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Influence of hydrogen concentration and defects introduced by neutron irradiation on hydrogen diffusion in tungsten has been investigated by molecular dynamics simulation at elevated temperatures. Hydrogen diffusion is shown to be significantly restrained at high concentrations due to spontaneous formation of platelet-like hydrogen clusters. For neutron irradiation defects, self-interstitials, mono-vacancies and vacancy clusters are considered. By clustering and acting as dislocation loops, self-interstitials show considerable trapping effects on hydrogen, leading to the suppression of hydrogen effective diffusion and the change of diffusion model in which hydrogen mainly diffuses along dislocation lines instead of hopping between tetrahedral interstitial sites. Moreover, an equation connecting hydrogen diffusion parameters and the total length of dislocation loops is empirically established. Different influences of mono-vacancies and vacancy clusters on hydrogen diffusion have been carefully identified. With the same vacancy concentration, hydrogen diffusivity is lower with mono-vacancies than that with vacancy clusters because more isolated trapping sites are provided by mono-vacancies. This work is not only helpful for understanding the synergistic effects of neutron irradiation and plasma interaction, but also potentially applicable for larger scale simulations as input data.

Atomistic simulations were employed to investigate the stress field spatial distributions, strain energy of the 1/2 〈1 1 1〉 {1 1 0} edge dislocation, and the binding energies between the edge dislocation and the point defects in tungsten, based on two different embedded-atom method (EAM) potentials. The basic static properties are consistent with the elastic theory of dislocations for these two potentials. Comparatively speaking, the map of the binding energy between the edge dislocation and the monovacancy illustrates that the results based on the Marinica’s potential fits better with the elastic theory. In addition, we obtained the radii of absorption between the point defects and the edge dislocation in the slip plane (the maximum value is 19 A for the monovacancy, while 34 A for the self-interstitial atom (SIA)) at 0 K. By calculating the binding energy and the interaction radii, we found the intensity of the interaction between the SIA and the edge dislocation is stronger than that of the vacancy and the edge dislocation.

Intervertebral disc degeneration (IDD) is a common orthopedic multifactorial disease associated with spine-related disorders, such as low back pain. Recent studies have shown that both platelet-rich plasma (PRP) and exosomes could be used to treat IDD, but the effects and mechanism of PRP-derived exosomes in the treatment of IDD are still unclear. This study showed that PRP-derived exosomes inhibited the polarization of M1 macrophages by regulating the NF-κB and MAPK pathways and affected the polarization of M2 macrophages by regulating STAT6 phosphorylation. Additionally, PRP-derived exosomes promoted the autophagic degradation of NLRP3 by increasing NLRP3 ubiquitination and reducing IL-1β and Caspase-1 production. Moreover, PRP-derived exosomes could reduce IL-1β-induced apoptosis of nucleus pulposus cells. Lastly, in vivo experiments confirmed that PRP-derived exosomes reduced the expression of inflammatory mediators and apoptotic factors, which could thereby alleviate the progression of IDD. Taken together, these data showed that PRP-derived exosomes could alleviate the IDD-associated inflammation by regulating the ubiquitination and autophagic degradation of NLRP3 inflammasome, providing new insights into the treatment of IDD.

We investigate the interaction between self-interstitial atoms (SIAs) and 1/2 self-interstitial dislocation loops in tungsten (W) via atomistic simulations. We explore the variation of the anisotropic distribution of binding energies with the shapes and sizes of the 1/2[111] loop and the nonequivalent configurations of SIAs. For an arbitrarily shaped loop, SIA can be more easily trapped in the concave region of the loop than the convex region, which forms a loop whose curvature is closer to that of a circular loop. The direction of SIAs can largely affect the interaction behaviors with the loop. The capture distance of an SIA by the edge of a circular-shaped 1/2[111] loop is clearly elongated along the direction of the SIA; however, it weakly depends on the size of the loop. Then, we analyze the slanted ring-like capture volume of SIAs formed by the circular loop based on their generated anisotropic stress fields. Furthermore, the binding energies obtained from the elastic theory and atomistic simulations are compared. The results provide a reasonable interpretation of the growth mechanism of the loop and the anisotropic interaction that induces irregular-shaped capture volume, affording an insight into the numerical and Object Kinetic Monte Carlo simulations to evaluate the long-term and large-scale microstructural evolution and mechanical properties of W.

