Atomic Potential Energy Research Articles

Linear dependence of grain boundary energy on structural orderliness in FCC metals

Establishing the correlation between grain boundary (GB) structure and energy is crucial for advancing the fundamental understanding of polycrystalline materials, yet this correlation remains elusive. In this study, systematic atomic simulations are performed to comprehensively explore numerous GBs in different FCC metals and alloys. The findings unveil a universal linear relationship between the GB energy and GB structural orderliness parameter (GB excess centrosymmetry or excess bond orientational order parameters). Remarkably, the identified linear relationship proves universally applicable across different metal systems, GB types, and length scales, rooted in the linear correlation between atomic potential energy and atomic structural orderliness. Furthermore, the ratio between GB energy and orderliness parameter emerges as a potent indicator for gauging the extent of GB relaxation, thereby providing valuable insights to inform the design of nanocrystalline materials with enhanced GB stability.

Atomistic simulation on the transformation behaviour of structural units at Σ27 tilt grain boundary

ABSTRACT Molecular dynamics simulations have been employed to investigate the transformation of structural units at the grain boundary under tensile conditions. A series of symmetric tilt-[110]-Σ27 bi-crystal Cu models are considered in this research. The findings indicate that atomic potential energy significantly influences the transformation of structural units. The potential energy of the common atom from two neighbouring structural units breaks through the threshold due to external loading, leading to its migration to adjacent atomic layers. This migration triggers the transformation of the structural unit and results in the nucleation of dislocations on the grain boundaries, which further leads to a precipitous decline in the potential energy of the grain boundaries.

Effect of biphasic interface on mechanical properties and deformation mechanism of nickel base alloy

The tensile properties of biphasic nickel-based nanopolycrystalline alloys (Ni–Ni3Al) at different temperatures were simulated by molecular dynamics. The results show that the tensile properties of the dual-phase nano-polycrystalline alloy are obviously better than those of the conventional nickel metal. The γ 'phase (Ni3Al) is LI2 type structure, which shows higher stability and reduces grain boundary migration during deformation due to the lower atomic potential energy of γ′ phase. As the temperature increases, the mechanical behavior of the biphasic alloy and nickel metal shows the same decreasing trend. Dislocation is an important factor affecting the strength of γ 'phase plastic deformation. The current study provides an atomic perspective on the deformation process and mechanism of biphasic nickel-based nanopolycrystalline alloys, and provides a new perspective for the design of novel superalloys with excellent mechanical properties.

Multiscale analysis method and experiments for the transverse mechanical property of unidirectional fiber-reinforced composites

Carbon fiber-reinforced epoxy matrix composite laminates have been widely used in the case of advanced aero-engines, due to their high specific mechanical properties and mature production technology. In order to predict accurately the transverse mechanical properties of composites, this paper developed a multiscale analysis method which consists of three scales: At the microscale, an interfacial cohesive zone model is established based on atomic potential energy to describe the interface. At the mesoscale, a unit cell model is established to represent the fiber, matrix, and interface. At the macroscale, the homogenization method, failure criteria, and damage degradation models are utilized to predict transverse mechanical properties. The results of predictions are compared with experimental tests, and it is found that the maximum errors are less than 3%. Furthermore, the predicted damage evolution path aligns with the experimental results, thus validating the accuracy of the multiscale analysis method. By employing the multiscale analysis method, the effects of interfacial strength on the macroscopic transverse mechanical properties are analyzed. The simulation results indicate that: The interfacial strength has a more pronounced influence on the transverse strength and ultimate strain compared with the transverse modulus. Reducing the interfacial strength has a greater influence on the transverse modulus, strength, and ultimate strain than increasing the interfacial strength. The interfacial cohesive zone model can reflect the nonlinearity of epoxy matrix composite materials. Overall, the multiscale analysis method proves to be effective in accurately predicting the transverse mechanical properties of composites, and provides insights into the interfacial strength’s role in these properties.

