Dislocation Core Spreading Research Articles

An atomistic study of the influence of carbon on the core structure of screw dislocation in BCC Fe and its consequences on non-Schmid behavior

In the present work, we report on the effect of introducing carbon atom in the core of the screw dislocations in body centered iron (Fe) using classical interatomic potentials available in open literature. Reconstruction of dislocation core from easy to hard core configuration is observed irrespective of the initial position of the carbon atom, which eventually settles in the dislocation core occupying the prismatic sites formed by the rows of Fe atoms in hard core. Loading is carried out parallel and perpendicular to the Burgers vector to understand the effect of carbon on the non-Schmid characteristics of the slip. Twinning–antitwinning (T/AT) asymmetry of CRSS is observed when the dislocation core symmetry of the easy core configuration is retained after the core reconstruction occurs. The CRSS of dislocation in both easy and hard core configuration is sensitive to the loading conditions. The asymmetry in CRSS and choice of glide planes are correlated with the dislocation core spreading onto the {1 1 0} planes in the 〈1 1 1〉 zone under the influence of the applied loads.

Thermodynamic model for polymorphic dislocation core spreading within hexagonal close packed metals

An exactly solvable thermodynamic model for the core structure of $\ensuremath{\langle}a\ensuremath{\rangle}$-type screw dislocations in $\ensuremath{\alpha}$-Ti is introduced and used to explore the role in which polymorphic core spreading contributes to the free energy of the dislocation. Each segment of the dislocation core is assumed to be in one of three possible configurations, and the free energy differences per segment between dislocation core spreadings are taken as parameters. It is shown that thermal fluctuations between the core spreadings should be common, even at room temperature, and that these fluctuations can contribute significantly to the free energy. It is also shown that under some circumstances, the core structures can display morphological switching wherein the stable core spreading changes morphology with a change in temperature. The implications for dislocation dynamics and for modeling finite temperature dislocation core properties using molecular dynamics simulations are considered.

Hardening induced by dislocation core spreading at disordered interface in Cu/Nb multilayers

Atomistic scale interface structures could tailor the dislocation behaviors at interface, as well as interface strengthening. In this work, we design a disordered interface by introducing an ultrathin amorphous layer in the Cu/Nb multilayers. Compared to the sharp interface, the disordered interface can smear the impinging dislocation with relaxing the in-plane and out-of-plane Burgers vectors, thus inducing a remarkable hardening in the Cu/Nb multilayers. The findings not only prove the hardening effect induced by dislocation interface smearing, but also shed light on developing advanced materials with ultrahigh hardness via disordered interface.

Hydrogen distribution induced screw dislocation core spreading in tungsten

The underlying physical mechanisms of the retention and self-clustering of hydrogen(H) in tungsten(W) are important in understanding the radiation damage and plasma trapping of W-based components in divertors, in which the influence of dislocations has not been sufficiently addressed. For this purpose, quantum mechanics/molecular dynamics(QM/MD) simulations are performed to investigate the distribution of hydrogen(H) clusters along screw dislocation lines in W, where the spreading of the screw dislocation core is first observed introduced by the H clusters. This core spreading constitutes an essential factor that could account for the hydrogen embrittlement phenomenon.

Unraveling the dislocation core structure at a van der Waals gap in bismuth telluride

Tetradymite-structured chalcogenides such as bismuth telluride (Bi2Te3) are of significant interest for thermoelectric energy conversion and as topological insulators. Dislocations play a critical role during synthesis and processing of such materials and can strongly affect their functional properties. The dislocations between quintuple layers present special interest since their core structure is controlled by the van der Waals interactions between the layers. In this work, using atomic-resolution electron microscopy, we resolve the basal dislocation core structure in Bi2Te3, quantifying the disregistry of the atomic planes across the core. We show that, despite the existence of a stable stacking fault in the basal plane gamma surface, the dislocation core spreading is mainly due to the weak bonding between the layers, which leads to a small energy penalty for layer sliding parallel to the van der Waals gap. Calculations within a semidiscrete variational Peierls-Nabarro model informed by first-principles calculations support our experimental findings.

Comparing modeling predictions of aluminum edge dislocations: semidiscrete variational Peierls-Nabarro versus atomistics.