Thermal desorption of deuterium in tungsten pre-irradiated with the inert gas (helium, neon, argon) is modelled by TMAP (Tritium Migration & Analysis Program). Each TMAP model incorporates three activation energies linked with three types of traps respectively. First-principles method is performed to calculate the activation energies of deuterium detrapping from different kinds of defects. By comparing the activation energies modelled in TMAP with that obtained from first-principles method, the types of traps are determined. The pre-irradiation of helium can suppress the deuterium retention in vacancy clusters. Neon and argon pre-irradiation suppresses deuterium retention in mono-vacancies due to decreasing the number of mono-vacancies caused by the occupation of the inert gas atoms into mono-vacancies. The pre-irradiation of neon can facilitate deuterium retention in vacancy clusters through generating vacancy clusters during the injection process. Argon pre-irradiation facilitates deuterium retention mainly through generating voids, which are of a larger scale than vacancy clusters and can serve as effective traps.

Atomistic simulations have been used to investigate the core structures, static properties of isolated 1/2 screw dislocations, and their interaction with vacancies in bcc tungsten (W) based on three empirical interatomic potentials. Differential displacement maps show that only one embedded atom method potential is able to reproduce the compact non-degenerate core as evidenced by ab initio calculations. The obtained strain energy and stress distribution from atomistic simulations are, in general, consistent with elasticity theory predictions. In particular, one component of the calculated shear stress, which is not present according to elasticity theory, is non-negligible in the core region of our dislocation model. The differences between the results calculated from three interatomic potentials are in details, such as the specific value and the symmetry, but the trend of spatial distributions of static properties in the long range are close to each other. By calculating the binding energies between the dislocations and vacancies, we demonstrate that the dislocations act as vacancy sinks, which may be important for the nucleation and growth of hydrogen bubbles in W under irradiation.

Molecular dynamic simulation is utilized to study the nanocluster and the fuzz structure on the PFM surface of tungsten. The polyhedral and linear cluster structures based on the icosahedron, cuboctahedron and rhombic dodecahedron are built up. Three interatomic potentials are used in calculating the relationship between the cluster energy and the number of atoms. The results are compared with first-principles calculation to show each potential’s best application scale. Furthermore, the transition between the icosahedral and the cuboctahedral clusters is observed in molecular dynamic simulation at different temperatures, which follows a critical curve for different numbers of atoms. The linear structures are proved to be stable at experimental temperatures by thermodynamics. The work presents a selection of interatomic potentials in simulating tungsten cluster systems and helps researchers understand the growth and evolution laws of clusters and the fuzz-like structure formation process in fusion devices.

International audience; We apply the Object Kinetic Monte Carlo method to simulate the hydrogen (H) clustering behaviour with large vacancy (V) clusters in tungsten. The main advantage of this method is to consider the temperature effects, large V clusters, and long-time evolution. It is thus a very useful tool to study the microstructure evolution of defects in irradiated materials covering from the atomic scale to mesoscale. Simulation results show that the number of trapped H atoms by V and V clusters decreases with the increase of temperature. This is expected because the de-trapping ability of H atoms increases when the temperature is increased. With the correction of the zero-point-energy in calculating binding energies, the present results show that the number of H atoms trapped by monovacancy decreases for all studied temperatures. We also found that the trapping ability of V clusters for H decreases with the increase of the sizes of V clusters, independent on temperature. The H concentration can largely affect the clustering behaviour of HmVn. When the H saturation concentration is reached, the clustering behaviour of HmVn is mainly determined by the sizes of V clusters.

We have developed an object kinetic Monte Carlo (OKMC) code and simulated hydrogen-vacancy clustering behavior and dependence on temperature and hydrogen-vacancy ratio in tungsten. For each of the temperatures we simulated from 300 K to 1000 K, H n V clusters with smaller n form before those with larger n . The elevating temperature leads to a decrease in hydrogen vacancies: H 10 V and H 9 V clusters dominate at 300 K and 600 K, whereas H 5 V , H 6 V , and H 7 V clusters dominate when the temperature reaches 1000 K. Furthermore, only H n V clusters with smaller n formed when a lower hydrogen-vacancy ratio was used due to insufficient availability of hydrogen atoms to occupy vacancies. The results suggest hydrogen emission occurs very rarely at lower temperatures, while higher temperatures facilitate the dissociation of hydrogen from H n V clusters.

