Influence of Surface Oxide Films on Elastic Behaviors of Straight Screw Dislocations Parallel to the Surface of Pure Aluminum

The image stress of straight screw dislocations parallel to the medium surface covered by thin heterogeneous films was analyzed and deduced, in order to calculate the image shear stress. The relationship between image stress and distance from the screw dislocation to the interface of pure aluminum and its oxide covering was calculated based on the analysis. It was shown quantitatively that a sign conversion of the image stress appears in the case of thin oxide covering, while dislocation would pile up near the interface because of the possible slips of the screw dislocations induced by the image stress, which might break down the very thin oxide covering. Further investigation on edge dislocations or other dislocation configurations need to be done.     

Kinks in the a/2〈111〉 screw dislocation in Ta

We study the structure and formation energy of kinks in the 1/2a〈111〉 screw dislocations in metallic Tantalum (Ta) using molecular dynamics with a first principles based many-body interatomic potential. In our study, four a/3〈112〉 kinks are constructed in a quadrupole arrangement in the simulation cell. To the simulation cell, we impose periodic boundary conditions in the directions perpendicular to the [111] direction and fix boundary condition in the [111] direction. We find that two, energetically equivalent, core configurations for the 1/2a〈111〈 dislocation lead to 8 distinguishable single kinks and 16 kinds of kink pairs. The different mismatches of the core configurations along [111] direction attributed to the differences in the formation energy for various types of kinks. Formation energies for all possible kinds of isolated single kinks and kink pairs have been determined. It was found that 0.730 eV was the lowest energy cost to form a kink pair in the a/2〈111〉: screw dislocation in Ta.     

Effect of Austenite Ageing on Martensitic Transformation

The effect of austenite ageing on martensite transformation in an iron and nickel alloy was studied. Attention was paid to the variation of dislocation configurations in austenite before martensite transformation. The clusters or precipitates produced during austenite ageing and the alteration of dislocation configurations affect together the consequent martensite transformation.Dispersed fine clusters and precipitates cripple nucleus role of special dislocation configurations and drop M. Coarse precipitates make Ms rise again. Densely piled up dislocations in nuclei lead to rise of Ms. All the changes in austenite are of benefit to small martensite plates.     

Kinks in the a/2?111? screw dislocation in Ta

We study the structure and formation energy of kinks in the 1/2a〈111〉 screw dislocations in metallic Tantalum (Ta) using molecular dynamics with a first principles based many-body interatomic potential. In our study, four a/3〈112〉 kinks are constructed in a quadrupole arrangement in the simulation cell. To the simulation cell, we impose periodic boundary conditions in the directions perpendicular to the [111] direction and fix boundary condition in the [111] direction. We find that two, energetically equivalent, core configurations for the 1/2a〈111〈 dislocation lead to 8 distinguishable single kinks and 16 kinds of kink pairs. The different mismatches of the core configurations along [111] direction attributed to the differences in the formation energy for various types of kinks. Formation energies for all possible kinds of isolated single kinks and kink pairs have been determined. It was found that 0.730 eV was the lowest energy cost to form a kink pair in the a/2〈111〉: screw dislocation in Ta. This revised version was published online in June 2006 with corrections to the Cover Date.     

Geometrical effect on dislocation nucleation at crystal surface nanostructures

Crystal surface nanostructures such as steps and trenches in microelectronic layer structures have been considered as stress concentrators that may facilitate dislocation nucleation. Quantitative characterization of the critical condition of this atomic scale process is of considerable interest for the development of high performance electronic devices. This paper addresses this issue using a multiscale approach based on the variational boundary integral formulation of the Peierls–Nabarro dislocation model. By representing the profiles of embryonic dislocations as the relative displacements between the two adjacent atomic layers along the slip planes, the critical conditions for dislocation nucleation are obtained by solving the stress dependent activation energies required to activate embryonic dislocations from their stable to unstable saddle point configurations. The geometrical effect of surface nanostructures such as steps and trenches on dislocation nucleation is ascertained quantitatively. Our results show that the atomic scale surface nanostructures can reduce the critical stress for dislocation nucleation by nearly an order of magnitude and the trench configurations are more prone to dislocation nucleation than the step configurations. Nucleation of versatile dislocations in multiple slip systems at crystal surfaces may be attributed surface nanostructures of a variety of geometries.     