Multiple computational methods for modeling dislocations are implemented within a high-throughput calculation framework allowing for rigorous investigations comparing the methodologies. Focusing on aluminum edge dislocations, twenty-one classical aluminum interatomic potentials are used to directly model dislocation core structures using molecular dynamics, as well as provide input data for solving the semidiscrete variational Peierls-Nabarro dislocation model. The predicted dislocation core spreading obtained from both computational methods show similar trends across the potentials. Additionally, tests are done to rigorously determine if a recent correction to the Peierls-Nabarro model results in better agreement with the atomistic calculations.

Interface Dislocation Core-Spreading Simulation and Corresponding Response

Dislocation can spread its core at an interface especially at a weak shear interface associated with shearing the interface. Such core-spreading dislocation can significantly reduce stress/strain concentration compared with the compact dislocation and thus trap the dislocation in the interface, correspondingly strengthening materials. Employing the Green’s function for a compact dislocation, we derived analytical expressions for the elastic fields of a dislocation with core spreading in anisotropic bimaterials. We proposed a conic model to mimic the spreading core of a dislocation at an interface. The accuracy and efficiency of the conic model are validated by the boundary conditions of both traction and displacement across the interface. Numerical simulation is calculated in the Cu/Nb biomaterial. The results of displacement and stress fields show that: (1) core-spreading dislocation can greatly reduce the stress intensity near the dislocation compared with the dislocation with a condensed core; (2) dislocation core spreading has a great influence on the elastic fields near the core region, while the influence can be negligible when the distance of a field point from the center of the dislocation core is greater than 1.51 times the width of the spreading core; (3) near the core region, Peach-Koehler force induced by the core spreading dislocation is larger than that of the compact dislocation.

Dislocation plasticity of Al film on polyimide investigated by cross-sectional in situ transmission electron microscopy straining

Tensile straining of a cross-sectional Al/polyimide was performed in a transmission electron microscope. The tensile deformation of Al was accomplished mainly by dislocations emitted from the film surface, which glide and impinge on the Al/polyimide interface. During further straining the interfacial dislocations disappeared, indicating dislocation core spreading, whereas threading dislocations moved towards the film surface. While the Al/polyimide interface remains flat and becomes depleted of dislocations, the Al surface becomes increasingly rough accompanied by a noticeable increase in dislocation density.

The influence of interface shear strength on the glide dislocation–interface interactions

Interfaces with relatively low shear strengths can be strong barriers to glide dislocations due to dislocation core spreading within the interface plane. Using atomistic modeling we have studied the influence of interface shear strength on the interaction of lattice glide dislocations with fcc/bcc interfaces. “Tunable” interatomic potentials are employed to vary the interface shear strength for the same interface crystallography. The results show that: (1) the interface shear strength increases as the dilute heat of mixing decreases; (2) the interface shear mechanism involves the nucleation and glide of interfacial dislocations, which is dominated by the atomic structures of interfaces, regardless of the interface shear strength; (3) weak interfaces entrap lattice glide dislocations due to the interface shear and core spreading of dislocations within interfaces. Reverse shear displacement is needed to enable collapse of the spread core for slip transmission. This study provides an insight into the correlation between interface shear strength and glide dislocation trapping at the interface, which is a crucial unit mechanism in understanding the ultra-high strengths observed in nanoscale fcc/bcc multilayers.

Mechanism for shear banding in nanolayered composites

Recent studies have shown that two-phase nanocomposite materials with semicoherent interfaces exhibit enhanced strength, deformability, and radiation damage resistance. The remarkable behavior exhibited by these materials has been attributed to the atomistic structure of the bimetal interface that results in interfaces with low shear strength and hence, strong barriers for slip transmission due to dislocation core spreading along the weak interfaces. In this work, the low interfacial shear strength of Cu/Nb nanoscale multilayers dictates a new mechanism for shear banding and strain softening during micropillar compression. Our findings, supported by molecular dynamics simulations, provide insight on the design of nanocomposites with tailored interface structures and geometry to obtain a combination of high strength and deformability. High strength is derived from the ability of the interfaces to trap dislocations through relative ease of interfacial shear, while deformability can be maximized by controlling the effects of loading geometry on shear band formation.

