Screw Dislocations In Copper Research Articles

Investigation of the Dynamics of a Screw Dislocation in Copper

A modification of the mathematical model of forming the crystallographic shear band is proposed in which the strength of elastic interaction between all dislocations of the forming dislocation pileups is taken into account in addition to the Peach–Keller force; lattice, impurity, and dislocation friction; linear tension; viscous braking; and intensity of generation of point defects behind kinks. The model is used to investigate the influence of the dislocation density on the time characteristics of the formation of dislocation loops in copper.

A complete absorption mechanism of stacking fault tetrahedron by screw dislocation in copper

It was frequently observed in experiments that stacking fault tetrahedron (SFT) can be completely absorbed by dislocation and generate defect-free channels in irradiated materials, but the mechanism is still open. In this paper, molecular dynamics (MD) was used to explore the dislocation mechanism of reaction between SFT and screw dislocation in copper. Our computational results reveal that, at high temperature, the SFT is completely absorbed by screw dislocation with the help of Lomer–Cottrell (LC) lock transforming into Lomer dislocation. This complete absorption mechanism is very helpful to understand the defect-free channels in irradiated materials.

Nonplanar core and dynamical behavior of screw dislocations in copper at high velocities

Nonplanar core and dynamical behavior of screw dislocations in copper at high velocities

Cooperative atomic motions and core rearrangement in dislocation cross slip

Atomistic study of cross slip of a screw dislocation in copper is presented using the action-optimization numerical technique which seeks the most probable dynamic pathway on the potential-energy surface of the atomic system during the cross-slip process. The observed mechanism reveals features of both competing mechanisms postulated in literature, i.e., the Fleischer mechanism and the Friedel-Escaig mechanism. Due to cooperative atomic motions and complex core rearrangement during the process, the activation energies of the current cross-slip mechanism are around 0.5eV less than the lowest ever reported in corresponding studies using atomistic numerical techniques.

Atomistic simulations of cross-slip of jogged screw dislocations in copper

We have performed atomic-scale simulations of cross-slip processes of screw dislocations in copper, simulating jog-free dislocations as well as different types of jogged screw dislocations. Minimum-energy paths and corresponding transition state energies are obtained using the nudged-elastic-band path technique. We find low barriers and effective masses for the conservative motion along the dislocations of elementary jogs on both ordinary {111}<110> and nonoctahedral {110}<110> slip systems. The jogs are found to be constricted and therefore effectively act as pre-existing constrictions; the cross-slip activation energy is thereby dramatically reduced, yielding values in agreement with experiment.

Atomic structure and energetics of constricted screw dislocations in copper

Atomistic simulations of cross slip of a dissociated screw dislocation have been performed. Shapes and energetics of different dislocation configurations relevant to cross slip in an f.c.c. metal (Cu) are determined. The minimum stress-free activation energy and activation length in the Friedel-Escaig cross-slip mechanism are determined. The simulations reveal a new energetically favourable configuration of a dissociated screw dislocation not previously considered. The importance of this configuration to surface nucleated cross slip is discussed.

Simulations of the atomic structure, energetics, and cross slip of screw dislocations in copper

Using nanoscale atomistic simulations it has been possible to address the problem of cross slip of a dissociated screw dislocation in an fcc metal (Cu) by a method not suffering from the limitations imposed by elasticity theory. The focus has been on different dislocation configurations relevant for cross slip via the Friedel-Escaig (FE) cross-slip mechanism. The stress free cross-slip activation energy and activation length for this mechanism are determined. We show that the two constrictions necessary for cross slip in the FE cross-slip mechanism are not equivalent and that a dislocation configuration with just one of these constrictions is energetically favored over two parallel Shockley partials. The effect of having the dislocation perpendicular to a free surface is investigated. The results are in qualitative agreement with transmission electron microscopy experiments and predictions from linear elasticity theory showing recombination or repulsion of the partials near the free surface. Such recombination at the free surface might be important in the context of cross slip because it allows the creation of the above-mentioned energetically favorable constriction alone. In addition we observe a strong preference for the partials to be in a glide plane parallel to the surface step. We have performed simulations of two screw dislocations of opposite signs, one simulation showing surface nucleated cross slip leading to subsequent annihilation of the two dislocations. It was possible to monitor the annihilation process, thereby determining the detailed dislocation reactions during annihilation.

