Dislocation Arrangements Research Articles

Evaluation of Mechanical Properties of Medium Carbon Steel by Dislocation Microstructure Analysis

Carbon steels realize high strength by quenching, which are used for mechanical elements such as gears and bearings. Martensitic transformation makes harder due to high dislocation density and fine crystal grains. The mechanical performance increase in proportion to their hardness. However, in the case of medium carbon steels, steels quenched and tempered at 290℃ have better mechanical properties than as quenched steels. There are some factors for decrease of mechanical properties such as dislocation density and retained austenite, this study focused on dislocation microstructure. To obtain their dislocation microstructure, line profiles from neutron diffraction were analyzed using convolution multiple whole profile (CMWP) fitting method. It is known that there are as proportional relationships between the square root of dislocation density and yield stress, but the analysis results show no proportional relationship. Therefore, we employed dislocation arrangement parameter to take into account mobile and immobile dislocations. Thus, effective dislocation density was introduced and may be able to explain the relationship with yield stress.

Mechanical tailoring of dislocation densities in SrTiO3 at room temperature

AbstractDislocation‐tuned functional properties such as electrical conductivity, thermal conductivity, and ferroelectric properties in oxides are attracting increasing research interest. A prerequisite for harvesting these functional properties in oxides requires successful introduction and control of dislocation density and arrangement without forming cracks, which is a great challenge due to their brittle nature. Here, we report a simple method to mechanically tailor the dislocation densities in single‐crystal perovskite SrTiO3. By using a millimeter‐sized Brinell indenter, dislocation densities from ∼1010 to ∼1013 m−2 are achieved by increasing the number of indenting cycles. Depending on tip radius and indenting load, large and crack‐free plastic zones over hundreds of micrometers are created. The dislocation multiplication mechanisms are discussed, and the work hardening in the plastic zone is evaluated by micro‐hardness measurement as a function of dislocation density. This simple approach opens many new opportunities in the area of dislocation‐tuned functional and mechanical studies.

Strengthening and Weakening by Dislocations in Monolayer MoS2

Dislocations govern the properties of crystals. Yet, how pentagon–heptagon (5|7) pairs in grain boundaries (GBs) affect the mechanical properties of MoS2 remains poorly known. Using atomistic simulations and the continuum disclination dipole model, we show that depending on the tilt angle and 5|7 dislocation arrangement, MoS2 GB strength can be enhanced or reduced with the tilt angle. For zigzag-tilt GBs primarily composed of Mo5|7 + S5|7 dislocations, GB strength monotonically increases as the square of the tilt angle. For armchair-tilt GBs with Mo5|7 or S5|7 dislocations, however, the trend of GB strength breaks down, as dislocations are unevenly spaced. Moreover, mechanical failure initiates at the bond shared by 5|7 rings, in contrast to graphene in which failure occurs at the bond shared by 6|7 rings. This work provides new insights into the mechanical design of synthetic transition metal dichalcogenide crystals via dislocation engineering.

Five-second STEM dislocation tomography for 300\xa0nm thick specimen assisted by deep-learning-based noise filtering

Scanning transmission electron microscopy (STEM) is suitable for visualizing the inside of a relatively thick specimen than the conventional transmission electron microscopy, whose resolution is limited by the chromatic aberration of image forming lenses, and thus, the STEM mode has been employed frequently for computed electron tomography based three-dimensional (3D) structural characterization and combined with analytical methods such as annular dark field imaging or spectroscopies. However, the image quality of STEM is severely suffered by noise or artifacts especially when rapid imaging, in the order of millisecond per frame or faster, is pursued. Here we demonstrate a deep-learning-assisted rapid STEM tomography, which visualizes 3D dislocation arrangement only within five-second acquisition of all the tilt-series images even in a 300 nm thick steel specimen. The developed method offers a new platform for various in situ or operando 3D microanalyses in which dealing with relatively thick specimens or covering media like liquid cells are required.

