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Mechanisms that make dislocation motion irreversible are associated with the formation of dislocation junctions and cross-slip, leaving dislocations trapped inside the specimen. Using Discrete Dislocation Dynamics simulations, we identify another mechanism that produces irreversible plastic deformation and leaves no or only very few dislocations inside the sample: Under cyclic loading, dislocations which pass the neutral plane during loading (pile-up formation), generate a slip step upon unloading. The explanation is an intrinsic asymmetry between the backward and forward motion. An additional bias may be introduced by the geometry of the specimen due to the shortening of the line length of dislocations. [ABSTRACT FROM AUTHOR]

Dislocation junctions are considered to control the hardening behavior of crystalline materials during plastic deformation. Here the influence of the glissile junction on the plastic deformation of microscale samples is investigated, based on discrete dislocation dynamics simulation results. It is found that with increasing dislocation density ρ , sample size d , which can be collapsed into a single dimensionless parameter d ρ , and an increasing number of activated slip systems due to different global crystallographic orientations, the glissile junction forms frequently and can bow out easily, acting as an effective source. The resulting new dislocations are mobile and contribute to the macroscopic plastic deformation on the order of 30–60%. In the size regime from 0.5 to 2 μm and dislocation densities in the range of 10 12 – 10 14 m - 2 , the glissile junction is therefore an important source for generating mobile dislocation density. Furthermore a significant correlation between stress drops and activity of dislocations originating from glissile junctions is found. A rate formulation is proposed to include these findings in crystal plasticity or continuum dislocation density frameworks. [ABSTRACT FROM AUTHOR]

Abstract: Grain growth is the simplest phenomena related to the evolution of a population of grains in crystalline materials. Some typical results obtained with vertex dynamics, a deterministic technique applied to the simulation of grain growth in polycrystalline materials, mostly in two dimensions (2D), are presented: (i) the dynamics of a population of grains interacting with various distributions of pinning obstacles; (ii) bulging as a possible recrystallization mechanism; and (iii) the influence of a confined geometry on grain growth as those found in electronic devices. Finally recent developments in 3D are presented. [ABSTRACT FROM AUTHOR]

In small dimensions, the flow stress of metallic samples shows a size-dependence such that smaller is stronger, even in nominally strain gradient-free loading conditions. However, the role of the boundary conditions in miniaturised tension or compression tests on the mechanical response and dislocation structure has not been studied in detail. In simulations performed with a three-dimensional discrete dislocation dynamics tool, initial, well-defined dislocation microstructures are loaded in tension with different boundary conditions including superimposed torsion moments. The influence of the loading conditions on details of the evolving dislocation microstructure was investigated by using identical starting configuration. An additional torsion moment significantly influences the dislocation activity since forest-dislocations are generated, but size effect of the flow stress is found to be unchanged. [ABSTRACT FROM AUTHOR]

Abstract: 3D finite element simulations of 9-grain multicrystalline aggregates are performed within the framework of the classical continuum crystal plasticity and discrete dislocation dynamics. The results are processed in a statistical way by ensemble averaging. The comparison is made at three levels: macroscopic stress–strain curves, average stress values per grain, local values of stress and plastic strain. The comparison shows that some similarities are observed in the stress and strain distributions in both simulations approaches. But there are also large discrepancies caused by the discrete nature of plasticity in DDD. The DDD simulations provide higher stress levels in the aggregate due to the small number of dislocation sources and to the stress field induced by individual dislocations. [Copyright &y& Elsevier]

Abstract: The atomic structure and energy of small-angle tilt grain boundaries in bcc tungsten and fcc aluminum are determined by atomistic simulations. The results show the dependence of the arrangement of grain boundary dislocations and the grain boundary energy on the tilt axis, the misorientation angle between the two grains and the orientation of the grain boundary plane for investigated small-angle grain boundaries. Comparison is made with a dislocation model for grain boundaries. The response of the small-angle grain boundaries to external loading is simulated. It is found that the boundary may either migrate easily or not at all at the same applied loading, depending on the characteristics and the arrangement of the grain boundary dislocations. [Copyright &y& Elsevier]

