Common Neighbor Analysis Research Articles

The interaction of nanoparticulate Fe with vacancies during melting and sintering: A molecular dynamics simulation

A molecular dynamics simulation of the mechanisms responsible for the atomic migration and neck growth during the sintering process remains significant due to the nanoparticle size and the vacancy defect in the Fe structure. In this work, we have designed five sets of single Fe nanoparticles with different vacancy concentrations (0%, 2.5%, 5%, 7.5%, and 10%), which were used to investigate the melting and sintering processes. The existence of the vacancy introduced a vacancy proliferation mechanism, which later reduced the bonding energy of the surface atoms and made the surface diffusion easier. As a result, the melting and sintering temperatures slightly reduce according to the vacancy concentration. Atomic vector diagrams and root mean square displacement have been employed to describe the law of atomic motion; common neighbor analysis and coordination analysis have been used to investigate the structural changes within nanoparticles. The consistent results reveal the influence of vacancy defects on the melting and sintering of nanoparticles and provide a new method to reasonably control the sintering process.

Local surface crystal structure fluctuation on Li, Na and Mg metal anodes

Dendrite formation occurs on Li, Na, and Mg metal anodes in rechargeable batteries, and is a safety challenge, as well as a limiting factor for increasing energy- and power density. However, the behaviour of the dendrites differs depending on the anode material. In this study, we investigate the local bulk and surface crystal structure of Li, Na, and Mg surfaces to shed light on how differences in the morphology and structure of the anode surface and its metal deposits can explain differences in dendrite formation on Li, Na, and Mg anodes. The local bulk- and surface structure are found using molecular dynamics simulations in combination with the surface adaptive common neighbour analysis, and indicate that Li and Na surfaces are more prone to surface instabilities and formation of protrusions than Mg surfaces, which remain flat and hexagonal close-packed even near room temperature. Additionally, the equilibrium shapes of the Mg deposits obtained from density functional theory assume more flat and hexagonal shapes than the Li and Na deposits. Together, these results shed light on atomic mechanisms that may contribute to the different propensities of Li, Na, and Mg metal anodes to form dendrites.

Matrix controlled structural phase transformations in embedded metallic nanoparticles

Non-equilibrium synthesis and post-processing methods may result in unique and unexpected microstructures. In arc-melted and subsequently laser-glazed Al-0.5 at% Co alloys, we show via high-resolution transmission electron microscopy lattice-imaging that cobalt forms coherent face-centered cubic nanoprecipitates. Using first-principles methods and a common-neighbor analysis, we explain the reasons for this and then expand the model by introducing nanoprecipitates of 4th-row transition metals into aluminum and titanium supercells. Our results indicate that the lattice strain and the physio-chemical interactions across the matrix-particle interface play significant roles in the crystal structure of the precipitate. Our computations make surprising predictions; for example, a hexagonal close packed copper can be stabilized in a titanium matrix and cubic zinc can be formed in an aluminum matrix. Extrapolating from these findings, we hypothesize that new allotropic forms of transition metals can be accessed through non-equilibrium processing and careful design of lattice strains.

Computational study of nanoscale mechanical properties of Fe–Cr–Ni alloy

ABSTRACT Mechanical properties of Fe–Cr–Ni alloy nanowires have been investigated using molecular dynamics simulation with embedded atom method and first principles approach. Various cases of uniaxial tension, compression and shear deformations have been performed and studied in this work. From the first principles calculations, the higher magnitudes of uniaxial and shear deformations resulted in higher probability of martensitic transformations. Before the first yielding, nanowires preserved the elastic stage and then the mechanical deformation proceeded in alternating quasi-elastic and yielding stages. The plastic behaviour was not observed in compression while both tensile and shear deformations showed apparent plastic behaviour. In shear deformation, due to the martensitic phase transformation, the plastic behaviour persisted for total strain of 0.6 which was much larger than that during tensile and compression. This validated the previous experimental observations. In the studied Fe–Cr–Ni nanowires, deformations were controlled by dislocations. Dislocation-mediated twinnings were captured by common neighbour analysis. Twin quantification showed that the twin activity increased with increasing strain rate. Twinnings originated from stacking faults led by 1/6 <112> Shockley partial dislocations. At elevated temperature (beyond 500 K), the materials softening happened, and 316L nanowire became more plastic under a lower stress.

