Cross-slip and annihilation of screw dislocations in Cu: a molecular dynamics study (original) (raw)
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Short-range dislocation interactions using molecular dynamics: Annihilation of screw dislocations
Journal of Materials Research, 1998
We present results of a large-scale atomistic study of the annihilation of oppositely signed screw dislocations in an fcc metal using molecular dynamics (MD) and an Embedded-Atom-Method (EAM) potential for Cu. The mechanisms of the annihilation process are studied in detail. From the simulation results, we determined the interaction energy between the dislocations as a function of separation. These results are compared with predictions from linear elasticity to examine the onset of non-linear-elastic interactions. The applicability of heuristic models for annihilation of dislocations in large-scale dislocation dynamics simulations is discussed in the light of these results.
Dislocation initiation in copper—a molecular dynamics study
Nanostructured Materials, 1999
Dislocation initiation was studied in copper systems under shear deformation with the molecular dynamics method. The shear strain in the system was generated by decreasing the external temperature and a mismatch in the coefficients of thermal expansion between the materials surrounding the system. Temperature varied between 230K and 460K. These conditions commonly occur in electronic packages.
Atomistic Determination of Cross-Slip Pathway and Energetics
Physical Review Letters, 1997
The mechanism for cross slip of a screw dislocation in Cu is determined by atomistic simulations that only presume the initial and final states of the process. The dissociated dislocation constricts in the primary plane and redissociates into the cross-slip plane while still partly in the primary plane. The transition state and activation energy for cross slip as well as the energies of the involved dislocation constrictions are determined. One constriction has a negative energy compared to parallel partials. The energy vs splitting width for recombination of parallel partials into a perfect dislocation is determined. The breakdown of linear elasticity theory for small splitting widths is studied.
Wear, 2005
Scratching of a Cu surface using a nanoindenter was studied by molecular dynamics simulation using embedded atom potentials. The simulation was carried out using a rigid Ni tip (radius = 2.5 nm) scratching on a Cu{0 0 1} surface. The Cu substrate contained 200,000 atoms and various scratching conditions were simulated by the rigid body translation of the tip at different indentation depths (0-0.36 nm), speeds (4.2-49.8 m/s), and temperatures (4-300 K). Atomic scale stick-slip was observed during the scratching simulation and it was associated with dislocation nucleation and propagation. The stick-slip accompanies the slow increase of friction force during the elastic deformation and is followed by an abrupt decrease of the friction force due to plastic yielding. The stick-slip was repeated with an approximately same period and it was pronounced at certain ranges of sliding conditions in terms of scratching depth, speed, and temperature. The simulation also exhibited that the V-shaped dislocations consisting of Shockley partial dislocations were constructed and they propagated near the free surface along the slip system of an fcc crystal. Detailed analysis of friction oscillation suggested that the nucleation of the dislocation during scratching plays more important roles in determining the abrupt drop during stick-slip than subsequent propagation of partial dislocations.
2005
For single phase metals, both the stress exponent n and activation energy Q of stationary deformation significantly change from low to high deformation temperature. This paper illustrates a recently developed model of dislocation density evolution, which is able to explain that transition. Important model components in this respect are the consideration of dislocation dipoles (represented as distribution of dipole heights) as well as of dislocation annihilation mechanisms of different kinetics: glide-induced dislocation-dislocation reactions and climb of edge dipole constituents. The model exhibits a transition with decreasing temperature from dislocation climb to glide as the decisive annihilation kinetics, which in turn is reflected in the steady-state deformation behavior.
Materials Science and Engineering: A, 2001
Using QM-Sutton-Chen many-body potential, we have studied the 1/2a 1 1 0 screw dislocation in nickel (Ni) via molecular dynamics (MD) simulations. We have studied core energy and structure using a quadrupolar dislocation system with 3D periodic boundary conditions. The relaxed structures show dissociation into two partials on {1 1 1} planes. The equilibrium separation distance between the two partials is 2.5 nm, which is larger than the derived value according to experimental data, due to low stacking fault energy given by the QM-Sutton-Chen force field. From our calculations, the core energy for the 1/2a 1 1 0 screw dislocation is 0.5 eV/b. We also studied motion and annihilation process of opposite signed dislocations. We build the dipole system with two combinations of dissociation planes: (a) two dislocations dissociated on intersecting slip planes and (b) two on parallel planes. The process of cross-slip and associated energy barriers are also calculated from these simulations.
