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Metastable precipitates of solute atoms, known as Guinier–Preston (GP) zones, play a significant role in the age hardening of Al–Cu alloys. In this study, we utilized the classical nucleation theory (CNT) to obtain time-temperature-transformation (TTT) diagrams for steady-state nucleation over a wide temperature range. The CNT model, which is based on atomistic simulations using artificial neural network potentials, was constructed to predict the nucleation kinetics of GP zones in the Al–Cu system with solute concentrations ranging from 1.3 to 2.5 at% (3.0 to 5.7 wt%). The nose temperature at which the steady-state nucleation time for GP zone formation is the shortest was calculated. The nose temperature increased and the minimum nucleation time decreased as the solute concentration increased. These predictions are comparable to previous theoretical results and are in good qualitative agreement with experimental observations. Furthermore, two formation scenarios of double-layer GP (GP2) zones, considering synchronous and asynchronous attachments of solute atoms to clusters, were compared in terms of nucleation efficiency. This provides new insights into the nucleation pathways of the GP2 zone in Al–Cu alloys. [Display omitted] • Classical nucleation theory was applied to predict nucleation times of the GP zone. • This method is based on atomistically informed parameterization. • Time-temperature-transformation diagrams over a wide temperature range were obtained. • The predicted nose temperatures agreed well with the experimental results. • Two formation scenarios of the GP2 zone were compared regarding nucleation efficiency. [ABSTRACT FROM AUTHOR]

In order to reproduce the atomic interaction at the interface between iron (Fe) and iron fluoride (FeF2, FeF3) and liquid sodium atoms, we created a cluster model in which Na atoms were placed on the surface of each material. The electronic states of each cluster model were calculated using the DV-Xα method, one of first-principles calculations. Figure (a) shows the strength of the covalent bonding (BO: Bond Order) between the central Fe atom of each cluster models and liquid Na atoms. Figure (b) shows the strength of the covalent bonding between the central F atom of each cluster models and liquid Na atoms. It is shown that the covalent bonding of FeF2 with Na atom is relatively larger than that of Fe with Na atom. In contrast, the covalent bonding of FeF3 with Na atom was not much different from that of Fe with Na atom. Figure (c) shows the amount of the difference in ionicity transferred from Na atoms to the central atom of each cluster model. The correlation was found between the oxidation number of Fe atoms by F atoms and the amount of difference in ionicity. [Display omitted] Atomic interactions at the interface involving iron, iron fluorides (iron (II) fluoride and iron (III) fluoride), and sodium were investigated using first-principles calculations. The wettability with liquid sodium was subsequently examined, focusing on the calculated atomic interactions. From a covalent bonding perspective, iron (II) fluoride exhibited weaker bonds between neighboring atoms in the substrate material than iron (III) fluoride. However, it exhibited relatively strong bonds between the substrate material and sodium atoms. In contrast, iron (III) fluoride had stronger bonds between neighboring atoms in the substrate material, with weaker bonds between the substrate material and sodium atoms. Additionally, in terms of ionic bonding, the contact of the sodium atom with the substrate material increased the charge transfer of the substrate material atom, correlating with the oxidation number of iron. Iron (III) fluoride exhibited the most substantial charge transfer among the substrate materials. The charge transfer from the sodium atom to the atom in contact with it was approximately 2.5 times that of iron. This result underscores the change in electronic states induced by the iron or iron fluoride interface, potentially influencing the wettability between the material surface and liquid sodium. [ABSTRACT FROM AUTHOR]