Gaussian mixture model for the unsupervised classification of AgCu nanoalloys based on the common neighbor analysis

In this short communication we describe the results obtained from the application of the Gaussian mixture model, a popular unsupervised learning algorithm, to some modified data sets gained after the global optimizations of three different AgCu nanoalloys. In particular we highlight both positive and negative aspects of such an approach to this kind of data. We show indeed that thanks to the Common Neighbor Analysis we are still able to describe nanoalloys well enough to exploit a physically meaningful separation in different structural families, even with a very low-dimensional representation. On the other hand, we show that the imposition of an energy cutoff over the data set is a delicate matter since it forces us to find a tradeoff between having a large set of data and having clean data.

The Influence of Grain Boundaries on Crystal Structure and Tensile Mechanical Properties of Al0.1CoCrFeNi High-Entropy Alloys Studied by Molecular Dynamics Method

The mechanical properties of high-entropy alloys are superior to those of traditional alloys. However, the key problem of finding a strengthening mechanism is still challenging. In this work, the molecular dynamics method is used to calculate the tensile properties of face-centered cubic Al0.1CoCrFeNi high-entropy alloys containing Σ3 grain boundaries and without grain boundary. The atomic model was established by the melting rapid cooling method, then stretched by the static drawing method. The common neighbor analysis and dislocation extraction algorithm are used to analyze the crystal evolution mechanism of Σ3 grain boundaries to improve the material properties of high-entropy alloys during the tensile test. The results show that compared with the mechanical properties Al0.1CoCrFeNi high-entropy alloys without grain boundary, the yield strength and Young’s modulus of a high-entropy alloy containing Σ3 grain boundary are obviously larger than that of high-entropy alloys without grain boundary. Dislocation type includes mainly 1/6&lt;112&gt; Shockley partial dislocations, a small account of 1/6&lt;110&gt; Stair-rod, 1/2&lt;110&gt;perfect dislocation, and 1/3&lt;111&gt; Hirth dislocations. The mechanical properties of high-entropy alloys are improved by dislocation entanglement and accumulation near the grain boundary.

Effects of extrusion ratio and direction on extruded Au nanowires investigated using molecular dynamics simulation

The effects of the extrusion ratio (diameter of the extrusion mold inlet divided by that of its outlet) and direction on extruded Au nanowires (NWs) are studied using molecular dynamics simulations based on the embedded-atom method. The effects are investigated in terms of atomic trajectories, common neighbor analysis, flow field, system pressure, and the extruded NW length-pressing ram displacement curve. The simulation results show that for the forward extrusion process, a smaller extrusion ratio leads to a larger diameter of the extruded NW and a smoother cross-sectional profile. Extruded NWs obtained at a smaller extrusion ratio have a higher face-centered cubic fraction and a less disordered structure due to lower system pressure. The adhesion effect between extruded NWs and mold outer walls increases with increasing extrusion ratio, resulting in an uneven appearance of extruded NWs and a decrease in their length. The backward extrusion process can produce amorphous NWs from crystal NWs. The forward extrusion process can produce NWs that are longer than those produced with the backward extrusion process.

A molecular dynamics study of the influence of nucleation conditions on the phase selection in Fe50Mn30Cr10Co10 high entropy alloy

Non-equiatomic high entropy alloys with single or multiphase microstructures offer opportunities to optimize the mechanical properties through compositional variation. The Fe-Mn-Cr-Co alloys have shown the potential to activate nanotwinning at large strains even at room temperatures, thereby enhancing both ductility and strength. This paper presents a molecular dynamics study of phase formation during solidification of Fe50Mn30Cr10Co10 using a modified embedded atom method potential. The evolution of the phases has been investigated using adaptive common neighbour analysis, composition analysis, and radial distribution function. In contrast to the reported microstructures containing FCC and HCP phases, a BCC phase with the average composition of the alloy has resulted under homogeneous conditions. On the other hand, heterogeneous nucleation from an initial FCC seed resulted in a dual-phase microstructure of FCC and HCP at 300 K in agreement with the experiments. Interestingly, annealing at 900 K did not alter the phase fractions in samples from both nucleation conditions. Analysis based on the empirical thermodynamic parameter suggested by Ye et al. [Materials Today (2016), Vol19, p349–362] and valence electron concentration (VEC) supports the formation of both types of microstructures observed here. Hence, biasing the nucleation process is believed to strongly influence the type of phases formed in this alloy.