The strength and hardening of metallic materials are dictated by the motion and interactions of dislocations. Individual dislocations intersect, react, and frequently form junctions, defining a defect topology that is the basis of subsequent deformation. While immobilized dislocation locks are intuitively considered as potent strengthening structures, simulations suggest that glissile reactions are a predominant contributor to hardening among the four types of dislocation reactions in face-centered cubic crystals, even though the resulting dislocations are inherently mobile. To date, the prevailing understanding of glissile reactions has been primarily based on classical geometric models of perfect dislocations and simulations thereof. Understandings of the reaction pathways and detailed experimental characterization of glissile reactions are lacking, leaving the potential topological variations shrouded in mystery. This study details molecular dynamics simulations of glissile reaction involving dissociated partial dislocations, and the direct experimental characterization of the dislocation configurations resulting from glissile reactions in deformed pure aluminum using transmission electron microscopy. The experimentally-determined structure was reconstructed in 3D and parametrically studied in discrete dislocation dynamics simulations, revealing varying topological evolutions under different loading conditions. Further statistical analyses on an ensemble of simulated dislocations revealed the essential role of stress states and cross-slip in affecting the probability of glissile reaction and the fraction of mobile dislocation nodes. These findings point to avenues for the development of dislocation-based constitutive theories of plasticity. [Display omitted] [ABSTRACT FROM AUTHOR]

The microstructural origin of strain hardening during plastic deformation in stage II deformation of face-centered cubic (fcc) metals can be attributed to the increase in dislocation density resulting in a formation of dislocation networks. Although this is a well known relation, the complexity of dislocation multiplication processes and details about the formation of dislocation networks have recently been revealed by discrete dislocation dynamics (DDD) simulations. It has been observed that dislocations, after being generated by multiplication mechanisms, show a limited expansion within their slip plane before they get trapped in the network by dislocation reactions. This mechanism involves multiple slip systems and results in a heterogeneous dislocation network, which is not reflected in most dislocation-based continuum models. We approach the continuum modeling of dislocation networks by using data science methods to provide a link between discrete dislocations and the continuum level. For this purpose, we identify relevant correlations that feed into a model for dislocation networks in a dislocation-based continuum theory of plasticity. As a key feature, the model combines the dislocation multiplication with the limitation of the travel distance of dislocations by formation of stable dislocation junctions. The effective mobility of the network is determined by a range of dislocation spacings which reproduces the scattering travel distances of generated dislocation as observed in DDD. The model is applied to a high-symmetry fcc loading case and compared to DDD simulations. The results show a physically meaningful microstructural evolution, where the generation of new dislocations by multiplication mechanisms is counteracted by a formation of a stable dislocation network. In conjunction with DDD, we observe a steady state interplay of the different mechanisms. [ABSTRACT FROM AUTHOR]

Over the past decades, discrete dislocation dynamics simulations have been shown to reliably predict the evolution of dislocation microstructures for micrometer-sized metallic samples. Such simulations provide insight into the governing deformation mechanisms and the interplay between different physical phenomena such as dislocation reactions or cross-slip. This work is focused on a detailed analysis of the influence of the cross-slip on the evolution of dislocation systems. A tailored data mining strategy using the 'discrete-to-continuous (D2C) framework' allows to quantify differences and to quantitatively compare dislocation structures. We analyze the quantitative effects of the cross-slip on the microstructure in the course of a tensile test and a subsequent relaxation to present the role of cross-slip in the microstructure evolution. The precision of the extracted quantitative information using D2C strongly depends on the resolution of the domain averaging. We also analyze how the resolution of the averaging influences the distribution of total dislocation density and curvature fields of the specimen. Our analyzes are important approaches for interpreting the resulting structures calculated by dislocation dynamics simulations. [ABSTRACT FROM AUTHOR]

Abstract: Abnormal grain growth is a commonly observed phenomenon in barium titanate. It is usually associated with grain boundaries of different mobility and energy present in the microstructure. The influence of interfaces with variable mobility and energy on grain growth is investigated by a combined experimental and numerical approach in a transition region where growth behaviour strongly deviates from Arrhenius behaviour. Abnormal growth occurs between 1275 and 1325°C, with normal grain growth occurring above and below this temperature range. The overall grain growth rate of the small matrix grains in the transition region is found to increase nonlinearly with inverse temperature between the high- and low-temperature states. A similar behaviour is found in simulations using a 3-D mesoscale grain growth model under the assumption of fractions of grain boundaries being in the high- or low-temperature state. The transition at the grain boundary is in agreement with the complexion model. Additionally, the simulation is used to map the nucleation probability for abnormal grains in the transition region as a function of combined energy and mobility advantages. The energy advantage of the grain boundaries is found to be of greater importance for the nucleation of abnormal grains compared to results from mean field models. [Copyright &y& Elsevier]

Abstract: In Ni-base superalloys with low Al and/or Ti content the precipitation and subsequent coarsening of γ′ particles at intermediate temperatures contribute to the degradation of the mechanical properties of the alloy. In the present paper the coarsening process is modelled and the change of the critical resolved shear stress of the alloy due to coarsening of the γ′ particles is calculated by means of statistical analysis of the depinning of a single gliding edge dislocation. It is found that the contribution of γ′ hardening to the critical resolved shear stress at 973K reduces to more than half of its original value in less than one year. [Copyright &y& Elsevier]