[Display omitted] • Structural basis of yielding of metallic glasses under deformation was analyzed. • The icosahedral network plays critical roles in yielding of metallic glasses. • Nodes with degree k > 1 in the network essentially determine the yielding behavior. • The larger the node degree of the network is, the higher the yield strain is. • A quantitative relation between atomic structures and yielding is established. Metallic glasses exhibit yielding behavior under mechanical deformation. However, the underlying atomic structural basis is still elusive. Here we investigated atomic structural evolution in both average fraction of atomic clusters and their spatial distribution for CuZr metallic glasses during deformation via molecular dynamic simulation. It is found that while the icosahedral clusters in the initial structures of CuZr glassy samples exhibit unique mechanical response to deformation, compared to other types of atomic clusters, its fraction is not key to the yielding of metallic glasses. We further explored the spatial distribution of icosahedral clusters, i.e., icosahedral medium-range order by analyzing the network formed via their interpenetrating connections. It is found that the topology of the icosahedral network plays the essential roles in the yielding of CuZr metallic glasses. Within the icosahedral networks, nodes with degree k > 1 mainly determine the yield strain, which is general in CuZr metallic glasses and independent of the fraction of icosahedral clusters. Moreover, it is also found that the larger the node degree of the interpenetrated icosahedral network is, the higher the yield strain is. Therefore, the disruption of the icosahedral network corresponds to the yielding of metallic glasses. The roles of nodes with degree k = 0 and 1 in the yielding were also elucidated. In addition, the newly formed icosahedral clusters during deformation are found to have almost no impact on the yielding of metallic glasses. Our findings establish a quantitative relation between atomic structures and yielding in CuZr metallic glasses based on the perspective of structural topology, and may shed light on the deformation mechanism of amorphous solids. [ABSTRACT FROM AUTHOR]

[Display omitted] • Structural relaxations of M-C (M = Nb,Ti) clusters in ferrite reveal growth paths with monotonically decreasing Gibbs energy. • Single (1 0 0) layer M-C clusters remain stable up to temperatures of 1100 K. • A precursor stage, i.e., a 'proto-precipitate', for NaCl-structured MC carbides is identified. • New understanding of the early stages of the formation of MC carbides. In this study possible routes from dissolved M and C atoms to a M-C (M = Ti, Nb) cluster are studied. Using atomistic modelling to perform relaxation simulations and molecular dynamics (MD) simulations for the Fe-M-C ternary system, the formation of clusters is studied for M. Additionally the stability of M-C clusters is assessed. The clustering of M and C atoms as observed in experiments is also found in simulations. The initial clusters found in this work have a (Fe,M)C composition with a large Fe fraction. Moreover, structurally relaxed clusters reveal that there are growth pathways with a monotone decrease in Gibbs energy, suggesting that the highest energy barrier in the formation of M-C clusters is the diffusion barrier for the atoms forming the cluster. The development of M-C clusters as found in this study suggests a formation mechanism for nano-precipitation of carbides consisting of several steps; first a C cluster forms, then M atoms attach to the C cluster forming a (Fe,M)C cluster, and in the final step the (Fe,M)C cluster transforms to a NaCl-structured carbide. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Cu-Bi clusters form core-shell structures under ion bombardment. • The dense of the shell and its synthesis rate can be controlled. • Energies slightly less than 300 eV are optimal for core-shell structures creation. Abstract Using the molecular dynamics method, 100 and 500 ps evolutions of 390-atom Cu-Au and Cu-Bi free clusters under 0.05–1.4 keV Ar 1 and Ar 13 projectiles' bombardment are simulated. Initial clusters have the Janus-like metastable construction of two mono-component parts with their insignificant overlap. Excluding low energy impacts, Ar 13 projectiles induce a few times higher temperatures in the clusters and an order of the magnitude larger sputtering yields with a contribution of the thermal mechanism than Ar particles do. In both clusters, Cu atoms show the resultant trend of displacements into the cluster depth, while Au and Bi atoms tend to increase their near-surface concentrations that lead to various distributions of the components depending on external conditions. In particular, at 300 eV Ar 13 impacts, an explicit core-shell structure with a mainly copper core coated by a bismuth-enriched near-surface layer in the Cu-Bi cluster is revealed. Along with that, an approximately homogeneous mixture with weak spatially fuzzy segregation of quantitatively dominating Cu atoms inside the Cu-Au cluster and Au atoms in the outer region is formed. At Ar 1 impacts, transfer of atoms is less intense and leads to eccentric arrangements of the copper-enriched core in the Cu-Bi cluster and an inhomogeneous mixture in Cu-Au cluster with still higher inner gold concentration despite the relocation trends. [ABSTRACT FROM AUTHOR]

