Extended Screw Dislocation Research Articles

Studying hydrogen effect on the core structure and mobility of dislocation in nickel by atomistically-informed generalized Peierls–Nabarro model

Hydrogen (H) atoms in the metallic crystalline lattice interact with the pre-existing dislocations and then remarkably affect the plastic deformation of metals. Thereby quantitatively characterizing the H-dislocation interaction is of great importance for understanding H-induced plasticity and failure. Most of the previous studies have focused on the long-range interaction between hydrogen and dislocation, but rarely considered the short-range interaction, especially the hydrogen effect on the dislocation core structure. Here, with the aid of the H-affected γ-surface calculated from atomistic modeling, an atomistically-informed generalized Peierls–Nabarro model is employed to study the hydrogen effect on the core structure of dislocation, the recombination energy of the extended screw dislocation, and the Peierls stress in nickel. Our results show that, on the one hand, hydrogen can decrease the stable stacking fault energy, leading to the increase of the stacking fault width for both extended edge and screw dislocations. Consequently, the recombination energy of extended screw dislocation is increased, indicating that hydrogen can suppress the cross-slip of screw dislocation and facilitate the slip planarity observed frequently in experiments. On the other hand, hydrogen can increase the unstable stacking fault energy, implying that hydrogen can inhibit the nucleation of partial dislocations. Moreover, hydrogen in the dislocation core increases the Peierls stress and thus increases dislocation slip resistance. Finally, quantitative relations between the Peierls stress and hydrogen concentration are given. These results are of great significance for understanding the H-affected dislocation plasticity mechanisms, and can be used for quantifying the hydrogen effect on dislocation dynamics.

Modeling the temperature and high strain rate sensitivity in BCC iron: Atomistically informed multiscale dislocation dynamics simulations

Multiscale discrete dislocation plasticity (MDDP) simulations are carried out to investigate the mechanical response and microstructure evolution of single crystal BCC iron subjected to high strain rate compression over a wide range of temperature. The simulations are conducted at temperatures ranging between 300 K and 900 K and strain rate ranging between 102 to107 s−1. Atomistically informed generalized mobility law was incorporated in MDDP to account for the effects of temperature and strain rate on dislocation mobility, lattice friction and elastic constants.MDDP based constitutive equations interrelating temperature and strain rate with the flow stress at high strain rate shock-less and shock conditions are proposed. The simulation results of the temperature and strain rate dependent yield strength and Hugoniot elastic limit are in good agreement with reported experimental results. Detailed investigations of the dislocation microstructure evolution show the formation of extended screw dislocation lines at temperatures below 340 K due to the large value of the lattice friction of the pure screw segments. Moreover, small sessile loops of radius in the order of few nanometers are formed. The formation of these sessile loops is facilitated by the easiness of multiple cross slip on available slip planes.

New role of screw dislocation in twin lamella during deformation: An in situ TEM study at the atomic scale

Two dynamic processes were revealed in Cu by in situ tensile tests at the atomic scale: a lattice screw dislocation forming by the combination of two twinning dislocations; and two 60° full dislocations evolving from an extended screw dislocation. The results indicate that screw dislocation can trigger the transition of dislocation slip mode by its formation and dissociation, and also nucleate non-screw dislocations.

Screw dislocation equations in a thin film and surface effects

The fundamental equations of screw dislocations in a thin film are derived for arbitrary boundary conditions: free surface, fixed surface and gradient loading imposed on the surface. The new equations make it possible to study the modification in dislocation structure and other surface effects. According to the modification of interaction between dislocations, boundary conditions can be essentially classified into two types: the stressed boundary (fixed stress on the surface) and the strained boundary (fixed strain on the surface). The interaction between dislocations decreases near stressed boundary and increases near strained boundary. The external stress (strain) field is separated as an independent superimposition with the modification of dislocation interaction. Surface effects on a single extended screw dislocation behavior in a thin film are investigated quantitatively. We show that the dislocation becomes narrower near a free surface, and wider near a fixed surface, where the reduction and increase of dislocation width scales proportionally to the reciprocal of distance between dislocation and surface. Correspondingly, the Peierls stress increases and decreases respectively. The image force on dislocation is attractive for the free surface and repulsive for the fixed surface. The amplitude of image force is smaller than that given in elastic theory, and the reduction is reversely proportional to the square of distance to the surface. Furthermore, the dislocation in a free standing thin film is considered and the distance between the dislocation and surface affect the amplitude of surface effect mostly.

