Hcp Clusters Research Articles

Atomistic simulation of chemical short-range order on the irradiation resistance of HfNbTaTiZr high entropy alloy

High entropy alloys (HEAs) have been considered as one of the potential structural material candidates for fourth-generation nuclear reactors and fusion reactors due to their excellent irradiation resistance. Current studies have shown that the chemical short-range order (CSRO) usually exists in HEAs, which has a significant effect on the mechanical properties and irradiation resistance of HEAs. Refractory high entropy alloys (RHEAs), as a new class of HEAs have better mechanical properties at high temperatures than face-centered cubic (FCC) HEAs, and therefore have better prospects of application in the nuclear field. In this study, CSRO and its effect on the irradiation resistance of HfNbTaTiZr are analyzed via molecular dynamics (MD) and Monte Carlo (MC). The primary cascade simulations, multi-cascade simulations and surface bombardment simulations are carried out to simulate the generation and accumulation of irradiation damage. The results of the primary cascade simulations and surface bombardment simulations of CSRO models show that the presence of CSRO induces cascade splitting into subcascades. The presence of subcascades reduces the thermal peak enhancement effect and thus lowers the recombination rate of Frenkel pairs (FPs) in the damage zone when FPs concentrations are low. However, the creation of subcascades increases the size of the damage zone caused by the cascade. Thus, when the concentrations of FPs are high, the larger area of damage zone allows more of the already existing FPs to be included, thus promoting their recombination, i.e., impedes their accumulation when concentrations are high. These subcascades lower the recombination of FPs at low FPs concentrations but inhibit their accumulation at high FPs concentrations. The presence of CSRO is also beneficial in inhibiting the growth of point defect clusters, which further improves the resistance of HfNbTaTiZr to dislocation generation. Furthermore, the presence of CSRO facilitates the irradiation-induced phase transition. But it is found that HfNbTaTiZr shows suppression of HCP cluster growth. And the tendency to break down large HCP clusters into smaller ones is demonstrated in the CSRO model. From our calculations we also find that the irradiation-induced HCP atoms have a higher potential energy relative to the matrix. The potential energy difference between those energetic HCP atoms and the matrix can lead to generating a great number of insurmountable barriers pervading the matrix and largely suppressing the long-term mobility of FPs, thus limiting their aggregation and growth into clusters.

Effect of the deformation temperature and strain on the strain rate sensitivity of fcc medium-entropy alloys

The primary objective of the present investigation is to elucidate the operative micromechanisms influencing the strain rate sensitivity and activation volume in (FeCrNi)99Si1 and FeMnNi medium-entropy alloys. Room-temperature nanoindentation experiments at different loading rates were performed to study the evolution of the strain rate sensitivity and activation volume in (FeCrNi)99Si1 and FeMnNi medium-entropy alloys. The (FeCrNi)99Si1 samples were subjected to plane strain deformation by rolling at 77 and 300 K to study the effect of temperature on the strain rate sensitivity, while the FeMnNi and (FeCrNi)99Si1 samples were subjected to simple shear deformation by high-pressure torsion at 300 K to examine the effect of strains. Contrary to the well-documented trend observed in fcc metals and alloys, where the strain rate sensitivity typically increases with decreasing grain size, the present study reveals a distinct behavior for the current alloys. Similarly, these alloys are characterized by extremely low activation volumes of a few tens of b3 compared to 100–1000 b3 for conventional fcc metals and alloys in the microcrystalline grain size regime. Unlike conventional fcc metals and alloys, there is an insignificant change in the activation volume of the current high-/medium-entropy alloy (H/MEA) with decreasing grain size from the microcrystalline to nanocrystalline regime. The unique evolution of strain rate sensitivity and activation volume in H/MEAs is explained in terms of the evolution of distinct dislocation structures as well as synergistic operation of additional mechanisms such as twinning, phase transformation from fcc to hcp phases, cluster strengthening, and short-range ordering due to the aperiodic energy landscape existing in MEAs.

