Molecular dynamics study of edge dislocation motion in a bcc metal

The motion of an edge dislocation gliding in a bcc lattice under shear loading is simulated by molecular dynamics. The time evolution of the dislocation profile, derived by analyzing the spatial disregistry between two adjacent atomic rows, reveals frequent nucleation of double kinks and little activity in kink migration in association with the dislocation motion. An essentially linear stress variation is obtained, while the temperature dependence suggests the mechanism of phonon drag. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the nature of attractive dislocation crossed states

Attractive non-coplanar dislocations that cannot react to form junctions can, nevertheless, form crossed states, i.e., junctions of null length. Such configurations have recently been described by Wickham and co-workers as an output of numerical simulations. The physical origin of the crossed states is cleared out and their conditions of occurrence are calculated within a simplified elastic frame. The results are further discussed by comparison with mesoscopic simulations of intersecting dislocations in fcc and bcc crystals.     

The flow stress of fractal dislocation arrangements

In metals deforming under multislip conditions, fractal dislocation arrangements may develop. The composite model of the flow stress of a dislocation cell structure is generalized for dislocation arrangements which exhibit a broad spectrum of local hardness. Both forest strengthening (cell walls) and the Orowan stress (cell interiors) are taken into account. Relations between the flow stress and the characteristic parameters of the dislocation arrangement such as fractal dimension, maximum and minimum cell size are established. The model is used to calculate stress dependences of fractal dimension and cell wall volume fraction of dislocation patterns in high-symmetry oriented f.c.c. single crystals. The results are in good agreement with experimental observations on Cu and Cu-rich Cu–Mn.     

Defect hardening modeled in 2D discrete dislocation dynamics

Two-dimensional discrete dislocation dynamics simulations are used to model the plastic deformation of an fcc metallic material containing large densities of defects. An obstacle model is proposed, based on the line tension concept. Increasing yield strength and hardening are obtained when the obstacle density is increased and destroyable junctions are included. A high dislocation source density is used to obtain a good dissemination of dislocations. Over 30% of the total density is stored as junctions. Slip is shown to be localized within a few intense slip bands, whatever the obstacle density. This localization is quantified as a function of the density of obstacles.     

Simulation of dislocation glide in precipitation hardened materials

Precipitation hardening is a widely used method for increasing the critical resolved shear stress (CRSS) of a material. Our simulations offer a flexible means to calculate the CRSS as a function of many parameters involved, e.g. the average precipitate size. For this, one or more dislocations are simulated while gliding through obstacle fields of arbitrary type or spatial arrangement. The elastic self-interaction is fully allowed for. Unlike analytical approaches and simulations known from literature, our method covers both shearing and circumventing of obstacles in a single model. To start with, the obstacles used in this contribution were chosen to be spheres with a constant obstacle stress inside; the distribution of the radii and the spatial arrangement suit the case of Ostwald-ripened particles. This corresponds to the case of the nickel base superalloy NIMONIC PE16 where a dislocation has to create an antiphase boundary in order to shear the long range ordered precipitates. Typical examples for dislocation arrangements are presented, and the results for various obstacle concentrations and mean radii are compared with published results.     

The brittle-to-ductile transition and dislocation activity at crack tips

Dislocation activity in the vicinity of a crack tip and the brittle-to-ductile transition (BDT) are analysed using discrete dislocation dynamics simulations. The comparison of these simulations with fracture experiments on tungsten single crystals helps to identify the decisive mechanisms for the BDT of this material. Dislocation nucleation and the availability of active sources are shown to be limiting plasticity at low temperatures and partly in the semi-brittle regime. At elevated temperatures, fracture toughness, crack tip plasticity and the BDT itself can all be viewed as thermally activated processes, which can all be scaled by the same activation energy. It is concluded that they must be controlled by dislocation mobility. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stress fields of tilted dislocation walls

Dislocation walls of Persistent Slip Bands are modelled in a two-dimensional plane strain approximation as arrays of plastically sheared inclusions. Parallelogram and elliptical cross-sections of the walls tilted at different angles up to 45° are considered. A simple numerical approach is proposed to compute the long-range internal stresses using a combination of stress function calculation and discrete Fourier transform. The computed results are compared with those predicted by the mean field theory.     

Overview on dislocation-point defect interaction: the brownian picture of dislocation motion

The interactions between dislocations (D) and point defects (PD) are one of the most important causes of mechanical damping in metals. In the past 40 years, many experimental results have been obtained and published, from which it appears that two fundamental behaviors can be observed when dislocations interact with motionless point defects: thermally activated behaviors and athermal behaviors. In this paper, it is shown that these two observed behaviors can be consistently explained by a “brownian picture” of the motion of dislocations interacting with PD distributed at different distances from the dislocation glide plane.     

The study of crack-propagation behaviors and dislocation structures in cyclically deformed polycrystalline IF steel

Strain controlled fatigue experiments were employed to evaluate automotive-grade interstitial-free ferrite steels (IF steels). This study demonstrates that the loop-patch structure, dislocation walls and dislocation cells larger than 2 μm have no significant effect on fatigue failure. The cracks prefer to grow near grain boundaries while dislocation cells smaller than 2 μm tend to develop along the grain boundaries and triple junction of the grains. Once the dislocation cells smaller than 2 μm develop from grain boundaries to grain interior, the cracks will propagate near the grain boundaries and through the grain interior simultaneously.     

A screw dislocation in a piezoelectric bi-material wedge

The electro-elastic stress investigation for a dislocation located near the apex of two bonded wedge-shaped dissimilar piezoelectric materials has been carried based on the complex variable method and perturbation technique. The dislocation has the Burger's vector normal to the isotropic basal plane, with a line force and a line charge being applied at the core of the dislocation. An exact analytical solution has been presented for the stress and electric displacement fields by means of conformal mapping method. Forces on the dislocation have been calculated. Examples for different material property combinations and geometric configurations are given and discussed.     

