Cross-slip Process Research Articles

Shear stress- and line length-dependent screw dislocation cross-slip in FCC Ni

Screw dislocation cross-slip is important in dynamic recovery of deformed metals. A mobile screw dislocation segment can cross slip to annihilate an immobile screw dislocation segment with opposite Burgers vector, leaving excess dislocations of one kind in a crystal. Previous studies have found that the cross-slip process depends on both the local stress state and dislocation line length, yet a quantitative study of the combined effects of these two factors has not been conducted. In this work, we employ both dynamic concurrent atomistic-continuum (CAC) [L. Xiong, G. Tucker, D.L. McDowell, Y. Chen, J. Mech. Phys. Solids 59 (2011) 160–177] and molecular dynamics simulations to explore the shear stress- and line length-dependent screw dislocation cross-slip in face-centered cubic Ni. It is demonstrated that the CAC approach can accurately describe the 3-D cross-slip process at a significantly reduced computational cost, as a complement to other numerical methods. In particular, we show that the Fleischer (FL) [R.L. Fleischer, Acta Metall. 7 (1959) 134–135] type cross-slip, in which a stair-rod dislocation is involved, can be simulated in the coarse-grained domain. Our simulations show that as the applied shear stress increases, the cross-slip mechanism changes from the Friedel-Escaig (FE) [B. Escaig, J. Phys. 29 (1968) 225–239] type to the FL type. In addition, the critical shear stress for both cross-slip mechanisms depends on the dislocation line length. Moreover, the cross-slip of a screw dislocation with a length of 6.47 nm analyzed using periodic boundary conditions occurs via only the FL mechanism, whereas a longer dislocation with length of 12.94 nm can cross-slip via either the FE or FL process in Ni subject to different shear stresses.

Effect of manganese content on hot deformation behaviour of Fe–(20/27)Mn–4Al–0.3C non-magnetic steels

The influence of Mn content on flow behaviour and constitutive equations of Fe–(20/27)Mn–4Al–0.3C austenitic steels was investigated by uniaxial hot compression tests. The recrystallisation fraction was also measured using electron backscattered diffraction. Although dynamic recrystallisation is slightly delayed by the increased Mn content, the peak stress still has a pronounced decline especially during the deformation at relatively low temperature. The reason should be ascribed to the enhanced stacking fault energy, by which workhardening rate before the onset of dynamic recrystallisation is diminished significantly with the reinforced cross-slip process. The hot deformation activation energy is decreased under the condition of higher Mn addition, while the strain rate sensitivity of flow stress almost remains unchanged.

Effect of stacking fault energy on mechanical properties and strengthening mechanisms of brasses processed by cryorolling

In the present study, contribution of individual strengthening mechanisms has been analyzed to corroborate the improvement of the mechanical properties of cryorolled brasses of Cu–3.8% Zn and Cu–9.6%11Zn content is in weight % until unless stated. Zn compositions. The samples were cryorolled up to the maximum possible reduction in area after homogenized annealing (at 800°C for 4h). After the cryodeformation, a remarkable ~12 times improvement of yield strength (YS) was observed for both the alloys compared to that of the homogenized samples. The improvement of the YS is found to be much higher compared to that reported in our earlier work [Mater. Des 67 (2015) 637–643], although alloying content is much less in the present study. This is attributed to the change in rolling design, which allowed deformation of the samples under LN2 during rolling. The short time ageing (20min, 225–300°C) followed by cryorolling was found to enhance ductility without affecting the YS significantly. The solid solution strengthening by Zn in Cu leads to the decrease in the stacking fault energy which prevents the dynamic recovery and restricts the process of cross slip. Thereby, it helps to increase twining activity for further deformation. The TEM investigation confirmed that the formation of subgrains and nanotwins is responsible for the simultaneous improvement of strength and ductility. The analysis of different strengthening mechanisms revealed that the grain size refinement played the pivotal role in the improvement of the mechanical properties.

