Interaction Of Edge Dislocation Research Articles

Dislocation Pinning in Helium-Implanted Tungsten: A Molecular Dynamics Study

Using molecular dynamics simulations, we investigate the interaction of edge dislocations with He-filled Frenkel pairs (He2V-SIA), the predominant defect type in helium-implanted tungsten. Clusters of 3-10 He2V-SIA are seen to be stable with their pinning strength increasing with size. For all cluster sizes, the dislocation bows around the cluster and unpins while carrying SIAs with it. The helium-vacancy complex and new vacancies left behind, have little pinning effect, explaining the “defect-clearing” and experimentally observed deformation softening. The predicted solute hardening for 3000 appm helium-induced defect distribution of varying sizes, is in excellent agreement with previous experimental observations.

Atomistic simulations of the interaction of edge dislocations with β-Nb precipitates in Zr-Nb alloys

Experiments have shown that precipitation can affect the mechanical properties of zirconium alloy, but the interaction mechanism between dislocations and Nb precipitates in zirconium alloys is still unclear. Thus, a systematic molecular dynamics study was performed to investigate the interaction between edge dislocations and Nb precipitates. It was found that the dislocation passed through Nb precipitate by shear mechanism or bypass mechanism of forming jogs, and the critical resolved shear stress increased with the diameter of the precipitate. After completion of the interaction, dislocations formed jogs due to climb when the precipitates were larger than 2 or 3 nm. Some atoms in the precipitate were more disordered after dislocation shearing, and dislocation fragments were generated around the precipitate, both of which lead to the precipitate hardening. The calculation of obstacle strength further confirmed that unsheared Nb precipitates until hundreds of nanometers were the weak obstacle for dislocations.

Atomistic Simulations of Dislocation-Void Interactions in Concentrated Solid Solution Alloys

This paper investigates the interaction of edge dislocations with voids in concentrated solid solution alloys (CSAs) using atomistic simulations. The simulation setup consists of edge dislocations with different periodicity lengths and a periodic array of voids as obstacles to dislocation motion. The critical resolved shear stress (CRSS) for dislocation motion is determined by static simulations bracketing the applied shear stress. The results show that shorter dislocation lengths and the presence of voids increase the CRSS for dislocation motion. The dislocation–void interaction is found to follow an Orowan-like mechanism, where partial dislocation arms mutually annihilate each other to overcome the void. Solute strengthening produces a ‘friction stress’ that adds to the Orowan stress. At variance with classical theories of solute pinning, this stress must be considered a function of the dislocation line length, in line with the idea that geometrical constraints synergetically enhance the pinning action of solutes. Modifying the equation by Bacon, Kocks and Scattergood for void strengthening to account for the solute hardening in CSAs allows one to quantitatively predict the CRSS in the presence of voids and its dependency on void spacing. The predictions show good agreement with the simulation data without invoking any fit parameters.

Interaction of an edge dislocation with a 〈110〉 tilt boundary in nickel: molecular dynamics simulation

The interaction of a lattice edge dislocation with a 〈110〉 tilt boundary in nickel was studied by the molecular dynamics method in the case when the dislocation glide plane is parallel to the grain tilt axis. The dependence of the local threshold stress on the grain misorientation angle is obtained. It is found that with increasing misorientation angle, the local threshold stress increases, but the growth rate for low- and high-angle boundaries is different: for high-angle boundaries, this growth is slower. When studying the mechanism of overcoming the low-angle 〈110〉 tilt boundary by an edge dislocation in the considered orientation, it was found that the dislocation, in fact, does not pass through the boundary, but exchanges places with a grain-boundary dislocation, which are ordinary perfect edge dislocations in the low-angle 〈110〉 tilt boundaries. When studying the interaction of a dislocation with high-angle boundaries, a peculiarity was noticed, which consists in pushing two or three grain-boundary dislocations out of the boundary at once.

Deformation mechanisms of a NiTi alloy under high cycle fatigue

In this paper, high-cycle fatigue mechanisms of a pseudoelastic NiTi shape memory alloy (SMA) under different mean strains are investigated. The uni-axial cyclic tensile tests with small amplitude of 0.25% are operated at three different states, i.e. austenite/elastic region, austenite and martensite coexisted region and martensite/elastic region with a pre-strain. Mechanical results indicate that the cyclic deformation strain is mainly in the inhomogeneous deformation band when the sample cycling in the B2 + B19′ phase coexist status. Cyclic deformation in the B2 + B19’ phase showed cyclic hardening effect. With increasing of the mean strain, the saturation stress is decreased, and saturation rate is reduced. The mean strain effect on the high-cycle fatigue life of NiTi is attributed to the inhomogeneous transformation of martensite. High resolution electron microscopy (HREM) observations and atomic simulations showed that partial dislocations with the Burgers vectors of 1/2<110> are induced on (001)M twin planes, the interaction of dislocations and edge dislocations in martensite results in cyclic hardening in martensite. While cyclic softening in martensite at the upper plateau of stress–strain (S–S) curve is attributed to the grain reorientation/rotation phenomenon. It suggests that newly generated stress-induced martensite at the interface of martensite and austenite could increase the high-cycle fatigue of NiTi SMA.

