Interaction Of Edge Dislocation Research Articles

Atomistic simulations of interaction of edge dislocation with twist grain boundaries in Al-effect of temperature and boundary misorientation

Grain boundaries play an important role in characterizing the mechanical behavior of polycrystalline materials, since they can act as sites for absorption or nucleation of dislocations, which are the main carriers of plastic deformation. In view of this, we performed Molecular Dynamics (MD) simulations to study the interaction of edge dislocations with twist grain boundaries. FCC Al was selected as the model material. Simulations were carried out under different temperature conditions (up to 0.9Tm, where Tm is the melting point of Al) on 〈110〉 twist grain boundaries of different misorientation angles. In this way, we were able to quantify the resistance of the grain boundary to dislocation absorption at different temperatures ranging from 10K to 900K . Our results reveal that the dislocation absorption resistance varies considerably with the grain boundary misorientation angle, and that the resistance first increases to a maximum before it starts decreasing again as the melting point is achieved. The ramifications of this study towards crystal plasticity finite element modeling are discussed.

Propagation and Interaction of Edge Dislocation (Kink) in the Square Lattice**Supported by the National Magnetic Confinement Fusion Science Program of China under Grant No 2014GB104002, the Strategic Priority Research Program of Chinese Academy of Sciences under Grant No XDA03030100, the National Natural Science Foundation of China under Grant Nos 11275156 and 11304324, the Open Project Program of State Key Laboratory

The propagation of kink or edge dislocations in the underdamped generalized two-dimensional Frenkel–Kontorova model with harmonic interaction is studied with numerical simulations. The obtained results show that exactly one line of atoms can be inserted into the lattice, which remains at standstill. However, if more than one line of atoms are inserted into the lattice, then they will split into several lines with α = 1, where α presents the atoms inserted. In other words, only the kink with α = 1 is stable, while the other kinks are unstable, and will split into α = 1 kinks, which remain at standstill.

Interaction between mobile dislocations and perfect dislocation loops in Fe-Ni-Cr austenitic alloy systems

The classical molecular dynamics method is employed to simulate the interaction of screw and edge dislocations with interstitial perfect dislocation loops (of 2 and 5 nm in diameter) in the austenitic model alloy Fe70Ni10Cr20 at temperatures of T = 300–900 K. Perfect loops can be created from Frank loops during the plastic deformation of irradiated austenitic steels applied in nuclear reactors. As a result, the dislocation-defect interaction mechanisms are established and classified. The loop absorption mechanisms, which are related to the formation of free channels capable of enhancing radiation-induced steel embrittlement, are revealed. The effectivenesses of loop absorption observed during their interaction with screw and edge dislocations, as well as unpinning stresses required for a dislocation to overcome the defect acting as an obstacle, are compared versus the material temperature, defect size, and interaction geometry.

Interaction of Dislocations with Carbides in BCC Fe Studied by Molecular Dynamics

Iron carbide (Fe3C), also known as cementite, is present in many steels and has also been seen as nanosized precipitates in steels. We examine the interaction of edge dislocations with nanosized cementite precipitates in Fe by molecular dynamics. The simulations are carried out with a Tersoff-like bond order interatomic potential by Henriksson et al. for Fe-C-Cr systems. Comparing the results obtained with this potential for a defect free Fe system with results from previously used potentials, we find that the potential by Henriksson et al. gives significantly higher values for the critical stress, at least at low temperatures. The explanation was found to be the difference in the core structure of the edge dislocation. The results show that edge dislocations can unpin from cementite precipitates of sizes 1 nm and 2 nm even at a temperature of 1 K, although the stresses needed for this are high. On the other hand, a 4 nm precipitate is not sheared by edge dislocations at low temperatures (≤100 K) on our simulation timescale.