Atomistic study on the cross-slip process of a screw dislocation in magnesium

The cross-slip process of a screw <a> dislocation from the basal to the prismatic plane in magnesium was studied using the density functional theory and the molecular dynamics calculations. An atomistic method for calculating the total Peierls energy map has been devised to track the transition path of a dissociated and/or constricted screw <a> dislocation in the cross-slip process. The barrier of a screw <a> dislocation from the basal to the prismatic plane is estimated by the density functional theory for the first time to be meV per Burgers vector length. The activation enthalpy for the cross slip is calculated using a line tension model based on the density functional theory to be 1.4–1.7 eV, which is in reasonable agreement with experiments. On the basis of the results, the effect of temperature on the cross-slip process of the dissociated screw <a> dislocation on the basal plane is studied in detail using the molecular dynamics method with the embedded-atom-method (EAM) interatomic potential, in which the critical resolved shear stress for the cross slip is evaluated. It is confirmed that the bowed-out dislocation line on the prismatic plane consists of slightly dissociated rectilinear segments with connecting jogs at low temperatures and, as the temperature rises, the curved dislocation line becomes smooth with many segments. The motion of an <a> dislocation on the prismatic plane is jerky in the low temperature region, while it is retarded by the formation of the largely dissociated plateau segment above the room temperature. A large reduction of the critical shear stress for the cross slip is obtained when the <a> screw dislocation interacts with a hard-sphere particle placed on the basal plane in the low temperature region.

Cross-slip process in model Ni(Al) solid solution: An embedded-atom method study

The cross-slip process of a screw dislocation in model Ni(Al) random solid solution is studied using the climbing image nudged elastic band method with an embedded-atom method potential. The average stacking fault energy of model Ni(Al) solid solution decreases with increasing Al concentration from 0 to 10 at.%. However, the average activation energy under zero stress shows an initial increase and then plateaus with increasing concentration of Al. The excess activation energy is determined by comparing with results from linear-elastic continuum theory. The short-range repulsive solute atom pair and its interaction with the dislocation provide the excess activation energy in the cross-slip process of model Ni(Al) solid solution.

Cross slip of dislocation loops in GaN under shear

AbstractThis work explores possible cross‐slip mechanisms of gliding type &lt;a &gt; = a /3[1 –2 1 0] dislocation loops in wurtzite gallium nitride (GaN) as a function of slip plane. A modified form of the dislocation dynamics code ParaDiS was employed using isotropic linear elasticity and dislocation mobilities estimated in part from molecular dynamics (MD) simulations. Under an externally applied uniform stress, the occurrence of cross slip events is highly dependent on the initial dislocation slip plane. The basal plane is the preferred active plane, owing to the greater mobility of &lt;a &gt; type segments on that plane, over the other planes considered including the prismatic (–1 0 1 0) and two equivalent pyramidal planes (–1 0 1 1) and (1 0 –1 1). For an applied stress state, cross slip processes are more readily seen from the prismatic‐to‐basal planes or the pyramidal‐to‐basal planes, and neither is found to occur in reverse. Cross slip by climb is not presently considered. In all cases, cross‐slip events occur after the loop expands until a greater number of screw‐oriented segments are able to access the cross slip plane and the resolved stresses on the plane become sufficiently large. In comparison to dislocations found in GaN previously, the calculations suggest that some threading dislocations along the [0001] direction that have edge character may have been formed from loops whose screw segments slip and escape on basal planes leaving only the edge segments.

The ternary Ni—Al—Co embedded-atom-method potential for γ/γ′ Ni-based single-crystal superalloys: Construction and application

An Ni—Al—Co system embedded-atom-method potential is constructed for the γ(Ni)/γ′(Ni3Al) superalloy based on experiments and first-principles calculations. The stacking fault energies (SFEs) of the Ni(Co, Al) random solid solutions are calculated as a function of the concentrations of Co and Al. The calculated SFEs decrease with increasing concentrations of Co and Al, which is consistent with the experimental results. The embedding energy term in the present potential has an important influence on the SFEs of the random solid solutions. The cross-slip processes of a screw dislocation in homogenous Ni(Co) solid solutions are simulated using the present potential and the nudged elastic band method. The cross-slip activation energies increase with increasing Co concentration, which implies that the creep resistance of γ(Ni) may be improved by the addition of Co.

