Atomistic Simulation of kinks for 1/2a Screw Dislocation in Ta

Materials Science and Engineering: A, 2001

Using a new, first principles based, embedded-atom-method (EAM) potential for tantalum (Ta), we have carried out molecular dynamics (MD) simulations to investigate the core structure, core energy and Peierls energy barrier and stress for the 1/2 a 1 1 1 screw dislocation. Equilibrated core structures were obtained by relaxation of dislocation quadrupoles with periodic boundary conditions. We found that the equilibrium dislocation core has three-fold symmetry and spreads out in three 1 1 2 directions on {1 1 0} planes. Core energy per Burgers vector b was determined to be 1.36 eV/b. We studied dislocation motion and annihilation via molecular dynamics simulations of a periodic dislocation dipole cell, with 1 1 2 and 1 1 0 dipole orientation. In both cases the dislocations move in zigzag on primary {1 1 0} planes. Atoms forming the dislocation cores are distinguished based on their atomic energy. In this way, we can accurately define the core energy and its position not only for equilibrium configurations but also during dislocation motion. Peierls energy barrier was computed to be ∼0.07 eV/b with a Peierls stress of ∼0.03µ, where µ is the bulk shear modulus of perfect crystal. The preferred slipping system at low temperature is 1 1 2 directions and {1 1 0} planes.

Physical Review B, 2003

Two types of equilibrium core structures ͑denoted symmetric and asymmetric͒ for 1/2a͗111͘ screw dislocations in bcc metals have been found in atomistic simulations. In asymmetric ͑or polarized͒ cores, the central three atoms simultaneously translate along the Burgers vector direction. This collective displacement of core atoms is called polarization. In contrast, symmetric ͑nonpolarized͒ cores have zero core polarization. To examine the possible role of dislocation core in kink-pair formation process, we studied the multiplicity, structural features, and formation energies of 1/3a͗112͘ kinks in 1/2a͗111͘ screw dislocations with different core structures. To do this we used a family of embedded atom model potentials for tantalum ͑Ta͒ all of which reproduce bulk properties ͑density, cohesive energy, and elastic constants͒ from quantum mechanics calculations but differ in the resulting polarization of 1/2a͗111͘ screw dislocations. For dislocations with asymmetric core, there are two energy equivalent core configurations ͓with positive ͑P͒ and negative ͑N͒ polarization͔, leading to 2 types of ͑polarization͒ flips, 8 kinds of isolated kinks, and 16 combinations of kink pairs. We find there are only two elementary kinks, while the others are composites of elementary kinks and flips. In contrast, for screw dislocations with symmetric core, there are only two types of isolated kinks and one kind of kink pair. We find that the equilibrium dislocation core structure of 1/2a͗111͘ screw dislocations is an important factor in determining the kink-pair formation energy.

Journal of Nuclear Materials, 2009

We have studied the structure and the formation and migration energies of single kinks in ½h1 1 1i screw dislocations in body-centered cubic iron, by performing static calculations using the Ackland-Mendelev empirical potential, which correctly accounts for the non-degenerate core structure. The methodology for constructing simulation cells with fully periodic boundary conditions based on the quadrupolar arrangement of dislocation dipoles, with a single kink on each dislocation line is presented. The two types of kinks-left and right-are found to have similar widths, namely $20 Burgers vectors. The convergences of the formation energies with cell-size along the dislocation line, as well as with the distance between the two dislocations are investigated. A dependence proportional to the inverse of the distance between kinks along the dislocation line is found when kinks overlap. The formation energies of the left and right kinks are significantly different: 0.57 and 0.08 eV, respectively. The Peierls potentials of the second kind are evaluated with the drag method: the energy barriers are found to be lower than 0.1 meV for both kinks.

Using a Quantum Mechanics (QM) based, many body force field for tantalum with Molecular Dynamics (MD), we calculate the core structure, core energy, Peierls energy barrier and stress for ½ a<111> screw dislocation in bcc Ta. We study static properties such as core energy and structure using quadrupole arrangement of dislocations in simulation cells with periodic boundary. We found that the equilibrium dislocation core has three-fold symmetry and spreads out in three <112> directions on {110} planes. We calculate the core energy of dislocation in two new ways. We obtained core energy to be 1.404 eV/b from fitting strain energy of dislocation quadruples with elastic theory with the choice of core radius to be 2.287b and 1.400 eV/b by summing atomistic strain energies of 12 atoms close to dislocation center. We also distinguished the core of dislocation based on atomistic strain energy of atoms and analyzed dislocation dipole (both in <110> and <112> orientations) annihilation processes that were simulated via molecular dynamics at 0.001K. Peierls energy barrier was computed to be ∼0.073 eV/b and a Peierls stress 1.8 GPa. From studies of dislocation dipole annihilation at temperatures from 20K to 300K, we determined the activation energy of dislocation jumps to be 0.0053 eV/b in Arhenius plot. This activation energy is 14 times less than one at 0.001 K. It implied that dislocation moved differently from Peierls-Nabarro model at high temperatures.

