Atomistic sliding mechanisms of the Σ=5 symmetric tilt grain boundary in bcc iron

Atomistic computer simulations were performed to investigate the mechanisms of grain-boundary sliding in bcc Fe using molecular statics and molecular dynamics with embedded-atom method interatomic potentials. For this study we have chosen the Σ?=?5, (310)[001] symmetrical tilt boundary with tilt angle θ?=?36.9°. Sliding was determined to be governed by grain-boundary dislocation activity with Burgers vectors belonging to the displacement shift complete lattice. The sliding process was found to occur through the nucleation and glide of partial grain-boundary dislocations, with a secondary grain-boundary structure playing an important role in the sliding process. Interstitial impurities and vacancies were introduced into the grain boundary to study their role as nucleation sites for the grain-boundary dislocations. While vacancies and H interstitials act as preferred nucleation sites, C interstitials to not.     

Molecular dynamics simulation of surface melting behaviours of the V（110） plane

The modified analytic embedded-atom method and molecular dynamics simulations are applied to the investigation of the surface premelting and melting behaviours of the V（110） plane by calculating the interlayer relaxation, the layer structure factor and atomic snapshots in this paper. The results obtained indicate that the premelting phenomenon occurs on the V（110） surface at about 1800K and then a liquid-like layer, which approximately keeps the same thickness up to 2020K, emerges on it. We discover that the temperature 2020K the V（110） surface starts to melt and is in a completely disordered state at the temperature of 2140K under the melting point for the bulk vanadium.     

Molecular dynamics of MgSiO3 perovskite melting

The melting curve of MgSiO分子动力学 MgSiO3钙钛矿 熔化温度 高压melting temperature, molecular dynamics, high pressureProject supported by the National Natural Science Foundation of China (Grant Nos 10274055 and 10376021),the Natural Science Foundation of Gansu Province, China (Grant No 3ZS051-A25-027) and the Scientific Research Foundation of Education Bureau of Gansu Province, China (Grant No 0410-01).2005-01-125/8/2005 12:00:00 AMThe melting curve of MgSiO3 perovskite is simulated using molecular dynamics simulations method at high pressure. It is shown that the simulated equation of state of MgSiO3 perovskite is very successful in reproducing accurately the experimental data. The pressure dependence of the simulated melting temperature of MgSiO3 perovskite reproduces the stability of the orthorhombic perovskite phase up to high pressure of 130GPa at ambient temperature, consistent with the theoretical data of the other calculations. It is shown that its transformation to the cubic phase and melting at high pressure and high temperature are in agreement with recent experiments.     

Molecular dynamics simulations of the melting of Al–Ni nanowires

Molecular dynamics (MD) simulations were performed to investigate the influence of nickel (Ni) composition and nanowire thickness on the thermal properties of Al-x%Ni (at%) nanowires using the embedded atom model (EAM) potential. The melting of the nanowire was characterised by studying the temperature dependence of the cohesive energy and mean square displacement. The effect of the nanowire thickness on the cohesive energy, melting temperature, heat capacity as well as latent heat was studied in canonical ensemble. Moreover, the crystal stability of Al, Al-20%Ni, Al-40%Ni, Al-60%Ni, Al-80%Ni, Al₃Ni, Ni₃Al and Ni nanowires was studied at different temperatures using mean square displacement and cohesive energy.     

HREM analysis of structure and defects in a Σ5 (210) grain boundary in rutile

The structure of a 5 (210) boundary in rutile was investigated by high resolution electron microscopy (HREM). The boundary was stepped with an average inclination of about 5° from the symmetrical (210) plane. The steps were associated with 1/5[210] DSC lattice dislocations accommodating a deviation of about 2° from the exact 5 misorientation of 53.1°, and resulting in a misorientation of 51°. The boundary topography, the location of structural units and the local symmetry were determined using pattern recognition techniques. Flat terraces between steps had a periodic 5 (210) structure which exhibited mirror glide symmetry. Image simulations showed best agreement with experimental images for a model structure with a rigid body shift of 0.21 nm parallel, and a 0.10 nm volume contraction normal to the interface. This structure requires a high density of defects or an excess of Ti ions, presumably of lower oxidation state.     