Research of the caged dynamics of clusters center atoms in the Pd82Si18 amorphous alloy

Abstract To date, there is still a lack of a comprehensive explanation for caged dynamics which is regarded as one of the intricate dynamic behaviors in amorphous alloys. This study focuses on Pd82Si18 as the research object to further elucidate the underlying mechanism of caged dynamics from multiple perspectives, including the cage’s lifetime, atomic local environment, and atomic potential energy. The results reveal that Si atoms exhibit a pronounced cage effect due to the hindrance of Pd atoms, resulting in an anomalous peak in the non-Gaussian parameters. An in-depth investigation was conducted on the caged dynamics differences between fast and slow Si atoms. In comparison to fast Si atoms, slow Si atoms were surrounded by more Pd atoms and occupied lower potential energy states, resulting in smaller diffusion displacements for the slow Si atoms. Concurrently, slow Si atoms tend to be in the centers of smaller clusters with coordination numbers of 9 and 10. During the isothermal relaxation process, clusters with coordination numbers 9 and 10 have longer lifetimes, suggesting that the escape of slow Si atoms from their cages is more challenging. The findings mentioned above hold significant implications for understanding the caged dynamics.

Effect of Al2O3 nanoparticle dispersion on the thermal properties of a eutectic salt for solar power applications: Experimental and molecular simulation studies

Thermophysical properties of molten salts aid in determining the storage efficiency of thermal energy storage (TES) systems in concentrating solar power plants. In this research, a novel ternary molten salt (4NaCl–37KCl–59LiNO3, mol%) was utilized as the phase change material (PCM), while Al2O3 nanoparticles (NPs) were employed as a thermal conductivity enhancer. Experimental measurements and molecular dynamics simulations were conducted to confirm that the introduction of Al2O3 NPs as dopants enhanced the specific heat, thermal conductivity, and energy storage density of the eutectic salt. Specifically, at an Al2O3 NP content of 0.5 wt%, the composite PCM exhibited a 44.23 % increase in liquid-state specific heat and a 40.82 % increase in liquid-state thermal conductivity, along with a 15.73 % increase in storage density. This study demonstrates that the enhanced thermal properties of eutectic salts by adding Al2O3 NPs are primarily attributed to the emergence of nanostructures and alterations in atomic potential energy, as evidenced by both experimental and simulated microstructural analyses. These findings offer valuable insight into the selection of thermal storage materials and additives, as well as their implementation in medium-temperature TES systems, with the objective of optimizing the efficient utilization of solar energy.

Formation and fracture of Mg88Al6Zn6 MGs analyzed by topologically close-packed cluster correlation

The microstructure of metallic glasses (MGs) significantly impacts glass transition and mechanical properties, but the current research on this topic is still not comprehensive enough. Therefore, the rapid solidification and tensile processes of Mg88Al6Zn6 MGs have been investigated in this paper by molecular dynamics simulations and their microstructure has been analyzed by the Largest Standard Cluster Analysis (LaSCA). The results show the transformation of other clusters to topological close-packed (TCP) and defective TCP (D-TCP) clusters during the rapid solidification process. The transformation rate slows down significantly near the glass transition temperature (Tg) 530 K, and the two finally reach 39.19%. These clusters can be divided into five groups (c1–c5) based on their correlation indexes, and their dynamic behavior and hereditary are discussed. Clusters with strong correlations tend to form larger-sized medium-range ordered nanoclusters, which serve as the “structural skeleton” of MGs. During the tensile process, TCP and D-TCP clusters with a lower average atomic potential energy (−0.57 eV) exhibit a higher packing density and structural stability, consequently enhancing the strength of MGs (3.29 GPa). Furthermore, the larger-sized nanoclusters formed by strongly correlated clusters also contribute to the strength of local regions, thereby improving the overall tensile performance of MGs. These findings offer valuable insights into the microstructural mechanisms involved in the formation and fracture of MGs.

Comparison of Taboo Search Methods for Atomic Cluster Global Optimization with a Basin-Hopping Algorithm.