Solute drag on perfect and extended dislocations

The drag force exerted on a moving dislocation by a field of mobile solutes is studied in the steady state. The drag force is numerically calculated as a function of the dislocation velocity for both perfect and extended dislocations. The sensitivity of the non-dimensionalized force–velocity curve to the various controlling parameters is assessed, and an approximate analytical force–velocity expression is given. A non-dimensional parameter S characterizing the strength of the solute-dislocation interaction, the background solute fraction , and the dislocation character angle , are found to have the strongest influence on the force–velocity curve. Within the model considered here, a perfect screw dislocation experiences no solute drag, but an extended screw dislocation experiences a non-zero drag force that is about 10 to 30% of the drag on an extended edge dislocation. The solutes can change the spacing between the Shockley partials in both stationary and moving extended dislocations, even when the stacking fault energy remains unaltered. Under certain conditions, the solutes destabilize an extended dislocation by either collapsing it into a perfect dislocation or causing the partials to separate unboundedly. It is proposed that the latter instability may lead to the formation of large faulted areas and deformation twins in low stacking fault energy materials containing solutes, consistent with experimental observations of copper and stainless steel containing hydrogen.

A dislocation dynamics analysis of the critical cross-slip annihilation distance and the cyclic saturation stress in fcc single crystals at different temperatures

Cross-slip is treated as a deterministic, mechanically activated process governed by the applied stress, by the interaction force between approaching screw dislocations of the opposite sign and by the line tension related to the persistent slip band channel width. The dislocations are modeled as moving polygons. In the evaluation of the critical cross-slip annihilation distance and the saturation stress in cycling, we accept two assumptions inspired by Brown [Brown L. Philos Mag A 2002;82:1691]: (i) the critical parameter associated with cross-slip is the spreading of the dislocation loop on the cross-slip plane, not the critical formation of a constriction in an extended screw dislocation core. (ii) The saturation stress in cycling is controlled by the stress required to separate two screw dislocations of opposite signs which are just on the point of mutual annihilation by cross-slip. The proposed model predicts the critical annihilation distance and the cyclic saturation stress in agreement with the available experimental data for Cu, Ni and Ag single crystals.

Stress orientation and relativistic effects on the separation of moving screw dislocations

The subsonic motion of a fast-moving, extended screw dislocation in an fcc metal under constant stress is studied using continuum linear elastic dislocation theory and molecular dynamics (MD) simulation. In this regime, many phenomena predicted by the theory are shown to prevail in simulation, in particular, relativistic effects and stress orientation effects. Due to the former, the fault width is found to contract as it moves faster until $\ensuremath{\approx}80%$ of the shear wave speed, beyond which a turning point occurs preventing it from constricting to a perfect one as speed increases further. The stress orientation effect, which is first introduced by Nabarro [Philos. Mag. 14, 861 (1966)], is demonstrated here to manifest when the shear stress resolved in the direction of motion and glide plane becomes high. A simple analytical expression for the steady-state fault width accounting for both stress orientation and relativistic effects is presented. In MD simulation under arbitrary stress states, both the dislocation velocity and separation width achieve a quasisteady state, about which they oscillate with an amplitude and frequency that reduces with speed. The separation width oscillates about a value close to that predicted by the new analytical expression. The drag coefficient is found to linearly increase with speed for speeds greater than 20% of the shear wave speed.

Atomistic simulations of jog migration on extended screw dislocations

We have performed large-scale atomistic simulations of the migration of elementary jogs on dissociated screw dislocations in Cu. The local crystalline configurations, transition paths, effective masses, and migration barriers for the jogs are determined using an interatomic potential based on the Effective Medium Theory. The minimum energy path through configuration space and the corresponding transition state energy are obtained using the Nudged Elastic Band path technique. We find very similar migration properties for elementary jogs on the (110){111} octahedral slip systems and the (110){110} non-octahedral slip systems, with energy barriers in the 15–19 meV range.

High-resolution transmission electron microscopy study of solid phase crystallized silicon thin films on SiO2: Crystal growth and defects formation

A high-resolution transmission electron microscopy study of the solid phase crystallization of amorphous silicon thin films deposited on SiO2 at 520 °C by low pressure chemical vapor deposition and annealed at 550 °C in a dry N2 ambient was carried out so that the grain growth mechanism, various types of defects, and the origins of defect formation could be understood on an atomic level. Silicon crystallites formed at the initial stage of the crystallization had a circular shape and grains had a branched elliptical or a dendritic shape. Many twins, of which {111} coherent boundaries were parallel to the long axis of a grain, were observed in the interior of all the elongated grains. In addition to twins, the following defects were observed in the grain: intrinsic stacking faults, extrinsic stacking faults, perfect dislocations, extended screw dislocations, and Shockley partial dislocations. These defects were formed by the following reasons: errors in the stacking sequence at the amorphous/crystalline interface; jumps of a twin plane; the intersecting of two crystal growth fronts slightly misoriented; and the intersecting of two twin planes at the amorphous/crystalline interface. Among those defects, twins and stacking faults provided a preferable nucleation site for an atomic step of a {111} plane. As a result, it was concluded that grain growth in the 〈112〉 direction along the {111} plane parallel to the long axis of a grain was accelerated by twins and stacking faults.