A lot of amorphous alloy deposits in the binary (Ni, Co, Cu)–(P, B) alloy systems fabricated by electroless plating (EP) have been reported up to date. But no one reported their theoretical modeling of the amorphous formation and calculated their concentration range of amorphous formation (RAF). Using Miedema model and subregular model scheme, the RAFs for the six EP (Ni, Co, Cu)–(P, B) alloys and three Ni–Cu, Ni–Co and Co–Cu alloys have been calculated systematically for the first time. The calculated results are in agreement with experimental observations. Experiments and calculations for the RAFs in the latter three alloy systems reveal that not any RAF formed except crystalline states. The huge difference between the six metal–metalloid alloys and three metal–metal alloys in RAF has been discussed in detail in the paper.

To study the behavior of noble gas atoms (He, Ne and Ar) in bulk tungsten, new DFT-based potentials for W–He, W–Ne and W–Ar interactions were developed by fitting the results obtained from density functional theory calculations. The new potentials adopt the embedded atom method (EAM) formalism, and the “s-band model” is used to describe the many-body interactions between each of the noble gas atoms and its neighboring W atoms. These potentials reproduce the formation energies of point defects and the migration barriers of single noble gas atoms. The simulations using these potentials successfully predict that the tetrahedral interstitial site is more stable than the octahedral interstitial site for X (= He, Ne or Ar) interstitials. Based on these new potentials, the binding interactions of a single X atom with the Xn and Xn–Vacancy clusters and the diffusion properties of Xn clusters in bulk W were studied using molecular dynamics (MD) simulations. The present results indicate that the binding energies obtained using the new potentials are good in agreement with the results of DFT calculations. The migration energies of the clusters increase with both the increase in the atomic radius of noble gases and the increase in the size of the clusters.

Using a first-principles method, we have investigated site preference and diffusion properties of carbon (C) in copper (Cu). A single C atom energetically prefers to occupy the octahedral interstitial site (OIS) instead of the tetrahedral interstitial site. Double C atoms tend to be paired up at the nearest neighboring OIS’s along the 〈1 0 0〉 direction mediated by a bridging Cu atom with a distance of 3.85 A and a binding energy of 0.15 eV. This suggests that an indirectly attractive interaction between C atoms exists, which might lead to a local higher concentration of C in Cu. We demonstrate that the OIS → OIS path is the optimal diffusion path of C in Cu with diffusion barrier of 0.93 eV. By the estimation of pre-exponential factor according to a classical thermodynamics model, the diffusion coefficient as a function of temperature has been determined, which is 3.68 × 10−15 m2/s at a typical temperature of 600 K. The results provide a good reference to understand the behavior of C in Cu.

Hydrogen (H) defect interactions have been investigated by molecular statics simulations in tungsten (W), including H-H interactions and interactions between H and W self-interstitial atoms. The interactions between H and small H-vacancy clusters are also demonstrated; the binding energies of an H, a vacancy and a self-interstitial W to an H-vacancy cluster depend on the H-to-vacancy ratio. We conclude that H bubble formation needs a high concentration of H in W for the H bubble nucleation and growth, which are also governed by the H-to-vacancy ratio of the cluster. The vacancy first combines with H atoms and a cluster forms, then the H-vacancy cluster goes through the whole process of vacancy capture, H capture, and vacancy capture again, and as a result the H-vacancy cluster grows larger and larger. Finally, the H bubble forms.

We have clarified the performance of two tungsten–helium analytical interatomic potentials, one of which, developed by Li et al., is a bond-order potential, and another, developed by Juslin et al., is a combination of embedded atom method potential and pair potential. Using these two potentials, we have simulated and made a full comparison of formation energy and migration energy of different defects including helium and vacancy, binding energies of helium and vacancy with helium-vacancy cluster, surface energy, as well as melting point, with reference to the corresponding results from the first-principles and experiments.