The basin-hopping algorithm (BHA) allows for the efficient exploration of atomic cluster potential energy surfaces by random perturbations in configuration space, followed by energy minimizations. Here, the taboo search method is incorporated to prevent the search from revisiting recently visited regions of the search space. Two taboo search modes are implemented, one mode resets the search to random coordinates upon encountering the taboo region, while the other simply rejects any proposed move into the taboo region. These two modes are tested and compared on a variety of potential energy surfaces─several clusters where atomic interactions are described by the Lennard-Jones potential, and Au55 where a semi-empirical tight binding potential is used to describe atomic interactions. Some differences in performance between the two taboo search modes were noted for LJ38 and Au55, with the mode that rejects all hops into the taboo region performing better, offering a means to improve the efficiency of the BHA for multifunnel systems. However, both taboo search modes failed to significantly improve performance on multifunnel systems where more than two funnels were present in the system.

Atomic potential energy uncertainty in machine-learning interatomic potentials and thermal transport in solids with atomic diffusion

Atomic potential energy uncertainty in machine-learning interatomic potentials and thermal transport in solids with atomic diffusion

Fitting the charged-optimized many-body potential for the Al-O-Se-Zn system

A charge-optimized many-body (COMB3) potential has been developed for the Al-O-Se-Zn system using first-principles data. This work updates COMB3 for zinc and extends it to include selenium as well as other pairwise and three-body interactions that were not developed previously. We tested the empirical potential for known crystalline phases and find reasonable agreement between the calculated structural, elastic, and vibrational properties and experimental data. We use the anisotropic coefficients of the thermal expansion method to calculate the temperature-dependent properties of a few selected materials. We then compare the total energy of the system and its first- and second-order positional derivatives, as well as the second-order strain derivative with respect to the atomic potential energy surface to thermal properties obtained from molecular dynamics simulations using the fitted COMB3 potential. The results of our temperature-dependent calculations show reasonable agreement with previous work for temperatures below the system’s melting point. With this fitting, we could utilize a versatile, charge-dependent empirical potential to model the interface between ZnSe and Al2O3 by molecular dynamics simulations.

Surface Effect on Phase Transformation of Single Crystal NiTi Shape Memory Alloys Studied by Molecular Dynamics Simulation

Surface effect on phase transformation of single crystal (SC) NiTi shape memory alloys (SMAs) with various thicknesses and various Ni contents is studied by molecular dynamics simulation. For the SMAs with various thicknesses considering surface effect, the residual strain, the average atomic potential energy, and the four characteristic phase transformation temperatures all increase, and fewer twinned martensite boundaries are left after cooling compared to the sample without considering surface effect. Moreover, with increasing thickness of the sample, the surface effect is gradually weakened, and there is no obvious regularity in phase transformation characteristics. For the SMAs with various Ni contents considering surface effect, segmental martensite phase transformation, phase transformation fluctuation, and wider phase transformation temperature ranges are observed, whereas these phenomena do not occur in the sample without considering surface effect. With the increasing of Ni contents, for both cases of considering surface effect or not, the critical stress of phase transformation, average atomic potential energy, the modulus of detwinned martensite increase, but the phase transformation temperature and modulus of twinned martensite decrease.

Structural mechanisms of enhanced mechanical property in ZrCu metallic glass at low temperatures

The application of traditional crystalline materials as structural materials cannot meet critical design challenges due to low-temperature brittleness. To address this issue, this study investigates the deformation behavior of Zr2Cu metallic glass (MG) model at ambient and low temperature states by molecular dynamics (MD) simulations to unravel the governing mechanisms associated with mechanical performance. Structurally representative amorphous models of the MG were developed and their evolutions under tensile loadings at ambient (300 K) and low temperatures (250 K, 200 K, 150 K, 100 K, 50 K) were captured. Notably, significant variations in maximum stress and corresponding strains characterized the stress-strain behavior as elastic-plastic properties increased with decreasing temperatures. Stress activations of the atomic mass were rapidly incipient at the ambient condition, but largely constrained at low temperatures by which the density variations of activated zones and the emerging strain of first shear transformation zones (STZ's) established a negative correlation with temperature as inferred by the wider strain range of deformation. Disparities in strain thresholds accounted for the higher free volume content at the ambient state, but as temperatures declined, fine structural differences stimulated the formation of densely-packed structures in the amorphous model at low temperatures by the spontaneous annihilation of excessive free volumes. Consequently, the higher growth rates of the average atomic potential energy and the retarded emergence of high-potential-energy atomic mass suggest that at low temperatures, there is a higher resistance to shear band nucleation and higher barrier for atomic migration. As a result, the generation of free volumes, formation of STZs and shear bands were retarded, leading to the enhanced ductility. Thus, at low temperatures, unique tuning mechanisms of structural performance were evolved in the amorphous model that suitably enhanced the constitutive properties of Zr2Cu MG.