Molecular dynamics simulation study on non-equilibrium melting of sodium crystals at high heating rates

ABSTRACT The non-equilibrium melting of Na crystals at high heating rates and the evolutions of the atomic clusters are studied by the MD method and PTM analysis. Results show that as the heating rate rises from 1 × 1011 K/s to 2 × 1013 K/s, the non-equilibrium melting temperature of Na increases non-linearly from 417 K to 480 K, the melting temperature difference experienced increases from 7 K to 27 K, while the melting time decreases from 70 ps to 1 ps. Through the PTM analysis, it is found that some BCC clusters transform into the disordered Other clusters through intermediate HCP and FCC clusters with increasing temperature, and the BCC cluster's reduction rate and the Other cluster's growth rate are getting slower with increasing heating rate. There is a good correspondence between the evolutions of the atomic clusters, the changes of macroscopic physical parameters, and the peaks and valleys of RDF with temperature.

Shed light on new growth pattern in rapid solidified Ta metal: MD simulation, DFT calculation and CALYPSO search

Aiming at the micro-structures of rapid solidified Ta system under cooling rate of 1×1010 K/s, the lowest-energy and meta-stable geometric structure of Tan (n = 19–30) clusters are systematically investigated via MD simulation, DFT calculation and CALYPSO search. It is revealed that the ground-state geometric structure of free Ta clusters plays a significant role in determining its solidified atomic structures and evolution laws in the early stage of nucleation. We explained why the hexagonal close packed (hcp) atomic clusters (Ta22) dominate the solid state of Ta when cooled at a rate of 1 × 1010 K/s. And found that the coexistence of slightly distorted icosahedral (Ta13) and hcp clusters (Ta22) at 300 K is due to the high energy stability of free Ta26–C2v (B) cluster. The larger size of Ta clusters with a perfect closed-shell geometric structure plays a key role in energetic effects, as demonstrated by their thermodynamic stability in terms of binding energy. Additionally, electronic structure stability is achieved when there are fewer electrons on the Fermi level (N(EF)), resulting in Ta atomic clusters that exhibit higher energetic stability and lower chemical activity. Our analysis of the interaction between different atoms in Ta clusters revealed that stronger electronic interactions of central(c)-central(c) and central(c)-shell(s) atoms contribute to the higher energy and chemical stability. Finally, we provide insights into understanding the growth patterns of nucleation in liquid metals through an examination of geometric and electronic structures of free Ta cluster.

Simulation of nucleation and evolution process of nuclei during solidification of Ti<sub>3</sub>Al alloy

The nucleation mechanism of Ti<sub>3</sub>Al alloy is simulated by the molecular dynamics method in this work. The atomic clusters on different spatial scales are identified in the solidification process by the cluster-type index method (CTIM), and the formation process and the growth process of critical nucleus are studied in depth. It is found that the solidification system contains ten thousands of different types of atomic cluster structures, but only 22 types play a key role in the nucleation process. In the nucleation and growth process of nuclei, the ICO-like cluster, the BCC-like cluster, and the defective FCC cluster and the defective HCP cluster respectively reach their saturation points at the characteristic temperature <i>T</i><sub>1</sub> (1110 K), <i>T</i><sub>2</sub> (1085 K) and <i>T</i><sub>3</sub> (1010 K). And the competition processes of these clusters are revealed according to the changes of their number and spatial distribution with temperature. By tracing the nucleation and growth process of the grain with parallel twin, it is found that the critical crystal nucleus is composed of single-phase FCC structures, and the preferent nucleation of metastable bcc structure is not observed. The twinned structure is formed by the layer-by-layer growth along the close-packed plane. It is also found that the CTIM is more accurate than other methods in revealing the microstructural characteristics during the solidification.

The effect of HCP on the formation of twin boundaries and dislocations in Ni–Co alloys

In this study, molecular dynamics (MD) was used to simulate the rapid solidification process of Ni47Co53 and Ni48Co52 alloys at a cooling rate of 1012 K/s. The effects of HCP on the formation of twin boundaries and dislocations in two Ni–Co alloys are studied. It is found that the difference of HCP clusters is the main effect that producing discrepancies on microstructure of two alloys. The number of HCP clusters accounted for 9.23% in Ni47Co53 alloy. They are regularly arranged to form the number of single-layer twin boundaries, and each twin boundary ends in a dislocation. The FCC and HCP structures coexist in the same atomic layers, which is easy to create dislocations. The relatively standard FCC crystal and only 0.32% HCP clusters are formed in Ni48Co52 alloy at 300 K. That small amount of HCP clusters are dispersed on the surface, and cause the formation of dislocation in the border with FCC clusters.