Plasticity of bcc micropillars controlled by competition between dislocation multiplication and depletion

Recent micropillar experiments have shown strong size effects at small pillar diameters. This “smaller is stronger” phenomenon is widely believed to involve dislocation motion, which can be studied using dislocation dynamics (DD) simulations. In the present paper, we use a three-dimensional DD model to study the collective dislocation behavior in body-centered cubic micropillars under compression. Following the molecular dynamics (MD) simulations of Weinberger and Cai, we consider a surface-controlled cross-slip process, involving image forces and non-planar core structures, that leads to multiplication without the presence of artificial dislocation sources or pinning points. The simulations exhibit size effects and effects of initial dislocation density and strain rate on strength, which appear to be in good agreement with recent experimental results and with a simple dislocation kinetics model described here. In addition, at the high strain rates considered, plasticity is governed mainly by the kinetics of dislocation motion, not their elastic interactions.

Dynamic Recrystallization in Zirconium Alloys

Abstract In this investigation, the characteristics of dynamic recrystallization (DRX) in five Zr-alloys, namely, commercial pure zirconium (CP-Zr), Zircaloy-2 (Zr–Sn), Zr-2.5Nb (Zr–Nb), Zr-2.5Nb-0.5Cu (Zr–Nb-Cu), and Zr-1Nb-1Sn-0.1Fe (Zr–Nb–Sn–Fe), have been examined. For this purpose, processing maps were constructed using strain rate sensitivity (m) data as a function of temperature and strain rate. This has been done using compression tests carried out at constant true strain rate. The processing maps revealed the bounds of DRX domain as iso-contour of m in strain rate–temperature plane. Optical microscopy and transmission electron microscopy (TEM) were used to analyze the evolution of the microstructure and to ascertain the nature of dislocation mechanisms in deformed and quenched samples from the DRX domain. For all the alloys studied, TEM revealed complete modification of β-transformed microstructure to an equiaxed morphology containing dislocation substructure within. The variation of grain size with strain rate and temperature, high ductility, and a linear variation of log(grain size) with log(Z) (where Z is the Zener–Hollomon parameter) are some important characteristic features associated with DRX domains of Zr-alloys. While Nb exhibited strong influence on lowering the strain rate for the occurrence of DRX of CP-Zr, Sn addition (Zr–Sn) did not alter DRX characteristics. TEM observations neither revealed the nature of nucleation event nor identified the dislocation mechanism involved in the DRX process. In order to ascertain the rate controlling dislocation mechanism, the flow stress data were analyzed using a model of thermally activated flow. On the basis of the magnitudes of activation energy and apparent activation area, the cross-slip of screw dislocation has been identified as the rate controlling mechanism involved in the DRX process. The activation energy has been found to increase by alloying elements, which is expected to lower stacking fault energy (SFE) and increase separation between the partials. Further analysis of the data using Friedel’s model revealed that the activation energy for the cross-slip process can be decomposed into two parts—one related to the process of forming a constriction in the dissociated dislocations and the other needed for recombining the dissociated dislocations. While the former component (related to SFE) is dominant in Nb containing Zr-alloys, both the components are involved in controlling the deformation behavior of CP-Zr and Zr–Sn alloys. The effects of alloying elements on DRX characteristics and on the evolution of dislocation structure have been rationalized on the basis of cross-slip as the rate controlling dislocation mechanism in a qualitative manner. The importance of dynamic recovery in DRX process has been emphasized in view of this.