Peierls stress in a simple model of a screw dislocation

A simple model of a screw dislocation with first neighbours interactions is presented. The interaction energy of atoms is a piece-wisely parabolic function of distortions. The equilibrium configurations of weak bonds as a function of the model parameter γ and the quasi-static motion of the dislocation are examined. Sessile and glissile configurations of weak bonds and symmetric and asymmetric equilibrium positions of the dislocation are found. The dependence of the Peierls stress on the model parameter γ is discussed.     

Dislocations and stacking fault energy in silicon ditelluride

The stacking fault energy in silicon ditelluride single crystals has been determined for different dislocation configurations observed by electron microscopy. The configurations studied are dislocation ribbons and dislocation nodes, and the ratio of the stacking fault energy to the shear modulus is estimated as / = 1.6×10^–11 cm. Some common features observed in electron microscopy are also discussed in terms of dislocation networks of extended and contracted nodes and symmetrical or asymmetrical three-fold ribbons, as well as various dislocation interactions.     

Molecular dynamics simulations of motion of edge and screw dislocations in a metal

Motions of a straight edge dislocation and a kinked screw dislocation in BCC Mo, described by the Finnis–Sinclair potential, are studied in periodic simulation cells subjected to an applied shear stress. Procedures for setting up the initial atomic configurations in each case are described, and estimate is made of the local driving force due to the image interactions. Preliminary results show that at low temperature the edge dislocation moves primarily through kink nucleation, whereas the mobility of the screw dislocation is strongly facilitated by the presence of a kink.     

Two-dimensional crystals: Monte-Carlo simulations near the melting region

Monte-Carlo simulations of two-dimensional systems of several hundred interacting particles are studied in order to delineate some of the features of the melting process. Structural and thermodynamic properties are monitored for equilibrium phases above and below the melting region. Free energy constructions estimate the melting temperature.Realistic pair potentials and substrate mediated effects for a xenon on silver [Xe/Ag(111)] system provide the particle interactions. The results are compared to earlier Lennard-Jones computations. Lattice dynamics and cell theory calculations are reviewed and comparisons to the Monte-Carlo results show that the computer program predicts consistent thermodynamic properties. This work combined with earlier computations allow the construction of a phase diagram showing some of the features expected for similar systems. Graphic displays show the occurrence of dislocation pairs, voids, particle exchange, and mixed phases. The technology of simulation is discussed, i.e. number of averaging configurations, equilibration, super heating, size effects, boundary conditions, and fluctuations.     

动态加载条件下形成的位错组织特征及形成机制

综述了动态加载条件下影响位错组织的因素，位错组织特征及位错形成的三种模型，即Ｓｍｉｔｈ模型、Ｈｏｒｎｂｏｇｅｎ模型及Ｍｅｙｅｒｓ模型。     

A screw dislocation in a piezoelectric bi-material wedge

The electro-elastic stress investigation for a dislocation located near the apex of two bonded wedge-shaped dissimilar piezoelectric materials has been carried based on the complex variable method and perturbation technique. The dislocation has the Burger's vector normal to the isotropic basal plane, with a line force and a line charge being applied at the core of the dislocation. An exact analytical solution has been presented for the stress and electric displacement fields by means of conformal mapping method. Forces on the dislocation have been calculated. Examples for different material property combinations and geometric configurations are given and discussed.     

18-8型奥氏体不锈钢在低周疲劳变形下的微观组织特征

利用透射电镜系统地研究了18—8型奥氏体不锈钢在低周疲劳变形下的微观组织,包括奥氏体位错组态,形变孪晶和形变马氏体。发现奥氏体位错组态与应变幅和温度有关,形变马氏体主要在 SB 交截处形核,在室温和低温下由于母相变形方式的不同而产生不同的组织特征。     

Atomistic study of dislocation cores in aluminium and copper

The core structure of an edge dislocation, dissociated or not, has been investigated through atomistic calculations. The simulation relies on two N-body phenomenological potentials for aluminium and copper. The various dislocation configurations are characterized in terms of strains and energies for both materials. The limit of validity of the elastic theory of dislocations is then discussed by comparing its predictions to those obtained by simulation.     

Grain boundary influence on short fatigue crack growth rate

The influence from different grain boundary configurations on the crack growth rate of a microstructurally short edge crack, located within one grain and subjected to remote fatigue loading, is studied. The study is performed using a dislocation formulation, were the geometry is described by dislocation dipole elements in a boundary element approach and the plasticity by discrete dislocations, located along specific slip planes in the material. Plane strain and quasi-static conditions are assumed. The crack is assumed to grow in a single shear mechanism due to nucleation, glide and annihilation of discrete dislocations. Different grain boundary configurations in front of the growing crack are considered, including both high angle and low angle grain boundaries. It is shown that both grain boundary configuration and distance between the crack and a grain boundary has a pronounced influence on the crack growth rate.     