S44. NEUROBIOLOGICAL FINGERPRINTS OF COGNITIVE SUBTYPES IN RECENT ONSET PSYCHOSIS PATIENTS

BackgroundNeurocognitive impairment plays a central role in pathogenesis and course of psychotic disorders. Patients suffering from psychosis or psychosis-related diseases show general and specific neurocognitive impairments that often precede the onset of manifest symptoms. Neurocognitive impairments are frequently associated with reduced psychosocial functioning along with altered brain morphology and function. Quantifying neurocognitive heterogeneity in impairment in schizophrenia patients, cluster analytic approaches differentiated several cognitive subtypes, including a generally impaired and generally spared subgroup. An independent set of findings provided evidence that neurocognitive subgroups of cognitively spared and impaired SZ patients can also be differentiated based on their brain morphology. The current study aims to (1) subtype a first episode psychosis sample based on neurocognitive performance using cluster analysis and (2) investigate grey matter (GM) differences between the obtained cognitive subtypes and healthy volunteers.Methods121 recent onset-psychosis (ROP) patients and 201 healthy controls (HC) were recruited within the framework of the Personalized Prognostic Tool for Early Psychosis Management (PRONIA) project. Patients fulfilled DSM-IV-TR criteria of a first affective or non-affective psychotic episode in the last 3 month with onset in the last 24 months. Cognitive subtypes of the ROP patients were derived from 83 variables of 10 neurocognitive tasks using a kmeans clustering algorithm. A resampling approach implemented in R was used to assess the statistical stability of the results. Neuropsychological and clinical cluster characteristics were assessed using t tests. GM volume of the obtained patient clusters was compared by means of intracranial volume corrected t tests with age, sex and site matched HCs using SPM.ResultsThe analysis reveals a stable two-cluster solution: A higher cognitive performance cluster (HCP; N=71) and a lower cognitive performance cluster (LCP; N=39). Patients assigned to HCP performed significantly (p < 0.01) worse in 4 out of 10 neuropsychological tasks. Patients assigned to LCP performed significantly (p < 0.01) worse in 9 out of 10 neuropsychological tasks. LCP patients show significantly lower premorbid IQ (p < 0.001) and General Assessment of Functioning score (GAF; p < 0.01), lower occupational functioning (p < 0.01) and higher values on the negative component of the Positive and Negative Syndrom Scale (PANSS; p < 0.05). They show no demographical differences regarding age, sex, education years and study site. LCP patients reveal widespread GM reductions in bilateral temporal, parietal and frontal brain regions as compared to healthy controls. GM reductions of the HCP cluster are more restricted and evident in temporal, occipital regions and in the anterior cingulum.DiscussionTwo homogeneous, neurocognitive subgroups of patients suffering from a recent psychotic episode were identified and show distinctive psychopathological and neuroanatomical patterns. The obtained GM reductions in LCP go in line with the neuro-developmental origin of the disease affecting premorbid cognitive and occupational functioning as well as the STG which was reported to be decreased in the early course of the disease. Finding subgroups within early stage psychotic patients with minimal intake of antipsychotic medication, suggests different underlying patho(-physio)logical mechanisms predating manifest illness expression. This evidence offers the opportunity for personalizing interventions like neurocognitive trainings to enhance patients’ odds for recovery after psychosis.