Atomistic simulation of the interaction between mobile edge dislocations and radiation-induced defects in Fe-Ni-Cr austenitic alloys

The classical molecular dynamics method is employed to simulate the interaction of edge dislocations with interstitial Frank loops (2 and 5 nm in diameter) in the Fe-Ni10-Cr20 model alloy at the temperatures T = 300–900 K. The examined Frank loops are typical extended radiation-induced defects in austenitic steels adapted to nuclear reactors, while the chosen triple alloy (Fe-Ni10-Cr20) has the alloying element concentration maximally resembling these steels. The dislocation-defect interaction mechanisms are ascertained and classified, and their comparison with the previously published data concerning screw dislocations is carried out. The detachment stress needed for a dislocation to overcome the defect acting as an obstacle is calculated depending on the material temperature, defect size, and interaction geometry. It is revealed that edge dislocations more efficiently absorb small loops than screw ones. It is demonstrated that, in the case of small loops, the number of reactions accompanied by loop absorption increases with temperature upon interaction with both edge and screw dislocations. It is established that Frank loops are stronger obstacles to the movement of screw dislocations than to the movement of edge ones.

Atomic-level computer simulation of the interaction between dislocations and interstitial loops in α-zirconium

Zirconium is an important metal for internal components of nuclear reactors, yet there have been few computer simulation studies of the interaction between dislocations and interstitial dislocation loops that are created in this metal by radiation damage. Reaction mechanisms have been simulated in this work by using an interatomic potential developed by Mendelev and Ackland (2007 Phil. Mag. Lett. 87 349) that has been shown to provide a good description of the core structure and glide resistance of dislocations on the principal slip plane . The interaction of both edge and screw dislocations of the slip system with small prismatic loops containing up to 156 interstitial atoms has been considered. If a loop intersects the dislocation glide plane it becomes absorbed on the dislocation line in most situations. If it does not intersect the glide plane but has a Burgers vector inclined to that of the dislocation, it glides to and is absorbed by the line in most cases. The obstacle resistance of loops is relatively strong for screw dislocations in comparison with edges, but loop absorption by screws is only temporary.

Interaction of the Lissajous singularity and singular Lissajous line passing through an astigmatic lens

The interaction of the Lissajous singularity and singular Lissajous line in the presence of an astigmatic lens is studied. It is found that the singular Lissajous line is usually unstable, and disappears after passage through the lens. The creation and annihilation of one or two Lissajous singularities may take place by varying the astigmatic coefficient, off-axis parameter, and propagation distance. A comparison with the interaction of the Lissajous singularity and singular Lissajous line in the free-space propagation and in the presence of an ideal lens, and with the vortex–edge dislocation interaction is made. The results are illustrated numerically and interpreted analytically.

Interaction of dislocations with Frank loops in Fe–Ni alloys and pure Ni: An MD study

Variation of the stacking fault energy in FCC alloys and austenitic steels is well known to influence the evolution of radiation damage and its effect on deformation mechanisms. The primary defects observed in austenitic steels under neutron irradiation are mainly Frank loops. Here, we study the interaction of edge and screw dislocations with Frank loops in low stacking fault energy Fe–Ni alloys. The interatomic potentials employed were specially developed to reproduce a number of properties of real austenitic steels. The influence of temperature and loop morphology on the interaction mechanism and the critical resolved shear stress for dislocations to overcome loops has been investigated. All investigated reactions have been subdivided into three classes depending on temperature, loop size and interaction geometry. It is shown that by decreasing stacking fault energy below a certain value the formation of constrictions on dislocations is suppressed so that loop unfaulting becomes a less favorable mechanism in comparison with loop shear. Additional effect of solid-solution alloying, causing a non-negligible friction stress, is expressed in the impedance of the propagation of dislocations in the secondary glide planes, which is another factor limiting the unfaulting process.

Diffusionally modified elastic interaction of edge dislocations with circular inclusions

This research contains two relevant parts. In the first part, we investigate the problem of an edge dislocation interacting with a thickly coated circular inclusion. Both interface slip and diffusion occur on the inclusion/coating and coating/matrix interfaces. We further assume that all these stress relaxation processes on the two interfaces have come to an end. A closed-form solution to the problem is derived. It is observed that: (i) the dislocation will induce an internal uniform hydrostatic stress field within the three-phase circular inclusion; (ii) the dislocation will induce the same uniform rigid body rotation within the inclusion and within the coating layer; and (iii) when a compliant coating layer is thick enough, there exists no climb repulsion region. In the second part, an exact solution to the problem of an edge dislocation inside a circular inclusion with a fully relaxed interface is obtained. We observe that the climb force on the dislocation tends to keep it at the center of the inclusion.

