Dissociation Of Perfect Dislocations Research Articles

Atomistic dislocation core energies and calibration of non-singular discrete dislocation dynamics

The total energy of an atomistic dislocation includes contributions from the inelastic/large-distortion ‘core’ region. Capturing this inelastic ‘core’ energy is important, especially for dislocations with a curvature in the 10–100 nm scale. Current implementations of discrete dislocation dynamics (DDD) mesoscale simulations either approximate or neglect the core energy and so do not provide consistency with fully-atomistic studies. Using established interatomic potentials for FCC metals, the total dislocation energy is computed directly in atomistic simulations of straight dislocations and a core energy at any desired cut-off core radius is obtained as a function of dislocation character. A proper introduction of the atomistic core energy into the ParaDiS DDD code that uses a non-singular theory (Cai et al 2006 J. Mech. Phys. Solids 54 561–87) is then presented. The resulting atomistically-informed ParaDiS DDD is used to simulate the periodic bow-out of edge and screw dislocations in near-elastically-isotropic aluminum at various length and stress, with comparisons to fully-atomistic simulations. Generally good agreement is obtained between DDD and atomistics, with the best agreement achieved using a non-singular regularization parameter in the range of a = 5 – 10b. The analysis is then extended to compute the core energy of the Shockley partial dislocations that arise in the dissociation of perfect dislocations in fcc metals.

Effect of surface on the dissociation of perfect dislocations into Shockley partials describing the herringbone Au(1 1 1) surface reconstruction

In the framework of the continuum elastic theory, the equilibrium positions of Shockley partial dislocations have been determined as a function of their distance from the free surface. It is found that the dissociation width decreases with the decreasing depth, except for a depth range very close to the free surface for which the dissociation width is enlarged. A similar behaviour is also predicted when Shockley dislocation pairs are regularly arranged, whatever the wavelength. These results derived from the elastic theory are compared to STM observations of the reconstructed (1 1 1) surface in gold, which is usually described by a Shockley dislocations network.

Origin of tension-compression asymmetry in ultrafine-grained fcc metals

Ultrafine-grained Al exhibits remarkable tension/compression asymmetry of yield stress. Large-scale atomistic simulations and dislocation theory reveal that the yield event is not related to intragranular dislocations but caused by dislocation nucleation from the grain boundaries (GBs). Dislocation core associated with the stacking fault energy is strongly affected by the external stress in Al; the dissociation of perfect dislocation is stabilized by high tensile stress. These dislocations are more likely to be nucleated from GBs with lower yield stress. The mechanism, which is completely different from well-known mechanisms for nanocrystalline and amorphous metals, is unique to high-strength UFG metals.

Ab initio and classical atomistic modelling of structure and defects in crystalline orthorhombic polyethylene: Twin boundaries, slip interfaces, and nature of barriers

We study the stability of twin boundaries and slip in crystalline orthorhombic polyethylene by means of density functional theory (DFT), using a nonempirical, truly nonlocal density function, and by means of classical molecular dynamics (MD). The results show that, in accordance with experimental observations, there is a clear preference to chain slip over transverse slip for all considered slip planes. The activation energy for pure chain slip lies in the range 10–20 mJ/m2 while that for transverse slip corresponds to 40–280 mJ/m2. For the (11¯0)-slip plane the energy landscape is non-convex with multiple potential energy minima, indicating the presence of stable stacking faults. This suggests that dissociation of perfect dislocations into partials may occur. For the two low-energy twin boundaries considered in this work, {110} and {310}, we find that the former is more stable than the latter, with ground state energies corresponding to 8.9 and 28 mJ/m2, respectively. We have also evaluated how well the empirical MD simulations with the all-atom optimized potential for liquid MD simulations (OPLS-AA) and the coarse-grained united atom (UA) potential concur with the DFT results. It is found that an all-atom potential is necessary to partially capture the γ-surface energy landscapes obtained from the DFT calculations. The OPLS-AA predicts chain slip activation energies comparable with DFT data, while the transverse slip energy thresholds are low in comparison, which is attributed to weak close ranged monomer repulsion. Finally, we find that the H-H interaction dominates the slip activation. While not explicitly represented in the UA potential, its key role is revealed by correlating the DFT energy landscape with changes in the electron distributions and by MD simulations in which components of the OPLS-AA intermolecular potential are selectively silenced.

Deformation of Austenitic CrMnNi TRIP/TWIP Steels: Nature and Role of the ɛ−martensite

High-alloy CrMnNi TRIP/TWIP steels show a martensitic phase transformation, which is frequently described by the transformation sequence from ϒ-austenite (fcc) via ɛ−martensite (hcp) to α−martensite (bcc). During the plastic deformation, deformation bands in austenite are formed, which contain a high density of stacking faults on the lattice planes {111}. The formation of the stacking faults is enabled by the dissociation of perfect dislocations into the partial dislocations. In ɛ−martensite, the stacking faults are highly ordered. Diffraction experiments recognize the periodic arrangement of the stacking faults as a hexagonal phase. In this contribution, the structure of ɛ−martensite was described with the aid of the analysis and the DIFFaX simulation of X-ray diffraction patterns, electron back-scatter diffraction and high-resolution transmission electron microscopy. The faulting of austenite enhances the formation of α−martensite during deformation, thus the work hardening capability is significantly increased.