Simulation and analysis of melting behavior of local atomic structure of refractory metals vanadium

By using large-scale atomic/molecular massively parallel simulator (LAMMPS) code, a molecular dynamics simulation is performed in the NPT ensemble at zero pressure to investigate the influence of melting rates &lt;i&gt;γ&lt;/i&gt; on the evolutional characteristics of vanadium atomic structure such as body-centered cubic (BCC), hexagonal close-packed structure (HCP), face centered cubic (FCC), simple cubic (SC) and icosahedra (ICO) during the rapid melting of solid vanadium crystal at five different melting rates (&lt;i&gt;γ&lt;/i&gt;&lt;sub&gt;1&lt;/sub&gt; = 1 × 10&lt;sup&gt;11&lt;/sup&gt; K/s, &lt;i&gt;γ&lt;/i&gt;&lt;sub&gt;2&lt;/sub&gt; = 1 × 10&lt;sup&gt;12&lt;/sup&gt; K/s, &lt;i&gt;γ&lt;/i&gt;&lt;sub&gt;3&lt;/sub&gt; = 1 × 10&lt;sup&gt;13&lt;/sup&gt; K/s, &lt;i&gt;γ&lt;/i&gt;&lt;sub&gt;4&lt;/sub&gt; = 1 × 10&lt;sup&gt;14&lt;/sup&gt; K/s , &lt;i&gt;γ&lt;/i&gt;&lt;sub&gt;5&lt;/sub&gt; = 1 × 10&lt;sup&gt;15&lt;/sup&gt; K/s), in which 16000 atoms in a cubic box under the periodic boundary condition are considered, and their motion equations are solved by Verlet’s algorithm in the velocity form in time steps of 1 fs. Constant pressure &lt;i&gt;P&lt;/i&gt; and temperature &lt;i&gt;T&lt;/i&gt; are imposed by a modified Nose-Hoover method for both &lt;i&gt;P&lt;/i&gt; and &lt;i&gt;T&lt;/i&gt; variables, and an embedded-atom model (EAM) potential is utilized. For identifying the local atomic structures of liquid and solid vanadium at different temperatures, a polyhedral template matching method (PTMM) is used by measuring the root-mean square deviation (RMSD), in which clusters are classified as the topology of the local atomic environment without any ambiguity in the classification. Subsequently, the variation of the potential energy, entropy and Gibbs free energy of FCC, HCP, BCC and ICO vanadium clusters are calculated through &lt;i&gt;ab initio&lt;/i&gt; MD simulation in the canonical ensemble (NVT) at selected temperatures, and the lowest-energy dynamic structure and its corresponding static heating structure are also shown in this paper. Based on the above calculated results, it is found that the melting point of refractory metal vanadium increases obviously with the increase of heating rate, but the heating rate only presents a limited effect on the population of atomic structure for each of BCC, HCP, FCC, SC and ICO. Namely, the temperature still plays a dominant role in the rapid melting process of V rather than heating rate. Moreover, the &lt;i&gt;ab initio&lt;/i&gt; MD simulation and thermodynamics analysis further reveal that lots of ICO clusters of vanadium can exist stably in the liquid region rather than in solid crystal, which is not only due to its higher stability and longer lifetime than those of crystalline atomic clusters, but also because ICO possesses higher entropy and lower Gibbs free energy in high temperature liquid region.

Molecular dynamics simulation of phase transformation and mechanical behavior in Al2O3 model

Molecular dynamics simulations of Al2O3 model have been carried out to study the structural evolution of Al2O3 model under the cooling process and the high-pressure of 45 GPa. The phase transition occurs in the temperature range of 2530 K down to 2280 K. Not only almost AlO4 and AlO5 units transform to AlO6 units but also almost OAl3 linkages transform to OAl4 linkages below the temperature of 2280 K. Both hcp and fcc oxygen clusters are detected during the cooling process. The formations of hcp and fcc oxygen clusters in Al2O3 model is in accordance to Ostwald's rule of stages. Firstly, both the γ- and κ-Al2O3 are formed at high temperature. Then, the γ-Al2O3 transforms to the δ-Al2O3 and the κ-Al2O3 transforms to the α-Al2O3 with decreasing temperate to 300 K. The mechanical behavior of Al2O3 model at 300 K, consisting of dominant α-Al2O3 and minor δ-Al2O3, has been investigated by the uniaxial tension test. Young modulus of this Al2O3 model is 396 GPa. That the hcp and fcc oxygen clusters are maintained at the large strain, at which these clusters are embedded by the poly-cluster of disordered atoms, indicates the plastic deformation in this Al2O3 model is inhomogeneous.

