Dislocation Arrangements Research Articles

{[formula omitted]} twin–twin intersection-induced lattice rotation and dynamic recrystallization in Mg–6Al–3Sn–2Zn alloy

This study investigated the formation mechanism of new grains due to twin–twin intersections in a coarse-grained Mg–6Al–3Sn–2Zn alloy during different strain rates of an isothermal compression. The results of electron backscattered diffraction investigations showed that the activated twins were primarily {101¯2} tension twins, and 60° <101¯0> boundaries formed due to twin–twin intersections under different strain rates. Isolated twin variants with 60° <101¯0> boundaries transformed into new grains through lattice rotations at a low strain rate (0.01 s−1). At a high strain rate (10 s−1), the regions surrounded by subgrain boundaries through high-density dislocation arrangement and the 60° <101¯0> boundaries transformed into new grains via dynamic recrystallization.

Low cycle fatigue behaviour study of a nano precipitate strengthened ferrite-bainite 780 steel

Low-cycle fatigue behaviour of a newly developed ferrite-bainite 780 steel with the ferrite phase, strengthened by nanosized precipitates was investigated. The LCF tests were conducted in fully reversed strain-controlled mode to determine the cyclic stress response and strain life. The steel showed predominantly cyclic softening behaviour at lower strain amplitudes (up to 0.25%). However, at higher strain amplitudes (≥ 0.35%), cyclic hardening was observed with increase in the number of loading cycles. EBSD study done on the fatigue tested samples revealed grain fragmentation and texture weakening during cyclic deformation. TEM study elucidated the evolution of dislocation structure responsible for cyclic softening/hardening behaviour during cyclic deformation. At lower strain amplitude (0.2%), concurrent presence of random arrangement of dislocations and dislocation tangles were observed with some grains having weakly developed persistent slip bands (PSBs) and veins. At high strain amplitude (0.55%), subgrain structure was found to be the dominant feature along with occurrence of PSBs and veins at few locations. Further to this the role of nano-precipitates in influencing the fatigue behaviour was also investigated.

Effects of Borehole Arrangement on Methane Migration and Implications for High Efficiency Extraction in Intact and Tectonic Combined Coal Seams

Gas extraction by bedding boreholes is a key means to realize safe mining of coal mine and protect the environment. The traditional bedding boreholes are usually constructed in the center of the coal seam, which may not obtain a high methane extraction rate because of the existence of tectonic coal sublayer with low permeability. In this paper, the gas migration characteristics of combined coal seams composed of tectonic and intact coal sublayers are investigated, and the efficient gas extraction method is further explored. The results indicate that only 2.676 kg/m3 of gas can be extracted under the traditional bedding borehole arrangement in 300 days. The tectonic coal sublayer with low permeability restricts the gas in the intact coal sublayers to flow to the extraction boreholes, which is the main reason for the low gas extraction rate. By arranging boreholes up and down alternately with a vertical spacing of 2.74 m, the maximum gas amount of 3.526 kg/m3 can be extracted, showing an increased rate of 31.8%. The optimal vertical spacing between adjacent boreholes (VSAB) raises with the relatively increasing thickness or decreasing permeability of the tectonic coal sublayer. The optimal VSAB has a linear relationship with the thickness ratio of the tectonic coal sublayer to the combined coal seams. The optimal VSAB can be obtained referring to the numerical relationship, and then, the extraction boreholes can be dilled to achieve efficient gas extraction of combined coal seams. In engineering, the dislocation arrangement of borehole can greatly improve the gas extraction efficiency and coal mine safety.