Temperature dependence of the structure and shear response of a Σ11 asymmetric tilt grain boundary in copper from molecular-dynamics

We investigate the effect of temperature on the structure and shear response of a Σ11 asymmetric tilt grain boundary in a classical embedded-atom model of elemental copper using molecular dynamics simulations. As the temperature is increased the structure of the boundary disorders considerably, but with a boundary width that remains finite at the melting point. The disordering of the boundary structure becomes significant for homologous temperatures above 0.83 (1100?K). As temperature increases above this point the boundary width and roughness increases monotonically. Near the temperature where the boundary starts to disorder we observe a change in the temperature dependence of the ideal shear strength of the boundary, as well as the value of the coupling parameter β, defined as the ratio of the velocity of relative translation of the grains parallel to the boundary plane to that corresponding to the motion of the boundary normal to its plane.     

Molecular dynamics simulation of the vapour → liquid transition of argon and the reaction coordinate of condensation

We have investigated the n-dependences of the rate constants of absorption and emission of monomers that are attached to and detached from the cluster of n monomers, and have determined n*, the number of monomers that form the critical nucleus of the homogeneous condensation of the Lennard-Jones?Ar vapour. The dynamics was clearly separated into two regions at the critical nucleus; n* did not, however, give the unique dividing point. The observed strong dependences of both the nucleus size and the barrier height of the nucleation on the induction time of the condensation suggest that a slowly changing variable instead of the cluster size is necessary as the reaction coordinate of the nucleation. Although the slow variable is not well characterised at the present stage of the investigation, it seems to be related to the local density of the gas atoms around the liquid-like clusters. Both the slow evolution of the radial distribution function of the gas atoms around the liquid-like atoms, and the correlation between n* and the onset of the condensation indicate that Gibbs energy curves represented in n-space change significantly with the activation of the slow variable.     

Tight-binding calculations of the {211} Σ=3 boundary in diamond

The atomic and electronic structure of the {211} =3 boundary in diamond has been calculated by using the transferable tight-binding method. Several atomic models with symmetry consistent with the electron microscopy observation have been dealt with. The four-fold coordinated model is the most stable, although its interfacial energy is fairly high as compared with four-fold coordinated configurations in Si and other boundaries in diamond. Thus the models with three-fold coordinated sites may exist partially as defects. The electron energy-loss spectra of this boundary have been calculated by the tight-binding method for the first time. The K-edge spectra of the models with three-fold coordinated sites have small peaks below the bulk conduction-band peak caused by unoccupied gap states. These can explain the increases at the position of the * peak below the * peak in the observed spectra of this boundary.     

Grain boundary migration in Σ=5 bicrystals of an Fe-3%Si alloy

Migration of differently oriented grain boundaries was studied in the =5, 36.9°[100] tilt bicrystals of an Fe-3mass%Si alloy by the modified reversed-capillary technique. The principles of this method are outlined and discussed in connection with the application of multiple annealing of a single sample. It is shown that the errors introduced by both heating and cooling periods and by possible existence of an incubation period do not exceed the scatter of experimental data. A linear dependence between grain boundary migration velocity and driving force was found in most cases. The measured values of the product of grain boundary mobility and energy for individual grain boundaries differ substantially. The values of activation energy of migration of 332 kJ/mol, 392 kJ/mol, and 97 kJ/mol were found for the, {01}, {02} and (001)/(0 4) grain boundaries, respectively.     