We employ a classic molecular dynamics method to investigate the dissolution and diffusion properties of hydrogen (H) in a Σ5(3 1 0) tilt grain boundary (GB). A maximum binding energy of 2.5 eV and a diffusion barrier of 1.65 eV indicate that GB plays an important role in H trapping. Dynamic simulations with temperature ranging from 600 K to 1200 K verify the diffusion and the aggregation of H in the GB are closely associated with the temperature. Pair radius distribution function analysis suggests a high local GB concentration of H such as 30% at 900 K can lead to a disordered GB structure.

Molecular dynamics (MD) simulations have been performed to investigate the diffusion behavior of helium (He) clusters in tungsten (W), because their diffusion properties provide basic knowledge in understanding the He bubble formation. The binding energy between He and He cluster is shown to be positive, and thus, He is easy to form bubbles by self-trapping. The mean squared displacements (MSDs) were employed to determine the diffusivities of He clusters with different sizes at different temperatures. The He bubble formation at different temperatures with 1% He was also investigated. It is revealed that the formation of He bubbles is strongly associated with the temperature and the diffusivities of the He clusters in W. The results demonstrate the initial stage of the He bubble formation and growth in W.

The diffusion behavior of hydrogen (H) in tungsten (W) has been studied by molecular dynamics simulations. The diffusivities from 200 K to 3000 K are calculated and the diffusion equation is fitted to D = 5.13 × 10 −8 exp(−0.21 eV / kT) m 2 /s . The diffusion equations in different temperature ranges of 200–500 K, 500–2400 K and 2400–3000 K are also given separately and the results imply different H diffusion modes in different temperature ranges, which is proved by analyzing equilibrium H positions at low and high temperatures. The H atom jumps between different tetrahedral interstitial sites (TISs) at lower temperatures, but the transition to octahedral interstitial sites (OISs) is also observed at high temperatures. Moreover, with a vacancy present in the W system, vacancy trapping of H is observed, and it is shown that the vacancy trapping reduces with the increasing temperature. The H binding energy to monovacancy is obtained using three different methods including NEB and fitting both H diffusivity and time for H to detrap from the vacancy, which provides more information of the H behaviors with the existence of defects.

Molecular dynamics simulations have been performed to investigate the interactions between helium (He) and defects in tungsten (W). The binding energy between He and He cluster is shown to be positive, which increases with increasing He cluster size. Both the W self-interstitial atoms (SIAs) and the vacancy can promote the He cluster formation. The binding energies of a He, a vacancy and an SIA W to a He–vacancy cluster (He n V m ) in W are also investigated, which depend on the n / m ratio. According to these results, we propose the formation and growth mechanism of He bubbles, which involves the procedures of He–vacancy cluster formation, the capturing of vacancies, then He atoms, and vacancies again. The mechanism provides a good reference to understand the initial stage of the He bubble formation and growth in W.

Tungsten (W) is considered to be a promising candidate for plasma facing materials in fusion reactors because of its high melting point, high thermal diffusivity and low sputtering yield. To investigate the multiple hydrogen (H) diffusion behaviours at the W(001) surface, molecular dynamics simulations were used to study the H distributions after diffusion of 1 ns under different conditions. The results of using different H concentrations and temperatures showed the preference of bulk to surface diffusion, and the H distribution patterns of the H/W ratio of 0·1 are analysed in detail with an ‘empty’ region near the surface. The effect of temperature on H diffusion is also investigated, which gives the highest adsorption peak at a mediate temperature of 900 K. The trapping effect of vacancies on H atoms is demonstrated at different vacancy concentrations. The effect of H embedded depth on H distribution is also investigated together with the effect of temperature, which gives an increasing trend of...

