查看更多>>摘要：The Activation-Relaxation Technique (ARTn) is an efficient technique for finding the minima and saddle points of multidimensional functions such as the potential energy surface of atomic systems in chemistry. In this work we detail and illustrate significant improvements made to the algorithm, regarding both preprocessing and the activation process itself. As showcased, these advances significantly reduce ARTn computational costs, especially when applied with ab initio description. With these modifications, ARTn establishes itself as a very efficient method for exploring the energy landscape and chemical reactions associated with complex mechanisms.
Wen, PengTonks, Michael R.Phillpot, Simon R.Spearot, Douglas E....
13页
查看更多>>摘要：A phase-field model is parameterized to study the effect of elastic stresses on the migration of He gas bubbles in Fe under a temperature gradient. Stresses caused by the gas bubble pressure and residual stress in the Fe matrix are considered. The dependence of He bubble migration velocity on the magnitude of the residual stress, average temperature, temperature gradient, and bubble size is measured. In agreement with a theoretical model based on surface diffusion, simulation results demonstrate that He bubbles move towards the high temperature region with velocities in Fe that are orders of magnitude faster than previously reported in UO2. It is found that local stresses in the matrix caused by the He bubble have negligible effect on the bubble migration process; however, residual stresses in the Fe matrix, potentially caused by processing or irradiation, can modestly modify bubble kinetics through pressure dependence of the He diffusion coefficients. Compressive residual stress decreases diffusion coefficients for bulk and surface diffusion mechanisms, thus reducing the migration velocity of the gas bubble. In contrast, tensile residual stress increases the diffusion coefficients, resulting in an increase in the gas bubble migration velocity. This pressure dependence is also consistent with a theoretical model. This phase field model lays the foundation for analysis of bubble coalescence-induced fracture in He bubble-containing steels.
查看更多>>摘要：In this paper, we use multiscale modeling to explore the coupling of the atomic orbital and magnetic moment and their evolutions under the excitations of external stress and magnetic fields. Our results of density functional theory (DFT) calculations show that strain-induced spin reorientation and redistribution of orbital coupling are the fundamental reasons for altering the atomic magnetic moment, magnetic anisotropy, and exchange in-teractions. Also, magnetic anisotropy and exchange interactions are found to play a more crucial role in the magnetization reversal process than the atomic magnetic moment. By using micromagnetics approach and the finite element method (FEM), the macroscopic magnetic properties such as coercivity, magnetic permeability, and remanence alter as a consequence of modified electronic structure, which eventually leads to the distortion of the macroscopic magnetic field of the stress concentration area. Through DFT, micromagnetics and FEM approaches, the cross-scale simulations herein highlight the remarkable short-range orbital ordering and coupling in ferromagnetic materials which provide the fresh insight in the stress-magnetic coupling for MMM applications.
查看更多>>摘要：In this study we have reported electronic structure, lattice dynamics, and thermoelectric (TE) transport properties of a new family of pentanary substituted TiNiSn systems using the 18 valence electron count (VEC) rule. We have modeled the pentanary substituted TiNiSn by supercell approach with the aliovalent substitution, inspite of the traditional isoelectronic substitution. We have performed the detailed analysis of electronic structure, lattice dynamics, and TE transport properties of selected systems from this family. From our calculated band structures and density of states we show that by preserving the 18 VEC through aliovalent substitutions at Ti site of TiNiSn semiconducting behavior can be achieved and hence one can tune the band structure and band gap to maximize the thermoelectric figure of merit (ZT) value. Two approaches have been used for calculating the lattice thermal conductivity (kappa(L)), one by fully solving the linearized phonon Boltzmann transport (LBTE) equation from first-principles anharmonic lattice dynamics calculations implemented in Phono3py code and other using Slack's equation with calculated Debye temperature and Gruneisen parameter using the calculated elastic constant values. At high temperatures the calculated kappa(L) and Z(T) values from both these methods show very good agreement. The calculated kappa(L) values decreases from parent TiNiSn to pentanary substituted TiNiSn systems as expected due to fluctuation in atomic mass. The substitution of atoms with different mass creates more phonon scattering centers and hence lower the kappa(L) value. The calculated kappa(L) for Hf containing systems La0.25Hf0.5V0.25NiSn and non Hf containing system La0.25Zr0.5V0.25NiSn calculated from Phono3py (Slack's equation) are found to be 0.37 (1.04) and 0.16 (0.95) W/mK, at 550 K, respectively and the corresponding ZT value are found to be 0.54 (0.4) and 0.77 (0.53). Among the considered systems, the calculated phonon spectra and heat capacity show that La0.25Hf0.5V0.25NiSn has more optical-acoustic band mixing which creates more phonon-phonon scattering and hence lower the kappa(L) value and maximizing the ZT. Based on the calculated results we conclude that one can design high efficiency thermoelectric materials by considering 18 VEC rule with aliovalent substitution.