Martensitic transformation within nanotwins enhances fatigue damage resistance of a nanotwinned austenitic stainless steel

We investigated the low-cycle fatigue behavior of a novel nanotwinned 316L stainless steel consisting of nanotwinned austenitic grains embedded in the matrix with characteristic dislocation arrangements. We excitingly found that the α\\prime-martensitic transformation is activated traversing twin lamellae to undertake the localized deformation of nanotwinned grains at high strain amplitudes. These α\\prime-martensite nuclei are mainly originated from the intersections between “zigzag” slip bands and twin lamellae. Thanks to the effective release of stress/strain localization by the α\\prime-martensitic transformation within nanotwins, the fatigue strength and fatigue life in the low-cycle regime are simultaneously enhanced.

Evaluation of dislocation activities and accumulation in cold swaged CoCrFeMnNi high entropy alloy

The CoCrFeMnNi high entropy alloys (HEAs) performs large work strengthening with excellent deformability. The work strengthening and deformation mechanism are facilitated by dislocation activities and dislocation accumulation. In this study, the dislocation density and microstructure of the CoCrFeMnNi HEA were characterized by the neutron line profile analysis using convolution multiple whole profile (CMWP) method and EBSD in a comparison with a binary FeNi alloy. The CoCrFeMnNi HEA and FeNi alloy were plastically deformed by rotary swaging until 85% area reduction. The characteristics (e.g., low stacking fault energy, local variation with different atom species) rising from the high compositional complexity of the CoCrFeMnNi HEA cause different dislocation activities and levels of dislocation accumulation from the binary FeNi alloy. The dislocation density of the CoCrFeMnNi HEA continuously increased during the cold swaging and was significantly larger than the FeNi alloy. The larger dislocation accumulation of the CoCrFeMnNi HEA is facilitated by higher compositional complexity, extensive dislocation arrangement, and strong grain fragmentation. The increasing heterogeneity of dislocation distribution in the CoCrFeMnNi HEA was contributed by dislocation cell formation and increasing geometrically necessary dislocations (GNDs). The larger work strengthening in CoCrFeMnNi HEA is correlated with large total dislocation density during cold swaging.

Subsurface Microstructural Evolution during Scratch Testing on Bcc Iron

Subsurface microstructures influence the friction and wear behavior of metallic tribological systems, among other factors. To gain a basic understanding of the microstructural changes occurring during sliding processes, face-centered cubic model systems, for example a copper system with a sapphire sphere sliding against it, were previously characterized. Such systems showed the evolution of the dislocation self-organization phenomenon called the dislocation trace line. To test the occurrence of this dislocation arrangement in bcc metals, in this study a ruby ball was slid against electropolished bcc iron under an increasing normal load. The wear track topography and subsurface microstructure were characterized using white light interferometry and scanning transmission electron microscopy. The analysis suggested that at least for bcc iron, the evolution of a dislocation trace line is connected with the onset of pronounced plastic deformation.

Effect of Mg content on the dislocation characteristics and discontinuous dynamic recrystallization during the hot deformation of Al-Mg alloy

The effect of Mg content on the dynamic recrystallization behavior during the hot deformation of Al-Mg alloys were investigated. Three different Mg contents of Al-6Mg, Al-8Mg, and Al-9Mg were hot-torsioned under the deformation temperature of 723 K and a strain rate of 0.1 s−1. Using and electron backscatter diffraction, the dislocation characteristics during the hot deformation of each alloy were evaluated. The results of X-ray line profile analysis showed that the strain anisotropy parameter and dislocation arrangement parameter decreased when the strain increased, and when the fraction of edge dislocation and screening of dislocation field (interaction between dislocations) increased. However, the variation in Mg content affected the strain dependence of the dislocation parameters, which means that the Mg content in the Al-Mg alloy affected dislocation characteristics and dynamic recrystallization. In addition, the fraction of low-angle boundaries was accelerated when the strain increased at lower Mg content, while the formation of serrated grain boundaries was remarkably observed. Consequently, considering the absence of twinning, at all strains, higher Mg content increased the dynamic recrystallization rate and induced the flow softening at relatively low strains.

