Edge dislocation mobilities in bcc Fe obtained by molecular dynamics

Queyreau, Sylvain; Marian, Jaime; Gilbert, Mark R.; Wirth, Brian D.

doi:10.1103/physrevb.84.064106

Cited by 135 publications

(91 citation statements)

References 36 publications

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“…In particular, the friction parameter for a dislocation is found to be temperature-independent, contradicting decades of theoretical work since Liebfried 5 . But this temperature-independence is seen in many other investigations of dislocations 2,3,6 with a large lattice resistance and in the stochastic motion of point defects 8,9 , although to the best of our knowledge it has not been highlighted before.…”

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confidence: 88%

“…In this paper we investigate the motion of kink-limited screw and edge dislocations in bcc Fe, where the kink formation energy is much larger than the thermal energy. To obtain dislocation motion on the time-scales accessible to molecular dynamics (MD) simulations some researchers have resorted to inducing kink formation by applying stresses some six orders of magnitude greater than those pertaining experimentally 2,3 .…”

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Theory and simulation of the diffusion of kinks on dislocations in bcc metals

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Isolated kinks on thermally fluctuating (1/2) 111 screw, 100 edge and (1/2) 111 edge dislocations in bcc iron are simulated under zero stress conditions using molecular dynamics (MD). Kinks are seen to perform stochastic motion in a potential landscape that depends on the dislocation character and geometry, and their motion provides fresh insight into the coupling of dislocations to a heat bath. The kink formation energy, migration barrier and friction parameter are deduced from the simulations. A discrete Frenkel-Kontorova-Langevin (FKL) model is able to reproduce the coarse grained data from MD at ∼10 −7 of the computational cost, without assuming an a priori temperature dependence beyond the fluctuation-dissipation theorem. Analytic results reveal that discreteness effects play an essential rôle in thermally activated dislocation glide, revealing the existence of a crucial intermediate length scale between molecular and dislocation dynamics. The model is used to investigate dislocation motion under the vanishingly small stress levels found in the evolution of dislocation microstructures in irradiated materials.Dislocation motion is limited by two general processes, the formation and migration of kinks and pinning by impurities and other defects 1 . In this paper we investigate the motion of kink-limited screw and edge dislocations in bcc Fe, where the kink formation energy is much larger than the thermal energy. To obtain dislocation motion on the time-scales accessible to molecular dynamics (MD) simulations some researchers have resorted to inducing kink formation by applying stresses some six orders of magnitude greater than those pertaining experimentally 2,3 .But dislocation core structures and Peierls barriers are known to be highly stress-dependent 4 , making it difficult to relate simulation to the vanishingly low stress conditions found in thermally activated evolution of dislocation microstructures.To avoid the problem of nucleating kinks in MD simulations we use periodic boundary conditions which enforce the existence of an isolated kink on each dislocation line in the supercell. This allows us to study the detailed dynamics of kink migration in isolation, a crucial and previously unexplored aspect of kink-limited dislocation motion, as seperate from the kink nucleation process. Under no applied stress, kinks are seen to undergo stochastic motion in a potential landscape that varies with the dislocation character and Burgers vector. A coarse graining procedure is introduced, that facilitates statistical analysis yielding many properties of the kink motion, and which provides physical insight into the coupling of dislocations to the heat bath. In particular, the friction parameter for a dislocation is found to be temperature-independent, contradicting decades of theoretical work since Liebfried 5 . But this temperature-independence is seen in many other investigations of dislocations 2,3,6 with a large lattice resistance and in the stochastic motion of point defects 8,9 , although to the best of our ...

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Theory and simulation of the diffusion of kinks on dislocations in bcc metals

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“…This normalization condition eliminates any potential arbitrariness associated with the choice of the argument of delta functions in (18). To emphasize that the projection is a conditional expectation value, we will also employ the notationP…”

Section: B Removing the Vibrational Coordinates By The Mori-zwanzig mentioning

confidence: 99%

Phonon drag force acting on a mobile crystal defect: Full treatment of discreteness and nonlinearity

Swinburne

Dudarev

2015

Phys. Rev. B

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Phonon scattering calculations predict the drag force acting on defects and dislocations rises linearly with temperature, in direct contradiction with molecular dynamics simulations that often finds the drag force to be independent of temperature. Using the Mori-Zwanzig projection technique, with no recourse to elasticity or scattering theories, we derive a general Langevin equation for a crystal defect, with full treatment of discreteness and non-linearity in the defect core. We obtain an analytical expression for the drag force that is evaluated in molecular statics and molecular dynamics, extracting the force on a defect directly from the inter-atomic forces. Our results show that a temperature independent drag force arises because vibrations in a discrete crystal are never independent of the defect motion, an implicit assumption in any phonon-based approach. This effect remains even when the Peierls barrier is effectively zero, invalidating qualitative explanations involving the radiation of phonons. We apply our methods to an interstitial defect in tungsten and solitons in the Frenkel-Kontorova model, finding very good agreement with trajectory-based estimations of the thermal drag force.

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“…In this section we investigate the motion of kink-limited screw and edge dislocations in bcc Fe, where the kink formation energy is much larger than the thermal energy. To obtain dislocation motion on the time-scales accessible to molecular dynamics (MD) simulations some researchers have resorted to inducing kink formation by applying stresses some six orders of magnitude greater than those pertaining experimentally [7,8]. But dislocation core structures and Peierls barriers are known to be highly stress-dependent [9], making it difficult to relate simulation to the vanishingly low stress conditions found in thermally activated evolution of dislocation microstructures.…”

Section: The Drift and Diffusion Of Kinksmentioning

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Atomistic Simulations in bcc Metals

Swinburne

2015

Stochastic Dynamics of Crystal Defects

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It is clear that numerical methods essential when trying to understand the detailed atomic structure and dynamics. The most popular quantum mechanical approach is density functional theory [1] (DFT), which exploits an isomorphism between the ground state wave function and ground state electron density to approach the many-body Schrödinger equation variationally, often allowing efficient evaluation of complex structures and energy landscapes. However, even with significant simplifications to the underlying Hamiltonian such techniques are currently limited to less than 500 metallic atoms, meaning that they only be used to inform coarse grained models of atomic interaction. At present, short segments of screw dislocation cores can be simulated in DFT allowing accurate parametrisation of the Peierls barrier and dislocation formation energies, along with similar properties for point defect formation energies, as well as bulk properties such as the lattice parameter and bulk modulus [2]. It is the aim of any coarse grained model to quantitatively reproduce these features as accurately as possible. Born and Oppenheimer [3] were the first to realise that much insight into atomic rather than electronic motion can be gained if one works on a coarse grained timescale of femtoseconds, where the electronic co-ordinates are assumed to instantaneously assume the minimum energy configuration consistent with the given atomic positions. In this way one can argue that only the atomic positions need be retained along with some phenomenological interaction potential which attempts to recreate the energy landscape generated by the full quasistatic quantum mechanical system.In practice, molecular dynamics (MD) simulation aims to use an interatomic potential that best reproduces the features from DFT. The simplest choice is some isotropic pair potential φ(r ) such thatwhere the factor of 1/2 accounts for the exchange symmetry between i, j. Despite the appeal of this simple and efficient form, it can be shown that this can only produce

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Edge dislocation mobilities in bcc Fe obtained by molecular dynamics

Cited by 135 publications

References 36 publications

Theory and simulation of the diffusion of kinks on dislocations in bcc metals

Theory and simulation of the diffusion of kinks on dislocations in bcc metals

Phonon drag force acting on a mobile crystal defect: Full treatment of discreteness and nonlinearity

Atomistic Simulations in bcc Metals

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