The diffusion process of multiple He atoms in W is simulated by a molecular dynamics (MD) method with the W-H-He analytic bond-order potential. The diffusivities of different number of helium (He) atoms in W are determined by the mean squared displacement (MSD) method at different temperatures. The diffusivity-temperature (D-T) relationship is fitted to the Arrhenius equation to obtain the pre-factor and the diffusion barrier. Under the temperature of 1200K He atoms diffuse together, and above 1200K they separate from each other. When the number of He atoms is greater than three, all He atoms oscillate at the tetrahedral interstitial site (TIS) instead of diffusing under 400K. In the temperature range of 400-1200K, the diffusion barriers of He atoms, the number of which is from two to five, are 0.098, 0.170, 0.125 and 0.112eV, respectively. Contrasting with one He atom (0.058eV), the higher diffusion barriers reflect a greater difficulty in diffusion of multiple He atoms in W. In addition, when the number of He atoms is over five, vacancies are formed in W, and He atoms occupy the vacancies.

The interstitial helium (He) atom diffusion in tungsten (W) was studied by the Molecular Dynamics Simulation with the drag method, the Nudged Elastic Band method (NEB) and the mean square displacement (MSD) method. The diffusion barriers and the possible microscopic diffusion path were calculated by the drag method. It has the characteristics of simple, intuitive, and occupies less computer resources, but can't get the diffusion equation. The NEB method is more reasonable than the drag method to calculate the diffusion barriers, and determine the diffusion path which, but the former spends more computer resources than the latter, and it also can't get the diffusion equation. The diffusion equation is obtained by MSD method, including the diffusion per-factor and diffusion barriers. It is suggested that the mechanism of He diffusion changes with difference temperature, which spends the most computer resources among the three methods.

Tungsten (W) dust generated from the interactions between the energetic edge plasma and W-based first-wall materials in future nuclear fusion reactors can significantly affect the plasma stability. As the precursor of dust, nanoscale clusters could aggregate and grow to become dust and are essential to the understanding of dust formation. Here, atomistic simulations have been used to explore the structure, energetics and melting properties of nanoscale W clusters. Comparisons of cluster configurations determined by two methods (high-temperature simulated annealing of a random configuration and low-temperature simulated annealing of a body-centered cubic configuration) reveal that the former technique gives more stable configurations for small clusters (N

The size and the electronic factor are used to analyze the occupation of doped Co atom in L10 FePt alloy as well as its influence on crystal structure and magnetic property. The Co atom located in Fe sublattice site is determined by the variance of the calculated structure and the doping energy results investigated by the first-principles calculations. The charge-density difference contour plot and Bader charge analysis can quantitatively distinguish the effects of the size and the electronic factor on physical properties of the doping alloy. The effect of the electronic factor on the doping energy is more dominant than that of the size factor. The size factor almost has no effect on saturation magnetization.

Shear-coupled grain boundary (GB) migration can be an efficacious mechanism to accommodate plastic deformation when the grain size of polycrystalline materials goes small. Nevertheless, how this kind of GB motion comes into play at the atomic level has not been fully revealed. Here, we have investigated the shear-coupled migration (SCM) of typical [100] group symmetrical tilt GBs in bcc W using atomistic simulations. Depending on GB character, the SCM is found to proceed via dislocation slipping in the 〈100〉 or 〈110〉 mode with striking shear strength difference between them. We demonstrate that there exists an unusual atomic shuffling along the tilt axis, which greatly assists SCM to operate in the easier 〈110〉 mode instead of the 〈100〉 one. The present results highlight the significant role of GB character in the atomistic SCM process and contribute to the future design and fabrication of high-performance materials in GB engineering.

We have constructed W–He and H–He analytical bond-order potentials for a W–H–He system. In combination with the previously self-developed W–H potential [X.-C. Li, X. Shu, Y.-N. Liu, F. Gao, G.-H. Lu, J. Nucl. Mater. 408 (2011) 12] and the Hartree–Fock-dispersion pair potential (Aziz-potential) for He−He interactions, we demonstrate that such potentials behave well for reproducing various properties of the W–H–He system such as defect formation energies, structural properties, and diffusion barriers. Such potentials can be employed to model both the He behaviours and the H–He synergetic effects in the W–H–He system.