Моделювання поля зсувних напружень в площині ковзання в твердих розчинах заміщення

The formation of stochastic shear stress field in the glide plane in the substitutional solid solution was investigated by computer simulation. If the atoms in the crystal lattice nodes of the substitutional solid solution are considered as a kind of point defects in the virtual solvent medium, the shear stress distribution in the glide plane can be calculated based on the interaction of edge dislocation and such defects. For concentrated solid solutions, the shear stress will be a normally distributed random value with zero mathematical expectation. The standard deviation of this distribution will be the greater the greater the effective distortion of crystalline lattice of the alloy. In the case of dilute solid solution, where one of the components has a predominant content, the simulation gives shear stress distribution in the glide plane, where large peaks are separated from each other by wide areas of near-zero stresses. Thus, there are separate discrete obstacles in the form of large stress peaks for the edge dislocation in the glide plane in dilute solid solution, and the space between the peaks is practically stress-free. The average distance between large peaks correlates with the average distance between the atoms of those components that are few in solution, if total atomic fraction of these components is considered. Thus, the proposed modeling gives a very realistic shear stress distribution in the glide plane for concentrated and dilute substitutional solid solutions with fcc and bcc structures. This can be useful in further modeling the yield strength in multicomponent alloys. Keywords: dislocation, distorsion, shear stresses.

Interaction of edge dislocation with copper atoms in an aluminum crystal

In this work, the interaction of the moving edge dislocation with obstacles in form of copper atoms is studied using the molecular dynamics simulations. The samples are aluminum monocrystals of 52 × 60 × 15 nm3 with axes oriented along directions [110], [111], [112]. The structure of copper solid solution is reproduced with following procedure: aluminum atoms are randomly selected and replaced by copper atoms. The concentration of copper atoms varies from 0.25% to 1%. The dislocation movement occurs under action of shear deformation. It is found that zones with a low concentration of copper atoms only slow down dislocation in an aluminum matrix, and the zones with a high local concentration of copper atoms not only produces stronger resistance to dislocation movement, but also they cause the change in the slip plane of the dislocation segment. When a significant part of a dislocation line moves to a neighboring slip plane, the complete transition of the dislocation to this slip plane can occur. It is also noted that such transitions of dislocation segments from one slip plane to another are accompanied by the formation of vacancies. Also the maximum value of the shear stress σxy is estimated-its value is approximately 250 MPa.

Molecular dynamics simulation of strengthening dependence on precipitate Cr composition in Fe-15at.%Cr alloy

Molecular Dynamics simulation was employed to study precipitate composition dependence on strengthening. Edge dislocation interaction with pure, 80at.%, and 60at.% Cr precipitates of different sizes in a matrix of Fe-15at.%Cr was investigated. The precipitates were found to be relatively hard. This is evident from the absence of shearing mechanism after the dislocation has bypassed them, the formation of an Orowan-like dislocation shape, and comparatively higher stress values. Precipitates with higher Cr content were found to greatly impede dislocation glide as indicated by the time taken by the dislocation to bypass them. The composition dependence on critical unpinning stress was also observed. The interaction of edge dislocation with precipitates having higher Cr composition leads to higher critical unpinning stress. The extent of critical unpinning stress dependence on precipitate composition is, however, not as high as was reported previously. Besides, the study has also confirmed the fact that α’ precipitation results in the hardening of high Cr ferritic/martensitic steels.