Effect of P impurity on mechanical properties of NiAl Σ5 grain boundary:From perspectives of stress and energy

In this paper, we employ the first-principle total energy method to investigate the effect of P impurity on mechanical properties of NiAl grain boundary(GB). According to energy, the segregation of P atom in NiAlΣ5 GB reduces the cleavage energy and embrittlement potential, demonstrating that P impurity embrittles NiAlΣ5 GB. The first-principle computational tensile test is conducted to determine the theoretical tensile strength of NiAlΣ5 GB. It is demonstrated that the maximum ideal tensile strength of NiAlΣ5 GB with P atom segregation is 144.5 GPa, which is lower than that of the pure NiAlΣ5 GB(164.7 GPa). It is indicated that the segregation of P weakens the theoretical strength of NiAlΣ5 GB.The analysis of atomic configuration shows that the GB fracture is caused by the interfacial bond breaking. Moreover, P is identified to weaken the interactions between Al–Al bonds and enhance Ni–Ni bonds.     

Molecular dynamics simulation of latent track formation in α-quartz

The latent ion track in α-quartz is studied by molecular dynamics simulations. The latent track is created by depositing electron energies into a cylindrical region with a radius of 3nm. In this study, the electron stopping power varies from 3.0keV/nm to 12.0keV/nm, and a continuous latent track is observed for all the simulated values of electron stopping power except 3.0keV/nm. The simulation results indicate that the threshold electron stopping power for a continous latent track lies between 3.0keV/nm and 3.7 keV/nm. In addition, the coordination defects produced in the latent track are analyzed for all the simulation conditions, and the results show that the latent track in α-quartz consists of an O-rich amorphous phase and Si-rich point defects. At the end of this paper, the influence of the energy deposition model on the latent track in α-quartz is investigated. The results indicate that different energy deposition models reveal similar latent track properties. However, the values of the threshold electron stopping power and the ion track radius are dependent on the choice of energy deposition model.     

Constant electrical resistivity of Zn along the melting boundary up to 5 GPa

We measured the electrical resistivity of high purity Zn along the melting boundary, up to 5?GPa in a large volume press. The electrical resistivity remained constant on the melting boundary, as predicted in a thermodynamics-based model for simple metals. The effects of pressure and temperature on the electrical resistivity of the solid and liquid states are interpreted in terms of their antagonistic effects on the electronic structure of Zn. Within the error of measurements, our melting temperature data agree well with those of the previous studies. The electronic thermal conductivity was calculated from resistivity data using the Wiedemann–Franz law and shows a decrease with temperature in the solid state and an increase in the liquid state, with a large decrease on melting. Comparison of calculated electronic and measured total thermal conductivities indicates that the electronic component dominates over the phonon component in Zn.     

Decreasing electrical resistivity of silver along the melting boundary up to 5 GPa

The electrical resistivity of Ag was experimentally measured at high pressures up to 5?GPa and at temperatures up to ～300?K above melting. The resistivity decreased as a function of pressure and increased as a function of temperature as expected and is in very good agreement with 1 atm data. Observed melting temperatures at high pressures also agree well with previous experimental and theoretical studies. The main finding of this study is that resistivity of Ag decreases along the pressure- and temperature-dependent melting boundary, in conflict with prediction of resistivity invariance. This result is discussed in terms of the dominant contribution of the increasing energy separation between the Fermi level and 4d-band as a function of pressure. Calculated from the resistivity using the Wiedemann–Franz law, the electronic thermal conductivity increased as a function of pressure and decreased as a function of temperature as expected. The decrease in the high pressure thermal conductivity in the liquid phase as a function of temperature contrasts with the behavior of the 1 atm data.     