[Display omitted] • VC formed under a hydrogen environment can nucleate into PDL. • The high-speed vacancy transport from the PDL-VC transition. • PDLs can diffuse to a region near a TJGB or GB and transform to a VC. • VCs can accumulate, with high-density VCs forming in the vicinity of GBs/crack tips. Plastic deformation and the presence of hydrogen enhance the concentration of vacancy-type defects in α-iron, wherein monovacancies can accumulate as a planar vacancy cluster (VC). The planar VC can then be nucleated into a corresponding prismatic dislocation loop (PDL), which can also be directly generated through cross-slip mechanisms of a screw dislocation. Inversely, a PDL can be converted to a corresponding VC. High densities of VCs and PDLs can significantly influence the mechanical properties of iron-based materials. Here, a quantitative energy barrier is established for the transition between nano-sized 1/2 〈1 1 1〉 PDLs and VCs using the nudged elastic band method for different applied strains. It was concluded that a cluster size of more than 19 vacancies can easily nucleate into a PDL if a VC with planar configuration is formed in α-iron. Further, the nano-sized VC to PDL transition is enhanced under compressive strain while the nano-sized PDL to VC transition is enhanced under tensile strain. This result indicates that nano-sized VCs generated by tensile deformation are converted to a PDL after unloading and diffusing away. Inversely, nano-sized PDLs that form by deformation can easily diffuse to high tensile stress regions near stress singularities, such as crack tips and triple-junctions of grain boundaries, and then accumulate as high-density VCs. [ABSTRACT FROM AUTHOR]

Electronic and magnetic properties of two dimensional (2D) cluster-assembled materials based on TM@Si 12 (TM = 3d transition metal) clusters were systematically investigated by using the density functional method. Taking the hexagonal prism TM @Si 12 as a building block, we constructed four different kinds of 2D crystal structures, each with a higher stability than the corresponding individual clusters. The hexagonal honeycomb and porous structures are proved to be thermodynamically stable at room temperature by first-principles molecular dynamics simulations, and the honeycomb structure is more favorable in energy than the porous structure. The magnetic coupling properties of the honeycomb and porous structures based on TM@Si 12 were further studied in detail. The results show that almost all of the hexagonal TM@Si 12 2D lattice exhibit a certain degree of magnetic ordering. These studies provide insights into the effective design of 2D spintronic materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • Expanding timescale of simulations with on-the-fly kinetic Monte Carlo while maintaining atomistic fidelity. • Activation energy threshold for events used to control event selection. • Enhanced metastable-to-stable transformations, e.g., perfect or Frank loops. On-the-fly kinetic Monte Carlo (kMC), a computational technique for atomistic simulations, has attracted attention because it increases the simulation timescale beyond that of molecular dynamics (MD) simulations while maintaining atomistic fidelity. However, for most kMC methods, when events with high and low activation energies coexist in the event list, trivial events with extremely low activation energies that do not essentially affect the phenomena of interest, so-called flicker events, are frequently selected, making it challenging to observe the key dynamics. In this study, we use Self-Evolving Atomistic kMC (SEAKMC), one of the on-the-fly kMC methods, to model the unstable-to-stable transformations of irregular three-dimensional self-interstitial-atom (SIA) clusters in Cu generated through collision cascade. By setting an activation energy threshold once every five steps, transformations into stable configurations are enhanced. The algorithm renders the simulation timescales one or two orders of magnitude longer than those possible with MD simulations. Further, the probability of transformations into stable configurations is increased by 40 times compared to that of the original SEAKMC method. In addition, we find that the stable configurations obtained by the transformation of the SIA clusters are mostly Frank loops. In summary, this new algorithm for the SEAKMC method helps to resolve the inefficiency of kMC methods resulting from the selection of flicker events and will aid the study of meso-timescale atomistic dynamics. [ABSTRACT FROM AUTHOR]