查看更多>>摘要：Understanding irradiation effects on the structure and properties of materials is crucial for the development of advanced nuclear technologies. Molecular dynamics (MD) simulations are suitable for the study of irradiation induced damage in materials with atomistic resolution. However, MD simulation of structural damage through collision cascades is computationally expensive, and therefore it is impractical for the study of cumulative irradiation-induced effects in materials due to long-term exposure to irradiation. To overcome this limitation, we propose a stochastic core-shell (SCS) approximation of collision cascades, which is capable of reproducing the defect structures produced by primary radiation damage in MD simulations. The SCS retains the characteristic damage for a given primary knock-on atom energy and system temperature, but without the explicit simulation of the preceding collision cascade. Thus, the method provides an efficient approach to investigate irradiation-induced damage in materials using MD simulations. We demonstrate the application of the SCS method to the investigation of irradiation-induced microstructural effects in -zirconium.
查看更多>>摘要：Rapid solidification kinetics of dilute Al-Cu alloys is simulated using a quantum mechanics based bond-order potential (BOP), in free solidification conditions, to determine kinetic and thermodynamic properties of solidification, as well as point defects and chemical ordering of the solidified structures. We measure the anisotropic kinetic coefficient, anisotropic solid-liquid interface energy, as well as solute trapping kinetics in terms of partition coefficient versus velocity and solute drag coefficient. Furthermore, solid-liquid interface free energy and its anisotropy are measured in equilibrium simulations, showing reasonably good agreement with previous studies. We also verified the self-consistency of the MD simulations, by comparing the interfacial temperature vs. velocity to that predicted by the continuous growth model. These solid-liquid interface properties are important for quantitative parametrization of larger scale solidification modeling techniques such as phase field models. We also investigated the point defect content, local chemical ordering, and local crystalline structures in the rapidly solidified samples. We found clustering of solute with vacancies, whereas copper atoms repelled each other in these dilute alloy simulations. In addition to vacancies, a large number of interstitials were found. In solidification velocities approaching the complete solute trapping regime, we found that the vacancies and interstitials formed in conjunction, i.e. as Frenkel pairs. Finally, in addition to FCC, we detected BCC and HCP phases, where the latter two were accompanied by an increase in local copper content. Understanding the formation of point defects and their relationship to chemical ordering is an important step towards controlling the formation of pre-precipitates and precipitates, which are an important strengthening mechanism for aluminum-copper alloys.
查看更多>>摘要：Non-conservative processes play a fundamental role in plasticity and are behind important macroscopic phenomena such as creep, dynamic strain aging, loop raft formation, etc. In the most general case, vacancy induced dislocation climb is the operating unit mechanism. While dislocation/vacancy interactions have been modeled in the literature using a variety of methods, the approaches developed rely on continuum descriptions of both the vacancy population and its fluxes. However, there are numerous situations in physics where point defect populations display heterogeneous concentrations and/or non-smooth kinetics. Here, a kinetic Monte Carlo (kMC) approach for modeling vacancy transport in response to arbitrary stress fields is used. Vacancies are treated as point particles and are coupled to the dislocation substructure representing a deformed material via an advection term defined by the local stress gradients. The stress fields and the dislocation substructure are evolved using a discrete dislocation dynamics (DDD) module. To extend the coupled model to the treatment of large systems, we have implemented it in the massively-parallel DDD code ParaDiS. To avoid numerical incompatibilities associated with merging deterministic (DDD) and stochastic (kMC) integration algorithms, we cast the entire elasto-plastic-diffusive problem within a single stochastic framework, taking advantage of a parallel kMC algorithm to evolve the system as a single event-driven process. The large-scale implementation enables the study of the evolution of a variety of dislocation-defect scenarios governed by non-conservative transport kinetics. After carrying out an exhaustive numerical and computational analysis of our parallel algorithm, we show results that emphasize situations where inhomogeneous vacancy dynamics are of relevance, and compare discrete kinetics to continuum solutions for several cases.
