Edge Dislocation Loops Research Articles

Molecular dynamics simulations of interaction between a super edge dislocation and interstitial dislocation loops in irradiated [formula omitted]-Ni3Al

The study employed MD simulations to investigate the interactions between a 〈11¯0〉 super-edge dislocation, consisting of the four Shockley partials, and interstitial dislocation loops (IDLs) in irradiated L12-Ni3Al. Accounting for symmetry breakage in the L12 lattice, the superlattice planar faults with four distinct fault vectors have been considered for different IDL configurations. The detailed dislocation reactions and structural evolution events were identified as the four partials interacted with various IDL configurations. The slipping characteristics of Shockley partials within the IDLs and the resultant shearing and looping mechanisms were also clarified, revealing distinct energetic transition states determined by the fault vectors after the Shockley partials sweeping the IDL. Furthermore, significant variations in critical resolved shear stress (CRSS) required for the super-edge dislocation to move past the IDL were observed, attributed to various sizes and faulted vectors of enclosed superlattice planar faults in the IDLs. The current study extends the existing dislocation-IDL interaction theory from pristine FCC to L12 lattice, advances the understanding of irradiation hardening effects in L12-Ni3Al, and suggests potential applicability to other L12 systems.

Formation mechanism of ultrafine α lamellae with high-density dislocations in Ti-5Al-2Sn-2Zr-4Mo-4Cr alloy fabricated by laser powder bed fusion

This study employed laser powder bed fusion (LPBF) to fabricate Ti-5Al-2Sn-2Zr-4Mo-4Cr (Ti17) alloy, and the microstructural characteristics and mechanism of α phase formation were investigated. Results revealed that the as-deposited sample, after undergoing stress relief annealing, exhibited a microstructure characterized by a high volume fraction of ultrafine nanoscale α phases dispersed throughout. Abundant edge dislocations and dislocation loops were observed within the α phase, while dislocations were scarcely detected in the β phase. The formation of ultrafine nanoscale α phases occurs during the subsequent low-temperature stress relief annealing process following additive manufacturing. After stress relieving, the Ti17 sample exhibited a superhigh tensile strength of nearly 1500 MPa.

Shielding effect of ring dislocation dipole on penny-shaped crack

Mechanical behavior of strongly crystalline materials can be significantly influenced by the internal dislocation structure and their iteration with micro crack-like defects. Thus, study on the crack-dislocation interaction problems can help to understand the failure mechanism of crystalline materials. Previous theoretical analysis have been mostly limited to plane problems, where the cracks are idealized as straight slit with infinite or zero out plane dimension and the dislocations slip along an inclined straight path starting from the crack tip. In this paper, we explore a new interaction mechanism under axisymmetric deformation state. An unconventional case of a penny-shaped crack interacts with a ring dislocation dipole is considered. Both the ring dislocation dipole and crack are modelled as axisymmetric edge dislocation loops. Solutions for the dislocation loops are obtained using the GKS based method. The mixed boundary value problem is converted into Abel integral equations, which can be solved analytically. Exact closed form solutions for the stress intensity factors (SIFs) in terms of elementary functions are obtained. Numerical studies are conducted to investigate the shielding effect of the ring dislocation dipole on the crack tip SIFs. Conditions for the dislocation to shield the crack are discussed. The present solutions can also be used as weight functions to study the axisymmetric interaction between penny shaped crack and other types of eigenstrain sources.

BIAS FOR PRISMATIC DISLOCATION LOOPS IN ZIRCONIUM. NUMERICAL ANALYSIS

Using the analytical expression for the energy of elastic interaction of radiation point defects with a prismatic edge dislocation loop of zirconium (Burgers vector bD 1/ 3 <1120>, {11 2 0 }occurrence plane), the bias of loops of different nature (vacancy and interstitial) was calculated by the finite difference method. The toroidal geometry of the reservoir was used. It allowed one to calculate biases for loops of any size without any correction of the elastic field in its area of influence. In the dilatation center approximation the dependences of the loop bias on the loop radius were obtained. The principal possibility of coexistence of loops of different nature in the prismatic plane of zirconium is shown. A qualitative concept of the radiation growth (RG) mechanism was formulated within the framework of the classical elastic ideology.