We present a species-conserving Monte Carlo (MC) method, motivated by systems such as high-entropy alloys. Current fast local-structure MC methods do not conserve the net concentration of atomic species, or are inefficient for complex atomic systems. By coarse-graining the atomic lattice into clusters and developing a renormalized MC method that takes advantage of the local structure of the atoms, we are able to significantly reduce the number of iterations required for MC simulations to reach equilibrium. In addition, the structure of the method enables easy parallelizability for the future. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • There are multi-structural changes from disordered to ordered packing for the Ih clusters having different compositions. • The clusters containing a small number of the Cu atoms can form the perfect core–shell icosahedral geometry in which the Ag atoms segregate on the surface at room temperature. • The substitution of the Cu atoms for the Ag atoms is beneficial to the stability of the alloying Cu@Ag cluster. • The freezing process of the clusters is the result of the interaction between the kinetic trapping effect and the entropic effect. Molecular dynamics simulations were performed to investigate the freezing process of Cu n Ag 55-n (n = 0–55) alloying cluster at the atomic scale. Potential energy and shape factors indicate the structural transformation accompanying with the atom motion. Pair-distribution function gives different atomic packing characteristics of some typical structures. Entropy combined with BP neural network is used to analyze the disorder degree of atomic packing in the binary alloying clusters as well as the influence of the number of Cu atoms on the entropy values. The simulation results show that the clusters having fewer or quantity Cu atoms prefer the perfect core–shell icosahedral structures at room temperature. Significant composition effects can be observed during cooling the small size alloying clusters. With changing compositions and decreasing the temperature, the packing structures change from disorder to order accompanying with the changes of atomic energy, shape factor, pair distribution functions, and entropy. In the restricted composition space having 55 atoms, the structure of the melt has an effect on the structural transformation during cooling and the room temperature structure. [ABSTRACT FROM AUTHOR]

[Display omitted] • In situ dual beam experiments showed the conversion of SFTs into helium bubbles. • Molecular dynamics confirmed the conversion of SFTs into helium bubbles. • The transition occurs by helium pushing the Cu atoms towards any of the vertices. Stacking fault tetrahedrons (SFTs) are commonly observed in irradiated face-centered-cubic metals with low-to-medium stacking fault energies. Several mechanisms were previously proposed for the removal of SFTs in irradiated solids, including high temperature annealing, interactions with interstitial atoms, dislocations, and twin boundaries, and transformation to dislocation loop under compressive stress. We propose a previously unreported mechanism for the removal of SFT in irradiated copper, supported by experiments and atomistic simulations. In situ experiments showed that helium bubble density increased at the expense of SFT density following the initial phase of dual 1 MeV Kr/12 keV He ion irradiation, suggesting a possible conversion of SFTs to helium bubbles. Atomistic simulations of the interactions of helium atoms with SFTs confirmed this possibility and revealed the collective effects of helium-induced shear stress that deformed the atomic planes of Cu leading to the destruction of the SFT and leaving behind helium atoms in vacancy clusters (bubbles). [ABSTRACT FROM AUTHOR]

[Display omitted] • Investigation of H atom(s) preferred interstitial sites in pristine α-Zr. • Two H atoms at tetrahedral sites spaced two sites apart are most energetically stable. • Four H atom clusters are thermodynamically preferred to adopt a "box" or "zig-zag" formation comprising two TTn2 pairs. 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 Å, 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. [ABSTRACT FROM AUTHOR]

