Dislocation Microstructure Research Articles

Using Energy‐Dispersive Laue Diffraction to Study Dislocation Arrangements in Materials Showing Wavy and Planar Slip Behavior

ABSTRACTThe present work shows an approach to monitor the evolution of the dislocation arrangement of a metallic material caused by cyclic plastic strain using white X‐ray radiation in combination with an energy‐dispersive detector. The method is demonstrated by single‐shot experiments performed on polycrystalline nickel and α‐brass, representing the pure wavy and the pure planar dislocation slip behavior. To correlate the resulting diffraction patterns with various dislocation arrangements of both metals, fatigue tests were carried out up to certain numbers of cycles and at predetermined plastic strain amplitudes. The differences in dislocation microstructure and internal stress distributions give rise to an appreciable change in the peak shape of Laue reflections, leading to unique characteristics in the respective diffraction patterns. Nickel reflections are elongated due to the high amount of cell structures leading to bending and misorientation of the lattice, whereas the present stacking faults in α‐brass result in powder‐like diffraction.

Progressive change in dislocation microstructures in shocked calcite with pressure: Characterization of micrometeoroid bombardment on asteroid Ryugu

Progressive change in dislocation microstructures in shocked calcite with pressure: Characterization of micrometeoroid bombardment on asteroid Ryugu

Enhanced hot corrosion resistance of AlCoCrFeNi2.1 high entropy alloy coatings by extreme high-speed laser cladding

Extreme high-speed laser cladding (EHLC) and multiple laser remelting (EHLC-MR) are used to improve hot corrosion resistance of AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) coatings by refining their microstructures, introducing low angle grain boundaries (LAGBs) and high densities of dislocations as well as higher compressive residual stresses (CRSs) in the coatings. The experimental results show that finer microstructure, more LAGBs and high densities of dislocations are beneficial to increase Al2O3 nucleation sites and promote the uniform formation of the oxide layer on the coating surface. On the other hand, higher CRSs suppress the initiation and propagation of cracks as well as enhance the adhesion between the oxide layer and the substrate. Thus the hot corrosion resistance of the EHEA coatings under a molten salt of 75 % Na2SO4 + 25 % NaCl at 900°C is improved significantly. These novel results provide effective approach for designing elevated-temperature materials.

Cyclic deformation behavior and damage mechanism of 316H stainless steel under low-cycle fatigue loading at 650 °C

Low-cycle fatigue tests were conducted at 650 °C to investigate the cyclic deformation behavior and damage mechanism of 316H stainless steel at various strain amplitudes ranging from 0.2 % to 1.0 %. The relationship between macroscopic deformation properties and microscopic physical mechanisms was revealed based on the comprehensive analysis of local deformation misorientation and dislocation microstructure. Results showed that the 316H stainless steel presented non-Masing behavior at strain amplitudes lower than 0.6 %, and Masing behavior at strain amplitudes larger than 0.6 %, which was primarily attributed to the evolution of dislocation configurations. The crack initiation and propagation behavior at 0.2 % strain amplitude, characterized by the transgranular mode, was dominated by fatigue. However, the intergranular damage became dominant at the strain amplitude larger than 0.2 %, due to the intensified strain localization at grain boundaries. Moreover, the plastic strain energy was used for life assessment with the consideration of non-Masing behavior.

Simulated TEM imaging of a heavily irradiated metal

We recast the Howie–Whelan equations for generating simulated transmission electron microscope (TEM) images, replacing the dependence on local atomic displacements with atomic positions only. This allows rapid computation of simulated TEM images for arbitrarily complex atomistic configurations of lattice defects and dislocations in the dynamical two beam approximation directly from standard atomistic simulation output files. Large-scale massively-overlapping cascade simulations, performed with molecular dynamics, are used to generate representative high-dose nanoscale defect and dislocation microstructures in tungsten at room temperature. We then compare the simulated TEM images to experimental TEM images with similar irradiation and imaging conditions. The simulated TEM shows ‘white-dot’ damage in weak-beam dark-field imaging conditions, in agreement with our experimental observations and as expected from previous studies, and in bright-field conditions a dislocation network is observed. In this work we also compare the simulated images to the nanoscale lattice defects in the original atomic structures, and find that at high dose the white spots are not only created by small dislocation loops, but rather arise from nanoscale fluctuations in strains around curved sections of dislocation lines.

