Interaction Of Screw Dislocation Research Articles

Interaction of O, N, C and H interstitials with screw dislocations in hexagonal titanium

Interaction of interstitial elements and screw dislocations in hexagonal titanium is determined with ab initio calculations. O, N and C are repulsive obstacles while H exhibits attraction to dislocation, especially to pyramidal stacking faults forming the dislocation core. Studied solutes thus provoke different strengthening mechanism, i.e. dislocation cross slip or binding.

Fundamental insights on ductile to brittle transition phenomenon in ferritic steel

Ductile to brittle transition (DBT) is a well-known phenomenon responsible for deteriorating the low temperature fracture toughness of ferritic steels. It is unambiguous to state that the transition phenomenon is a direct consequence of the strong temperature and strain rate sensitivity of flow stress, which in turn is regulated by the thermally activated motion of screw dislocation at the atomic scale. Due to very high Peierls stress, screw dislocation motion is accompanied by the nucleation and propagation of kink-pairs along the dislocation line. Thus, the activation energy required for kink-pair nucleation is an essential ingredient for correctly predicting the flow stress sensitivity and hence the ductile to brittle transition temperature (DBTT) of ferritic steel. However, experimental determination of kink-pair activation energy is rather tedious and there exists a strong discrepancy between atomistic simulation results (using Density Functional Theory and empirical potentials). Therefore, the present review is aimed at understanding the mechanism of screw dislocation motion in BCC Fe lattice and comparing the results obtained for kink-pair activation energy through experiments and simulations. The effect of different alloying elements on the overall flow stress sensitivity of Fe is also discussed with reference to the interaction of screw dislocation with solute atoms or clusters. Finally, the importance of screw dislocation on predicting the DBTT is explored to identify the present knowledge gaps and the future research directions.

Discovery of multimechanisms of screw dislocation interaction in bcc iron from open-ended saddle point searches

Dislocation motion and interactions determine mechanical properties in body-centered cubic (bcc) metallic materials. However, studying mechanisms for the screw dislocation interaction is fundamentally challenging since many underlying processes involve mesotimescales and atomistic resolution, currently inaccessible by either experimental techniques or continuum theoretical methods. In this paper, we develop a computational capability based on self-evolving atomistic kinetic Monte Carlo (SEAKMC) to sample the critical events and saddle point energies related to screw dislocations and their junctions. The method is first validated by calculating the stress dependence of Peierls barriers and formation energies of kink pairs and cross-slip kink pairs on a single screw dislocation in bcc iron. Then the method is applied to a binary junction of a pair of intersecting screw dislocations, the structure of which is crucial for low-temperature plastic deformation. We identify three important mechanisms: coplanar cross-slipping, jog-pinning, and a previously unknown unzipping mechanism during the evolution of the binary junction. The mechanisms are then further validated using classical molecular dynamics simulations. The computational capability developed in this paper provides an effective tool to evaluate screw dislocation related thermally activated events in complex stress conditions. The mechanisms discovered in this paper provide critical insights into temperature dependence of the anomalous slip, a breakdown of the Schmidt law, during the plastic deformation in bcc iron and can be generalized to other bcc metals.

Diffractive analysis of spiral checkerboard zone plate

This work studies the unique behavior of spiral checkerboard zone plates under plane wave illumination. The element is constructed by superimposing two orthogonal helical linear zone plates. Based on their topological charge, various structured beams are generated whose structures display fascinating dynamics. Structure transforming, vortex pair production, annihilation, and the interaction of dipole and singular vortices, are some of the observed phenomena during the beam propagation produced by the element. On top of all that, we see the interaction of screw dislocations in the way that they get closer together when the diffracted beam propagates toward the focus, and they move away from each other as the beam moves away from the focus.