Decreasing electrical resistivity of gold along the melting boundary up to 5 GPa

ABSTRACT

The electrical resistivity of gold was experimentally measured at high pressures from 2 to 5?GPa and temperatures ～300?K above melting. The resistivity decreased as a function of pressure and increased as a function of temperature as expected. The temperature dependence of resistivity in the solid and liquid phases are comparable to 1?atm results. The observed melting temperatures at each pressure agree well with previous experimental and theoretical studies. The essential result of this study is that resistivity decreases along the pressure-dependent melting boundary, conflicting with a prediction of invariant behavior as reported in the literature. This result is discussed in terms of the interaction between s and d-bands as both pressure and temperature increase along the melting boundary. The thermal conductivity of gold was calculated from the measured electrical resistivity using the Wiedemann-Franz law. The temperature-induced effect on the thermal conductivity at high temperatures is as expected in both the solid and liquid phase while the pressure-effect shows some variability.     

Geometry of Σ9 and Σ11 asymmetrical grain boundaries

It is shown how the decomposition scheme into structural units can be developed for rational asymmetrical tilt grain boundaries without computer simulations. The method is illustrated on the 9 and 11 grain boundaries in the f.c.c. lattice for which experimental observations of the boundary structures are available. Besides short-period boundaries, which may serve as elementary structural units, the irrational aperiodic asymmetrical boundaries are also considered. There are clear experimental indications that some of them, namely those composed of dense-packed atomic planes, and called special asymmetrical boundaries, behave differently from the other more general boundaries.     

Solute segregation at 46.8°(111) twist grain boundary of a phosphorus doped Fe–2.3%V alloy

The Auger electron spectroscopy study on chemistry of the 46.8°(111) twist grain boundary of an Fe–2.3%V alloy showed an extended phosphorus enrichment at temperatures in range of 500 °C and 800 °C. Simultaneously, slight but nearly independent segregation of vanadium was also detected. The standard enthalpy and entropy of grain boundary segregation of phosphorus and vanadium were determined according to the Guttmann model of multicomponent interfacial segregation. Obtained data clearly show that this Σ = 19 coincidence boundary is special (i.e. low energy interface). The data also fit well with the predictive model of grain boundary segregation and confirm that phosphorus segregates interstitially at the grain boundary while vanadium substitutes iron atoms in the interface structure.     

Hydrogen sorption in titanium alloys with a symmetric Σ5(310) tilt grain boundary and a (310) surface

The hydrogen sorption in intermetallic B2 TiM (M = Ni, Co, Pd) with a symmetric ??5(310) tilt grain boundary and a (310) surface is studied by density functional theory methods. The effect of hydrogen on the electronic characteristics of the alloys is analyzed as a function of a sorption position at the interfaces. The hydrogen sorption energy is shown to depend on the local environment of hydrogen; on the whole, hydrogen at the interfaces prefers titanium-rich positions. The hydrogen sorption energy in metal-rich positions decreases when the d shell of the second alloy component is filled with electrons. The grain-boundary energy, the surface energy, and the hydrogen segregation energies to the interfaces are calculated. Hydrogen sorption in titanium alloys is shown to decrease Griffith work and to favor brittle fracture along tilt grain boundaries.     

Effect of element Re on the grain boundary cohesion of α-Fe

The effect of Re segregation on the α-Fe ∑5 [001] （010） grain boundary （GB） is investigated by using a software called DMol and discrete variational method （DVM）. Based on the Rice Wang model, the calculated segregation energy and defect formation energy show that Re is a strong cohesive enhancer. We also calculated the interatomic energy （IE） and bond order （BO） of several atomic pairs to investigate the mechanism of the cohesive effect of Re microscopically and locally. The results show that IEs of atomic pairs formed by those atoms which cross the plane of GB are strengthened due to the segregation of Re, while the BOs of the corresponding pairs are slightly decreased. This discrepancy demonstrates that IE which contains the Hamiltoniaa of interaction between atoms is a good quantity to describe the bonding strength. The analysis suggests that the electronic effect between atomic pair which comes directly from Hamiltonian is the key factor, The charge density is also presented, and the result indicates that the bonding strength between the Fe atoms on the GB is enhanced due to the segregation of Re, which is consistent with the analysis of IE.