We have performed a molecular dynamics study on the growth of helium (He) clusters in the subsurface of tungsten (W) (1 0 0) at 300 K, focusing on the role of cluster depth. Irregular 'stick–slip' behavior exhibited during the evolution of the He cluster growth is identified, which is due to the combined effects of the continuous cluster growth and the loop punching induced pressure relief. We demonstrate that the He cluster grows via trap-mutation and loop punching mechanisms. Initially, the self-interstitial atom SIA clusters are almost always attached to the He cluster; while they are instantly emitted to the surface once a critical cluster pressure is reached. The repetition of this process results in the He cluster approaching the surface via a 'stop-and-go' manner and the formation of surface adatom islands (surface roughening), ultimately leading to cluster bursting and He escape. We reveal that, for the Nth loop punching event, the critical size of the He cluster to trigger loop punching and the size of the emitted SIA clusters are correspondingly increased with the increasing initial cluster depth. We tentatively attribute the observed depth effects to the lower formation energies of Frenkel pairs and the greatly reduced barriers for loop punching in the stress field of the W subsurface. In addition, some intriguing features emerge, such as the morphological transformation of the He cluster from 'platelet-like' to spherical, to ellipsoidal with a 'bullet-like' tip, and finally to a 'bottle-like' shape after cluster rupture.

The point defects and their related physical properties in L10 FePt are investigated by molecular dynamics simulations based on an analytic bond-order potential. The calculated results agree well with the experimental value, indicating that the analytic bond-order potential is suitable to describe the structural properties and surface energies of the FePt alloy in the L10 phase. However, the calculated vacancy formation energy of an Fe atom is higher than that of a Pt atom, which disagrees with some other previously calculated results. This result indicates that the analytic bond-order potential is unable to describe the related point defect properties. The analytic bond-order potential needs to be modified in order to study these defect properties of an FePt alloy. analytic bond-order potential, FePt, point defects, molecular dynamics simulation

The preference site and magnetic property of doped Cr atom in L10 FePt alloy are investigated by the first-principles calculations. Due to the energy of Cr located in Fe and Pt sublattice are -0.15 and 0.48 eV, respectively, the Cr atom occupying Fe site is more energetic favorable than in Pt site in L10 FePt alloy. The calculated saturation magnetizations for Cr additive replacing Fe or Pt atom in L10 FePt are 959 or 989 emu/cm3, respectively. The results are lower than that of L10 FePt (1085emu/cm3). The additive of Cr in L10 FePt will reduce the saturation magnetization.

We construct modified W–H and W–W analytical bond-order potentials for a W–H system. In combination with Brenner’s H–H potential, we demonstrate that such potentials can reproduce energetics and structural properties of W and W–H systems, including defect formation energies, surface energies and diffusion barriers as well as melting point determined from first-principles or experiments. The present potentials can be employed for modelling the behaviour of H in W containing defects such as vacancies and surfaces.

An embedded-atom method potential has been developed for modeling hydrogen in body-centered-cubic (bcc) tungsten by fitting to an extensive database of density functional theory (DFT) calculations. Comprehensive evaluations of the new potential are conducted by comparing various hydrogen properties with DFT calculations and available experimental data, as well as all the other tungsten-hydrogen potentials. The new potential accurately reproduces the point defect properties of hydrogen, the interaction among hydrogen atoms, the interplay between hydrogen and a monovacancy, and the thermal diffusion of hydrogen in tungsten. The successful validation of the new potential confirms its good reliability and transferability, which enables large-scale atomistic simulations of tungsten-hydrogen system. The new potential is afterward employed to investigate the interplay between hydrogen and other defects, including [1 1 1] self-interstitial atoms (SIAs) and vacancy clusters in tungsten. It is found that both the [1 1 1] SIAs and the vacancy clusters exhibit considerable attraction for hydrogen. Hydrogen solution and diffusion in strained tungsten are also studied using the present potential, which demonstrates that tensile (compressive) stress facilitates (impedes) hydrogen solution, and isotropic tensile (compressive) stress impedes (facilitates) hydrogen diffusion while anisotropic tensile (compressive) stress facilitates (impedes) hydrogen diffusion.

The dislocation 1/2[111](1-10) in iron is constructed. The motion behaviors of dislocation 1/2[111](1-10) in iron, as well as the dislocation with the Carbon located in dislocation core, are simulated by the molecular dynamics method with a modified analytic embedded atomic method (MAEAM). The carbon atoms are easy captured by the dislocation core and they can block the dislocation moving. After the carbon atoms are separated from the dislocation, the moving velocities of the dislocation are similar as the case of single dislocation.