Probing neck growth mechanisms and tensile properties of sintered multi-nanoparticle Al-Cu systems via MD simulation

In the present work, a series of molecular dynamics simulations are conducted to figure out how sintering parameters would influence the atomic structure, sintering mechanisms, and elastoplastic properties of sintered Al-Cu nanoparticulate systems. For this purpose, first, utilizing the atomic potential energy diagrams, the suitable sintering temperatures for the introduced nanoparticles (NPs) have been calculated. NPs are then sintered at the temperatures of 410, 510, 600 and 680 K, and microstructural changes have been probed during the process. Finally, employing a uniaxial tensile test, the sintering temperature and holding time effects on the tensile behavior of the sintered products are studied in detail. Our simulation results indicate a strong correlation between the crystalline structure of the sintered NPs and the process temperature. It is also concluded that the main sintering mechanism at low-temperature conditions is dislocation slip, while at elevated temperatures, the sintering mechanism switches to diffusion-based phenomena. Moreover, it is revealed that increasing the sintering temperature from 410 to 680 K enhances the Young modulus of the samples monotonically, while the greatest yield strength is achieved at 600 K. Similar correlation is also found between the holding time and mechanical properties of the final product.

Effect of Intragranular Solute Atom Segregation on the Mechanical Properties of the Ni–Co Alloy

In the present contribution, a certain amount of Co atoms was added to the grains (GI) of nano-polycrystalline Ni by a molecular dynamics method to study the effect of solute atom content on the mechanical properties of the alloy. It was found that the tensile strength and yield strength of the alloy increased with the increase of Co atomic content. With the increase of Co content, the grain boundary atomic potential energy decreased and the grain boundary became more stable. As the strain increased, the Co atoms in the grain migrated to the grain boundary, the lattice distortion at the grain boundary increased, and the dislocation nucleation at the grain boundary increased, forming a stable dislocation network and hindering the grain boundary migration. The 1/6⟨112⟩ Shockley incomplete dislocation formed at the grain boundary became the stacking fault source, which was also the source of transformation from the internal stacking fault to the external stacking fault, finally transforming into deformation twins. As the strain increased, the content of Shockley incomplete dislocations in the crystal increased, interacting with other dislocations to dominate the deformation process.

Critical contact angle for triggering dynamic Leidenfrost phenomenon at different surface wettability: A molecular dynamics study

The microscopic mechanism of the dynamic Leidenfrost phenomenon involving the impact of droplets on high-temperature surfaces remains unclear, although many efforts have been devoted to the static Leidenfrost phenomenon of evaporative sessile droplets. In this work, we study the dynamic Leidenfrost phenomenon of droplets impacting high-temperature surfaces with different wettability by using water and copper. The simulations by the molecular dynamics method show that determined by the atomic potential energy, a hydrophilic surface favors a lower dynamic Leidenfrost temperature compared to a hydrophobic surface. It is found that there is a critical contact angle that can trigger the Leidenfrost phenomenon at different surface temperatures. The critical contact angles are about 44.06°, 87.93°, and 92.63° when the surface temperatures are 998 K, 1,098 K, and 1,198 K, respectively. That the critical contact angle becomes larger with elevated surface temperature is helpful for industrial designs, in which novel insulating materials with higher Leidenfrost temperatures are required.