Interaction of screw and edge dislocations with chromium precipitates in ferritic iron: An atomistic study

Binary (Fe, Cr) alloys and high-chromium ferritic–martensitic steels undergo α– α′ phase separation under thermal ageing or irradiation. The resulting Cr-rich precipitates ( α′ phase) are well known to cause hardening and embrittlement in such alloys and steels. In this work molecular statics (MS) and molecular dynamics (MD) simulations were applied to study the interaction between both a ½〈1 1 1〉 screw and ½〈1 1 1〉{1 1 0} edge dislocation with pure Cr precipitates in a bcc Fe matrix at various temperatures. After summarizing the interaction mechanisms for both types of dislocations, an analytical assessment of the interaction energy between a dislocation and precipitate is presented. The critical stress derived from the interaction energy is compared with MD data to reveal a possible correlation. For the edge dislocation and a precipitate of diameter less than 4 nm correlation with MD data is good, while for the screw dislocation no correlation was found.

Critical shear stress produced by interaction of edge dislocation with nanoscale inhomogeneity

According to the Mott and Nabarro’s model, the contribution to the critical shear stress of the material caused by the interaction between edge dislocations and nanoscale cylindrical inhomogeneities with interface stresses is obtained. The influence of the radius and the volume fraction of the inhomogeneity as well as the interface stresses on the critical shear stress is investigated. The important result is that, if the interface stress is considered, a maximum of the contribution to the critical shear stress produced by this interaction may be obtained when the radius of the inhomogeneity reaches a critical value.

Resonant interaction of edge dislocations with running acoustic waves

The interaction of equilibrium clusters of straight edge dislocations with a monochromatic sound wave having a non-zero wavevector is investigated taking into account the dislocation mass. We report on a significant increase in drift velocities of clusters when the sound wave frequency approaches a cluster's eigenfrequency. Of practical importance is the increase in the drift velocity observed for clusters with a non-zero topological charge interacting with small frequency sound waves.

Vortex-edge dislocation interaction in the presence of an astigmatic lens

The vortex-edge dislocation interaction in the presence of an astigmatic lens is studied both analytically and numerically, where the effect of astigmatism is stressed. It is shown that for the aberration-free case the edge dislocation bending and break up into a pair of oppositely charged vortices and the shift of the initial vortex appear. The astigmatism leads to some richer vortex evolution behavior. By suitably varying the astigmatic coefficient of the lens, the motion, creation, annihilation and shift to infinity of vortices take place. The off-axis distance additionally affects the vortex evolution behavior for the case of the on-axis vortex and off-axis edge dislocation interaction. In the vortex evolution process the total topological charge is not conserved in general.

Molecular dynamics study of the interactions between dislocation and imperfect stacking fault tetrahedron in Cu

The microstructure of irradiated face centered cubic alloys with low stacking fault energy is distinguished by the formation of a high number density of nanometer size stacking fault tetrahedra (SFT). A recent transmission electron microscopy investigation of high-energy proton irradiated copper [16] has shown that nearly 50% of the visible SFT population are not perfect SFTs, but rather consist of truncated SFT and/or groups of overlapping SFT. This paper presents the results of atomistic molecular dynamics simulations of the interaction between gliding dislocations, of either edge or screw character, and truncated SFT or overlapping SFT. The most common result of the edge dislocation interaction with a truncated SFT is defect shearing, ultimately leading to complete separation into two smaller defect clusters. Partial absorption of the truncated SFT is the most common result of the interaction with a screw dislocation, resulting in the formation of super-jog (or helical) segments as the defect is absorbed into the dislocation core. The resulting non-planar screw dislocation is self-pinned with reduced mobility and is re-emitted as a similar truncated SFT as the applied shear stress is increased. The re-emitted truncated SFT is often rotated and translated relative to the original position. These observations are consistent with the hypothesis that shearing (decreased defect cluster size) and dislocation dragging of the defect clusters by partial absorption into the dislocation core contributes to the formation of defect-free channels.