The yield and flow stress of Cu1% Cd alloy

The tensile properties of a commercial copper-1% cadmium alloy have been investigated at 77, 300 and 420 K. The presence of cadmium produces a sharp yield point similar to that observed in mild steel and in aluminium-magnesium alloys. The yield and flow stresses of the material were found to obey the Hall-Petch relation and it is concluded that cadmium strengthens copper mainly by increasing the grain-size-dependent component of the strength. The alloy exhibits strain ageing. The interaction between a cadmium atom and dislocations in the alloy is considered and it is shown that the atomic size misfit interaction is dominant. It is concluded that cadmium atoms can strongly pin both dissociated edge and screw dislocations in copper. The value of the grain-size-dependent component of the yield strength at low temperatures and its temperature dependence suggest that the critical event at yield is the unpinning of dislocation sources within the grain boundaries. It also appears that this process is still important in the region of homogeneous deformation at higher strains, in contrast to what occurs in copper, where the grain-size-dependent component of the flow stress diminishes markedly with increasing plastic strain. A possible mechanism for the persistence of a strong grain-size effect in the alloy is suggested.

Computer simulation of edge and screw dislocations in copper

A computer simulation was carried out of the edge and screw dislocations in copper. The calculated separation of the Schockley partials is in good agreement with recent observations using weak beam electron microscopy. However, the results demonstrate that the splitting of the partials is considerably greater than would be expected from anisotropic elasticity theory so the use of this theory to deduce stacking fault energies is questionable.

Theory of Electric field Gradient due to Conduction Electron Charge Density Redistribution around Screw Dislocations in Metals

AbstractThe electric field gradient due to conduction electron charge density redistribution around screw dislocations in metals is calculated. It is found that the electric field gradient behaves as r−5/2 cos(2 kF r + ϕ), where kF is the Fermi wave number and ϕ is the phase. The Bloch enhancement factor is computed using the orthogonalized plane wave procedure, and is found to be 7.62 for a screw dislocation in copper. These results are in agreement with nuclear magnetic resonance experiments in plastically deformed copper.

Energy and Atomic Configurations of Complete and Dissociated Dislocations. II. Screw Dislocation in an fcc Metal

The arrangement of atoms around a screw dislocation in copper has been calculated by a variational method. The pairwise interaction between discrete atoms was represented by a Morse potential function. The displacements parallel to the dislocation line agree well with those given by linear elastic theory except for atoms within a distance of about 5.3 \AA{} of the center of the dislocation. Because of this, there is a disparity between the atomistic and elastic energies inside a core radius of 5.3 \AA{} for a complete $〈110〉$ screw dislocation ($\mathrm{Burgers}\mathrm{vector}=(\frac{{a}_{0}}{2})〈110〉$), where ${a}_{0}$ is the lattice parameter. The corresponding core energy is 1.0 eV per nearest-neighbor distance. In the calculation of the complete dislocation, the atoms were not permitted to relax in a direction perpendicular to the dislocation line. This prevented dissociation. When this constraint is removed, dissociation into two partial dislocations occurs spontaneously. If the core is replaced by a hole of radius ${r}_{\mathrm{eh}}$ (the equivalent hole radius), the inside of which is hollow and outside of which linear-elastic theory holds at all points, this radius is 1.1 \AA{}. The energy of the screw dislocation varies as $\mathrm{ln}r$ at large radii, in agreement with elastic-continuum theory. By comparing this asymptotic behavior with the corresponding curve for the edge dislocation, atomistic values of the shear modulus and Poisson's ratio were obtained.

Energy and Atomic Configuration of Complete and Dissociated Dislocations. I. Edge Dislocation in an fcc Metal

The arrangement of atoms around a screw dislocation in copper has been calculated by a variational method. The pairwise interaction between discrete atoms was represented by a Morse potential function. The displacements parallel to the dislocation line agree well with those given by linear elastic theory except for atoms within a distance of about 5.3 \AA{} of the center of the dislocation. Because of this, there is a disparity between the atomistic and elastic energies inside a core radius of 5.3 \AA{} for a complete $〈110〉$ screw dislocation ($\mathrm{Burgers}\mathrm{vector}=(\frac{{a}_{0}}{2})〈110〉$), where ${a}_{0}$ is the lattice parameter. The corresponding core energy is 1.0 eV per nearest-neighbor distance. In the calculation of the complete dislocation, the atoms were not permitted to relax in a direction perpendicular to the dislocation line. This prevented dissociation. When this constraint is removed, dissociation into two partial dislocations occurs spontaneously. If the core is replaced by a hole of radius ${r}_{\mathrm{eh}}$ (the equivalent hole radius), the inside of which is hollow and outside of which linear-elastic theory holds at all points, this radius is 1.1 \AA{}. The energy of the screw dislocation varies as $\mathrm{ln}r$ at large radii, in agreement with elastic-continuum theory. By comparing this asymptotic behavior with the corresponding curve for the edge dislocation, atomistic values of the shear modulus and Poisson's ratio were obtained.