Interaction of dislocation with GP zones or θ" phase precipitates in aluminum: Atomistic simulations and dislocation dynamics

We investigate the interaction of edge dislocation in aluminum matrix with GP zones and θ″ phase with molecular dynamics (MD) simulations. All these copper-containing precipitates have a form of single or multilayer parallel copper disks separated by planes of aluminum atoms. The maximal number of copper layers of θ″ phase considered in the present work is equal to six; the diameter of precipitates varies from 3 to 15 nm. It is shown that all types of precipitates are initially overcome by dislocation with formation of the Orowan loop around them. The most fragile precipitate is GP zone of first type with a 3 nm diameter; it is sheared during the first closure of the Orowan loop by magnitude of leading partial dislocation, and obtains an additional displacement of trailing partial after the dislocation detachment. The remaining inclusions, even GP zones of a larger diameter, demonstrate the process of accumulation of surface deformation over time due to the shift of the upper half of the copper atom disks relative to the lower one. The moments of separation of the shifted halves are determined for different types of inclusion. Cutting of the strongest six-layer θ″ phase is not observed during our MD simulations. The evolution of local stress acting on copper atoms is traced over the several interactions required for the complete shearing of obstacle. The maximal local stress reached during deformation of GP of the first type of the smallest size is equal to 1.5 GPa, while the substantial increase in acting local stress is registered for higher number of copper disks. The local stress of 5.7 GPa is observed during the sequential interactions of dislocation with six-layer θ″ phase. The entire MD system demonstrate analogical growth of the average level of acting stress simultaneously with the increase in local stress on inclusion; it varies from about 125 MPa in the case of smallest GP zone to about 670 MPa for the hardest θ″ phase. The MD data is generalized in the form of the model of dislocation dynamics at interaction with precipitate. The model was previously proposed by Krasnikov and Mayer (2019) for the case of θ′ phase in aluminum. In the present work, it is modified by accounting the change in the energy of the dislocation core during contact with different types of inclusions. The energy values are fitted from a comparison of the time dependences of average stress in system with MD data. Variation of the core energy for the dislocation line segments attached to precipitate allows us to take into account specific features of precipitates with different number of layers.

Strengthening of Al–Cu alloys by Guinier–Preston zones: Predictions from atomistic simulations

A scale bridging strategy based in molecular statics and molecular dynamics simulations in combination with transition state theory has been developed to determine the flow stress of Al–Cu alloy containing Guinier–Preston zones. The athermal contribution to the flow stress was determined from the Taylor model, while the thermal contribution was obtained from the obstacle strength and the free energy barrier. These two magnitudes were obtained by means of molecular statics and molecular dynamics simulations of the interaction of edge dislocations with Guinier–Preston zones in two different orientations. The predictions of the model were compared with experimental data and were in reasonable agreement, showing the potential of atomistic simulations in combination with transition state theory to predict the flow stress of metallic alloys strengthened with precipitates.

Dislocation dynamics in aluminum containing θ’ phase: Atomistic simulation and continuum modeling

We investigate the interaction of edge dislocation with θ′ phase in aluminum matrix using atomistic simulations. The thickness of θ′ phase is chosen to be constant of 2.2 nm and the diameter is varied in range from 3 to 10 nm. It is shown that first interactions of dislocation with θ′ phase occur according to the mechanism of Orowan loop formation around the obstacle. In this case, the cross-slip processes, the formation of dislocation jogs in adjacent slip plane and the emission of vacancies upon the return of a segment to the initial slip plane are possible. During these interactions, the material of θ′ phase is subjected to high shear stresses up to 3 GPa in a layer of 2 nm thickness. Such a high stress leads to a θ′ phase cutting on the third or fourth overcoming of the obstacle by dislocation. A study is carried out of the dependence of the average stress in the system on the size of inclusion and the distance between inclusions. It is shown that an increase in the diameter of inclusion causes an increase in the average stresses in the system in proportion close to the square root of the diameter of the inclusion. Increasing the distance between inclusions causes the inversely proportional reduction of the average stress. The conducted investigation of the strain rate sensitivity showed that in the case of high shear rates, the average stresses in the system continuously increase that does not allow applying the time averaging procedure to them. The described effect is also registered in the case of pure aluminum. The existence of two regions on the temperature dependence of the average stresses in the system on the strain rate in the case of θ′ phase, previously described by Yanilkin et al. (2014) for the Guinier-Prestone zones, is confirmed. In the case of high strain rates, heating of the system leads to a decrease in the dislocation velocity, while at low strain rates the dislocation velocity increases with increasing temperature at a fixed shear rate. An interesting result obtained with long-term molecular-dynamics simulations when the tracing time is up to 2 ns, there is a tendency to reduce the average stress in the system through time. This result can be explained by destruction of the structure and form of θ’ phase.The law of motion of a dislocation in the approximation of the constancy of θ′ phase properties is proposed to describe the response of the system to shear deformation. The model contains a dislocation mass, phonon friction, and takes into account the effect of the inclusion of θ’ phase through an increase in the elastic energy of a dislocation during the formation of the Orowan loop around an obstacle.