We study and compare four coarse-grained models of cis -1,4-polyisoprene distinguished by mapping schemes for locating superatoms. First, coarse-grained potentials are obtained by iterative Boltzmann inversion method for the polymer melt. For all the coarse-grained models considered, time-scale factors based on translational and rotational motion were found to be different. However, coarse-grained potentials were unable to reproduce stress–strain behaviour of the underlying detailed model due to weak attractive nature of the nonbonded part of the coarse-grained potential. Consequently, the nonbonded potentials were optimized using particle swarm optimization to match the tensile behaviour of a detailed model of the polymer below glass transition temperature. While the modified potentials seemed to better predict the mechanical behaviour, the ability to accurately predict simultaneously the structural distributions also, depends on the mapping scheme. • Four mapping schemes were used to coarse-grain cis -1,4-polyisoprene using IBI technique. • The time-scale factors were sensitive to the mapping schemes and the method used to calculate. • Potentials from IBI did not reproduce stress–strain response of the detailed level model. • Modifying the nonbonded portion of the potential better predicts the stress–strain response. [ABSTRACT FROM AUTHOR]

To investigate the formation and evolution mechanisms of nano-clusters formed during the rapid solidification process, a molecular dynamics (MD) simulation study has been performed for a system consisting of 10 6 atoms of Cu 64.5 Zr 35.5 alloy. Adopting a new cluster-type index method (CTIM-3) to analyse the local atomic clusters, it is found that the dominant microstructural configurations in the supercooled liquid and amorphous are the icosahedrons and their combinations, especially the intercross-sharing (IS) clusters linked of two icosahedrons (IS-ICO-clusters) can be used to build some larger IS-ICO nano-clusters with different sizes. Highly interesting, in the system the number of atoms in various IS-ICO nano-clusters demonstrates a magic number sequence of 19, 23, 25, 27, 29, 31, 33, 35, 37, 41, 43, and so on. The magic number clusters can be classified into three main types: chain-like, triangle-tailed and quadrilateral-tailed. The degree of tightness for IS-ICO nano-cluster can be expressed by formula n I / n IS . And some IS-ICO nano-structures have stronger mechanical stability and can bear higher load. [ABSTRACT FROM AUTHOR]

Geometry, electronic structure, and magnetic properties of TM@Na 8 (TM = 3d, 4d and 5d transition metal atoms) clusters have been systematically investigated by using scalar relativistic density functional theory with generalized gradient approximation. Calculations demonstrate that the superatom nature and the magnetic moments of TM@Na 8 (TM = 3d, 4d and 5d) clusters vary from 0 to 6 μ B . The stability and origins of these magnetic properties were fully analyzed. Interestingly, we found the magnetic properties of several TM@Na 8 clusters have little relation with d electrons of the TM atom. [ABSTRACT FROM AUTHOR]

[Display omitted] Classical molecular dynamics simulations were performed for five model metallic glass-forming liquids to investigate the structure correlation with both short-time β and long-time α relaxation dynamics. It is found that Debye-Waller factor characterizing the short-time β relaxation decreases with increasing five-fold local symmetry in atomic clusters, following a power-law behavior. It is also found that the equation of Debye-Waller factor and α -relaxation time, which is widely studied in polymer systems, is applicable in metallic glass-forming systems as well. Furthermore, a new equation is derived for the five-fold local symmetry and α -relaxation time, which describes the simulated data very well. These findings indicate that there exist a common structural basis for both the short-time β -relaxation and the long-time α -relaxation dynamics. [ABSTRACT FROM AUTHOR]