Discrete dislocation analysis of pre-existing cracks

The discrete dislocation method is employed to determine the dislocation configurations associated with a continuous plastic crack subjected to both loading and unloading. The concept of anticrack dislocations is introduced in order to satisfy the free surface boundary conditions on the surfaces of the unloaded crack associated with lattice dislocations in its plastic zone. Various configurations of the unloaded crack, which incorporate a zipping action of the crack surfaces to reduce surface energy, are illustrated schematically wherein the discrete dislocation analysis is utilized. The results are obtained for both a tensile and a shear crack.

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Résumé On utilise la méthode de dislocations discrètes pour déterminer la configuration des dislocations associées à une fissure plastique continue soumise à une charge et à déchargement. Le concept de dislocation d'anti-fissure est introduit en vue de satisfaire les conditions aux limites de surface libre sur les faces de la fissure non chargée, associées aux dislocations des réseaux dans la zône plastique. Des configurations variées de fissures non soumises à sollicitation qui comportent une action de fermeture des surfaces de fissure en vue de réduire l'énergie de surface, sont schématiquement illustrées dès lors que l'analyse par dislocations discrètes peut être utilisée. Des résultats sont obtenus pour des fissurations sous tension et sous cisaillement.

     

Distribution of dislocations at a mode I crack tip and their shielding effect

Distribution of dislocations at a finite mode I crack tip is formulated. Closed form solutions for the dislocation distribution function, the dislocation-free zone (DFZ), the local stress intensity factor and the crack tip stress field are obtained. The dislocation distribution has similar features to a mode III crack model. Under a given applied stress, there may exist different configurations of plastic zone and DFZ. Crack tip shielding by dislocations depends on both applied stresses and the configuration.     

Dislocation dipoles in nanocrystalline films

A theoretical model is suggested that describes the behavioral features and energetic characteristics of dipoles of grain boundary dislocations in nanocrystalline films. Such dislocation dipoles in nanocrystalline films are shown to play the role of misfit defect configurations that compensate, in part, for misfit stresses that occur due to a mismatch between crystal lattice parameters of films and substrates. Ranges of parameters (misfit parameter, grain size, etc.) are revealed at which the formation of dislocation dipoles is energetically favorable in nanocrystalline films. It is demonstrated that dislocation dipoles are typical structural elements of nanocrystalline films fabricated at highly nonequilibrium conditions.     

A triggering criterion of dislocation sources under Mode-I singular stresses

A triggering criterion is suggested to determine the instability of a dislocation source under Mode-I singular stresses. Two kinds of configurations are considered, including a crack and a wedge formed by climbing dislocations pileup. Numerical results are given and discussed for both the cases.     

Electro-elastic stress analysis for a wedge-shaped crack interacting with a screw dislocation in piezoelectric solid

The electro-elastic stress investigation on the interaction problem of a screw dislocation near the tip of a semi-infinite wedge-shaped crack in piezoelectric material has been carried out. Explicit closed-form analytical solutions are obtained for the stress intensity factor (SIF) and the electric displacement intensity factor (EDIF) of the crack, as well as the force on dislocation. The derivation is based on the conformal mapping method and the perturbation technique. The dislocation has Burger's vector normal to the isotropic basal plane, with a line force and a line charge being applied at the core of the dislocation. The influence of the location and the wedge angle of the crack on the image force of the dislocation has been discussed in detail. At the same time the effect of the dislocation on the crack behavior has been also examined under different configurations. Two types of PZT materials are used to numerically illustrate the influences of the wedge angle and the location of the dislocation on the image force and the crack intensities. Results obtained in the current study can be fully reduced to various special cases available in the literatures.     

Long-term dynamic stability of discrete dislocations in graphene at finite temperature

We present an assessment of the finite-temperature dynamical stability of discrete dislocations in graphene. In order to ascertain stability, we insert discrete dislocation quadrupole configurations into molecular dynamics calculations as initial conditions. In calculations we use Sandia National Laboratories Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) and the Adaptive Intermolecular Reactive Empirical Bond-Order (AIREBO) potential. The analysis shows that the core structures predicted by discrete dislocation theory are dynamically stable up to temperatures of 2,500 K, though they tend to relax somewhat in the course of molecular dynamics. In addition, we find that discrete dislocation theory accurately predicts energies, though it exhibits a slight overly-stiff bias.