Solid solution softening and hardening in binary BCC alloys

Solute-dislocation interaction energies are calculated using Density Functional Theory and the 2-D lattice Greens function approach for Al, Ti, Cr, Fe, Zr, Nb, Mo, Ru, Hf, Ta, W, Re solutes in BCC Nb, Ta, Mo, and W. The predictions are then used in an extension of the Rao-Suzuki model to analyze the experimentally observed solid solution strengthening and softening in Nb-X, Fe-X, Ta-X, Mo-X and W-X crystals, X= Mo, W, Ta, Si or Re. DFT predictions for the solute dislocation interaction energies for Fe-Si, taken from the literature are also analyzed. The original Rao-Suzuki model considers kink migration controlled mobility of screw dislocations in body-centered cubic (BCC) structures and is valid for high solute concentrations (>5 at%) as well as above a critical temperature. Here, we extend the Rao-Suzuki model to include kink-pair nucleation statistics in the presence of solutes which is expected to be the rate limiting process at low temperatures (<0.2Tmelt). Such an extension should be valid up to low concentration of solutes as well as fairly low temperatures. The key parameters required for the model are solute – ½[111] screw dislocation interaction energies in pure Nb, Ta, Mo or W. These parameters were derived in this paper using first principles calculations. Using the calculated interaction energies, the model satisfactorily reproduces experimentally observed solution strengthening data in Nb-Mo, Nb-W, Nb-Re crystals at temperatures from 77 to 600K, for solute concentrations in the range 8.5 to 19 at% for Mo, 6 to 15 at.% for W and 5 to 9 at.% for Re. Similar agreement is found for strengthening/softening data for Ta-W, Ta-Re, Ta-Nb, Fe-Si, Mo-Re and W-Re crystals in the temperature range 77 – 600K. We also explore sensitivity of these predictions to the variation in the solute-dislocation energies. In Ta-W, a 10% variation (±0.015 eV) produces a modest change in solution hardening, which is within experimental error. The refined Rao-Suzuki model can be used in designing high strength, multi-component BCC alloys for high temperature applications.

A screw dislocation interacting with a semi-infinite crack partially penetrating a parabolic elastic inhomogeneity

We consider the interaction of a screw dislocation with a semi-infinite crack partially penetrating a parabolic elastic inhomogeneity embedded in an infinite elastic matrix subjected to anti-plane mechanical loading. Analytical solutions are derived in the case when the screw dislocation is located either in the matrix or inside the parabolic inhomogeneity. In addition, the local mode III stress intensity factor at the crack tip and the image force acting on the screw dislocation are obtained and illustrated accordingly. Moreover, we establish a dislocation emission criterion from the crack.

Interaction of a screw dislocation with a parabolic cavity and a semi-infinite crack

We derive an analytic solution to the problem of a screw dislocation interacting with a parabolic cavity and a semi-infinite sharp crack using conformal mapping techniques and the method of images. Closed-form expressions for the image force acting on the screw dislocation, the mode III stress intensity factor at the crack tip and the generalized mode III stress intensity factor for the parabolic cavity are obtained. The correctness of the solution is validated by comparison with existing solutions in the literature.

Helical dislocations: Observation of vacancy defect bias of screw dislocations in neutron irradiated Fe–9Cr

We have analysed the microstructure of a model alloy of Fe9Cr irradiated with neutrons to a dose of 1.6 dpa at 325 °C. Helical dislocations comprise a major part of the damage; these formed from the interaction of pre-existing screw dislocations with irradiation-induced defects. We have investigated the process behind how these helices form, and how they cause local clustering of dislocation loops. Specifically, we have shown experimentally that the interaction of vacancy defects with pre-existing screw dislocations causes the formation of mixed screw-edge helical dislocations. Interstitials and vacancies were generated in equal numbers, which shows that the screw dislocations must have acted as vacancy-biased sinks.Helical dislocations in general were analysed from a theoretical perspective, and three Dimensional Discrete Dislocation Dynamics (3D-DDD) was used to develop a model for the formation and growth of a vacancy-fed helical dislocation.Since the helical dislocations cause the removal of vacancies from the local microstructure, this leaves a higher supersaturation of interstitials close to the dislocations. We argue that this supersaturation is responsible for enhanced interstitial loop coarsening, leading to a higher proportion of visible interstitial clusters in the vicinity of helical dislocations. These findings offer a new perspective on how dislocations affect the spatial homogeneity of radiation damage.

Influence of solute hydrogen on the interaction of screw dislocations with vicinal twin boundaries in nickel

The influence of solute H on the interaction between screw dislocations and vicinal twin boundaries in Ni bicrystals is investigated using molecular dynamics (MD) simulations. Several interaction mechanisms such as dislocation transmission, nucleation and reflection are reported for different glide planes and grain boundary (GB) structures. The presence of H tends to transform these interactions into ones involving dislocation absorption. This disorders the atomic structure of GBs and establishes a local stress state, which promotes the ultimate failure of GBs due to the formation of vacancies. These findings will deepen our understanding on the experimentally-observed H embrittlement in metallic materials.