Microstructure evolution and mechanical performance of ternary Zn-0.8Mg-0.2Sr (wt. %) alloy processed by equal-channel angular pressing

In this study, we prepared a Zn-0.8Mg-0.2Sr (wt. %) alloy and processed it by ECAP. The evolution of the microstructure during the processing was observed and discussed in detail. The obtained results revealed the continuous dynamic recrystallization as the prevailing recrystallization mechanism. It affected all the aspects of the microstructure, namely the grain size, residual stresses, and dislocation arrangement. The obtained grain size was in good agreement with both empirical and theoretical relations predicting the minimal (0.4–0.6 μm) and average (2.5 μm) grain size. The compressive tests revealed the relations between alignment of the intermetallic regions, texture of the Zn matrix, and resulting mechanical performance of the material. The compressive yield strength of the material ranged from 230 to 250 MPa in the individual directions, and the tensile yield strength reached the value of approximately 200 MPa. The resulting mechanical properties were almost isotropic in the individual directions and fulfilled the basic requirements for applications in implantology, particularly, for maxillofacial, cranial or orthopaedic implants.

In situ diffraction characterization on microstructure evolution in austenitic stainless steel during cyclic plastic deformation and its relation to the mechanical response

Diffraction line profile analysis was performed to qualitatively evaluate the change in the microstructure of austenitic stainless steel during 250 cycles of plastic deformation. The dislocation density increased with increasing number of cycles until 50 cycles but thereafter decreased. The cycle number corresponding to this maximum point differed depending on whether it was evaluated as the total dislocation density or was deconvoluted into edge and screw dislocation densities. At the initial state, edge dislocations were predominant; however, screw dislocations greatly increased at the first stage of cyclic loading. Afterwards, edge dislocations formed cell walls and screw dislocations annihilated. The cycle number at which the singularity was reached depended on the development of the dislocation structure. When the cell structure developed and the cell wall became sharp, the arrangement of dislocations on average decreased and the crystallite size increased. The flow stress of the austenite phase increased and decreased, reflecting the dislocation density during cyclic loading. However, after αʹ-martensite was generated as the number of cycles increased, the contribution of the transformed martensite to the total flow stress could increase.

Characterizing dislocation loops in irradiated polycrystalline Zr alloys by X-ray line profile analysis of powder diffraction patterns with satellites.

This work extends the convolutional multiple whole profile (CMWP) line profile analysis (LPA) procedure to determine the total dislocation density and character of irradiation-induced dislocation loops in commercial polycrystalline Zr specimens. Zr alloys are widely used in the nuclear industry as fuel cladding materials in which irradiation-induced point defects evolve into dislocation loops. LPA has long been established as a powerful tool to determine the density and nature of lattice defects in plastically deformed materials. The CMWP LPA procedure is based on the Krivoglaz-Wilkens theory in which the dislocation structure is characterized by the total dislocation density ρ and the dislocation arrangement parameter M. In commercial Zr alloys irradiation-induced dislocation loops broaden the peak profiles, mainly in the tail regions, and occasionally generate small satellites next to the Bragg peaks. In this work, two challenges in powder diffraction patterns of irradiated Zr alloys are solved: (i) determination of the M values from the long tail regions of peaks has been made unequivocal and (ii) satellites have been fitted separately, using physically well established principles, in order to exclude them from the dislocation determination process. Referring to the theory of heterogeneous dislocation distributions, determination of the total dislocation density from the main peaks free of satellites has been justified. The dislocation loop structure has been characterized by the total dislocation density of loops and the M parameter correlated to the dipole character of dislocation loops. The extended CMWP procedure is applied to determine the total dislocation density, the dipole character of dislocation loops, and the fractions of 〈a〉- and 〈c〉-type loops in proton- or neutron-irradiated polycrystalline Zr alloys used in the nuclear energy industry.

The relationship between slip behavior and dislocation arrangement for large-size Mo-3Nb single crystal at room temperature

The slip behavior and mechanism of large-size Mo-3Nb single crystal have been investigated and disclosed comprehensively at room temperature by quasi-static compression with various strains. With the increase of deformation, the slip traces change from shallow non-uniform slip lines to dense and uniform slip bands. Different slip traces in the same deformation condition were observed, suggesting that the slip traces in the single crystal are controlled by different types and arrangement mechanisms of mobile dislocation. To clarify the relationship between slip behavior and dislocation arrangement, TEM and AFM analyses were performed. Significant discrepancy between the mobility of screw segments and edge segments caused by double cross-slip multiplication mechanism is the reason why different slip features were witnessed. During the whole slip deformation process, screw dislocations play a dominant role and they are inclined to form wall-substructures by interaction and entanglement. With the development of dislocation accumulation, the entangled dislocation walls evolve into dislocation cells with higher stability.