Atomistic simulations of the shear strength and sliding mechanisms of copper–niobium interfaces

Interfaces play a prominent role in the deformation behavior of high-strength Cu–Nb layered composites by acting as barriers to slip transmission due to core spreading of glide dislocations within interfaces. The dislocation core spreading along the interfaces implies these interfaces are weak in shear. In this investigation, we have used atomistic simulations to explore the shear resistance and sliding mechanism of interfaces of Cu–Nb layered composites, as a function of applied in-plane shear direction and different interface atomic structures. The simulation results indicate that the shear strengths of Cu–Nb interfaces are: (i) lower than the theoretical estimates of shear strengths for perfect crystals, (ii) strongly anisotropic, (iii) spatially non-uniform and (iv) strongly dependent on the atomic structures of interfaces. The mechanism of interface sliding involves glide of interfacial dislocation loops that nucleate in the weakest regions of interfaces.

Modeling dislocation cores inSrTiO3using the Peierls-Nabarro model

We have recently determined dislocation core structures in complex minerals relevant to the Earth's mantle using the Peierls-Nabarro model. In this approach, the original Peierls-Nabarro model is coupled with first-principles calculations of generalized stacking fault. In order to test the reliability of such calculations, we study here the dislocation core properties in a perovskite-structured material, $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$, for which a lot of experimental information are available. Four different slip systems have been investigated in $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$: ⟨100⟩{010}, ⟨100⟩{011}, ⟨110⟩{001}, and $⟨110⟩{1\overline{1}0}$. $⟨110⟩{1\overline{1}0}$ exhibits the lower lattice friction due to core spreading, and the next easiest slip system is found to be ⟨100⟩{010}. It is shown that our results (dislocation core spreading and Peierls stress values) are in perfect agreement with experiments (transmission electron microscopy model of core structure and mechanical properties), providing an interesting validation of the Peierls-Nabarro model on complex materials.

Dislocation transmission across the Cu/Ni interface: a hybrid atomistic–continuum study

The strengthening mechanisms in bimetallic Cu/Ni thin layers are investigated using a hybrid approach that links the parametric dislocation dynamics method with ab initio calculations. The hybrid approach is an extension of the Peierls–Nabarro (PN) model to bimaterials, where the dislocation spreading over the interface is explicitly accounted for. The model takes into account all three components of atomic displacements of the dislocation and utilizes the entire generalized stacking fault energy surface (GSFS) to capture the essential features of dislocation core structure. Both coherent and incoherent interfaces are considered and the lattice resistance of dislocation motion is estimated through the ab initio-determined GSFS. The effects of the mismatch in the elastic properties, GSFS and lattice parameters on the spreading of the dislocation onto the interface and the transmission across the interface are studied in detail. The hybrid model shows that the dislocation dissociates into partials in both Cu and Ni, and the dislocation core is squeezed near the interface facilitating the spreading process, and leaving an interfacial ledge. The competition of dislocation spreading and transmission depends on the characteristics of the GSFS of the interface. The strength of the bimaterial can be greatly enhanced by the spreading of the glide dislocation, and also increased by the pre-existence of misfit dislocations. In contrast to other available PN models, dislocation core spreading in the two dissimilar materials and on their common interface must be simultaneously considered because of the significant effects on the transmission stress.

Modeling dislocation by coupling Peierls–Nabarro and element-free Galerkin methods

A new method to describe dislocations within the framework of the element-free Galerkin method is presented. Based on the Peierls–Nabarro concept of balancing the elastic energy with an inter-planar potential, the method naturally leads to dislocation core spreading on the glide planes. A weak formulation compatible with the Time-Dependent Ginzburg–Landau relaxation mechanism is presented and its implementation discussed. As an example, the modeling of the complex core structure of a screw dislocation in tantalum by using 15 glide planes is presented and its Peierls stress calculated.