Stacking faults in B2-structured magnesium alloys from first principles calculations

The stacking faults of 17 B2-structured magnesium alloys have been studied systematically by means of first-principles calculations. After structure optimization and stability analysis, the generalized stacking fault energy surfaces for two possible slip planes {001} and {110} have been calculated using a super-cell tilling technique, and the main feature of generalized stacking fault energy surfaces was analyzed. Then the most likely slip directions were determined, and the stable and unstable stacking energies were obtained. The dissociation of perfect dislocation was further discussed. The electronic structures were also investigated to reveal the underlying mechanism for stability and stacking faults.

Multiscale analysis of defect initiation on the atomistic crack tip in body-centered-cubic metal Ta

The quasi-continuum method (QC), a multiscale method, is used to analyze body-centered-cubic (bcc) metal tantalum (tantalum, Ta) type Ⅱ crack-tip dislocation nucleation. Based on the relationship curves between dislocations emission position and stress intensity factor, the processes of dislocation defect initiation and development are investigated. Dislocation travels forward with different characteristics in different stages and the new nucleated dislocations expedite the already nucleated dislocation to move away from the crack tip. The analysis of initiation of the crack tip defects shows that they are the local defects that first appear, and with loading, more local defects emerge, which eventually move to the boundary, and lead to the type Ⅱ fracture. Furthermore, dislocation dissociation as well as extended dislocation is discussed. The partial dislocation nucleating before the perfect dislocation nucleation and emission is full proof that the dissociation of perfect dislocation takes place step by step, which means that the two minimum points on the energy curve have different formation mechanisms.

Determination of stacking fault probability in fcc Fe–Mn–Si–Al alloy by electron diffraction

The stacking disorder in the fcc structure of Fe–23Mn–2Si–2Al alloy after tensile testing at −75°C was investigated by electron diffraction, using a relationship between the stacking fault probability and the shift of diffraction spots deduced in the present work (and based on Kakinoki’s theory [Acta Crystallogr. 23, 875 (1967)] and Kajiwara’s previous work [Jpn. J. Appl. Phys. 9, 385 (1970); J. Phys. Soc. Jpn. 22, 795 (1967)]). Shifts of diffraction spots along the [111] direction in the fcc structure were observed, and the stacking fault probabilities in two selected areas with different densities of stacking faults were determined as α=0.15 and α=0.35. The stacking fault probabilities measured by electron diffraction are much larger than average values determined by x-ray diffraction, indicating that the distribution of stacking faults is localized. A mechanism for the γfcc→εhcp transformation is suggested, whereby during deformation the following sequences take place: dissociation of perfect dislocation→localization of stacking faults→evolution from stacking disorder to stacking order→the formation of the perfect hcp martensite (α=1) or the hcp martensite with stacking faults (α→1). This proposed mechanism for strain-induced hcp martensite formation should be also applicable to the thermally induced hcp martensite.

Grain Scale Assessment of Variant Selection in a Thermomechanically Processed TRIP Steel

Orientations of both the α and γ phases in a multi-phase commercial steel were measured by means of electron backscatter diffraction (EBSD) techniques. Using the average orientation of each austenite grain as the reference frame, the orientation relationships between the two lattices were compared with the common orientation relationships (i.e. the Kurdjumov-Sachs and Nishiyama-Wassermann) in Rodrigues-Frank space. The occurrence of variant selection in individual austenite grains was examined using a recent dislocation-based model. This model considers the role of the slip systems that were active during prior deformation, as well as those of in-plane reactions, cross-slip and the partial dislocations that are linked to specific variants. It also unites the competing K-S and N-W relationships through the dissociation of perfect dislocations. Reasonably good agreement was observed between the predictions and the observations. Possible explanations for some of the discrepancies are also presented.

In situ TEM observations of dislocation processes in iron-aluminides with 25 and 30 at.% aluminium

Thin single crystal foils of long-range ordered binary iron—aluminium alloys with 25 and 30 at.% aluminium have been tensile tested inside of a transmission electron microscope. The motion of dislocations has been recorded under full load at temperatures ranging from 300 to 970 K. These experiments yield information pertaining to: i) the activated glide planes; ii) the mode of glide (viscous versus jerky and planar versus cross-slip); iii) the dissociation of perfect dislocations into partials and the magnitude of the partial Burgers vectors; and iv) the widths and specific energies of the antiphase boundaries between the partial dislocations. The results are discussed with a view of elucidating the well-known anomalous, positive temperature dependence of the critical resolved shear stress of these intermetallic compounds between 500 and 780 K.