Identification and tracking of different types of crystalline nucleiduring isothermal crystallization of amorphous Ag

The isothermal crystallization of amorphous Ag is investigated by a molecular dynamics (MD) simulation, and the heredity and evolution of different types of crystalline clusters aretracked and analyzed by a reverse tracking method of atom trajectories with the help of cluster-type index method (CTIM) based on Honeycutt-Anderson (H-A) bond-type index. According to the difference in the type of crystalline cluster and the linkage mode, i.e., vertex-sharing (VS), edge-sharing (ES), face-sharing (FS) and intercross-sharing (IS), a cluster analysis method which can efficiently characterize fcc single-crystal, fcc poly-crystal and fcc hydrid-crystal, is proposed. That is, the IS-linkage of fcc basic clusters, i.e., a fcc medium range order, is defined as a fcc single-crystal cluster. The extended cluster of fccbasic clusterslinked by ISand FS modes is named fcc poly-crystal clusters. In the case of IS-linkages, if the majority of core atoms arefcc atoms, the extended cluster composed of fcc, hcp and bcc basic clusters will be regarded as a fcc hydrid-crystal cluster. Moreover, a structural analysis method of critical nuclei distinguishing embryosis also developed in terms of the hereditary characteristics of various crystalline clusters. In this scheme, the extended cluster which has only transient heredity and no continuous heredity is defined as an embryo, while it will be named nuclei if part of core atoms in extended clusters can keep cluster type of atoms unchanged and be continuously passed down in the early stage of crystallization. Thus, corresponding to the onset time/temperature of continuous heredity, the critical nuclei of fcc singe-crystals, fcc poly-crystals and fcc hybrid-crystals can be identified and characterized. It is found that the nuclei of fcc crystalsemerge after the steep drop of total energy of system and before the abrupt increase of sizesof tracked clusters. And regardless of critical sizes or geometric configurations, an evident difference exists among fcc singe-crystal, hybrid-crystal clusters and fcc poly-crystal clusters, of which the fcc single-crystal nucleus is the smallest (~1.6 nm ×1.0 nm × 1.1 nm), followed by poly-crystal nucleus (~1.7 nm × 1.0 nm × 1.6 nm) and hydrid-crystal nucleus (~2.3 nm × 2.0 nm × 2.4 nm) in sequence. There are a few hcp and bcc atoms at surfaces, i.e. shells, of single-crystal and poly-crystal nucleus, but neither hcp nor bcc atom can be detected at the shell of fcc hydrid-crystal nucleus. And theconfiguration of fcc single-crystal, poly-crystal and hydrid-crystal critical nuclei are all non-spherical.

Structural and bonding transformation of Al0.67CrCoCuFeNi high-entropy alloys during quenching

Structural and bonding transformation of the Al0.67CrCoFeNi high-entropy alloys (HEA) during quenching is investigated by molecular dynamics simulations. At a high cooling rate, some short-ranged ordered clusters, such as FCC, HCP and BCC crystalline clusters are already present in the almost amorphous HEAs. When the cooling rate decreases, the atoms become packed more orderly and ultimately form a nano-polycrystalline structure dominated by FCC structures. The BCC structures appear only as an intermediate state acting on the course of crystallization, while the HCP structure can be viewed as the precursor of the malposed FCC structure due to the identical first neighbor distances. In liquid HEAs, the low-symmetry and low-coordination bond pairs, either transform to high-symmetry and high-coordination 1551 bond pairs, or transform to 1(5,4)41 bond pairs for FCC structure and 1661 bond pairs for an HCP structure, depending on the cooling rates. This study will contribute to a better understanding of the essential phase change in HEAs.

Three Hcp homologs with divergent extended loop regions exhibit different functions in avian pathogenic Escherichia coli