In situ synchrotron diffraction study of a crack-free additively manufactured Ni base superalloy

The tensile deformation behaviour of a crack-free additively manufactured (AM) Ni base superalloy and its wrought (W) counterpart has been investigated by in-situ synchrotron X-ray diffraction. In order to understand the evolution of dislocation character, dislocation density (ρ), coherent scattering domain size (D) and dislocation arrangement parameter (M), the diffraction data corresponding to the first six{hkl} reflections have been analyzed using modified Williamson-Hall and modified Warren-Averbach methodologies. The ρ was found to be relatively higher in AM superalloy as compared to W superalloy during both the regimes of elastic and plastic deformation. In the plastic deformation regime, while the fraction of edge and screw dislocations was found to be nearly equal in proportion in W superalloy, the fraction of screw dislocations remained higher in AM superalloy. In view of the relatively high initial ρ and inefficacy of D in AM superalloy, heat treatment in as-built parts is suggested.

Dislocations arrangements in magnesium matrix composites

Magnesium matrix composites reinforced with TiC particulates was successful synthesized by in-situ synthesis method. The dislocation arrangements in the composites have been investigated. Compared to magnesium alloy, there is high dislocation density in the composites. Besides dislocation network, dislocation pile-ups have been found in α-Mg and β-Mg17Al12 phases. They are confined in the slip planes, and some of them pile up against the grain boundaries. The dislocation arrangements in the composites are due to the addition of TiC particulates. Moreover, the compound twin reveal in the casting composites, which is beneficial to the deformation behavior of the composites.

Dynamic recrystallization and solute precipitation during friction stir assisted incremental forming of AA2024 sheet

Friction stir assisted incremental sheet forming (FS-ISF) has been proved to further improve the formability of aluminum alloy sheets. Under such thermal forming condition, it is quite necessary to explore if the recrystallization and solute precipitation would happen, which might have potential in further improve the strength of aluminum alloy conventionally realized by heat treatment aging. In the present work, FS-ISF experiments are conducted for AA2024 sheet with proper parameters to achieve more solute precipitates and fine recrystallized grains with stable boundaries, of which microstructure evolutions are characterized by Transmission Electron Microscopy (TEM) and Electron Backscattered Diffraction (EBSD). It's found that cyclic and incremental plastic deformation during FS-ISF induces dislocation jogs and dislocation loops, and finally makes solute move into vacancies and generates solute precipitates. And friction stir also contributes to more solute precipitates than conventional incremental sheet forming (CISF) at room temperature. Meanwhile, precipitation zone inside grains helps to resist ductility damage and stable boundary with pinning contributes to higher strength. Besides, irregular sub-boundaries are merged and sub-boundaries consume dislocations to form new boundaries by the slip and arrangement of dislocations during dynamic recrystallization so that fine equiaxed grains with high angle boundaries are obtained, which results in higher yield strength (YS), ultimate tensile strength (UTS) and better ductility than those by CISF. Furthermore, investigations also demonstrate that static recrystallization cannot arise during FS-ISF.

Parallel Dislocation Networks and Cottrell Atmospheres Reduce Thermal Conductivity of PbTe Thermoelectrics

AbstractDislocations play an important role in thermal transport by scattering phonons. Nevertheless, for materials with intrinsically low thermal conductivity, such as thermoelectrics, classical models require exceedingly high numbers of dislocations (>1012 cm–2) to further impede thermal transport. In this work, a significant reduction in thermal conductivity of Na0.025Eu0.03Pb0.945Te is demonstrated at a moderate dislocation density of 1 × 1010 cm–2. Further characteristics of dislocations, including their arrangement, orientation, and local chemistry are shown to be crucial to their phonon‐scattering effect and are characterized by correlative microscopy techniques. Electron channeling contrast imaging reveals a uniform distribution of dislocations within individual grains, with parallel lines along four <111> directions. Transmission electron microscopy (TEM) shows the parallel networks are edge‐type and share the same Burgers vectors within each group. Atom probe tomography reveals the enrichment of dopant Na at dislocation cores, forming Cottrell atmospheres. The dislocation network is demonstrated to be stable during in situ heating in the TEM. Using the Callaway transport model, it is demonstrated that both parallel arrangement of dislocations and Cottrell atmospheres make dislocations more efficient in phonon scattering. These two mechanisms provide new avenues to lower the thermal conductivity in materials for thermal‐insulating applications.