Measurement of the extent of strain relief in InGaAs layers grown under tensile strain on InP(100) substrates

High resolution x-ray diffraction has been used to investigate the structural properties of InxGa1−xAs epitaxial layers grown under tension on InP(100) substrates. The nominal indium composition (x=0.42) corresponds to a small lattice mismatch and a two dimensional growth mode. We have also included for comparison two samples grown under compression covering the mostly strained and the mostly relaxed regimes. Our results show that the residual strain and the asymmetry in strain relaxation along 〈011〉 directions are always larger for layers under tension. This can be explained by the difference in dislocation glide velocity induced by a different indium content, by the dissociation of perfect dislocations and partially by the difference in thermal expansion coefficients between substrate and epilayer. The larger asymmetry in strain relaxation for tensile strain layers is interpreted by the existence of microcracks aligned in the [011] direction.

Effect of the homogenization treatment in an electric field on T 1, precipitation in 2091 AlLi alloy

The T{sub 1} phase is an equilibrium phase and is known to nucleate heterogeneously in Al-Li-Cu-Mg alloys, probably via a stacking fault mechanism or via the dissociation of perfect dislocations in the vicinity of jogs. During aging treatment, precipitation of T{sub 1} phase becomes possible and has been reported to occur heterogeneously either at GPzones in copper-rich alloys or at matrix dislocations in lithium-rich alloys. The previous paper shows that the homogenization treatment in an electric field accelerates the dissolution of second phase particles containing copper and magnesium atoms, so leading to high solubility of alloy elements on T{sub 1} precipitation and tensile properties, in this paper, the effect of the homogenization treatment in an electric field on T{sub 1} precipitation is investigated. With increase in homogenization time in an electric field, the T{sub 1} precipitates distribute more homogeneously and their volume fraction increases because the high speed diffusion in an electric field results to a high solubility of copper element, which make the strength of 2091 Al-Li alloy increase.

Effect of additional elements on the mechanical and microstructural properties of copper polycrystal under dynamic compression

The effect of additional elements (Si and Co) on the behavior of copper polycrystalline alloy under static and dynamic compression was studied. Relations between the deformation microstructure and the mechanical behavior have been investigated. In the Cu–Si alloys, the SFE depends on additional element content and the SFE level controls the dissociation of perfect dislocations into partial ones. In the Cu–Co alloy, Co induces precipitation and the dislocation repartition is uniform if the precipitates are coherent. When the precipitates become semicoherent, there is formation of dislocation walls around the precipitates.

Stress Orientation Dependence of Dislocation Glide in Epitaxial Films

ABSTRACTThe effect of different stresses on the glide of the commonly observed dislocations in epitaxial films is systematically investigated. The possibility of planar dissociation of perfect dislocations and the conditions for their subsequent glide are explored.Slip transfer from one plane to another is affected by the crystal orientation relative to the stress axis. This process is quantitatively analyzed by calculating iso-Schmid factors on a stereographic projection. The results of this model are correlated with those found in the literature.

Dislocation dissociation in low-angle spinel boundaries

It is well known that in the spinel structure a perfect dislocation can dissociate into two partial dislocations with collinear Burgers vectors :(1)The two partial dislocations, which have the same character as the perfect dislocation, are separated by a stacking fault in the cation sublattice only. In ceramic materials, unlike fcc metals, dissociation of dislocations can occur by a climb mechanism in addition to, or instead of, dissociation by glide. Climb dissociation of perfect dislocations into two or more partials with collinear Burgers vectors has been observed in e.g. yttrium iron garnet and sapphire. Previous studies in spinels, deformed at high temperatures, emphasize climb dissociation and dissociation of mixed character. Dissociation of screw dislocations in stoichiometric MgAl2O4 spinel could not be resolved and hence no experimental measurements of the dissociation width have been reported.

Dislocations in polyethylene crystals: line energies and deformation modes

Abstract Explicit expressions are obtained for the line energy of straight dislocations in orthorhombic polyethylene crystals. It is found that the ⟨0, 0, c⟩ screw dislocation is the perfect dislocation with the lowest line energy whilst the ⟨0, 0, c⟩ edge has negative line tension and is therefore unstable. The possible dissociation of perfect dislocations into partials is discussed, and it is shown that both (110) and (310) twin dislocations can arise from such dissociations. Numerical calculations also show that homogeneous nucleation of ⟨0, 0, c⟩ dislocations, and twin dislocations, has a very high probability of occurrence under a stress of the order of the yield stress of polyethylene crystals. These results help to explain the readiness of polyethylene crystals to deform by c axis slip, twinning, and monoclinic transformation modes. The supression of c axis slip by irradiation and cross-linking is also explained by the very high line tension of the ⟨0, 0, c⟩ screw dislocations.

Elastic Strain Energy of Dislocations in Ice

The energy stored in the elastic strain field of dislocations in hexagonal ice is calculated using anisotropic elasticity and the most complete set of elastic constants available. Ice is elastically fairly isotropic, and it is proposed that the high mobility of dislocations on the basal plane is due to dissociation of perfect dislocations on this plane.