Under stress, many crystalline materials exhibit irreversible plastic deformation caused by the motion of lattice dislocations. In plastically deformed microcrystals, internal dislocation avalanches lead to jumps in the stress-strain curves (strain bursts), whereas in macroscopic samples plasticity appears as a smooth process. By combining three-dimensional simulations of the dynamics of interacting dislocations with statistical analysis of the corresponding deformation behavior, we determined the distribution of strain changes during dislocation avalanches and established its dependence on microcrystal size. Our results suggest that for sample dimensions on the micrometer and submicrometer scale, large strain fluctuations may make it difficult to control the resulting shape in a plastic-forming process. [ABSTRACT FROM AUTHOR]

Three-dimensional dislocation networks control the mechanical properties such as strain hardening of crystals. Due to the complexity of dislocation networks and their temporal evolution, analysis tools are needed that fully resolve the dynamic processes of the intrinsic dislocation graph structure. We propose the use of a graph database for the analysis of three-dimensional dislocation networks obtained from discrete dislocation dynamics simulations. This makes it possible to extract (sub-)graphs and their features with relative ease. That allows for a more holistic view of the evolution of dislocation networks and for the extraction of homogenized graph features to be incorporated into continuum formulation. As an illustration, we describe the static and dynamic analysis of spatio-temporal dislocation graphs as well as graph feature analysis. [ABSTRACT FROM AUTHOR]

Abstract The aim of this paper is to elucidate the mechanisms controlling the brittle-to-ductile transition (BDT) in pre-deformed, textured, polycrystalline body-centred cubic (bcc) metals by the example of cold rolled tungsten (W). For this purpose, five sheets were rolled out from one and the same sintered ingot, by various levels, representing degrees of deformation of 1.8, 2.5, 3.0, 3.4, and 4.1 (this refers to 83.5%, 91.8%, 95.0%, 96.7%, and 98.3% in the technical notation). Toughness tests show that the BDT temperature decreases with increasing degree of deformation from 115 °C ± 15 °C (388 K ± 15 K) down to −65 °C ± 15 °C (208 K ± 15 K). This is an improvement of >600 K compared with a sintered ingot. In this paper we perform an in-depth analysis of the microstructure of the five sheets mentioned above. This analysis includes the assessment of (i) crystallographic texture, (ii) grain size and (iii) dislocation density. A comparison between microstructural features and experimental data confirms our working hypothesis which states that the BDT is controlled by the glide of screw dislocations and that the transition temperature decreases with decreasing spacing, λ , of dislocation sources along the crack front. Sources for dislocations may be the intersection points of grain boundaries with the crack front (BDT-temperature-grain-size-relation) or dislocation multiplication processes such as e.g., the expansion of open and closed loops (impact of dislocation density). Graphical abstract Unlabelled Image Highlights • Five tungsten sheets were rolled out from one and the same sintered ingot. • The evolution of texture, grain size and dislocation density is displayed. • A correlation was found between microstructural features and BDT temperatures. • The brittle-to-ductile transition is controlled by the glide of screw dislocations. • BDT temperature = f(spacing, λ , of dislocation sources along the crack front). [ABSTRACT FROM AUTHOR]

Depending on the process parameters, the directional solidification (DS) of eutectic alloys leads to a fibrous or lamellar microstructure. A physically motivated creep model which was evaluated for a DS-eutectic with a fibrous microstructure is applied to a DS-eutectic with a lamellar microstructure. Creep curves are simulated and compared to experimentally measured ones. It is shown, that the simulation is in good agreement with the experiment. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

A small-deformation strain gradient plasticity (GP) model for single-crystals has been proposed in [1], including a grain boundary (GB) yield condition without hardening. It has been extended by a hardening term for the GBs after a comparison to discrete dislocation dynamics (DDD) results in [2]. Differences between the strain gradients of the GP results and the DDD results motivate the consideration of a non-quadratic defect energy [3] in the GP model. It is shown that the gradients in the GP model can be improved using an exponent different from two. Remaining discrepancies in the strain profiles, compared to the DDD results, are attributed to the neglect of the individual gradients of plastic slip and due to the lack of a mechanism for the misorientation-dependent elastic interactions of dislocations across GBs [4] in the GP model. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