Influence of kinetic effect on interaction between edge dislocation and irradiated dislocation loops in BCC Tantalum

In this paper, the interaction between an edge dislocation and irradiation <100> sessile self-interstitial atom (SIA) type dislocation loops (DLs) is studied by atomistic simulations in BCC Ta at 0 and 300 K. Both molecular dynamics (MD) and statics (MS) simulations are performed to highlight the dislocation kinetic effect (or dynamic effect) on the critical resolved shear stress (CRSS) for dislocation bypassing the DLs and on the dislocation-SIA DLs interaction mechanisms. The simulation results show that the dislocation-DLs interaction mechanisms are two-fold. When the DL size is lower than a critical value, the moving edge dislocation tends to completely absorb the DLs or fully transform the original DLs into other-type ones. However, when the DL size is higher than the critical value, the moving dislocation tends to absorb or transform the DLs partially or even marginally. Moreover, with increasing applied shear stress, the high-speed dislocation has no sufficient time to absorb SIAs from DLs, which also changes the dislocation-DLs interaction process remarkably. When the dislocation is accelerated to subsonic, an abnormal “pull forward” dislocation configuration may present upon the dislocation bypasses a small DL. More importantly, with the kinetic effect considered, the CRSS for dislocation overcoming the barriers of DLs decreases significantly for the dislocation-DLs interaction mechanisms. In order to appropriately describe the kinetic effect on the CRSS, amended dispersed barrier hardening (DBH) and Bacon-Kocks-Scattergood (BKS) hardening models for the latter dislocation-DLs interaction mechanism are proposed.

Atomistic study of internal stress effect on scratching behavior in titanium single crystal

The internal stress effect on the scratching behavior in titanium single crystal is investigated by molecular dynamics simulations. It is introduced by applying the compressive/tensile pre-strain in this work. The friction force in the model with compressive pre-strain is higher than that with tensile pre-strain, which is related to the atomistic spacing and the distribution of pile-up. The dislocation mechanism is systematically illustrated that three types of <a> dislocations of edge dislocation loop, prism edge dislocation and screw dislocation dominate the plastic deformation. The coupling of dislocation loop and screw dislocation in the model with tensile pre-strain causes much more defects to remain in the matrix. However, when the model is applied by small compressive pre-strain, the dislocations left in the matrix are obviously decreased, reducing the internal crystal damage. Overall, the compressive/tensile internal stress has a distinguishing effect on the scratching behavior, which can be used for altering the friction and wear property of titanium.

Interaction of Two Coaxial Penny-Shaped Cracks Near an Arbitrarily Graded Interface in Functionally Graded Materials: Exact and Approximate Solutions

Abstract Multiple cracks interaction is an important topic in fracture mechanics. The related solutions are helpful to understand the failure process and the toughening mechanism of brittle materials. Previous works on the topic were most for homogenous material. In this paper, we extend the analysis and examine the problem of interaction of two coaxial penny-shaped cracks near an arbitrarily graded interface in functionally graded materials (FGMs). The cracks are modelled as circular edge dislocation loops. An efficient dislocation solution for FGMs and Fredholm integral equation technique are used to solve the crack problem. Both exact solution using a system of integral equations and approximate solution by virtue of Kachanov’s method are presented. Unlike most existing analytical treatments to the crack problems in FGMs with the assumption of special gradation, i.e., graded shear modulus according to special functions and constant Poisson’s ratio, the present method is more flexible since it can consider arbitrarily graded shear modulus and Poisson’s ratio. The validity of the present solutions is checked by comparing to existing results in literatures for two stacked penny-shaped cracks in homogenous material and a penny-shaped crack near a graded interface with exponentially graded shear modulus. Finally, a practical example of double cracks interaction in a real epoxy-glass FGM with measured data of material properties is considered. The error due to the assumption of special gradation is also discussed.

Edge dislocations in Ni monocrystalline structure studied by McChasy 2.0 Monte Carlo code

The main goal of the McChasy code, in general, is to reproduce Rutherford Backscattering Spectrometry experimental spectra recorded in channeling direction (RBS/C) by simulating He-ions travelling inside crystalline structures and to calculate the probability of the backscattering process. The 2.0 version of the code provides the possibility to simulate channeling spectra in large (ca. 108 atoms) arbitrary structures that are created based on crystallographic data or Molecular Dynamic (MD) calculations.In this work, we present the current status of the code and the results of recent investigations of extended structural defects (edge dislocations and dislocation loops) formed inside nickel (Ni) single crystals. Two ways of modelling extended defects are described: one developed using the McChasy code (Peierls-Nabarro approach) and the other one obtained by modification and thermalization of Ni structures by MD (LAMMPS code). The atomic local environment was studied qualitatively and quantitatively by the local projectile-flux density distributions around the defects.

Complete solution for the axisymmetric problem of a penny-shaped crack near and parallel to an arbitrarily graded interface in FGMs

Previous analytical treatments to the crack problems in functionally graded materials (FGMs) were mostly based on the assumption that only the shear modulus is variable according to special functions while the Poisson’s ratio is constant. However, the material inhomogeneity in real FGMs is more complex than that can be well approximated using the assumption. This paper examines the axisymmetric problem of a penny-shaped crack near and parallel to a graded interface with arbitrarily variable shear modulus and Poisson’ s ratio. To consider the general material inhomogeneity, a new efficient solution of general axisymmetric edge dislocation loop in multilayered elastic medium is derived first using the General Kelvin’s Solution (GKS) based method. This method allows the approximation of general material inhomogeneity using large number of homogenous sublayers without the loss of computational efficiency, accuracy and stability. The challenging issue in multilayered elasticity associated with the calculation of near-source elastic field is also tackled, which enables the high precision evaluation of crack tip field. Exact solutions are given for the full elastic field. Numerical studies are conducted to explore the fracture mechanics of FGMs. The results show that the gradation pattern can have certain effect on the fracture response if the crack is very closed to the graded interface.