Burgers vector analysis by three-dimensional laser-scattering tomography

We propose a simple method to analyse the Burgers vector of dislocations in crystalline semiconductors or insulators by light scattering. The method is based on the analysis of cross-slip processes revealed by three-dimensional laser-scattering tomography. It permits the full analysis of the Burgers vector distribution in macroscopic volumes. Its application presupposes that (i) dislocations are decorated by precipitates and (ii) cross-slip took place in the sample.

Atomistic Simulations of Dislocation - Crack Interaction

The interaction of dislocations with a static mode I crack is studied by large scale molecular dynamics simulations. The model consists of a blunted [001](110) crack in nickel, to which after relaxation at K < K Ic the displacement field of a dislocation is added. The response of the system is monitored during its evolution in the micro-canonical ensemble. The three dimensional nature of the problem requires the simulation of many millions of atoms. The great demands on the computational resources and data storage can only be met by high performance computing platforms and by the development of appropriate simulation methods. The simulations allowed to identify different characteristic processes during the interaction of the impinging dislocation with the crack. In particular, stimulated dislocation emission and cross slip processes are observed to be important for the development of a plastic zone.

Atomistically informed dislocation dynamics in fcc crystals

We develop a nodal dislocation dynamics (DD) model to simulate plastic processes in fcc crystals. The model explicitly accounts for all slip systems and Burgers vectors observed in fcc systems, including stacking faults and partial dislocations. We derive simple conservation rules that describe all partial dislocation interactions rigorously and allow us to model and quantify cross-slip processes, the structure and strength of dislocation junctions, and the formation of fcc-specific structures such as stacking fault tetrahedra. The DD framework is built upon isotropic non-singular linear elasticity and supports itself on information transmitted from the atomistic scale. In this fashion, connection between the meso and micro scales is attained self-consistently, with all material parameters fitted to atomistic data. We perform a series of targeted simulations to demonstrate the capabilities of the model, including dislocation reactions and dissociations and dislocation junction strength. Additionally we map the four-dimensional stress space relevant for cross-slip and relate our findings to the plastic behavior of monocrystalline fcc metals.

Cross-slip process in fcc nickel with hydrogen in a stacking fault: An atomistic study using the embedded-atom method

An atomistic study is conducted on the effect of H, which is homogeneously distributed in a stacking fault, on the activation energy of the cross slip of a screw dislocation in Ni using the embedded-atom method. The global minimum energy configurations with H in the stacking fault and the cross-slip processes are determined by the nudged elastic band method. It is revealed that H in the stacking fault increases the activation energy of cross slip significantly, due to the increase in the separation between partial dislocations induced by H. The increase in the activation energy induced by H in the stacking fault is much larger than that induced by H bound to dislocation cores during cross slip. The present study provides direct evidence that H in the stacking fault is the controlling mechanism for H-inhibited cross slip, and thus, H-induced slip planarity in common fcc materials where dislocations dissociate into partial dislocations.

Cooperative atomic motions and core rearrangement in dislocation cross slip

Atomistic study of cross slip of a screw dislocation in copper is presented using the action-optimization numerical technique which seeks the most probable dynamic pathway on the potential-energy surface of the atomic system during the cross-slip process. The observed mechanism reveals features of both competing mechanisms postulated in literature, i.e., the Fleischer mechanism and the Friedel-Escaig mechanism. Due to cooperative atomic motions and complex core rearrangement during the process, the activation energies of the current cross-slip mechanism are around 0.5eV less than the lowest ever reported in corresponding studies using atomistic numerical techniques.

Investigation of Low Angle Grain Boundaries in Hexagonal Silicon Carbide

ABSTRACTInteraction between basal plane dislocations and single or well-spaced threading dislocations is discussed based on synchrotron white beam X-ray topographic studies carried out on physical vapor transport grown hexagonal silicon carbide single crystals. The basal plane dislocations are able to cut through single or well-spaced threading edge dislocations even if the formation of kinks/jogs is energetically unfavorable while threading screw dislocations were mostly observed to act as effective pinning points. However, basal plane dislocations can sometimes cut through a threading screw dislocation, forming a superjog and which subsequently migrates on the prismatic plane via a cross-slip process. Threading edge dislocation walls act as obstacles for the glide of basal plane dislocations and the mechanism by which this occurs is discussed. The character of low angle grain boundaries and their dislocation content are discussed.