Computer Simulation of the Interaction Between An Extended Dislocation and Radiation Defects in the fcc Lattice

The interaction between an extended dislocation and a radiation-induced defect, especially a self-intersistial atom (SIA), has been investigated in the model fcc lattice by computer simulation technique. An SIA was absorbed into the core of one of the two partial dislocations of the extended screw dislocation as a crowdion which extends along the dislocation line. Under the applied shear stress this crowdion acted as a pinning point, resulting in irradiation hardening. On the other hand, an SIA was absorbed at the jog site of the extended edge dislocation (at one of the two jog sites on two partial dislocations) and after some relaxation the total jog was shifted to one atomic distance through the spreading out of the strain due to an SIA from one partial side to the other side

Elastic interaction of 〈111〉 defects and extended screw dislocation in anisotropic fcc crystals

Elastic interaction between extended screw dislocations and tetragonal defects of the 〈111〉 type was studied by computing the energy-distance and the force-distance profiles for several metals using anistropic elasticity theory. The isotropic values of interaction energy and force as the limiting cases of the anistropic values were also obtained. It is shown that both the width of the dislocation and the anisotropy of the crystal significantly affect the energy-distance and the force-distance profiles. From the results obtained, the maximum interaction energy and the hardening coefficients for defect-dislocation interaction were also calculated. These values are compared with experimentally observed hardening in metals.

Elastic interaction of extended screw dislocations with 〈110〉 misfit defects in anisotropic F.C.C. crystals

Elastic interaction between extended screw dislocations and defects of the 〈110〉 type is studied by computing the energy–distance and the force–distance profiles for several metals using anisotropic elasticity theory. It is shown that anisotropy has a significant effect on the magnitudes of interaction energy and interaction force, and on the shape of the energy–distance and force–distance curves. From the results obtained, the maximum interaction energy between dislocations and divacancies of various orientation in copper is also calculated. Elastische Wechselwirkungen zwischen ausgedehnten Schraubenversetzungen und Defekten vom 〈110〉-Typ werden mittels Berechnung des Energie-Distanz- und der Kraft-Distanzprofile fur einige Metalle mit der anisotropen Elastizitatstheorie untersucht. Es wird gezeigt, das die Anisotropie einen betrachtlichen Einflus auf die Grose der Wechselwirkungsenergie und der Wechselwirkungskraft und auf die Form der Energie–Distanz- und Kraft–Distanz-Kurven haf. Aus den erhaltenen Ergebnissen wird auch die maximale Wechselwirkungsenergie zwischen Versetzungen und Doppelleerstellen verschiedener Orientierung in Kupfer berechnet.

Anisotropic elastic interaction between misfit defects and extended screw dislocations in fcc metals

The energy and force of interaction of an extended screw dislocation with spherical and tetragonal defects were calculated for several fcc metals using anisotropic and isotropic elasticity theories. It is shown that anisotropy has a large effect on the magnitude of interaction energy and interaction force, as well as on the shape of the energy-distance and force-distance curves. The maximum value of the interaction energy was always smaller in the anisotropic case than in the isotropic case. But the maximum resisting force to dislocation motion due to defect-dislocation interaction is not always smaller in the anisotropic crystal. Computation of the hardening coefficient at 0 °K for several fcc metals indicates that tetragonal defects cause significantly larger hardening than spherical defects.

Deformation of high purity tantalum single crystals at 4.2 K

High purity Ta single crystals of various orientations were deformed in tension and compression at 4.2 K. The expected (011) slip was the dominant mode of deformation in tension, except for orientations near [011], where twinning and some anomalous ( 1 01) slip were observed. In tension, stress-strain curves were always serrated owing to the occurrence of adiabatic slip. In compression, the dominant mode of deformation was twinning; slip was only observed to a small extent for orientations near [001]. The occurrence of adiabatic slip in tension obscured any possible effects of normal stress on the critical resolved shear stress for slip. However, a pronounced increase of twinning stress with increasing normal stress was observed in compression. This can be explained in terms of the effect of normal stress on the nucleation of twins by the transformation of the screw dislocation core into a three-layer twin configuration. The addition of up to 4% Re to Ta eliminated twinning at 77K, and promoted the occurrence of slip at 4.2 K for orientations displaying only twinning in unalloyed specimens. This effect is most probably related to effects of solute atoms on the extended screw dislocation core.

Kinetics of solute atom drift to dislocation dipoles

The time dependence of solute drift to an edge dislocation dipole is calculated for a dislocation-solute atom interaction characterized by the first-order size effect. It is found that the rate of solute arrival to dislocations at constant temperature is proportional to the square root of the recovery time, t, in contrast to the t 2 3 behavior characteristic of single straight edge dislocations as sinks. This effect can be a significant influence on solute segregation to extended dislocations, since an extended screw dislocation has a first order size effect interaction with a solute atom which is of the same form as the dipole considered. Thus t 1 2 kinetics are characteristic of solute drift to extended screw dislocations as well.