Mechanism of Surface Wettability of Nanostructure Morphology Enhancing Boiling Heat Transfer: Molecular Dynamics Simulation

In this paper, the interaction mechanism between the solid–liquid–gas interface phenomenon caused by nanostructure and surface wettability and boiling heat transfer is described, and the heat transfer theory of single wettable nanostructure surface and mixed wettable nanostructure surface is proposed. Through molecular dynamics simulation, the thermodynamic model of the wettable surface of nanostructures is established. The nanostructures are set as four rectangular lattice structures with a height of 18 Å. The solid atoms are platinum atoms, and the liquid atoms are argon atoms. The simulation results show that with the increase of surface hydrophilicity of nanostructures, the fluid temperature increases significantly, and the heat transfer at the interface is enhanced. With the increase in surface hydrophobicity of nanostructures, the atoms staying on the surface of nanostructures are affected by the hydrophobicity, showing a phenomenon of exclusion, and the evaporation rate in the evaporation area of nanostructures is significantly increased. In addition, the mixed wettable surface is influenced by the atomic potential energy and kinetic energy of the solid surface, and when compared with the pure wettable surface under the nanostructure, it changes the diffusion behavior of argon atoms on the nanostructure surface, enhances the heat transfer phenomenon compared with the pure hydrophobic surface, and enhances the evaporation phenomenon compared with the pure hydrophilic surface. This study provides insights into the relationship between the vapor film and the heating surface with mixed wettability and nanostructures.

Heredity of clusters in liquid Ta rapid solidification process and its correlation with local symmetry

Metallic glass (MG) has received intensive attention in the fields of amorphous physics and materials science, owing to its excellent mechanical properties, good corrosion resistance, and large elastic deformation limit. Comparing with traditional oxide glass, the limited glass-forming ability (GFA) seriously restricts the application of MG in engineering. Therefore, the GFA has been a hot scientific issue in the field of amorphous material research. Recently, scientists have fully realized that GFA is closely related to the local atomic structure in liquid as well as its evolution features. Since the MG is called the “freezing” liquid, exploring the correlation of local atomic structures between liquid phase and solid phase under rapid solidification conditions is helpful in understanding the microstructural mechanism of GFA. Therefore, the rapid solidification process of liquid Ta is investigated via molecular dynamics simulation. The pair correlation function (PDF), the largest standard cluster (LSC), and the reverse atomic trajectory tracking methods are used to characterize and analyze the microstructure and its evolution during the rapid solicitation of liquid Ta. The results show that the local atomic configurations of the rapidly solidified Ta are various Kasper clusters as well as their distorted configurations, among of which [1/444, 10/555, 2/666] deformed icosahedron (or Z13 cluster) accounts for the highest proportion. The trend of hereditary ability of clusters revealed by the onset temperature of continuous heredity is consistent well with that by the fraction of staged heredity. The geometric symmetry of clusters can be quantitatively characterized by using the local symmetry parameter (LSP). The hereditary ability of clusters is closely related to their LSP. The local five-fold symmetry is beneficial to enhancing hereditary ability, while local four- and six-fold symmetry are disadvantageous for that. The probability of clusters with the same LSC index emerging in the energy range follows the Gaussian distribution, and the expected average atomic potential energy <inline-formula><tex-math id="M3">\begin{document}$ {E}_{\rm exp}^{j} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20231153_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20231153_M3.png"/></alternatives></inline-formula> is almost linearly related to the LSP, and <inline-formula><tex-math id="M4">\begin{document}$ {E}_{\rm exp}^{j} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20231153_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="24-20231153_M4.png"/></alternatives></inline-formula> decreases with the increase of LSP<sub>5</sub>. The high local five-fold symmetry reduces the average atomic potential energy of LSC, thereby enhancing its configurational heredity. These findings have guiding significance in improving GFA through regulating the local symmetry of liquid monatomic metals or alloys.