During the plastic deformation of crystalline materials, 3d dislocation networks form based on dislocation junctions. Particularly, immobile Lomer junctions are essential for the stability of dislocation networks. However, the formed Lomer junctions can unzip and dissolve again, if the linked mobile dislocations of the Lomer junction - the Lomer arms - experience sufficiently high resolved shear stresses. To generate a better understanding of the dislocation network stability and to pave the way to a general stability criterion of dislocation networks, we investigate the Lomer arm length distribution in dislocation networks by analyzing discrete dislocation dynamics simulation data of tensile-tested aluminum single crystals. We show that an exponential distribution fits best to the Lomer arm length distribution in the systems considered, which is independent of the crystal orientation. The influence of the slip system activity on the Lomer arm length distribution is discussed. [ABSTRACT FROM AUTHOR]

Abstract: Micron-scale single-crystalline materials deform plastically via large intermittent strain bursts that make the deformation process non-deterministic. Here we investigate this stochastic phenomenon by analyzing the plastic response of an ensemble of specimens differing only in the initial arrangement of dislocations. We apply discrete dislocation dynamics simulations and microcompression tests on identically fabricated Cu single-crystalline micropillars. We find that a characteristic yield stress can be defined in the average sense for a given specimen ensemble, where the average and the variance of the plastic strain start to increase considerably. In addition, in all studied cases the stress values at a given strain follow a Weibull distribution with similar Weibull exponents, which suggests that dislocation-mediated plastic yielding is characterized by an underlying weakest-link phenomenon. These results are found not to depend on fine details of the actual set-up; rather, they represent general features of micron-scale plasticity. [Copyright &y& Elsevier]

Microstructure reconstructions resulting from diffraction contrast tomography data of polycrystalline bulk strontium titanate were reinvestigated by means of electron backscatter diffraction (EBSD) characterization. Corresponding two-dimensional grain maps from the two characterization methods were aligned and compared, focusing on the spatial resolution at the internal interfaces. The compared grain boundary networks show a remarkably good agreement both morphologically and in crystallographic orientation. Deviations are critically assessed and discussed in the context of diffraction data reconstruction and EBSD data collection techniques. [ABSTRACT FROM AUTHOR]

This review examines the size effects observed in the mechanical strength of thin metal films and small samples such as single-crystalline pillars, whiskers, and wires. Experimental results from mechanical testing and electron microscopy studies, as well as recent insights from discrete dislocation dynamics simulations, are presented. The size dependency of deformation may be separated into three regimes: the nanometer regime of roughly 100 nm and below, an intermediate regime between 100 nm and approximately 1 μm, and a bulk-like regime. We argue that there is no scaling law with one universal power-law exponent encompassing the entire range. Instead, there are a number of different mechanisms and underlying effects, e.g., the initial dislocation microstructure or loading conditions. The complex interaction of these mechanisms leads to the typically observed scaling behavior. [ABSTRACT FROM AUTHOR]

In this study, we present the recrystallisation behaviour of heavily cold rolled tungsten sheets and show that the material recrystallises towards a single texture component, i.e. the rotated cube component {001}<110>. This result is remarkable as it distinguishes from the classical concept of recrystallisation and is likely to have a strong impact in various fields in science and technology. The thermal stability of as-rolled tungsten sheets featuring a thickness reduction of 98% (i.e. 4.1 in the logarithmic notation) was investigated. Annealings were performed in a vacuum and in the temperature range of 400 °C (673 K) to 2000 °C (2273 K) for a time period of 6 min up to 500 h. The annealing-induced evolution of the microstructure is displayed by hardness (HV0.1) and electron backscatter diffraction (EBSD) measurements. The results show that after an annealing of 6 min at 800 °C (1073 K), the hardness drops from 666 ± 6.3 to 625 ± 7.9 HV0.1. The evolution of the hardness is fitted by classical kinetic models for recovery (logarithmic kinetics) and recrystallisation. The apparent activation energy of half of the hardness loss, E Δ HV /2 , was found to be 356 kJ/mol (3.69 eV), which can be associated with the activation energy of grain boundary diffusion in tungsten. Furthermore, the hardness data allows for the distinction between two stages. In stage I, the material rapidly develops an equiaxed grain structure by the break-up and spheroidisation of the high aspect ratio grains. We do not observe the formation of nuclei in the classical sense. Moreover, the recrystallisation processes can be described as grain growth. We suggest the dominant driving force to be the reduction of the internal stored energy. Finally, the transition from stage I to stage II is caused by the onset of abnormal grain growth. Unlabelled Image • Annealing studies were performed in the range of 400–2000 °C for 0.1–500 h. • The tungsten sheet recrystallises towards a single texture component, {001}<110>. • After 6 min at 800 °C, the hardness drops from 666 ± 6.3 to 625 ± 7.9 HV0.1. • The activation energy of half of the hardness loss was found to be 356 kJ/mol. • RX processes: break up of grains, spheroidisation followed by abnormal grain growth. [ABSTRACT FROM AUTHOR]