Evidence of Dislocation Mixed Climb in Quartz From the Main Central and Moine Thrusts: An Electron Tomography Study

AbstractIn this study we apply electron tomography to characterize 3D dislocation microstructures in two quartz mylonite specimens from the Moine and Main Central Thrusts, both of which accommodated displacements by dislocation creep in the presence of water. Both specimens show dislocation activity with dislocation densities of the order of 3–4 × 1012 m−2 and evidence of recovery from the presence of subgrain boundaries. 〈a〉 slip occurs predominantly on pyramidal and prismatic planes (〈a〉 basal glide is not active). [c] Glide is not significant. On the other hand, we observe a very high level of activation of 〈c + a〉 glide on the , , (n = 1,2) and even planes. Approximately 60% of all dislocations show evidence of climb with a predominance of mixed climb, a deformation mechanism characterized by dislocations moving in a plane intermediate between the glide and the climb planes. This atypical mode of deformation demonstrates comparable glide and climb efficiency under natural deformation conditions. It promotes dislocation glide in planes not expected for the quartz structure, probably by inhibiting lattice friction. Our quantitative characterization of the microstructure enables us to assess the strain that dislocations can generate. We show that glide systems indicated by the observed dislocations are sufficient to accommodate any arbitrary 3D strain by themselves. Although historically dislocation glide has been regarded as being primarily responsible for producing strain, activation of climb can also directly contribute to the finite strain. On the basis of this characterization, we propose a numerical modeling approach for attempting to characterize the local stress state that gave rise to the observed microstructure.

Mechanics of gradient nanostructured metals

The emergence of heterogeneous nanostructured materials (HNMs) offers exciting opportunities to achieve outstanding mechanical properties. Among these materials, gradient nanotwinned (GNT) Cu is a prominent class of HNMs, demonstrating superior strengths by gaining extra strengths compared to non-gradient counterparts. Its layered gradient structure provides a simplified quasi-one-dimensional model system for understanding the extra strengthening effects of structural gradients and resulting plastic strain gradients. This paper presents a comprehensive report for recent experimental and modeling studies on the mechanics of GNT Cu, covering advances in controlled material processing, back stress measurement, deformation field characterization, dislocation microstructure analysis, and strain gradient plasticity modeling. These studies unveil the spatiotemporal evolution of both plastic strain gradients and extra back stresses originating from structural gradients. Direct connections are established between the sample-level extra strength of GNT Cu and the synergistic strengthening effects induced by local nanotwin structures and their gradients. We emphasize the critical role of the size of the representative volume element in assessing the effects of plastic strain gradient and extra back stress. Moreover, lower-order strain gradient plasticity models are validated through experimental characterizations of GNT Cu, paving the way for future investigation into the mechanics of gradient nanostructured metals. Finally, we provide an outlook on research needs for understanding the mechanics of gradient nanostructured metals and, more broadly, HNMs, towards achieving exceptional mechanical properties.

Study on Characteristics of Ultrasound-Assisted Fracture Splitting for AISI 1045 Quenched and Tempered Steel.

Ultrasonic vibration-assisted con-rod fracture splitting (UV-CFS) was used to carry out the fracture experiment of 1045 quenched and tempered steel. The effect of ultrasonic vibration on the fracture properties was studied, the fracture microstructure and the evolution of dislocations near the fracture were analyzed and the microscopic mechanism was analyzed. The results show that in the case of conventional fracture splitting without amplitude, the dimple and the fracture belong to ductile fracture. With the increase in ultrasonic amplitude, the plasticity and pore deformation of the con-rod samples decrease at first and then increase; when the amplitude reaches a certain point, the load required for cracking is reduced to a minimum and the ultrasonic hardening effect is dominant, resulting in a decrease in the plasticity of the sample, a cleavage fracture, a brittle fracture, the minimum pore deformation and high cracking quality. The research results also show that with the increase in ultrasonic amplitude, the fracture dislocation density decreases at first, then increases, and dislocation entanglement and grain breakage appear, then decrease, and multiple dislocation slip trajectories appear. The changes in the dislocation density and microstructure are consistent with the above results.

Understanding dislocation plasticity of single crystalline Ta micropillars under dynamic loading

Recent experimental findings have shown that tantalum single crystals display strong anisotropy during Taylor impact testing in stark contrast to isotropic deformation in polycrystalline counterparts. In this study, a coupled dislocation dynamics and finite element model was developed to simulate the complex stress field under dynamic loading of a Taylor impact test and track the intricate evolution of the dislocation microstructure. Our model allowed us to investigate detailed motion of dislocations and their mutual interactions and the effect of varying simulation parameters, such as sample size, initial dislocation density, crystallographic orientation, and temperature. Simulation results show good agreement with experimental observations and shed light on the mechanical response at small-scale under extreme loading conditions. In addition, resolved shear stress analysis incorporating the effect of shear stress from impact was performed to quantitatively support and provide a means to understand the model predictions of the impact foot shape.