Core structure of a dissociated easy-glide dislocation in copper investigated by molecular dynamics
Physical Review B, 1990
The atomic structure in the core of two Schockley partial dislocations in copper, resulting from the dissociation of a perfect easy-glide dislocation, and its influence on the fault ribbon, have been investigated for the first time as a function of temperature using molecular dynamics. We employed a resonant model pseudopotential adapted to copper. Our results show that at increasing temperature, the core of the partial dislocations becomes increasingly extended and invades entirely the fault ribbon, but the separation distance between the partial dislocation pairs is not altered. It follows that the structure of the fault ribbon differs significantly from that of an infinitely extended stacking fault and for this reason experimental determinations of the stacking-fault energy, based on the measure of the separation distance between partial dislocation pairs, should be considered with caution. We found that the temperature dependence of the fault ribbon energy in our model is mainly due to the elastic-modulus variation. Moreover, at high temperatures vibrational amplitudes of atoms are much larger in the core of the partial dislocations than in the bulk of the perfect crystal and the local atomic structure becomes highly disordered. Although disordered, the core structure remains solidlike up to the melting point T. Above T the liquid nucleates always in the core region, thus qualitatively indicating that the nucleation barrier therein is lower than in the bulk.
Physical Review B, 2012
We develop a model of cross-slip in face-centered cubic (fcc) metals based on an extension of the Peierls-Nabarro representation of the dislocation core. The dissociated core is described by a group of parametric fractional Volterra dislocations, subject to their mutual elastic interaction and a lattice-restoring force. The elastic interaction between them is computed from a nonsingular expression, while the lattice force is derived from the γ surface obtained directly from ab initio calculations. Using a network-based formulation of dislocation dynamics, the dislocation core structure is not restricted to be planar, and the activation energy is determined for a path where the core has three-dimensional equilibrium configurations. We show that the activation energy for cross-slip in Cu is 1.9 eV when the core is represented by only two Shockley partials, while this value converges to 1.43 eV when the core is distributed over a bundle of 20 Volterra partial fractional dislocations. The results of the model compare favorably with the experimental value of 1.15 ± 0.37 eV [J. Bonneville and B. Escaig, Acta Metall. 27, 1477 (1979)]. We also show that the cross-slip activation energy decreases significantly when the core is in a particular local stress field. Results are given for a representative uniform "Escaig" stress and for the nonuniform stress field at the head of a dislocation pileup. A local homogeneous stress field is found to result in a significant reduction of the cross-slip energy. Additionally, for a nonhomogeneous stress field at the head of a five-dislocation pileup compressed against a Lomer-Cottrell junction, the cross-slip energy is found to decrease to 0.62 eV. The relatively low values of the activation energy in local stress fields predicted by the proposed model suggest that cross-slip events are energetically more favorable in strained fcc crystals.
Stress and temperature dependence of screw dislocation mobility inα-Fe by molecular dynamics
Physical Review B, 2011
The low-temperature plastic yield of α-Fe single crystals is known to display a strong temperature dependence and to be controlled by the thermally activated motion of screw dislocations. In this paper, we present molecular dynamics simulations of 1 2 111 {112} screw dislocation motion as a function of temperature and stress in order to extract mobility relations that describe the general dynamic behavior of screw dislocations in pure α-Fe. We find two dynamic regimes in the stress-velocity space governed by different mechanisms of motion. Consistent with experimental evidence, at low stresses and temperatures, the dislocations move by thermally activated nucleation and propagation of kink pairs. Then, at a critical stress, a temperature-dependent transition to a viscous linear regime is observed. Critical output from the simulations, such as threshold stresses and the stress dependence of the kink activation energy, are compared to experimental data and other atomistic works with generally very good agreement. Contrary to some experimental interpretations, we find that glide on {112} planes is only apparent, as slip always occurs by elementary kink-pair nucleation/propagation events on {110} planes. Additionally, a dislocation core transformation from compact to dissociated has been identified above room temperature, although its impact on the general mobility is seen to be limited. This and other observations expose the limitations of inferring or presuming dynamic behavior on the basis of only static calculations. We discuss the relevance and applicability of our results and provide a closed-form functional mobility law suitable for mesoscale computational techniques.
Journal of Physics: Condensed Matter, 2013
Screw dislocations in bcc metals display non-planar cores at zero temperature which result in high lattice friction and thermally activated strain rate behavior. In bcc W, electronic structure molecular statics calculations reveal a compact, non-degenerate core with an associated Peierls stress between 1.7 and 2.8 GPa. However, a full picture of the dynamic behavior of dislocations can only be gained by using more efficient atomistic simulations based on semiempirical interatomic potentials. In this paper we assess the suitability of five different potentials in terms of static properties relevant to screw dislocations in pure W. As well, we perform molecular dynamics simulations of stress-assisted glide using all five potentials to study the dynamic behavior of screw dislocations under shear stress. Dislocations are seen to display thermally-activated motion in most of the applied stress range, with a gradual transition to a viscous damping regime at high stresses. We find that one potential predicts a core transformation from compact to dissociated at finite temperature that affects the energetics of kink-pair production and impacts the mechanism of motion. We conclude that a modified embedded-atom potential achieves the best compromise in terms of static and dynamic screw dislocation properties, although at an expense of about ten-fold compared to central potentials.