Correlation between internal friction background and the concentration of carbon in solid solution in a martensitic steel

A martensitic carbon steel was tempered in successive steps in order to vary the concentration of carbon in solid solution in the martensite. The actual solute content was assessed by thermoelectric power and X-ray diffraction. Internal friction was measured in a vibrating reed apparatus and in a torsion pendulum. The internal friction level around room temperature shows a surprising linear correlation with the carbon content even in the absence of the Snoek peak. This internal friction level is attributed to long-range interaction of edge dislocations with carbon in solid solution.

The vacancy–edge dislocation interaction in fcc metals: A comparison between atomic simulations and elasticity theory

The interaction between vacancies and edge dislocations in face centered cubic metals (Al, Au, Cu, Ni) is studied at different length scales. Using empirical potentials and static relaxation, atomic simulations give us a precise description of this interaction, mostly in the case when the separation distance between both defects is small. At larger distances, elasticity theory can be used to predict this interaction. From the comparison between both approaches we obtain the minimal separation distance where elasticity applies and we estimate the degree of refinement required in the calculation. In this purpose, isotropic and anisotropic elasticity is used assuming a perfect or a dissociated edge dislocation and considering the size effect as well as the inhomogeneity interaction.

Atomistic simulation of dislocation interactions in a model crystal subjected to shear

The interaction of edge dislocations in a two-dimensional (2D) model crystal subjected to “simple shear” is studied using molecular statics simulations. An initial point defect is introduced in the model to trigger the dislocation activities in a controlled manner. We consider dislocations gliding towards one another on parallel slip planes separated by various distances. The overall load-displacement response of the crystal is obtained from the simulations, which can be correlated with the nano-scale atomistic mechanisms. Although the crystal is inherently anisotropic, the incipient dislocation plasticity is such that slip is parallel to the primary shear direction as clearly demonstrated in this work. It is also illustrated that dislocation annihilation, as well as dislocation encounter which leaves behind a point defect, can be unambiguously modeled. Throughout the deformation history, more dislocations capable of gliding in the crystal tend to generate a weaker mechanical response and more pronounced plasticity. The present study also offers mechanistic insight into experimentally observed small-scale crystal plasticity.

Atomistic simulation of the pinning of edge dislocations in Ni by Ni 3Al precipitates

Classical molecular dynamics simulations of the interaction of edge dislocations in Ni with chains of spherical Ni 3Al precipitates are performed using EAM potentials. The order hardening is investigated at temperature T = 0 K by determining the critical resolved shear stresses (CRSSs) for a superdislocation that is dissociated into four partial dislocations. The CRSS is computed as a function of the radius and the distance of the precipitates. It is found that for precipitates with a diameter smaller than the dissociation width of perfect edge dislocation in Ni, the CRSS of the trailing dislocation of the superdislocation is a fraction of about 0.4 of the CRSS of the leading dislocation.

Atomistic simulation of precipitation hardening in α-iron: influence of precipitate shape and chemical composition

Classical molecular dynamics simulations of the interaction of edge dislocations with precipitates in α-iron are performed. The critical resolved shear stress (CRSS) is determined for various morphologies of precipitates: pure copper and nickel precipitates, ordered and unordered copper/nickel precipitates, copper and nickel precipitates with substitutional iron atoms, copper precipitates of ellipsoidal shape, and copper precipitates with nickel shells. The dependence of the CRSS on the nature of the precipitate is explained by considering the Burgers vector distribution within the precipitates. It is shown that, except for the ordered precipitates, chemical inhomogeneities of the precipitates lower the CRSS with respect to the precipitates consisting of the pure phases.

The interaction of an edge dislocation with an inclusion of arbitrary shape analyzed by the Eshelby inclusion method

The interaction of an edge dislocation with an inclusion of arbitrary shape is analyzed based on the Eshelby equivalent inclusion method. A general solution to determine the force on the dislocation is obtained, from which a set of simple approximate formulae is also suggested.