Analytical methods for the calculation of the elastic interaction of point defects with dislocation loops in hexagonal crystals

The Green’s function method for hexagonal crystals within the Lifshitz–Rosenzweig (1947) and Kroner (1953) approaches has been used to obtain analytical expressions for the energy of elastic interaction of radiation-induced point defects with dislocation loops of three types: the basal edge dislocation loop (cloop), the basal shear dislocation loop, and the edge a-loop (bedding plane {11 20}, Burgers vector bD = 1/3〈11 20〉). In the case of the basal edge dislocation loop, a similar expression has been obtained independently by solving the equilibrium equations using the Elliott method. A numerical comparison of the derived expressions for zirconium has demonstrated a complete identity of the results obtained within the approaches considered in this study.

Annihilation of edge dislocation loops via climb during nanoindentation

In this work, we explore the role of dislocation climb in annihilating the dislocation microstructure produced during a nanoindentation test. We produce creep conditions in Molecular Dynamics (MD) simulations of nanoindentation and show that half prismatic edge dislocation loops annihilate via pipe diffusion of vacancies from the free surface. Inspired by the MD simulations, we develop a model for the annihilation rate of these loops via pipe diffusion and compare it with the contribution of bulk diffusion. The model allows us discussing the temperatures at which we expect climb to prevail in experimental conditions and explain why climb is imperative during nanoindentation experiments.

Elastic interaction of point defects with an edge dislocation loop within the Green’s function formalism

The universal expressions have been obtained for components of the tensor Green’s function of an elastically anisotropic hexagonal medium. In contrast to the classical expressions (the Lifshitz–Rosenzweig method), they do not contain uncertainties of the type 0/0 upon the transition to the isotropic approximation and hold true for any hexagonal crystal. As an example of their use, the displacement and strain fields created by an edge dislocation loop lying in the basal plane of the crystal have been calculated.

The topology of dislocations in smectic liquid crystals

The order parameter of the smectic liquid crystal phase is the same as that of a superfluid or superconductor, namely a complex scalar field. We show that the essential difference in boundary conditions between these systems leads to a markedly different topological structure of the defects. Screw and edge defects can be distinguished topologically. This implies an invariant on an edge dislocation loop so that smectic defects can be topologically linked not unlike defects in ordered systems with non-Abelian fundamental groups.

Microstructural modifications in tungsten induced by high flux plasma exposure: TEM examination

We have performed microstructural characterization using transmission electron microscopy (TEM) techniques to reveal nanometric features in the sub-surface region of tungsten samples exposed to high flux, low energy deuterium plasma. TEM examination revealed formation of a dense dislocation network and dislocation tangles, overall resulting in a strong increase in the dislocation density by at least one order of magnitude as compared to the initial one. Plasma-induced dislocation microstructure vanishes beyond a depth of about 10 μm from the top of the exposed surface where the dislocation density and its morphology becomes comparable to the reference microstructure. Interstitial edge dislocation loops with Burgers vector a0/2〈111〉 and a0〈100〉 were regularly observed within 6 μm of the sub-surface region of the exposed samples, but absent in the reference material. The presence of these loops points to a co-existence of nanometric D bubbles, growing by loop punching mechanism, and sub-micron deuterium flakes, resulting in the formation of surface blisters, also observed here by scanning electron microscopy.

Molecular dynamics investigation of the interaction of an edge dislocation with Frank loops in Fe–Ni10–Cr20 alloy

The inhibition of dislocations motion by irradiation-induced defects, such as dislocation loops, is one of the main mechanisms of irradiation hardening of austenitic stainless steels. In this work, Molecular Dynamics (MD) simulations of interaction between an edge dislocation and Frank loops in Fe–Ni10–Cr20 ternary alloy mimicking austenitic stainless steels are carried out to investigate and model dislocation behavior. An empirical interatomic potential developed recently for a ternary FeNiCr system is used for the MD calculations. The interactions are calculated at different temperatures, loop orientations, loop size and solute atom configurations. The results show that the loop strength and the interaction processes depend on the solute atom configuration, the geometrical configurations between the dislocation and the loop and temperature. It is also demonstrated that a small Frank loop is not so weak an obstacle in the alloy. The interaction leads microstructural change such as loop shearing, loop unfaulting and loop absorption in the dislocation. In the former two cases, the loop remains after the interaction, however in some cases an absorption of the remaining loop by subsequent interactions with successive dislocations is observed.