Efficiency in identification of internal structure in simulated monoatomic clusters: Comparison between common neighbor analysis and coordination polyhedron method

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.

Molecular dynamics and DFT study of 38-atom coinage metal clusters

The thermal behavior of 38-atom mono-, bi-, and trimetallic clusters consisting of Cu, Ag, and Au atoms, is analyzed employing molecular dynamics simulations and DFT calculations for selected cluster compositions. Low-energy structures were singled out to perform NVT molecular dynamics simulations at several temperatures, using the Andersen thermostat for temperature control. The caloric curve is used to estimate the melting temperature and the specific heat. The pair distribution function g(r) of the solid and liquid-phase clusters is examined at different temperatures. When comparing the estimated melting points (Tm) among the monatomic clusters, the order becomes TmCu38>TmAg38>TmAu38. For bimetallic clusters, an increase of Tm is observed for Cu-Au compared to their monatomic counterparts, while the opposite occurs for Cu-Ag clusters. For trimetallic clusters, two low-energy isomers of the Cu36Ag1Au1 cluster are investigated. In this case, Tm is estimated to be 475 K, for the two isomers with the lowest-energy and second-to-lowest energy, respectively. For all the clusters studied, the pair distribution function g(r) shows that the first peak position is not shifted as an effect of temperature and its maximum value varies with composition, while the second peak essentially vanishes upon melting. The common-neighbor analysis (CNA) technique is used to analyze the local structural changes for the trimetallic clusters, again demonstrating a clear structural change upon melting. The HOMO-LUMO energy gap indicates that the trimetallic isomers' behavior is metallic, while the average binding energy show these clusters' energetic stability to be similar.

Effect of water medium on material removal and sub-surface defect evolution in nano-cutting of single-crystal γ-TiAl alloy

ABSTRACT In this paper, molecular dynamics (MD) simulation has been applied to study the influence of water medium on the nano-cutting process and the evolution of sub-surface defects of single-crystal γ-TiAl alloy. The effect of water medium in the result of cutting force and cutting temperature have been analysed and discussed in the machining process, while the function of common neighbour analysis (CNA) method has been used to research the chip removal mechanism, sub-surface defects and crystal structure changes. The results showed that the extrusion and shear effects of tool removed atoms from the surface of the workpiece. In the meantime, the cooling and lubricating effect of the water medium reduces the cutting temperature and cutting force. Especially, the temperature of the workpiece has a diffusion effect in the cutting process in the absence of the water medium. Some heat was taken away by the water medium, which lead to insufficient nucleation and migration of the dislocation, and high compressive stress was formed during cutting of water-bearing medium.

Study on wear behavior of FeNiCrCoCu high entropy alloy coating on Cu substrate based on molecular dynamics

The high entropy alloy (HEA) is a potential material for coating because of its high hardness and wear resistance. In order to study the wear performance and protection ability of FeNiCrCoCu HEA coating on Cu substrate, the friction behavior of abrasive particle and material wear and damage were studied by molecular dynamics (MD). The cutting of depth and velocity of abrasive particle were used as variables for simulation. By analyzing the mechanical data of results, it was found that the friction coefficient of abrasive particle decreased gradually with the increase of the cutting of velocity. Dislocation extraction analysis (DXA) and Common neighbor analysis (CNA) were used to analyze the lattice structure and dislocations. By analyzing the above data, the wear and damage of the material was obtained, and the inhibition behavior of Hirth dislocation lock on dislocation nucleation was found. Cu substrate damage results showed that HEA coatings could effectively improve the wear resistance of the material surface. In the case of small cutting of depth, the high stress region in the material was concentrated below the abrasive particle, and the material subsurface damage would be intensified. This research can provide a theoretical basis for the application of HEA coatings.