Screw dislocation interaction with irradiation defect-loops in α-iron: Evaluation of loop-induced stress field effect using dislocation dynamics simulations

Plastic strain spreading in post-irradiated Fe grains takes the form of wavy shear bands, where mobile dislocations interact with the radiation defect dispersions. In actual Fe grains, dislocation/loop interactions involve several contributing factors including: screw dislocation cross-slip and loop-induced stress. The loop-induced stress effect is here evaluated by systematic simulation case comparisons, using adapted dislocation dynamics simulations. Namely, dislocation/loop simulation cases are systematically compared with equivalent dislocation/facet simulation cases, under room temperature straining conditions. The facets have exactly the same size and orientation as in the reference loop cases, however; the facets have no associated stress field. It is thereby found that in presence of cross-slip the total reaction time and reaction strain associated with various dislocation/facet cases is close to their dislocation/loop counterparts (within 15% and 12%, respectively). In the present investigation context, loop-induced stress field contribution is thus a second order effect, regardless of the considered loop type and orientation. In this situation, the calculation-intensive dislocation/loop interaction description can be replaced by the much faster dislocation/facet approach, for computationally intensive grain-scale simulations.

Helical Dislocations: Observation of Vacancy Defect Bias of Screw Dislocations in Neutron Irradiated Fe-9Cr

We have analysed the microstructure of a model alloy of Fe9Cr irradiated with neutrons to a dose of 1.6 dpa at 325oC. Helical dislocations comprise a major part of the damage; these formed from the interaction of pre-existing screw dislocations with irradiation-induced defects. We have investigated the process behind how these helices form, and how they cause local clustering of dislocation loops. Specifically, we have shown experimentally that the interaction of vacancy defects with pre-existing screw dislocations causes the formation of mixed screw-edge helical dislocations. Interstitials and vacancies were generated in equal numbers, which shows that the screw dislocations must have acted as vacancy-biased sinks.Helical dislocations in general were analysed from a theoretical perspective, and three Dimensional Discrete Dislocation Dynamics (3D-DDD) was used to develop a model for the formation and growth of a vacancy-fed helical dislocation. Since the helical dislocations cause the removal of vacancies from the local microstructure, this leaves a higher supersaturation of interstitials close to the dislocations. We argue that this supersaturation is responsible for enhanced interstitial loop coarsening, leading to a higher proportion of visible interstitial clusters in the vicinity of helical dislocations. These findings offer a new perspective on how dislocations affect the spatial homogeneity of radiation damage.

Screw dislocation interaction with irradiation defect-loops in α-iron: evaluation of cross-slip effect using dislocation dynamics simulations

Plastic strain spreading in post-irradiated ferritic steels usually takes the form of wavy shear bands, where the mobile dislocations randomly interact with the radiation-induced, dispersed defect populations. In these conditions, it is believed that the dislocation/defect interactions are significantly affected by the ubiquitous, stochastic cross-slip mechanism. The cross-slip effect is assessed by means of specific three-dimensional nodal DD simulation setups, consistent with available experimental evidence of cross-slip activity in post-irradiation straining conditions, at room temperature. A significant, cross-slip induced reduction of the effective dislocation/loop interaction strength is thereby evidenced, for various applied stress conditions and dislocation configurations. The corresponding, local interaction mechanisms are consistent with grain scale DD simulations results, in terms of post-irradiation plastic strain spreading.