Orientation Fragmentation in Copper, Nickel and Aluminum: A Comparative Study of the Nucleation Process

An attempt to model the nucleation of fragment boundaries during cold plastic deformation of f.c.c. metals is presented. The paper focuses on intrinsic nucleation in the grain interior due to elementary processes on the dislocation level. Since orientation fragmentation seems to be linked to slip banding and the underlying mechanisms should be the same, the model is based on the elementary process of double cross-slip. Simulations were carried out for Cu, Ni and Al. Fragment boundary spacings and misorientations could be predicted in reasonable agreement with experiment for Cu. For Ni, comparable results were obtained, when a stacking fault energy at the lower end of the range of literature data was chosen. The resulting rate equation for the generation of partial disclinations as carriers of orientation fragmentation can be implemented into an earlier model for the coupled substructure and texture development during cold plastic deformation.

Cross-slip and annihilation of screw dislocations in Cu: a molecular dynamics study

Cross-slip of screw dislocations and the annihilation of screw dislocation dipoles in Cu were simulated using constant-temperature constant-stress molecular dynamics. The cross-slip mechanism and annihilation process of flexible dislocations in a large dipole configuration was identified as a dynamic variant of the Friedel–Escaig mechanism. The cross-slip rate was found to exhibit exponential dependence on the temperature, from which the activation enthalpy for the cross-slip process was calculated by the Arrhenius relation.

Hydrogen-affected cross-slip process in fcc nickel

The effects of H on the cross-slip process of a dissociated screw dislocation in nickel are studied by atomistic simulations using a configuration-space-path technique. We find that H binding in the stacking fault exerts no effect on the activation energy of cross-slip. H that is bound to the cores of the partial dislocations and moves with the dislocations during cross-slip leads to an increase of the activation energy and thus induces slip planarity. Slip planarity is due only to a net decrease of the H-binding energy in the cross-slip process.

On stress assisted dislocation constriction and cross-slip

A novel semidiscrete Peierls–Nabarro model is introduced which can be used to study dislocation spreading at more than one slip plane, such as dislocation cross-slip and junctions. The strength of the model, when combined with an atomistic simulation for dislocation core properties, is without suffering from the uncertainties associated with empirical potentials. Therefore, this method is particularly useful in providing insight into alloy design when empirical potentials are not available or not reliable for such multi-element systems. The model is applied to study the external stress assisted dislocation cross-slip and constriction process in two fcc metals, Al and Ag, exhibiting different deformation properties. We find that the screw dislocation in Al can cross-slip spontaneously in contrast with that in Ag, where the screw dislocation splits into two partials that cannot cross-slip without first being constricted. The dislocation response to an external stress is examined in detail. The dislocation constriction energy and the critical stress for cross-slip are determined, and from the latter, we estimate the cross-slip energy barrier for straight screw dislocations.

TEM study of the friction stress acting on edge dislocations in Ni 3Al

Highly curved edge dislocations gliding on primary (111) planes are frequently observed in addition to the dominance of screw dislocations in L1 2 ordered Ni 3Al deformed at 400 °C, a temperature within the anomalous temperature regime. The detailed analysis of transmission electron microscopic (TEM) images using tilting experiments proves that the superlattice dislocations of near-edge character contain superjogs consisting of segments dissociated on both (111) planes and cube cross-slip planes. Based on the TEM observations and on stress calculations two processes are proposed for the formation of the superjogs: firstly, a double cross-slip process of the screws via the cube cross-slip plane and secondly, the reaction of primary dislocations gliding on (111) with primaries cross-slipped onto (010). Since these superjogs contain segments on the cube cross-slip plane their mobility is low causing a high friction stress acting on the edge dislocations gliding on (111); as a consequence work hardening will be affected.