttria-stabilized zirconia (YSZ) have been reported to elucidate its properties, most of them have employed simple pairwise potential functions to express interactions between atoms, which limits the transferability of the models. We have developed a Tangney-Scandolo dipole model potential for YSZ. Energy, stress and forces on atoms calculated by the ab initio (first-principles) density functional theory are provided as reference data for potential fitting. The developed potential successfully reproduces cubic-tetragonal phase transition at a range of yttria concentration relevant with SOFC application. The potential can well reproduce the barrier energy of oxygen vacancy migration. Molecular dynamics simulations of oxygen diffusion in bulk and at grain boundaries are demonstrated.

Abstract: A semi-empirical Modified Embedded Atom Method (MEAM) potential is developed for application to the high temperature body-centered-cubic uranium–zirconium alloy (γ-U–Zr) phase and employed with molecular dynamics (MD) simulations to investigate the high temperature thermo-physical properties of U–Zr alloys. Uranium-rich U–Zr alloys (e.g. U–10Zr) have been tested and qualified for use as metallic nuclear fuel in U.S. fast reactors such as the Integral Fast Reactor and the Experimental Breeder Reactors, and are a common sub-system of ternary metallic alloys like U–Pu–Zr and U–Zr–Nb. The potential was constructed to ensure that basic properties (e.g., elastic constants, bulk modulus, and formation energies) were in agreement with first principles calculations and experimental results. After which, slight adjustments were made to the potential to fit the known thermal properties and thermodynamics of the system. The potentials successfully reproduce the experimental melting point, enthalpy of fusion, volume change upon melting, thermal expansion, and the heat capacity of pure U and Zr. Simulations of the U–Zr system are found to be in good agreement with experimental thermal expansion values, Vegard's law for the lattice constants, and the experimental enthalpy of mixing. This is the first simulation to reproduce the experimental thermodynamics of the high temperature γ-U–Zr metallic alloy system. The MEAM potential is then used to explore thermodynamics properties of the high temperature U–Zr system including the constant volume heat capacity, isothermal compressibility, adiabatic index, and the Grüneisen parameters.

Abstract: Metastable high entropy alloy (HEA) with excellent properties have attracted extensive attentions recently. However, as a consequence of limited experiments of high-resolution transmission electron microscopy (HRTEM) and the difficulties of molecular dynamic (MD) simulations for the phase transformation process, the detailed atomic deformation mechanisms in the HEA is not well understood. We carry out the in situ HRTEM observation of the martensitic transformation process and find surprisingly wide phase interface between the parent and the martensite in a typical high strength and high elongation metastable HEA. One specific interatomic potential is developed for the metastable HEA and large-scale MD simulation is carried out to investigate the martensitic transformation process from body-centered cubic to hexagonal close packed structures. The whole processes of the stress-induced martensitic transformation (nucleation, incubation, bursting and propagating of the new phase) are well reproduced in the MD simulations, suggesting its good agreements with the HRTEM observations. The width of the phase interface mainly depends on the competition between interfacial energy and lattice distortion energy during the martensitic transformation process. This wide phase interface acts as a buffer to coordinate the martensitic transformation induced strain and as a buffer storage for dislocation gliding and pile-up. As a result, the metastable HEA achieves a high strength combined with a large tensile elongation. The revealed atomic-scale deformation and corresponding interatomic potential should be useful to guide the design in the new series of high-performance metastable alloy.