Elastic stress induced metastable local ordering in Cu-Zr-Al metallic glasses: A numerical simulation study

This work presents a study based on numerical simulations to describe the local structural evolution of Cu-Zr-Al amorphous alloys when moderate stresses are applied and removed. In particular, it will seek to recognize the formation of clusters with some degree of symmetry in the amorphous matrix and how these are altered depending on the mechanical stress experienced by them. The selected technique is classical molecular dynamics, using the ‘Embedded-atom method’ (EAM) and some other studies trying to identify local crystalline order (Common Neighbor Analysis, atomic centro-symmetry parameter, radial distribution functions and the reciprocal space pattern). The simulations confirm the local order/disorder mechanism based on the existence of ordered regions, which depends on the nature of the applied stress. Particularly, this work focused on performing simulations in time and stress regimes close to anelastic experimental conditions, which makes it different to other related computational reports that focus on regimes close to plastic deformation. The atomistic simulations provide strong evidence of the local ordering occurrence, in good concordance with early ultrasound assisted experiments on Cu-Zr-Al amorphous alloys, thus providing strong evidence of the local ordering occurrence in an elastic regime when uniaxial stress is applied. The reversibility of the system, when stress is removed, was also shown. The application of uniaxial stress favors the presence of different kind of local order structures, when compared with the situation of hydrostatic pressure.

Dislocation and deformation analysis of Cu-Ni thin films during Nano-indentation using molecular dynamics simulation approach

Aim of this paper is to investigate the effect of varying the size of indenter, speed of indenter and thickness of nano-coating on hardness and dislocations. The dislocation mechanism, common neighbour analysis in Cu-Ni, is analysed by changing the distinctive parameters of indentation using molecular dynamics simulation approach. Overall indentation impact and mechanisms are studied with indentation velocity, indenter radius, and Nickel layer thickness. The embedded atom method based potential is used to describe the interaction between atoms. From the analysis it was seen that by applying varying thickness of nano-coating of nickel over a copper substrate, the hardness value increases corresponding with force as the thickness of nickel coating increases. Cu-50Ni is having more value of harness, which is 43% higher than pure Copper. The total length of dislocations for Cu-50Ni was 5% and 20% less than Copper and Cu-20Ni, respectively, determined by dislocation analysis at full load. From varying loading rate analysis, is was found that hardness value for Cu-Ni bilayer with loading rate of 1.5 Å/ps is maximum compared to other loading rates and is 7.40% and 36% more than Cu and Cu-Ni at a loading rate of 0.5 Å/ps, respectively. Then from by varying indenter radius, it was seen that the hardness value decreases with an increase in indenter radius. The hardness value of Cu-Ni with an indenter radius of 30 Å is 31% and 30% more than Cu and Cu-Ni with an indenter radius of 50 Å, respectively. The total length of dislocations for Cu-50Ni was 37% and 25% less than Cu and Cu-Ni with an indenter radius of 50 Å, which was determined by dislocation analysis at full load.

Molecular dynamics study on the deposition of Ni/graphene composite film

The Ni/graphene composite films deposited on Fe (001) substrate were studied by molecular dynamics simulation. The effects of incident energy and graphene nanoplatelet (GNP) size on the growth of the films were studied. The surface roughness, growth mode, crystal structure and orientation of the films under different deposition conditions were compared and analyzed. The simulation results show that when the incident energy is low, the Stranski-Krastanov growth mode is dominant; when the incident energy reaches a certain value, the film begins to grow in the Frank-Vander Merwe mode. The surface roughness decreases with the increase of incident energy due to the enhancement of surface diffusion. With the addition of GNPs, the surface roughness of the films increased significantly. The crystal structure and orientation of the deposited films were studied based on the adaptive common neighbor analysis and centrosymmetric parameters. The incident energy affects the crystal structure and orientation of the films. GNPs affect the growth mode of Ni grains, which makes Ni grains grow along (111) direction on it.

Effect of variation in inclination angle of Ʃ5 tilt grain boundary on the shock response of Ni bicrystals

The intensity of damage production during shock wave propagation in polycrystalline metallic systems is mostly dependent on the shock-defect interactions. Traditionally, different coincidence site lattice (CSL) grain boundaries (GBs) are introduced in the polycrystalline structure through various processing techniques to enhance the strength or plasticity. However, their dynamic response to the impulsive loading condition has not been elaborately explored to date, which necessitates a detailed study on the interaction between CSL GBs and shock wave. In this study, we have employed molecular dynamics simulations to investigate the response of both symmetric and asymmetric ∑5[1 0 0] tilt GBs of nickel bicrystal under the influence of shock-wave. We have also analysed the role of piston velocities and inclination angles of the GBs on the shock response and the consequent deformation behaviour. This investigation gives insight into the mechanical response and the underlying mechanisms which are inspected through atomic strain analysis, common neighbour analysis and pressure–time–distance mapping. The results show that the stacking faults formation takes place when specimens are subjected to lower shock velocities, whereas higher velocities facilitate phase transformation along with amorphization. There is a stark contrast between the specimens with symmetric and asymmetric tilt GBs in the manner of plastic deformation behaviour in response to shock-wave and GB failure at lower and higher piston velocities.