Research on microplastic deformation of Inconel718 under the conditions of high temperature and high strain rate

The compression experiments of the nickel-based superalloy Inconel718 were carried out with the help of split Hopkinson pressure bar device, and the microstructure evolution law in the compression deformation of the alloy was studied by means of optical microscopy, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Based on the dislocation theory, a three-dimensional discrete dislocation dynamics model is established. By simulating the dynamic evolution of the discrete dislocation, the interaction of the dislocation itself and the formation of the dislocation microstructure were captured, and further explore the microplastic deformation process of Inconel718. The results show that deformation conditions significantly affect the rheological behavior and dynamic recrystallization behaviour. The cross slip of dislocations is closely related to temperature and strain rate, which in turn affect the dislocation configuration. The deformation mechanism of Inconel718 under the conditions of high temperature and high strain rate is jointly dominated by dislocation slip and twin deformation. As the temperature increases, the average Schmidt factor increases, and more dislocation slip systems are activated. As the strain rate increases, the number of deformation twins increases, while the thickness of deformation twins decreases.

Microstructure evolution and mechanical properties of high strength and high conductivity Cu[sbnd]Fe alloy wire prepared by cold drawing

CuFe alloy wire is widely used in the field of electronic information because of its high strength and high conductivity. In this study, Cu-5Fe-0.1Si-0.3 Mg-0.05RE (La, Ce) alloys were prepared by cold drawing and heat treatment. The microstructure evolution of the alloy during deformation was studied by optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). Three kinds of second phases with different sizes and shapes were successfully constructed in the designed alloys, including fibrous iron phase, submicron spherical iron phase and nano spherical phase. The strength of the alloy is greatly improved by drawing deformation, with only slight reduction of the conductivity. Through joint addition of Mg, Si and rare earth elements, the designed CuFe alloy wire rod has excellent overall properties. It's electrical conductivity, tensile strength, yield strength and elongation reached 63.8% IACS, 831 MPa, 730 MPa and 6.1%, respectively. Detailed characterizations of the alloys reveal that: the addition of Mg reduces solid solubility limit of iron in matrix and spaces between iron phases; the FeSi phase formed by Si and Fe promotes the precipitation of iron phase; and the addition of rare earth inhibits segregation of second phase, recovery and recrystallization of the alloy, and the coarsening of Fe phase. The mechanical properties of the alloy are improved by fiber strengthening, Hall-Petch strengthening and precipitation strengthening, respectively. The contributions of precipitation strengthening, fiber strengthening and Hall-Petch strengthening to yield strength are 65 MPa, 59 MPa and 277 MPa, respectively. The microstructure of dislocation and fibrosis is the main factor of alloy strengthening.

Effects of temperature and strain amplitude on low-cycle fatigue behavior of 12Cr13 martensitic stainless steel

12Cr13 martensitic stainless steel has been selected as the structural material for steam turbine blade which is frequently subjected to low-cycle fatigue (LCF) damage. The study involved conducting low-cycle fatigue (LCF) tests under strain control at various temperatures (25 °C, 250 °C, 350 °C, 450 °C) and strain amplitudes (±0.3 %, ±0.4 %, ±0.5 %, ±0.6 %). The cyclic deformation behavior, primarily indicating cyclic stress response and the Massing effect is thoroughly analyzed based on the characterization of dislocation microstructure and electron backscatter diffraction maps. The mechanism of crack initiation and the propagation behavior are discussed based on the SEM observations of microcracks on the specimen surface and the fracture surface morphology. The influence of temperature and strain amplitude on fatigue life behavior is determined, and the underlying mechanisms are revealed. Moreover, the life prediction is performed by using the classical Basquin-Coffin-Manson model at ambient temperature.