Edge dislocation nucleation mechanism of surface blistering in tungsten exposed to deuterium plasma

The mechanism of tungsten (W) blistering under deuterium (D) plasma exposure is still under investigation. To clearly demonstrate the microstructure and nucleation mechanism of blistering on W exposed to D plasma, special recrystallized W disc samples were prepared and electropolished with back-thinned method for TEM observation. D plasma exposure (2.0 × 1026 D m−2, 573 K) brought it numerous intra-granular blisters and protrusions on W with typical orientation dependence. TEM observation revealed the intra-granular blister microstructure that substantial dislocations were generated on the center and edge of blister via severe plastic deformation of blister cap. Dislocation tangles formed by dislocations from blisters were revealed and supposed to be the nucleation of intra-granular blister. By using the g · b = 0 and g · b × u = 0 criterion, 〈0 0 1〉 dislocations with edge component were identified which were generated by 〈1 1 1〉 edge dislocation interaction in dislocation tangles. A {0 0 1}〈0 0 1〉 edge dislocation nucleating and blistering mechanism based on previous works is proposed, and by applying which blistering in recrystallized W could be well explained. Dedicated experiments demonstrated that intra-granular blister formation depends on local dislocation density which validated the mechanism.

Simulation of interaction of edge dislocations with radiation defects in Fe-10Cr alloy

The interaction of edge dislocations with radiation-damaged regions and mobility of dislocation loops in Fe-10Cr alloy is studied. Calculations are carried out in the framework of the molecular dynamics method. Dislocation loops formed as a result of ion irradiation of the sample free surface and straight dislocations in the bulk of the material were considered. It is found that the size, direction of the Burgers vector, and the distance to the free surface significantly affect the mobility of dislocation loops. As a rule, only dislocation loops of small length escape to the free surface. It is shown that 〈100〉 dislocations are almost immobile at 300 K and begin to escape to free surface after heating the sample to 600 K. An increase of radiation induced defect concentration leads to an increase in the threshold shear stress at which the edge dislocations begin to slip.

Effect of Segregation of Ni and Cr at Dislocation Loops on Their Interaction with Gliding Dislocations in Irradiated Fe−Ni−Cr BCC Alloys

Ferritic–martensitic steels alloyed with chromium and containing nickel impurities are considered as promising structural materials for nuclear and thermonuclear power engineering. During operation, the plastic properties of such materials degrade under the influence of neutron irradiation caused by the generation of radiation defects of the crystal structure, in particular, dislocation loops and new phases (precipitates). In this paper, an atomistic computer simulation of the interaction of mobile edge dislocations with dislocation loops having the 〈100〉 and 1/2〈111〉 Burgers vectors forming a single extended defect with Ni−Cr precipitates is performed using the classical molecular dynamics method at various temperatures (300 and 600 K). Such composite radiation-induced defects cause a change in the plastic properties of the irradiated material due to radiation hardening. The results of studying the interactions of gliding dislocations with loops (both in pure iron and in the Fe−Ni−Cr alloy, taking into account the precipitation of Ni and Cr at dislocation loops) show that the presence of an increased concentration of chromium and nickel atoms near the dislocation-loop perimeter at 300 K either decreases the critical stress for passing the dislocation through a defect (by more than 50 MPa) for the 〈100〉 loops at 300 K or increases it for the 1/2〈111〉 loops at 300 K. At a high temperature (600 K), the presence of Ni and Cr impurities near the dislocation loop leads to an increase in the critical stress for both types of loops. It is shown that the presence of an increased concentration of Ni and Cr atoms near the loop perimeter facilitates or hinders (depending on the specific dislocation-loop configuration) the transverse gliding of dislocation segments, complicates the possibility of the resplitting of junction segments of the dislocation and loop in the plane of loop location, and causes the immobilization of the loop having the [111] Burgers vector parallel to the gliding plane of the dislocation at 300 K.

Interaction of Edge Dislocations with Graphene Nanosheets in Graphene/Fe Composites

Graphene is an ideal reinforcement material for metal-matrix composites owing to its exceptional mechanical properties. However, as a 2D layered material, graphene shows highly anisotropic behavior, which greatly affects the mechanical properties of graphene-based composites. In this study, the interaction between an edge dislocation (b = 1/2 (111)) and a pair of graphene nanosheets (GNSs) in GNS reinforced iron matrix composite (GNS/Fe) was investigated using molecular dynamic simulations under simple shearing conditions. We studied the cases wherein the GNS pair was parallel to the (1 1 ¯ 0), (11 2 ¯ ), and (111) planes, respectively. The results showed that the GNS reinforcement can effectively hinder dislocation motion, which improves the yield strength. The interaction between the edge dislocation and the GNS pair parallel to the (11 2 ¯ ) plane showed the strongest effect of blocking dislocations among the three cases, resulting in increases in the shear modulus and yield stress of 107% and 1400%, respectively. This remarkable enhancement was attributed to the Orowan “by-passing” strengthening mechanism, whereas cross-slip of dislocation segments was observed during looping around GNSs. Our results might contribute to the development of high-strength iron matrix composites.