Determination of dislocation density from hardness measurements in metals

For the first time the Nix-Gao model for indentation size effect (ISE) is used to estimate the dislocation density in a metal. The estimate of dislocation density obtained by this method, using Ni as a case study, is compared with the values obtained from direct observation by transmission electron microscopy. It is shown that the estimate of dislocation density from indentation hardness measurements, adjusted by the Nix-Gao model, gives values consistent with those obtained by TEM, provided that the proper procedures to minimize errors are adopted. Although the direct observation of dislocations by TEM gives additional structural information, the indirect method to estimate dislocation density based on hardness measurements is more efficient, since the sample preparation method, measurement procedure and analysis of results are easier and faster.     

Deformation at the nanometer and micrometer length scales: Effects of strain gradients and dislocation starvation

Nanomechanical devices are certain to play an important role in future technologies. Already sensors and actuators based on MEMS technologies are common and new devices based on NEMS are just around the corner. These developments are part of a decade-long trend to build useful engineering devices and structures on a smaller and smaller scale. The creation of structures and devices calls for an understanding of the mechanical properties of materials at these small length scales. Here we examine some of the effects that arise when crystalline materials are mechanically deformed in small volumes. We show that indentation size effects at the micrometer scale can be understood in terms of the hardening associated with strain gradients and geometrically necessary dislocations, while indentation size effects at the nanometer scale involve the concepts of dislocation starvation and the nucleation of dislocations. We also describe uniaxial compression experiments on micrometer size pillars of single crystal gold and find surprisingly strong size effects, even though no significant strain gradients are present and the crystals are not initially dislocation free. We argue that these size effects are caused by dislocation starvation hardening, with dislocations leaving the crystal more quickly than they multiply and leading to the requirement of continual dislocation nucleation during the course of deformation. A new length scale for plasticity, the distance a dislocation travels before it creates another, arises naturally in this treatment. Hardening of crystals smaller than this characteristic size is expected to be dominated by dislocation starvation while crystals much larger than this size should exhibit conventional dislocation plasticity.     

Modeling of dislocation patterns of small- and high-angle grain boundaries in aluminum

A molecular dynamics simulation approach to the investigation of grain boundary structures is presented. By the example of aluminum crystallization from the melt we demonstrate the formation of polycrystalline structures from coexisting nucleation seeds. The latter are used to induce specific crystallographic orientations and hence determine the tilting of the grains resulting from further crystal growth. This allows the systematic investigation of the evolution of grain boundary structures as a function of the tilt angle. On this basis, the transition from small- to high-angle grain boundaries in aluminum as obtained from rapid under-cooling of the melt is rationalized by arrays of two different sets of dislocation pairs.     

The dislocation structure of slip bands in deformed high entropy alloy nanopillars

Remarkable diversity is observed in dislocation interactions that are responsible for intermittent and sud-den crystal slips.While large crystal slips can be easily observed on the surface of deformed crystals,unraveling the underlying dislocation interaction mechanisms,however,has been a longstanding chal-lenge in the study of single-crystal plasticity.A recent study demonstrated that the sluggish dislocation dynamics in the high entropy alloy (HEA) of Al0.1CoCrFeNi enables the observation of slip bands for a direct link to dislocation avalanches in a nanopillar.Here,we further examined the dislocation structure of slip bands in the HEA nanopillars oriented for single slip.Experimental evidence was provided on the dislocation organization in a slip band based on groups of primary dislocations,secondary dislocations,and dislocation pileups.The results were compared with the previously proposed slip band models.The unique aspects of the HEA that enable such observations were also investigated through an examination of the dislocation microstructure and its response to applied forces in the HEA nanopillars.     

Extended dislocation barriers in tilt boundaries in fcc crystals

 Extended dislocation barriers at tilt boundaries in fcc crystals are considered. For boundaries with a high degree of lattice coincidence, stable extended dislocation barriers, analogous to those occurring within a single crystal, are shown to exist. Other boundaries can have components equivalent to the high coincidence boundaries. The boundary barriers are shown to be possible nucleation sources for mechanical twins or martensite phase transformations. Received: 10 October 1997 / Accepted: 13 December 1997     

Hall—Petch relation in two-Phase TiAl alloys

The Hall—Petch relation in lamellar—gamma duplex structures of TiAl alloys has been evaluated using Fan and Miodownik's model, by assuming that the friction stress varies linearly with the lamellar volume fraction. The results reasonably predict the strength of lamellar—gamma duplex structures at various grain sizes and volume fractions. The individual contributions from three constituting boundaries, lamellar—lamellar (-), gamma—gamma (γ-γ), and lamellar—gamma (-γ) boundaries to the overall strengthening have been calculated as a function of volume fraction: the -γ phase boundaries are not effective obstacles to dislocation motion. The - grain boundaries are rather the strongest obstacles to dislocation motion.     

Dislocation motion-induced strain in nanocrystalline materials: Overlooked considerations

The amount of plastic strain caused by the motion of a single dislocation across an individual nanosize grain is drastically higher than the amount recorded for larger grain sizes. As a result, in nanocrystalline materials, only a small number of dislocations would need to move within each individual grain in order to accommodate the plastic strain on the entire sample. This observation leads to a quantitative criterion for determining if observed dislocation activity is sufficient to accommodate realistic applied plastic strains. This new criterion is directly applicable to the interpretation of in situ TEM experiments and computational molecular dynamics simulations.