Duan, YuhuaAndolina, Christopher M.Saidi, Wissam A.Paudel, Hari P....
8页
查看更多>>摘要：Alpha-zirconium is a hydrogen host material used in nuclear energy and semiconducting applications due to Zr's excellent properties such as corrosion resistance, low thermal neutrons cross-section, and thermodynamically favorable interstitial sequestering of H atoms. Herein we use density functional theory simulations to systematically analyze the initial stages of H insertion in the Zr matrix, and determine optimal hydrogen localization, distribution, and clustering tendencies from a thermodynamics and kinetics perspective. Our results suggest that H occupation of tetrahedral sites is strongly favored, consistent with previous experimental and theoretical studies. Further, a cooperative effect in H localization is shown where H atoms favor clustering. The H pair is found to be the most stable by occupying tetrahedral sites with pairs at second nearest-neighbor positions (TTn2). Among the four H atom clusters, the most favorable configuration has two TTn2 pairs forming "box" or "zig-zag" shaped-cluster with the pairs separated by 3.03 or 3.25 angstrom, respectively. Nudged elastic band simulations suggest that the formation of TTn2 is facile and is limited by the small diffusion barrier of the monomer of less than 0.1 eV. Further, the formation routes for the 4H atom clusters are also presented. The current study can serve as a foundation to systematically model H atom clustering in metals and alloys.
查看更多>>摘要：In this study, the mechanical behavior of BNTs with distinct morphologies subjected to tensile loading is investigated through reactive molecular dynamics simulations. For this purpose, atomistic models of eight different BNT morphologies with zigzag and armchair configurations are generated to be utilized in tensile testing. Furthermore, a reactive force field, namely ReaxFF, allowing continuous bond formation/breaking, is employed in MD simulations to conduct more realistic tensile simulations. Simulation results indicate that ReaxFF potential can represent key structural properties of BNTs, such as surface buckling and elliptic cross-section. Furthermore, it is demonstrated that Young's modulus and tensile strength of BNT structures highly depend on the vacancy ratio. In this regard, empirical formulae for Young's moduli and tensile strength of BNTs with non-zero vacancy ratios are proposed based on the BNT structure with zero vacancies (i.e., 2-pmmn). According to the overall results, it can be underlined that BNTs, which have comparable mechanical proper -ties with carbon nanotubes, can be considered as an ideal analog of carbon nanotubes in diverse applications, including nanocomposites and nanoelectromechanical systems.
Valencia, Felipe J.Ortega, RobinsonGonzalez, Rafael I.Bringa, Eduardo M....
8页
查看更多>>摘要：Nanoporous metals, also known as metallic nanofoams, offer a wide range of functionalities and improved mechanical properties enabled by nanoscale effects. In particular, tungsten nanofoams are a novel class of materials with potential applications as radiation-resistant coating, and they share some similarities with the fuzz structure arising in fusion devices. We approach their study by nanoindentation tests using molecular dynamics simulations, for a single crystal nanofoam. To help understanding the foam mechanical behavior we also carry out simulations of W nanowire compression, finding elastic moduli of 375-450 GPa, and plastic yielding at asymptotic to 15 GPa. For the nanofoam, we obtain an elastic modulus of 64 GPa, in reasonable agreement with experiments, but our hardness value of ~15 GPa is higher, likely due to nanocrystalline effects in the experiment. Atomistic simulations reveal that plastic deformation is caused by a combination of 1/2 & lang;111 & rang; dislocations and twinning in the neighborhood of the indenter surface. It was found that twins also promote complete amorphization of some thin filaments in contact with the indenter tip. Dislocation activity also produces vacancies in the plastic region. Besides, the displacement induced by the indenter also drives changes of the network topology mainly due to densification, filament bending and twisting. Dislocation density is lower in the foam than in bulk indentation, due to the dislocation annihilation on the filament surfaces, but also because of changes in network topology help accommodate strain. Based on the simulation results, a nanoporous bcc foam behaves differently than a fcc foam, but still displays excellent mechanical properties for a low density material, and also offer additional technological advantages.
NSTL
Elsevier