Monte Carlo study of decorated dislocation loops in FeNiMnCu model alloys

Radiation-induced embrittlement of bainitic steels is the lifetime limiting factor of reactor pressure vessels in existing nuclear light water reactors. The primary mechanism of embrittlement is the obstruction of dislocation motion by nano-metric defects in the bulk of the material due to irradiation. Such features are known to be solute clusters that may be attached to point defect clusters. In this work we study the thermal stability of solute clusters near edge dislocation lines and loops with Burgers vector b=½[111] and b=[100] in FeNiMnCu model alloys by means of Metropolis Monte Carlo simulations. It is concluded that small dislocation loops may indeed act as points for heterogeneous nucleation of solute precipitates in reactor pressure vessel steels and increase their thermodynamic stability up to and above normal reactor operating temperatures. We also found that, in the presence of dislocation-type defects, the Ni content determines the thermodynamic driving force for precipitation, rather than the Mn content.

Au(111) films on W(110) studied by STM and LEED – Uniaxial reconstruction, dislocations and Ag nanostructures

Abstract The morphology of Au films on W(1 1 0) was studied as a function of the Au thickness by scanning tunnelling microscopy and low-energy electron diffraction. The stress originating at the fcc-Au/bcc-W interface was found to significantly affect the directionality of the reconstruction by favouring one particular rotational domain of the well-known Au herringbone pattern. The presence of characteristic defect bunches, interpreted to be edge dislocation loops, also contributed to the disturbances in the reconstruction pattern. The observed defects were strictly correlated with the dimensions of the structural domains of the reconstruction. To examine nucleation processes on an anisotropically reconstructed surface, the Au films were used as an adsorption template for Ag. On the hcp stacking regions of the reconstruction Ag showed a tendency to form nanowires.

Spreading and retraction dynamics of smectic liquid crystal domains at interfaces

The spreading of a liquid drop over liquid subphase can be driven by change in interfacial tension mediated through a surfactant, volatile solvent or photoinduced reaction. In contrast to the spreading dynamics of a liquid drop, a liquid crystal drop with anisotropic structure can lead to interesting behaviour due to its viscoelasticity and anchoring at the interfaces. Recently, we have reported studies on unusual spreading and retraction dynamics of a smectic domain doped with a fluorescent dye in the collapsed state of a Langmuir monolayer. Under epifluorescence microscope, during excitation, a stack of layers of the dye-doped smectic domain gets sheared causing the domain to spread asymmetrically. Further, due to line tension, the domain transforms into a circular shape. We also find the domain size to be about twice that of the initial size. Interestingly, in the absence of excitation, the domain retracts to a smaller area. During retraction of the domain, successive generation of edge dislocation loops arising from a nucleus results in an increase in the domain thickness. The dynamics of spreading and retraction of the domain can be understood by invoking changes in the spreading coefficient due to photoinduced modification of the interfacial tension.

Defect-mediated lamellar–isotropic transition of amphiphile bilayers

We report the observation of an isotropic phase of amphiphile bilayers, characterized by small average inter-bilayer spacing and short-range positional correlations. These bilayers form a weakly swollen lamellar (Lα) phase over a wide range of water content, which transforms into an isotropic (Li) phase on heating. This transition is not observed in samples without excess water, where the Lα phase is stable at higher temperatures. Small-angle X-ray scattering and ionic conductivity studies indicate that the Lα–Li transition is driven by the unbinding of edge dislocation loops, which has been shown to destroy the quasi-long-range positional order of the lamellar phase. Observation of similar behavior in other bilayer forming systems dominated by attractive inter-bilayer interactions suggests that this isotropic phase is generic to such systems.

Spreading and retraction dynamics of a dye doped smectic liquid crystal domain at the air–water interface

We have studied the spreading and retraction dynamics of a dye doped smectic multilayer domain in the collapsed state of a Langmuir monolayer at the air–water interface. We find that the domain undergoes shearing when excited with an appropriate wavelength in the epifluorescence setting of the microscope. The shearing leads to displacement of the stack of layers and results in asymmetric spreading of the domain. Eventually, due to line tension, the domain transforms into a circular shape. Here, the domain area was about twice that of the initial area. Under reflection setting of the microscope, with white light, we observe that the domain retracts to a smaller area. During the course of retraction, we find successive generation and evolution of edge dislocation loops leading to thickening of the domain. Our analysis on the variation of the normalized area of the domain with time yields different characteristic time constants for spreading and retraction. The spreading and retraction of the domain can be understood by invoking changes in interfacial tension due to photobleached surface active products and the depletion of photobleached products, respectively.