The effects of different amounts of segregated Ga (substitutional) on an Al grain boundary have been investigated by using a first-principles pseudopotential method. The segregated Ga is found to draw charge from the surrounding Al due to the electronegativity difference between Ga and Al, leading to a charge density reduction between Ga and Al as well as along the Al grain boundary. Such an effect can be enhanced by increasing the Ga segregation amount. With further Ga segregated, in addition to the charge-drawing effect that occurs in the Al–Ga interface, a heterogeneous α-Ga-like phase can form in the grain boundary, which greatly alters the boundary structure. These effects are suggested to be responsible for Ga-induced Al intergranular embrittlement.

The point defect properties of L12 Ni3Al are determined by MD with a simple modified analytic EAM model with the basic physical properties of pure constituents Ni and Al. On the basis of the calculated lattice constants, the formation energies of vacancy and antisite are calculated and the defect types are also discussed. The energies of alloying element Re replacing Ni site and Al site in Ni3Al are estimated. The occupational site of Re is found in Al site. The present calculations are in agreement with the experimental value and the theoretical results obtained from other authors.

Segregation of solute elements or impurities to the grain boundary (GB) may be impeding the dislocation movement to cause the embrittlement of materials. The diffusion behaviour of elements in alloy, especially in GB, has a crucial effect on the segregation of impurities. We calculated the migration energy of Fe, Cu and Ni atom in Fe by the molecular dynamics method with the Nudged Elastic Band (NEB) method. The self-diffusion migration energy of Fe is 0.628 eV. The migration energies of Cu and Ni in Fe crystal are 0.592 eV and 0.608 eV, respectively. These results are good agreement with other calculations. The migration energies of Fe atom, Cu and Ni solute diffusion in [0 0 1] and [3 1 0] direction in GB Σ5 [3 1 0] and diffusion out GB Σ5 [3 1 0] in [1 1 1] direction are calculated. The Cu solute is segregate easier to GB and more difficult diffusion out of GB than the Ni solute. The Fe atom, Cu and Ni solute are very difficult to diffuse in [0 0 1] and [3 1 0] in Σ5 [3 1 0] because of their large migration energies, respectively. They can jump out of GB in [1 1 1] and back to GB in other [1 1 1] to diffuse near GB.

The diffusion process of He in W is simulated by a molecular dynamics (MD) method with self-developed W–H–He analytic bond-order potentials. The mean squared displacement (MSD) method is employed to determine diffusivity at different temperatures, and then the diffusivity–temperature (D–T) relationship is fitted by the Arrhenius equation to obtain the prefactor and the diffusion barrier. We show the diffusivity of He and the corresponding prefactor and diffusion barrier are different at different temperature ranges. The diffusivities are 9.50 × 10−9 exp(−0.021 eV/kT) m2/s in the temperature range of 50–300 K, 3.61 × 10−8 exp(−0.057 eV/kT) m2/s in 300–1500 K, and 8.562 × 10−8 exp(−0.157 eV/kT) m2/s in 1500–3000 K, respectively, which correspond to different diffusion mechanisms. At a lower temperature, He diffuses directly from one tetrahedral interstitial site (TIS) to another TIS through the diagonal interstitial site with a lower diffusion barrier, while it can diffuse from one TIS to other TIS through the octahedral interstitial site with higher barrier at higher temperature.

The diffusion properties of iron and nickel in iron are investigated by the molecular dynamics method with a modified analytic embedded-atom method. In the monovacancy diffusion, the energy curve of self-diffusion and diffusion of nickel in iron is symmetric. However, the energy curve of self-diffusion and diffusion of nickel in iron is not symmetric in the divacancy diffusion mechanism. The self-diffusion mechanism is the monovacancy diffusion of nearest neighbor (NN) jump at low temperature. The activation energy, vacancy formation energy and migration energy of self-diffusion are 2.425, 1.657 and 0.768 eV, respectively. In high temperature, the divacancy diffusion must be considered. The solute nickel diffusion mechanism is that the nickel migrates to the NN vacancy. The activation energy and migration energy of diffusion of nickel in iron are 2.260 and 0.582 eV, respectively. The calculated results are in good agreement with the experimental values and other calculated results.