Type VI secretion systems (T6SSs) contribute to the pathogenicity of avian pathogenic Escherichia coli (APEC), one of the leading causative agents of sepsis and meningitis in poultry. The Hcp protein is a core component of the T6SS tail tube and acts as an exported receptor and a chaperone of effectors. In this study, four distinct Hcp types (Ia, Ib, IIa, and IIb) were designated in Gram-negative bacteria, three of which were widely distributed in APEC. We detected divergence in transcription levels among three hcp clusters in 50% duck serum and demonstrated that hcp1 was upregulated by relieving Fur repression. Further analyses revealed that the host serum could activate the hcp2B operon by H-NS derepression to transcribe the downstream xmtU/xmtV pair for inter-bacterial antagonism. Notably, in a structural analysis based on the genetic classification, Hcp proteins exhibited significant differences in the extended loop regions, suggesting that these regions were related to their functional properties. Indeed, the variant region Vs2 (Loop L2, 3) in Hcp1 and Hcp2B was essential for the delivery of antibacterial effectors and the inhibition of macrophage phagocytosis. Further analyses using a duck model indicated that these Hcps play different roles in the pathogenic processes of APEC and immunoprotection. These results indicated that the functional differentiation of Hcp homologs was driven by differences in transcriptional regulation, extended loop regions, and effector delivery.

Effects of high pressure on microstructure evolution and crystallization mechanisms during solidification of nickel

To deeply understand the effects of high pressure on microstructural evolutions and crystallization mechanisms of liquid metal Ni during solidification process, MD simulation studies have been performed under 7 pressures of 0 ∼ 30 GPa, at cooling rate of 1.0 × 1011 K s−1. Adopting several microstructural analyzing methods, especially the cluster-type index method (CTIM-2) to analyze the local microstructures in the system. It is found that the pressure has important influence on the formation and evolution of microstructures, especially of the main basic clusters in the system. All the simulation systems are directly solidified into crystal structures, and the 1421, 1422, 1441 and 1661 bond-types, as well the FCC (12 0 0 0 12 0), HCP (12 0 0 0 6 6) and BCC (14 6 0 8 0 0) clusters play a key role in the microstructure transitions from liquid to crystal structures. The crystallization temperature Tc is enhanced almost linearly with the increase of pressure. Highly interesting, it is found for the first time that there is an important phase transformation point from FCC to BCC structures between 20 ∼ 22.5 GPa during the solidification processes from the same initial liquid system at the same cooling rate. And the effect of increasing pressure is similar to that of decreasing cooling rate for the phase transformation of microstructures during solidification process of liquid metal Ni system, though they have different concrete effecting mechanisms.

Simulation study of effects of Ti content on microstructure evolution and elastic constants of immiscible Mg-Ti alloys during rapid quenching process

As for immiscible Mg-Ti alloys, how the microstructures and properties change during rapid quenching, which would finally decide their functions, is of significance. Thus, microstructure evolutions and elastic constants of immiscible Mg100-XTiX (X = 1, 10, 20, 30, 40 and 50) alloys have been studied by molecular dynamics (MD) simulation. The Energy-Temperature (E-T) curves and Cluster Type Index Method (CTIM-2) are adopted to analyze the evolution of microstructures. Results show that the crystallization temperature Tc is enhanced with the increase of Ti content X. As the content X of Ti is at 1 and 10, the main structures in system are predominant fcc (12 0 0 0 12 0) coexisting with minor hcp (12 0 0 0 6 6) clusters, and whose values of E and G are both close to 40GPa and 16GPa, respectively. As X is in the range from 20 to 50, the main structures are bcc (14 6 0 8 0 0) clusters. Their values of E and G are close to 70 GPa and 25 GPa. Especially, the G/B ratio reveals that these alloys are all ductile materials.

Solute-homogenization model and its experimental verification in Mg-Gd-based alloys

Composition homogenization in solid solution is important for industrial alloys. In the present work, a solute homogenization model is proposed based on the chemical short-range-order tendency in Mg-Gd-based alloys. After a calculation using the cluster-plus-glue-atom model, the stable Mg-Gd structural unit is derived, [Gd-Mg12]Mg6, where one solute Gd is nearest-neighbored with twelve Mg atoms to form the characteristic hcp cluster [Gd-Mg12] and this cluster is matched with six Mg glue atoms. Such a local unit is then mixed with [Mg-Mg12]Mg3, the stable unit for pure Mg. Assuming that the Gd-containing units are arranged in fcc- or bcc-like lattice points and the Mg units in their octahedral interstices, three proportions between the two units are obtained, 1:1, 2:3, and 1:3, which constitute three solute homogenization modes. The prevailing Mg-Gd-based alloys are consequently classified into three groups, respectively exemplified by GW103K (Mg-10Gd-3Y-0.4Zr, wt%), GW83K (Mg-8Gd-3Y-0.4Zr), and GW63K (Mg-6Gd-3Y-0.4Zr). Mg-Gd-Y-Zr alloys were designed following the model (where Y and Zr were also added in substitution for Gd) and prepared by permanent-mould casting. According to their mechanical properties, the 1:3 alloy (Mg-5.9Gd-1.6Y-0.4Zr) shows the best comprehensive properties (ultimate tensile strength 305MPa, yield strength 186MPa, elongation 9.0%) in solution plus ageing state.