Modelling and Simulation in Materials Science and Engineering, 2004

We introduce a novel approach to calculating the Peierls energy barrier (and Peierls stress) based on the analysis of the dislocation migration dynamics, which we apply to 1/2a 111 screw dislocations in bcc Ta. To study the migration of screw dislocations we use molecular dynamics with a first principles based embedded-atom method force field for Ta. We first distinguish the atoms belonging to the dislocation core based on their atomic strain energies, defining the dislocation core as the 12 atoms with higher strain energies per Burgers vector. We then apply this definition to the moving dislocations (following the dynamics of a [1-10] dipole of 1/2 111 screw dislocations at 0.001 K) and extract their Peierls energy barrier (E P ) and Peierls stress (τ P ). From the dynamics of a dislocation dipole, we determine E P = 0.032 eV (and τ P = 790 MPa) for twinning shear and E P = 0.068 eV (and τ P = 1430 MPa) for anti-twinning shear, in good agreement with the results by applying direct shear stresses. This dislocation dynamics method provides insights regarding the dislocation migration process, allowing us to determine the continuous path of dislocation migration. We find that under twinning shear the screw dislocation moves along a path at an angle of only 8.5˚with the [1-10] direction while for anti-twinning shear it moves along a path at an angle of 29.5˚with the [1-10] direction, documenting the magnitude of the violation of the Schmid Law.

Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 2005

Atomistic simulations of accelerating edge and screw dislocations were carried out to study the dynamics of dislocations in a face centered cubic metal. Using two different embedded atom potentials for nickel and a simple slab geometry, the Peierls stress, the effective mass, the line tension and the drag coefficient were determined. A dislocation intersecting an array of voids is used to study dynamic effects in dislocation-obstacle interactions. A pronounced effect caused by inertial overshooting is found. A dynamic line tension model is developed which reproduces the simulation results. The model can be used to easily estimate the magnitude of inertial effects in the interaction of dislocations with localized obstacles for different obstacle strengths,-spacings and temperatures.

Journal of the Mechanics and Physics of Solids, 2017

We use a real-space formulation of orbital-free DFT to study the core energetics and core structure of an isolated screw dislocation in Aluminum. Using a direct energetics based approach, we estimate the core size of a perfect screw dislocation to be ≈ 7 |b|, which is considerably larger than previous estimates of 1−3 |b| based on displacement fields. The perfect screw upon structural relaxation dissociates into two Shockley partials with partial separation distances of 8.2Å and 6.6Å measured from the screw and edge component differential displacement plots, respectively. Similar to a previous electronic structure study on edge dislocation, we find that the core energy of the relaxed screw dislocation is not a constant, but strongly dependent on macroscopic deformations. Next, we use the edge and screw dislocation core energetics data with physically reasonable assumptions to develop a continuum energetics model for an aggregate of dislocations that accounts for the core energy dependence on macroscopic deformations. Further, we use this energetics model in a discrete dislocation network, and from the variations of the core energy with respect to the nodal positions of the network, we obtain the nodal core force which can directly be incorporated into discrete dislocation dynamics frameworks. We analyze and classify the nodal core force into three different contributions based on their decay behavior. Two of these contributions to the core force, both arising from the core energy dependence on macroscopic deformations, are not accounted for in currently used discrete dislocation dynamics models which assume the core energy to be a constant excepting for its dependence on the dislocation line orientation. Using case studies involving simple dislocation structures, we demonstrate that the contribution to the core force from the core energy dependence on macroscopic deformations can be significant in comparison to the elastic Peach-Koehler force even up to distances of 10 − 15 nm between dislocation structures. Thus, these core effects, whose origins are in the electronic structure of the dislocation core, can play an important role in influencing dislocation-dislocation interactions to much larger distances than considered heretofore.

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.

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.

Materials Science and Engineering, 1981

Three different models for a pinned dislocation under an applied stress are compared in this paper: (i) the continuous string model of Koehler, Granato and Liicke; (ii) a twodimensional lattice model which represents the dislocation in its slip plane; (iii) a discrete string model which is obtained as an analytical approximation to model (ii). Good agreement is obtained for the quantities investigated for the three models. Comparison of models (i) and (iii) therefore provides an atomistic interpretation for the parameters appearing in the defining equation of the continuous string model. One result of interest is that these parameters cannot be expressed solely in terms of the characteristics of the interatomic law for atoms in the vicinity of their equilibrium sites for the perfect crystal but must also depend on aspects of the interatomic force law for atoms in configurations such as those which occur near the dislocation.