Molecular dynamics investigations of the strengthening of Al-Cu alloys during thermal ageing

Classical molecular dynamics simulations of the interaction of edge dislocations with solid soluted copper atoms and Guinier-Preston zones (I and II) in aluminium are performed using embedded atom method potentials. Hereby, the strengthening mechanism and its modulus are identified for different stages of thermally aged Al-Cu alloys. Critical resolved shear stresses are calculated for different concentrations of solid soluted copper. In case of precipitate strengthening, the Guinier-Preston zone size, its orientation and offset from the dislocation plane are taken as simulation parameters. It is found that in case of solid soluted copper, the critical resolved shear stress is proportional to the copper concentration. In case of the two subsequent aging stages both the dislocation depinning mechanism as well as the depinning stress are highly dependent on the Guinier-Preston zone orientation and to a lesser degree to its size.

Першопринципне моделювання взаємодії крайової дислокації з домішковими атомами кисню та вуглецю в кремнії

Першопринципне моделювання взаємодії крайової дислокації з домішковими атомами кисню та вуглецю в кремнії

Dynamic interaction of edge dislocations with point defects and prismatic dislocation loops at high-strain-rate deformation of crystals

The motion of an ensemble of edge dislocations at high-strain-rate deformation of a crystal with a high concentration of prismatic dislocation loops and point defects has been analyzed. It has been shown that, under certain conditions, the drag of an edge dislocation by prismatic dislocation loops has the character of dry friction, and the magnitude of the drag force of the dislocation is determined by the relationship between the concentration of prismatic dislocation loops and the density of mobile dislocations. An increase in the density of mobile dislocations leads to an enhancement of their collective interaction, thus facilitating the overcoming of prismatic dislocation loops by edge dislocations. The total drag force of an edge dislocation is a nonmonotonic function of the concentration of point defects, which, under certain conditions, has a minimum.

Molecular dynamic simulation of edge dislocation-void interaction in pure Al and Al-Mg alloy

In this study, the interaction of edge dislocations with nano-scale voids was investigated for the face centered cubic (FCC) structure of pure aluminum and Al-Mg alloy. The effect of Mg solute atoms on the Peierls stress (required for dislocation motion) at different temperatures, and the critical resolved shear stress (CRSS) for dislocation-void interaction was investigated in this study. In addition, the influences of void diameter (1, 2 and 3nm), inter-void distance (7.5 and 15nm) in a void array, and of temperature (300K) on resolved shear stress were determined. It was found that substitutional Mg atoms was highly effective on improving the mechanical behavior of the Al lattice and on the type of dislocation-void interaction (simultaneous or separate passing of partial dislocations). In addition, it was obtained that no void-induced climbing occurred during the interactions for these systems. Higher void diameter and in particular lower inter-void spacing led to a considerable increase in the CRSS, while the latter changed the type of dislocation-void interaction. Finally, it was shown that Peierls stress was decreased for pure aluminum from 0K to 10K, while different results were obtained for Al-Mg alloy that were discussed in detail.

Interaction of run-in edge dislocations with twist grain boundaries in Al-a molecular dynamics study

Grain boundaries play an important role in outlining the mechanical properties of crystalline materials. They act as sites for absorption/nucleation of dislocations, which are the main carriers of plastic deformation. In view of this, the interactions between edge dislocations and twist grain boundaries-dislocation pileup, dislocation absorption and dislocation emission were explored by performing molecular dynamics simulations in face-centered cubic Al using embedded atom method. The 1 1 0 twist grain boundaries with various misorientation angles were selected for this purpose. It was found that the misorientation angle of boundary and stress anomalies arising from repeated dislocation absorption at the grain boundaries are the important parameters in determining the ability of the boundary to emit dislocations. Complex network of dislocations results in later stages of deformation, which may have a significant effect on the mechanical properties of the material. The peculiarities of dislocation nucleation, their emission from twist grain boundaries and the ramifications of this study towards development of higher length scale material models are discussed.