Irreversible evolution of dislocation pile-ups during cyclic microcantilever bending

In single crystals, plastic deformations are predominantly governed by dislocation movement and interactions. The group of dislocations that creates strain gradients, known as geometrically necessary dislocations (GNDs), also deterministically contributes to strain hardening, micron-scale size effects, fatigue, and Bauschinger effect. During bending large strain gradients naturally emerge which makes this deformation mode exceptionally suitable to study the evolution of GNDs. Here we present bi-directional bending experiment of a Cu single crystalline microcantilever with in situ characterisation of the dislocation microstructure in terms of high-resolution electron backscatter diffraction (HR-EBSD). The experiments are complemented with dislocation density modelling to provide physical understanding of the collective dislocation phenomena. We find that dislocation pile-ups form around the neutral zone during initial bending, however, these do not dissolve upon reversed loading, rather they contribute to the development of a much more complex GND dominated microstructure. This irreversible process is analysed in detail in terms of the involved Burgers vectors and slip systems. We conclude that at this scale the most dominant role in the Bauschinger effect and corresponding strain hardening is played by short-range dislocation interactions. The in-depth understanding of these phenomena will aid the design of microscopic metallic components with increased performance and reliability.

Activation of multiple deformation mechanisms and HDI hardening devoting to significant work-hardening of gradient-dislocation structured TRIP steel

Combining hetero-structure and TRIP effect can be a feasible approach to enhancing the strength-ductility synergy. Here, varied gradient microstructures with gradual increase in dislocation density with or without martensite content from the center to the surface were constructed in a commercial austenitic stainless steel with strong TRIP effect by cyclic torsion (CT) processes. These designed gradient microstructures resulted in excellent strength-ductility synergies, for example, after CT processing with 20° torsion angle, the yield strength was enhanced 2.7 times, while 78% of the uniform elongation and 121% of the static toughness was achieved, compared to its CG counterpart. The high yield strength was caused by the introduced dislocations and the additional hetero-deformation induced (HDI) strengthening (>350 MPa) due to the mechanical incompatibility between different zones. The high ductility and toughness originated from the unexpectedly continuous work hardening from the soft center to the hard surface. Amazingly, the work hardening provided by the hard surface zones outperformed that by the soft central zones during the middle and late tensile deformation. The multiple-stage activation of stack faults, continuous γ→α′ martensitic transformation (i.e., TRIP effect), mechanical nanotwinning and Lomer-Cottrell locks combined with the HDI work-hardening devoted to the significantly enhanced work hardening capacity of the hard surface zones. This study provides a strategy of combining the gradient dislocation microstructure and TRIP effect to develop advanced metallic materials with high strength and ductility/toughness.

Deep learning of crystalline defects from TEM images: a solution for the problem of ‘never enough training data’

Crystalline defects, such as line-like dislocations, play an important role for the performance and reliability of many metallic devices. Their interaction and evolution still poses a multitude of open questions to materials science and materials physics. In-situ transmission electron microscopy (TEM) experiments can provide important insights into how dislocations behave and move. The analysis of individual video frames from such experiments can provide useful insights but is limited by the capabilities of automated identification, digitization, and quantitative extraction of the dislocations as curved objects. The vast amount of data also makes manual annotation very time consuming, thereby limiting the use of deep learning (DL)-based, automated image analysis and segmentation of the dislocation microstructure. In this work, a parametric model for generating synthetic training data for segmentation of dislocations is developed. Even though domain scientists might dismiss synthetic images as artificial, our findings show that they can result in superior performance. Additionally, we propose an enhanced DL method optimized for segmenting overlapping or intersecting dislocation lines. Upon testing this framework on four distinct real datasets, we find that a model trained only on synthetic training data can also yield high-quality results on real images–even more so if the model is further fine-tuned on a few real images. Our approach demonstrates the potential of synthetic data in overcoming the limitations of manual annotation of TEM image data of dislocation microstructure, paving the way for more efficient and accurate analysis of dislocation microstructures. Last but not least, segmenting such thin, curvilinear structures is a task that is ubiquitous in many fields, which makes our method a potential candidate for other applications as well.

Mechanism of enhanced room temperature ductility of cold–rolled tungsten under micro-bending test

The enhanced ductility of cold-rolled (CR) tungsten at room temperature is considered to be related to the activities of dislocations and the associated microstructures. However, the specific mechanism and the exact roles that dislocation activity and microstructure effects play in enhancing ductility in CR tungsten are still subjects of conjecture. In this context, three-dimensional electron backscatter diffraction reveals that a CR tungsten micro-cantilever fractures in a semi-brittle manner within the L-T fracture system. The ductile, blunted region shows high dislocation mobility away from the cracking tip, and its emission pathway is strongly correlated with low-angle grain boundaries. Meanwhile, strain and dislocations blocked at high-angle grain boundaries induces stress cracking and results in brittle intergranular fracture. These ductile and brittle failure modes trigger the crack bridging effect, ultimately leading to controlled crack propagation. This work would provide valuable idea for future investigations into the ductility enhancement of ultra-fine grain W.