Development of a Structural Comparison Method to Promote Exploration of the Potential Energy Surface in the Global Optimization of Nanoclusters.

A structural comparison method (SCM) was created to quantify the structural diversity of nanoclusters and was implemented into a global optimization algorithm to evaluate structural diversity between generated clusters on the fly and promote exploration of the potential energy surface. The SCM evaluated topological differences between clusters using the common neighbor analysis and provided a numerical measure of similarity between the two clusters. The SCM was implemented into a genetic algorithm by integrating it into a new structure + energy fitness operator such that structural diversity of clusters in the population and their energies were used to assign fitness values to clusters. The efficiency of the genetic algorithm with this new fitness operator was benchmarked against several Lennard-Jones clusters (LJ38, LJ75, and LJ98) known to be difficult cases for global optimization algorithms. For LJ38 and LJ75, this new structure + energy fitness operator performed equally well or better than the energy fitness operator. However, the efficiency of locating the global minimum of LJ98 decreased using this new structure + energy fitness operator. Further analysis of the genetic algorithm with this fitness operator showed that the algorithm did indeed promote exploration of the PES of LJ98 as desired but hindered refinement of clusters, preventing it from locating the global minimum even if the energy funnel of the global minimum had been located.

Defect dynamics and uniaxial tensile deformation of equi and non-equi-atomic configuration of multi-elemental alloys

In this article, molecular dynamics and statics-based simulations were carried out to study the effect of equi-atomic and non-equi-atomic compositions of multi-elemental alloys on defect dynamics and tensile deformation. High and medium entropy alloy configurations were subjected to uni-axial tensile loading, and deformation governing mechanism was identified with the help of dislocation extraction algorithm and common neighbour analysis. Due to low or negative stacking fault energy, phase transformation from FCC to HCP in conjunction with solid solution hardening was identified as the governing mechanism for the plastic deformation in most of the multi-elemental alloys. Atomistic snapshots of simulation box containing deformed configurations of medium and high entropy alloys also show heterogenous nucleation, pinning of dislocations, cross channelling of twin boundaries. It was predicted that in alloys with higher Cr content twinning and stacking fault is prominent mode of deformation. Nudged elastic band algorithm was used to predict the vacancy migration energies in each of the atomistic configurations. The results presented in the article are in qualitatively in agreement with the experimental results, and higher fidelity simulations helps in developing better understanding about the plastic deformation in equi and non-equi-atomic composition of multi-elemental alloys.

Melting dynamics of Gold thin film: A Molecular Dynamics study

We studied the melting dynamics of Gold in the form of thin film. The thin film thickness is 9.756 nm. The systems are heated from room temperature up to slightly above melting point, Tm = 1338 K. The evolution of the atoms in each system is followed using Molecular Dynamics (MD) scheme up to 20 ps. Thermodynamics analysis indicated that thin film is suppressed by the uniform heat while the temperature is increasing. Structural analysis compared with Common Neighbor Analysis (CNA) expressed the condition that melting conduction occurred at the end of simulation.

The Structure and Crystallizing Process of NiAu Alloy: A Molecular Dynamics Simulation Method

This paper studies the influence of factors such as heating rate, atomic number, temperature, and annealing time on the structure and the crystallization process of NiAu alloy. Increasing the heating rate leads to the moving process from the crystalline state to the amorphous state; increasing the temperature (T) also leads to a changing process into the liquid state; when the atomic number (N), and t increase, it leads to an increased crystalline process. As a result, the dependence between size (l) and atomic number (N), the total energy of the system (Etot) with N as l~N−1/3, and −Etot always creates a linear function of N, glass temperature (Tg) of the NiAu alloy, which is Tg = 600 K. During the study, the number of the structural units was determined by the Common Neighborhood Analysis (CNA) method, radial distribution function (RDF), size (l), and Etot. The result shows that the influencing factors to the structure of NiAu alloy are considerable.