Donor and acceptor-like self-doping by mechanically induced dislocations in bulk TiO2

Dislocations have been recently introduced as a novel tool to tailor the conductivity of functional ceramics. However, tuning strategies suffer from poor insight into the structural complexity of dislocations and their networks. Here, we demonstrate that dislocations can be used to both enhance and reduce the overall conductivity in the same ceramic material. Accurate control of the arrangement of dislocations within the dislocation network enables tailoring TiO2 bulk samples to behave like being chemically modified either with an acceptor or donor dopant. Our approach combines ultra-high voltage electron microscopy, oxygen partial pressure, and temperature dependent electrical conductivity measurements combined with time-of-flight secondary ion mass spectrometry. This allows us to focus on mechanically tailored interaction of next neighbor dislocations and to differentiate between percolating conductive pathways and separated charge carrier zones. This seemingly simple approach purposefully tailors the conductivity of TiO2, opening new avenues to engineer functional ceramics beyond common chemical doping strategies.

Large plastic deformability of bulk ferroelectric KNbO3 single crystals

Ferroelectrics have become indispensable electroceramic materials in modern electronic applications. While point-defect engineering is a common tool to modify the properties of ferroelectrics, dislocations have not been widely considered, although they were previously shown to alter electrical properties in other oxides. Here, we study the room temperature uniaxial deformation of the perovskite ferroelectric single crystalline KNbO3 in 010pc and 101pc orientations. We demonstrate a large plastic deformation in the range of 5%. Dislocation and domain structure were investigated across different length scales using optical microscopy, electron channeling contrast imaging, and transmission electron microscopy. Planar arrangement of dislocations in slip bands was observed, while the predominant appearance of edge dislocations indicates a dominant dislocation motion by the kink-pair mechanism. Increased dislocation density resulted in decreased ferroelectric domain size from 1.8 μm to 0.5 μm after deformation. Piezoresponse force microscopy highlights the effect of dislocations on domain nucleation and their mobility.

Atom scale study of cracks in the carbide phase of WC–Co alloys

ABSTRACT In this study, cracks propagating in WC grains are investigated at the atom scale along the [0001] axis. A brittle crack, characterised by cleavage along {1, 0, −1, 0} prismatic facets and the absence of dislocations is observed. The crack follows a zig-zag path allowing a higher deformation energy to be absorbed by the crystal. A ductile crack is also characterised. The plastic zone is limited and contains mainly multipoles consisting in unextended dislocations. The crack path is tortuous and delineated by dislocation arrangements sometimes involving extended dislocations. Close to the crack tip, the fracture is discontinuous and nanocracks occur at the level of dislocation arrangements. These dislocation configurations could provide crack nucleation sites for the crack propagation.

Quantitative Analysis of the Recovery Process in Pure Iron Using X-ray Diffraction Line Profile Analysis.

We conducted quantitative analysis of the recovery process during pure iron annealing using the modified Williamson-Hall and Warren-Averbach methods. We prepared four types of specimens with different dislocation substructures. By increasing the annealing temperature, we confirmed a decrease in dislocation density. In particular, screw-dislocation density substantially decreased in the early stage of the recovery process, while edge-dislocation density gradually decreased as annealing temperature increased. Moreover, changes in hardness during the recovery process mainly depended on edge-dislocation density. Increases in annealing temperature weakly affected the dislocation arrangement parameter and crystallite size. Recovery-process modeling demonstrated that the decrease in screw-dislocation density during the recovery process was mainly dominated by glide and/or cross-slip with dislocation core diffusion. In contrast, the decrease in edge-dislocation density during the recovery process was governed by a climbing motion with both dislocation core diffusion and lattice self-diffusion. From the above results, we succeeded in quantitatively distinguishing between edge- and screw-dislocation density during the recovery process, which are difficult to distinguish using transmission electron microscope and electron backscatter diffraction.