Length-scale-dependent deformation mechanisms in incoherent metallic multilayered composites

Nano-indentation hardness as a function of bilayer period has been measured for sputter-deposited Cu–Nb multilayers. For this face-centered cubic/body-centered cubic system with incoherent interfaces, we develop dislocation models for the multilayer flow strength as a function of length scale from greater than a micrometer to less than a nanometer. A dislocation pile-up-based Hall–Petch model is found applicable at the sub-micrometer length scales and the Hall–Petch slope is used to estimate the peak strength of the multilayers. At the few to a few tens of nanometers length scales, confined layer slip of single dislocations is treated as the operative mechanism. The effects of dislocation core spreading along the interface, interface stress and interface dislocation arrays on the confined layer slip stress are incorporated in the model to correctly predict the strength increase with decreasing layer thickness. At layer thicknesses of a few nanometers or less, the strength reaches a peak. We postulate that this peak strength is set by the interface resistance to single dislocation transmission, and calculate the transition from confined layer slip to an interface cutting mechanism.

Effect of Dislocation Core Spreading at Interfaces on Strength of Thin-films

Critical strain arguments are often used to model the thickness dependence of the strength of thin films on substrates. In these arguments, plastic deformation occurs when the stress in a film is high enough that the strain energy relieved by the introduction of a misfit dislocation is sufficient to generate the line energy of that misfit. Such models typically assume compact dislocation cores. However, experimental evidence suggests that, under certain circumstances, dislocation cores may spread out into the interface between the film and the substrate. If this happens, the energy of the misfit dislocation, and the critical stress needed for its propagation, will be lowered. In this paper, the effect of dislocation core spreading on the critical stress has been modeled. The effects of interface strength, film thickness, and misfit dislocation spacing are considered.

Microstructures and strength of nanoscale Cu–Ag multilayers

Cu–Ag multilayers were found to have lower peak hardness than Cu–Ni in spite of lower misfit dislocation spacing that is expected to increase the resistance of interfaces to glide dislocation transmission. This is attributed to misfit dislocation core spreading in the interface plane in Cu–Ag.

Dislocation core spreading at interfaces between metal films and amorphous substrates

Experiments with transmission electron microscopy have shown that in a strong electron beam the contrast of dislocations may gradually disappear at an incoherent interface between a metal thin film and an amorphous substrate. There are reasons to believe that this phenomenon is caused by radiation-induced dislocation core spreading at the interface. A quantitative model accounting for this effect will be necessary for a better understanding of dislocation structures and plastic deformation in metal thin films. As a first step toward this objective, we develop a number of mathematical solutions for dislocation core spreading at an incoherent interface. For simplicity, we consider screw dislocations, and consider the interface to be characterized by a shear adhesive strength, τ 0, below which no core spreading occurs, and above which spreading takes place in a viscous manner. We determine the final equilibrium core width and the rate of core spreading for single or planar arrays of dislocations in a homogeneous bulk material or at the interface between a thin film and a semi-infinite substrate where the film and substrate may have the same, or different, elastic constants. Some of our solutions are analytic and others are based on an implicit finite difference method with a Gauss–Chebyshev quadrature scheme. The phenomenon of dislocation core spreading is expected to have a dramatic effect on the strength of crystalline films deposited on amorphous substrates.

Incorporation processes of extrinsic dislocations in singular, vicinal and general grain boundaries in nickel

The relaxation of the stresses associated to grain boundary (GB) extrinsic dislocations is investigated by in situ transmission electron microscopy in nickel near- Σ=3 {1 1 1}, near- Σ=11 {3 1 1} and near- Σ=11 {3 3 2} GBs. The results show that the stress relaxation depends on the GB structure: in periodic GBs, it requires discrete interactions between intergranular dislocations; in non-periodic GBs, the widening of dislocation contrasts is attributed to a dislocation core spreading.

Dislocation Core Spreading at Interfaces between Crystalline and Amorphous Solids

ABSTRACTA fundamental question addressed here is concerned with the equilibrium structure of a dislocation core at an interface between a crystalline and an amorphous solid. This is motivated by experimental observations that the contrast of dislocations at an interface between a crystalline film and an amorphous substrate disappears under transmission electron microscopy. We have developed a mathematical moedl to describe the time-dependent behavior of dislocation core spreading as a function of the adhesive strength of the interface. The equilibrium core width and the rate of core spreading are determined in closed form solutions.