[Display omitted] • Identification efficiency of fcc and hcp local structures is similar for both methods. • Bad detection of deformed bcc structure by CPM attributed to mixing of atomic shells. • CPM detects decahedral units in tested and simulated clusters while CNA is blind. • CPM is more effective in detection of deformed icosahedral units. Efficiency of two methods for structural analysis, the Common Neighbor Analysis (CNA) from the program OVITO and the Coordination Polyhedron Method (CPM), was compared when applied to detect all possible structural atoms in monoatomic test clusters. The analyzed clusters are in the form of spherical fragments of a pure or disturbed crystal structure or they represent Mackay icosahedra. It was observed that the number of detected structural atoms for both methods is initially similar but decreases rapidly with the increase of applied random translations of atoms. CNA gives higher number of detected structural atoms representing bulk-centered cubic structure, while CPM has significant advantage in detection of icosahedral units. CNA is unable to identify decahedral units; therefore, this method fails to visualize clearly any global symmetry in internal cluster structure. Attempts are made to understand the reasons for the poor performance of both methods in the mentioned aspects and some ways are proposed to improve them. [ABSTRACT FROM AUTHOR]

[Display omitted] An effective molecular dynamics model to describe heterogeneous materials like glass-ceramics is proposed. Based on the model, we find that the velocity direction of the primary knock-on atom (PKA) is more prone to deflect sharply in glassy phases (GP) than that in crystalline phases (CP). And the high-stress region in CP is more concentrated, while the distribution of the high-stress region in GP is relatively discrete. Typical topologies of the Frenkel pairs (FPs) show that the direction of FPs propagation has an obviously turn at the boundary. In the hot peak period, the off-site atoms induced by the cascade collisions in CP could rearrange to avoid the number of defects coming to a high point. However, once the distorted clusters form in GP, the dislocated atoms of the distorted clusters are hard to move back to their site due to the high energy barriers. The annihilation of defects is also blocked in GP. The derived results could help to further understand the evolution mechanism of the heterostructure and defects at the nanoscale. And the method of model construction could contribute to more exciting simulations. [ABSTRACT FROM AUTHOR]

Abstract: The structural and magnetic stability of small NiCu N−1 clusters is determined by performing global structural optimizations within Kohn-Sham density-functional theory. The calculated structures for N ⩽5 atoms are compact and resemble, besides some interesting changes in bond length, the structures of the corresponding Cu N clusters. The Ni ion occupies the most-coordinated atomic position, which reflects the dominant role of the sd hybridizations. For the optimal NiCu N−1 geometry the ground-state corresponds to a minimum-spin configuration (S z =0 or 1/2). Varying the total spin moment of the cluster reveals interesting correlations between cluster structure and magnetism. Low-energy spin excitations are found which involve similar energies as isomerizations. Structure and magnetic behavior are found to depend strongly on each other. The possible consequences of electron-correlation and finite-temperature effects are briefly discussed in the framework of the single-impurity Anderson model. [Copyright &y& Elsevier]

In this work, the structural transformation from a crystalline to quasicrystalline symmetry in palladium (Pd) and palladium-hydrogen (Pd-H) atomic clusters upon thermal annealing and hydrogenation has been addressed by means of atomistic simulations. A structural analysis of the clusters was performed during the heating up to the melting point to identify the temperature for the phase transformation. It has been demonstrated that nanometric pure Pd clusters transform from cuboctahedral to icosahedral structures under heating. This transformation is thermally activated process and the activation barrier depends on the cluster size. The activation energy of the cubo-ico symmetry transformation was measured using the variable heating rate method and was found to increase with the cluster size from 0.05 eV for 55 atomic cluster up to 0.66 eV for 147 atomic cluster. Hydrogenation of the nanometric Pd clusters yields to the modification of the transformation barrier in a non-monotonic form. At low H concentration, the transformation barrier decreases, while by increasing H concentration above a certain threshold, the barrier grows again thus making a minimum around a specific hydrogen concentration. This behaviour was rationalized as a competition between two processes, namely: the structure symmetry breaking at low H concentrations and stabilization of cuboctahedral phase of the clusters at high H concentration. The obtained results provide an estimation of the temperature range at which the symmetry transformation should occur under thermal annealing with experimentally achievable heating rates. [ABSTRACT FROM AUTHOR]