In the present paper, the modified analytic embedded-atom method (MAEAM) many-body potentials are provided for fcc and hcp elements. The segregation energies of Mg in Al host are calculated with the MAEAM potentials. The composition depth profiles of Al–Mg alloys are obtained with Monte Carlo simulation method based on grand canonical ensemble. It is found that Mg segregates strongly to the surface layers at 600 K and the composition near the surface is monotonic to the bulk one. The simulation results are reasonable agreement with other experimental and theoretical data.

Modified analytic embedded atom method (MAEAM) type potentials have been constructed for seven binary immiscible alloy systems: Al–Pb, Ag–Ni, Fe–Cu, Ag–Cu, Cu–Ta, Cu–W and Cu–Co. The potentials are fitted to the lattice constant, cohesive energy, unrelaxed monovacancy formation energy and elastic constants for only pure metals which consist the immiscible alloy systems. In order to test the reliability of the constructed MAEAM potentials, formation enthalpies of disordered alloys for those seven binary immiscible alloy systems have been calculated. The calculated results are in general agreement with the experimental data available and those theoretical results calculated by other authors. As only very limited experimental information is available for alloy properties in immiscible alloy systems, the MAEAM is demonstrated to be a reasonable method to construct the interatomic potentials for immiscible alloy systems because only the properties of pure elements are needed in calculation.

Based on the Newton’s law of motion, the motion equation of the microscopic car-following model is introduced. The moving vehicles comply with the idea of Newton's equations of motion. But the interaction between vehicles is different from their traction. The inertia of the car is not its mass. We discuss the differences and present how to treat them. This article can deepen and widen the students' understanding of Newton's laws of motion. It can also provide the materials for university physical teaching. It is suitable for the students to exploratory study.

Using the analytic modified EAM potentials, the surface concentrations and concentration depth profiles of (111) and (10 0) faces of Pt-Pd and Pt-Ir binary alloys are studied with Monte Carlo simulation based on the grand canonical ensemble statistical rule. In Pt-Pd alloy, the topmost surface is enriched with Pd, and the subsurface layer is depleted of Pd. The simulation results show a damped oscillation of Pd concentration in the whole composition range. A strong Pd segregation is observed for both the orientation faces, while the amount of Pd segregation in the (I 11) face is significantly less than that in the more open (10 0) face. However, Pt segregates to the surface strongly in the Pt-Ir alloy, and the amount of Pt segregation in the (10 0) and (I 11) face is almost the same. The simulation results are in good agreement with the available experiment data and other theoretical values. (C) 2002 Elsevier Science B.V. All rights reserved.

Analytic embedded-atom method (EAM) interatomic potentials for body-centered cubic (bcc) transition metals have been constructed. The total energy is regarded as consisting of a pair potential part, an embedding interaction part and a modification term. All these parts are analytic functions of the atomic separations only and are fitted to various bulk properties, such as cohesive energy, vacancy formation energy, lattice constant and elastic constants. The present potentials are shown to predict a more realistic pressure–volume relationship so that interactions at separations smaller than that of the first-nearest neighbors can be treated. Interstitial formation energies are calculated for various configurations, using quenched molecular dynamics. Vacancy, surface and phonon spectra have been also studied and satisfactory agreement with available experimental data has been found.

We report a hydrogen induced vacancy formation mechanism in tungsten based on classical molecular dynamics simulations. We demonstrate the vacancy formation in tungsten due to the presence of hydrogen associated directly with a stable hexagonal self-interstitial cluster as well as a linear crowdion. The stability of different self-interstitial structures has been further studied and it is particularly shown that hydrogen plays a crucial role in determining the configuration of SIAs, in which the hexagonal cluster structure is preferred. Energetic analysis has been carried out to prove that the formation of SIA clusters facilitates the formation of vacancies. Such a mechanism contributes to the understanding of the early stage of the hydrogen blistering in tungsten under a fusion reactor environment.