Machine-learning approach for local classification of crystalline structures in multiphase systems.

Machine learning is one of the most popular fields in computer science and has a vast number of applications. In this work we will propose a method that will use a neural network to locally identify crystal structures in a mixed phase Yukawa system consisting of fcc, hcp, and bcc clusters and disordered particles similar to plasma crystals. We compare our approach to already used methods and show that the quality of identification increases significantly. The technique works very well for highly disturbed lattices and shows a flexible and robust way to classify crystalline structures that can be used by only providing particle positions. This leads to insights into highly disturbed crystalline structures.

Molecular dynamics simulations of pressure-induced structural and mechanical property changes in amorphous Al2O3

Molecular dynamics (MD) simulations of amorphous alumina have been carried out to investigate the pressure-induced structural transformation and mechanical properties. We found that not only the fraction of units AlOx (x=4, 5, 6) but also the density of each AlOx type change upon compression. The density of sample can be expressed through the fraction and partial density of units AlOx. With increasing pressure, O atoms are more ordered than Al atoms and form fcc and hcp clusters. The same units AlOx link together to form atomic clusters (AC) of type AC4, AC5 and AC6 (AC4, AC5 and AC6 consist of units AlO4, AlO5 and AlO6, respectively). The Young's modulus can also be expressed through the fraction and partial Young's modulus of units AlOx.

The potential energy landscape for crystallisation of a Lennard-Jones fluid

Crystallisation pathways are explored by direct analysis of the potential energy landscape for a system of Lennard-Jones particles with periodic boundary conditions. A database of minima and transition states linking liquid and crystalline states is constructed using discrete path sampling and the entire potential energy landscape from liquid to crystal is visualised. We demonstrate that there is a strong negative correlation between the number of atoms in the largest crystalline cluster and the potential energy. In common with previous results we find a strong bias towards the growth of FCC rather than HCP clusters, despite a very small potential energy difference. We characterise three types of perfect crystals with very similar energies: pure FCC, pure HCP, and combinations of FCC and HCP layers. There are also many slightly defective crystalline structures. The effect of the simulation box is analysed for a supercell containing 864 atoms. There are low barriers between some of the different crystalline structures via pathways involving sliding layers, and many different defective structures with FCC layers stacked at an angle to the periodic box. Finally, we compare a binary Lennard-Jones system and visualise the potential energy landscape from supercooled liquid to crystal.

Structural and magnetic properties of large cobalt clusters

We use a real-space implementation of pseudopotentials within the density functional theory to investigate the structural and magnetic properties of cobalt clusters with up to 365 atoms. We find from structural optimization that hcp and icosahedral clusters are lower in energy than bcc and fcc clusters. The calculated magnetic moments generally decrease as a function of increasing cluster size. For clusters of several hundred atoms the bulk limit becomes apparent. However, the decrease is not monotonic. It depends on the details of the interior structure of the cluster and the corresponding surface geometry. By analyzing the detailed evolution of the local magnetic moment, we find the spin moment is bulklike in the cluster interior and increases in the vicinity of the surface and can be correlated with coordination. The calculated behavior accounts for the observed variations in the measured moments.

Microstructure Transition during Rapid Solidification of Liquid Metal Zn

A molecular dynamics simulation study has been performed for a system consisting of 10,000 atoms to investigate the microstructure evolutions during the rapid solidification. Results indicate that the crystallization has not enough time to complete due to the high cooling rate; therefore, a part of crystal structure is formed, in which the hcp and fcc basic clusters and some other metallic type clusters coexist in the final solidified structure.