分子動力学法を用いたマグネシウム合金の双晶と転位の相互作用の解析

分子動力学法を用いたマグネシウム合金の双晶と転位の相互作用の解析

Influence of intrinsic kink-like defects on screw dislocation – coherent twin boundary interactions in copper

The interaction mechanisms between a dislocation and a twin boundary in nanotwinned face-centered cubic metals are well understood in terms of perfect coherent interfaces. Processes involving intrinsic incoherent twin boundary defects, however, remain largely unexplored, despite recent evidence suggesting that imperfect twin boundaries containing short kink-like step defects contribute notably to plastic deformation and twin stability in large nanotwinned grains. Here, molecular dynamics simulation is used to study the underlying interaction of screw dislocations with either 0° or 60° atomic-scale kink twin boundary defects, in order to understand how the presence of imperfect twin boundaries can alter hardening and ductility mechanisms in nanotwinned copper. It is found that kinked twin boundaries are effective in changing the mechanisms from direct dislocation transmission to dislocation absorption when the applied shear strain exceeds 1.06%, with pronounced hardening arising from such transformation. Hardening by dislocation pinning from individual kink steps is also manifest for 60° intersections. On the contrary, twin boundary defects produce no hardening at low applied strains when dislocation absorption at the twin boundary is already prevailing.

1115 AlとCuにおけるらせん転位とボイドの相互作用に関する原子シミュレーション(OS8-3 材料力学と計算力学の新展開)

1115 AlとCuにおけるらせん転位とボイドの相互作用に関する原子シミュレーション(OS8-3 材料力学と計算力学の新展開)

Elastic interaction of a screw dislocation with a double-coated cylinder

We study the problem of a screw dislocation interacting with a double-coated cylindrical inclusion in a dissimilar matrix. Using an elastostatic image method, we determine the displacement field in each material phase and obtain the force of interaction between the inhomogeneity and the dislocation in the form of a rapidly convergent infinite series. Numerical examples are presented to illustrate the effect of double coating on the force of interaction. It is found that, for certain material combinations, the presence of a double-coated cylinder gives rise to two equilibrium positions of the dislocation. This result is significant as it indicates that multiple equilibrium positions are possible for multiply-coated inclusions interacting with a screw dislocation.

A screw dislocation interacting with a bimaterial interface incorporating surface strain gradient elasticity

Abstract We present an analytical solution in terms of the exponential integral of the problem associated with the interaction of a screw dislocation near a bimaterial interface incorporating surface strain gradient elasticity. Three intrinsic material lengths are identified as a result of the introduction of the surface strain gradient elasticity for the interface. The size-dependent image force acting on the screw dislocation is also obtained. The stiffening effect of the interface can be clearly seen from the image force expression.

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.

Size-dependent elastic interaction of screw dislocations with semi-infinite coating materials revealed by atomistic simulation and two dimensional analysis

The aim of this work is to study the size-dependent elastic interaction of a screw dislocation with an elastic isotropic layered semi-infinite matrix using molecular dynamics analysis. The analytic solutions of complex functions of a screw dislocation in the matrix and in the thin layer are solved by means of applying complex potential approach in conjunction with the techniques of the image method. With the aid of the obtained stress fields and the Peach–Koehler formula, the explicit expressions for the image force acting on the screw dislocation are obtained. The results indicate that a large change in distance between dislocation and interface leads to a slight change in potential energy and stress. The material elastic dissimilarity, the thin layer thickness and the dislocation position play a crucial role in the image force acting on the dislocation. When a screw dislocation is in the matrix or in the layer, the image force approaches a constant value with the increasing thickness of thin layer. In addition, when the matrix is softer than the layer and the dislocation is in the matrix, there is a stable equilibrium position for dislocation; while, when the matrix is softer than the layer and the dislocation is in the layer, there exists a non-stable equilibrium position for dislocation.

Interaction of a screw dislocation with a nano-sized, arbitrarily shaped inhomogeneity with interface stresses under anti-plane deformations.

We propose an elegant and concise general method for the solution of a problem involving the interaction of a screw dislocation and a nano-sized, arbitrarily shaped, elastic inhomogeneity in which the contribution of interface/surface elasticity is incorporated using a version of the Gurtin-Murdoch model. The analytic function inside the arbitrarily shaped inhomogeneity is represented in the form of a Faber series. The real periodic function arising from the contribution of the surface mechanics is then expanded as a Fourier series. The resulting system of linear algebraic equations is solved through the use of simple matrix algebra. When the elastic inhomogeneity represents a hole, our solution method simplifies considerably. Furthermore, we undertake an analytical investigation of the challenging problem of a screw dislocation interacting with two closely spaced nano-sized holes of arbitrary shape in the presence of surface stresses. Our solutions quite clearly demonstrate that the induced elastic fields and image force acting on the dislocation are indeed size-dependent.