Mesoscale dislocation dynamics modeling of incipient plasticity under nanoindentation

A mesoscale dislocation dynamics model which couples three-dimensional dislocation dynamics (DD), and finite element method (FEM) was used to investigate the mechanical response of Aluminum (Al) single crystal under spherical nanoindentation. Together with an atomistically informed nucleation model, the dislocation dynamics model can capture both the dynamic evolution of dislocations under the complex stress state of indentation and the corresponding constitutive response of material. The resulting load-displacement curves and the evolution of dislocation microstructures were analyzed to provide insights into the underlying deformation mechanism for incipient plasticity under nanoindentation. Our model could show the transition of the governing mechanism for plasticity from nucleation-controlled plasticity to pre-existing source-driven plasticity with increasing indenter size, as shown in recent experiments. In addition, it could capture a clear picture on incipient plasticity including the formation of prismatic loops through successive cross-slips of nucleated dislocations, such as rhombus loop and prismatic helical structures, which agrees well with atomistic modeling results.

The Deformation Behavior of Niobium Microalloyed Steel during Lüders Band Formation

In microalloyed steels, plastic instabilities often appear which have been found to be associated with changes in the microstructure. In this paper, research was carried out on the influence of the microstructure in different areas of the deformation zone during the formation of Lüders bands in niobium microalloyed steel. Thermography and digital image correlation during static tensile testing were used to research deformation behavior and the area before and during the formation of the Lüders band. Different local values of temperature changes, i.e., stress changes, and strains in the examined areas during the formation of the Lüders band were determined. The highest values of the temperature changes and strains during the formation of the Lüders band were measured in the area of the initial appearance of the Lüders band. In order to clarify the observed changes, a microstructural analysis, using X-ray diffraction, scanning electron and transmission electron microscopy methods, was used. It was established that the observed temperature, i.e., stress, and strain changes are related to changes in the microstructure. The analyses of changes in the microstructure, arrangement and interaction of dislocations with precipitates revealed significant changes in the movement of dislocations and their interaction with fine niobium-containing precipitates during the formation of Lüders bands. The influence of microstructural parameters on the Lüders band formation in niobium microalloyed steel has been established based on this research.

Effects of laser surface remelting on damage resistance of oxide scales on T91 exposed to flowing oxygen-saturated lead-bismuth eutectic

Forming suitable oxide scales on the surface of T91 steel by controlling the oxygen concentration has been considered as an effective method to restrain the liquid metal corrosion of T91 steel exposed to lead-bismuth eutectic (LBE). Notably, the successful precondition of this method is that the oxide scales on T91 steel can not be damaged easily, especially in flowing LBE environment. In the present study, laser surface remelting (LSR) treatment is employed to enhance the damage resistance of oxide scales on T91 steel under the flowing (∼3 m/s) oxygen-saturated LBE conditions at 500 ℃. The experimental results reveal that although the T91 steel specimens subjected to LSR treatment still process a double-layered oxide scales comprising an outer magnetite layer and an inner Fe-Cr spinel layer, their damage resistance ability is remarkably stronger compared the as-received T91 steel specimens in flowing LBE environment. Further observations indicate that such phenomenon should be ascribed to fewer microstructural defects, such as LBE-filled holes within the oxide scales, resulting from the refined microstructure and high density of dislocations activated in LSR specimens. Accordingly, the possible reasons behind these phenomena were proposed, and a corresponding damage model of oxide scales in flowing LBE condition was established.

Confluent effects of initial dislocation microstructures on yield behavior in single-crystal tungsten at high temperature

Though it is well known that initial dislocation microstructures play an important role in the deformation behavior of materials, how dislocation microstructures affect the macroscopic properties is still subjected to intensive research due to their complexity. In this work, we use discrete dislocation dynamics (DDD) to understand the effect of initial dislocation microstructures on the dynamic yield behavior of single-crystal tungsten above the critical temperature (800 K). DDD results suggest that the dislocation source length and the dislocation density are responsible for the initial yield stress and the flow stress of tungsten under dynamic loading, respectively. As and increase, the plastic yield mechanism transforms from dislocation-source activation into dislocation-dislocation interactions, resulting in the initial yield stress decreasing and the flow stress increasing. The confluent effects of and on the steady-state flow stress can be unified by a linear relationship between and Our results could be a valuable piece that connects dynamic yield behavior to the initial dislocation microstructures.