A revised Lennard-Jones potential for bubble nucleation study of argon based on the molecular dynamics simulation method

Bubble nucleation in boiling heat transfer is a microscale phenomenon, which is usually investigated by the molecular dynamics simulation method with the study object of argon. The simple potential model of argon favors revealing the bubble nucleation mechanism. However, the published works indicate that the required heating temperature for achieving bubble nucleation inside liquid argon is unreasonable and usually over the critical temperature. Therefore, in this study, a revised Lennard-Jones (L-J) potential for bubble nucleation study of argon is proposed based on the “PK” norm that bubble nucleation happens when the average kinetic energy of a group of liquid atoms exceeds their potential barrier (absolute value of atomic potential energy). The parameters α and β are added to the original L-J potential to weaken the phase transition barrier and reduce atomic balance distance, corresponding to the functions of lowering onset nucleation temperature and avoiding the unreasonable change of liquid argon properties, respectively. Besides, a moving top wall based on the force balance is conducted to maintain the simulation system pressure at the expected value. Simulation results indicate that based on the revised L-J potential and pressure controlled at one atmosphere, only the evaporation phenomenon happens when the heat source temperature is fixed at 85 K within the simulation time of 10 ns, while it converts into the bubble nucleation phenomenon as the thermostat temperature is raised to 90 K, which is very close to the onset boiling temperature of liquid argon (87.2 K). In addition, the relative differences between the original and revised potentials in the density, thermal conductivity, and thermal diffusion coefficient are 1.31%, 33.50%, and 233.33%, respectively. The difference in the third property is significant, but compared with the benchmark value obtained by the popular property database RefProp, the computational accuracy of the revised potential is higher than the original potential. Hence, the revised L-J potential is more suitable for the bubble nucleation study of argon by using the molecular dynamics simulation method.

Fluctuation–dissipation analysis of heat and mass flow in energy transport at different CO2 hydrate dissociation interfaces

Understanding the mechanism of kinetic behaviors is necessary for gas hydrate application. In this paper, the system of three-phase and two-interface was established, and the kinetic dissociation characteristics in different CO2 hydrate interfacial areas were compared using molecular dynamic simulations. It is found that the dissociation rate in the right interfacial region with relatively more carbon dioxide molecules is faster than that with relatively more water molecules. The mechanism of different kinetic behaviors at different dissociation interfaces was analyzed from the perspective of mass transfer, heat transfer and energy transport. The results show that the fluctuation–dissipation theory holds well for the number of hydrate-like CO2 molecules in the whole and part of the bulk hydrate at the near-equilibrium state, the interfacial latent heat flow in both interfacial areas, the interfacial average atomic potential energy difference, and the interfacial sensible heat flow in the left interfacial area at the non-equilibrium state. In which, the potential energy transport from the adjacent liquid to hydrate-like water molecules has an inherent correlation with the interfacial latent heat flow, and both have the similar fluctuation period, 900 ps and 1000 ps. The interfacial thermal resistance (ITR) was used to represent the dissipation of the net heat flow fluctuation amplitude, the values for the left and right interfacial regions are 5.57 × 10-10 m2·K/W and 5.46 × 10-10 m2·K/W, which reveals the faster heat transfer (including latent and sensible heat) in the right interfacial regions with relatively more carbon dioxide molecules. In addition, it was inferred that the mass transfer plays a leading role when there is no initial temperature gradient between the hydrate and the water (gas) phase, and in contrary, the heat transfer dominants.

‘Anti-sluggish’ Ti diffusion in HCP high-entropy alloys: Chemical complexity vs. lattice distortions

Self-diffusion of Ti in HCP HfTiZr and AlScHfTiZr multi-principal element alloys is measured using the radiotracer technique and applying the 44Ti isotope. In the temperature interval from 973 K to 1373 K, no systematic deviations from linear Arrhenius temperature dependencies are observed. Alloying equiatomic HfTiZr with Al and Sc enhances Ti diffusion rates and the effect becomes more pronounced with increasing Al content. The Ti diffusivities in the present multi-principal element alloys are found to exceed the values predicted by a simple geometric mean of the Ti diffusion coefficients in the pure metals by orders of magnitude, a phenomenon which we refer to as ‘anti-sluggish’ diffusion. Lattice distortions are speculated to dominate the relative enhancement of Ti diffusion in these HCP high-entropy alloys, inducing the ‘anti-sluggish’ behavior. The experimental findings are supported by ab initio-derived mean squared atomic displacements and potential energy fluctuations in these alloys.