Evolution of dislocation structure determined by neutron diffraction line profile analysis during tensile deformation in quenched and tempered martensitic steels

The role of the dislocation structure on the work-hardening behavior during the tensile deformation of quenched and tempered martensite was studied. The evolution of the dislocation structure during tensile deformation at room temperature in ultralow-carbon 18 mass%Ni martensitic steels under the conditions of as-quenched by sub-zero-treatment (SZ) and quenched-and-tempered at 573 and 773 K (T573 and T773, respectively) was monitored using in situ time-of-flight neutron diffraction combined with the convolutional multiple whole profile (CMWP) procedure. The changes in the dislocation parameters due to tempering and deformation obtained by the CMWP procedure were explained by the metallurgical phenomena of body-centered cubic iron. The elastic limit increased in the order SZ, T573, and T773, whereas the dislocation density decreased in the opposite order, indicating that the elastic limit is not always dependent on the total dislocation density of martensite. The dislocation density in SZ, which showed a high level of work hardening after yielding, hardly changed during tensile deformation, whereas that in T573 and T773 increased with tensile straining. The dislocation arrangement parameter that represents the interaction among the dislocations was high before deformation and decreased during deformation in materials SZ and T573, whereas the parameter was maintained at a low value during the entire deformation in material T773. Large and small values of the dislocation arrangement parameter indicate weak and strong interactions, respectively. The dislocation arrangement parameter is considered helpful for estimating the increment in strength of martensitic steels by dislocation strengthening, as the coefficient α in Taylor’s equation, for both the as-quenched and tempered conditions.

Microstructure characterization and damage coupled constitutive modeling of nickel-based single-crystal alloy with different orientations

The mechanical properties of nickel-based single-crystal superalloys are sensitive to temperature and orientation. The present work focuses on the stress and strain responses of a second-generation nickel-based superalloy at different temperatures and with different orientations. Microstructure and dislocation arrangements are observed by scanning and transmission electron microscopy of samples after tensile testing. The results reveal the tensile fracture failure modes and dislocation morphologies (braids, cells, stacking faults, etc.) for different temperatures and orientations. Although the [001] sample shows higher yield strength than the other two orientations, the strain hardening was more significant at [111] orientation and stress softening at [011] orientation due to their different dislocation mechanisms. In addition, a new hardening model and damage evolution law are proposed and coupled with a crystal plastic constitutive model. Numerical analysis results show that the proposed constitutive model can accurately fit the full stress-strain response from tensile experiments for different temperatures and orientations.

Elevated-temperature cyclic deformation mechanisms of CoCrNi in comparison to CoCrFeMnNi

We report the cyclic deformation behavior of CoCrNi at 550 °C under a strain amplitude of ± 0.5% and compare it to that of CoCrFeMnNi. CoCrNi manifests cyclic hardening followed by minor softening and a near-steady state until failure. Transmission electron microscopy investigations of CoCrNi revealed that increasing the number of cycles from 10 to 2500/5000 leads to a transition of dislocation arrangements from slip bands to tangles. Compared to CoCrFeMnNi, CoCrNi exhibits higher strength, longer lifetime and persistent serrated flow. Owing to its lower stacking fault energy (even at 550 °C), planar slip is more pronounced in CoCrNi than CoCrFeMnNi, which additionally shows wavy slip.

Characterization of the Isothermal and Thermomechanical Fatigue Behavior of a Duplex Steel Considering the Alloy Microstructure

Isothermal and thermomechanical fatigue behavior of duplex stainless steel (DSS) X2CrNiMoN22-5-3 was investigated. The aim of this work was to understand the fatigue behavior by correlation of the isothermal and thermomechanical fatigue behavior with microstructural observations. Fatigue tests at plastic-strain-amplitude of 0.2% were carried out at 20, 300 and 600 °C, while in-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) experiments were performed between 300 and 600 °C. During the 20 °C fatigue test, a continuous softening was observed. Transmission electron microscopy examinations reveal pronounced planar slip behavior in austenite. At 300 °C, deformation concentrates in the ferrite, where strong interactions between CrxN and dislocations were observed that explain the pronounced cyclic hardening. DSS studied exhibits softening throughout the whole isothermal fatigue test at 600 °C. In ferrite, during the 600 °C fatigue test, the G phase, γ′ austenite precipitated, and an unordered dislocation arrangement was observed. The stress responses of the TMF tests can be correlated to those of the isothermal fatigue tests. In IP mode, a positive mean stress resulted in premature failure. No γ′ austenite but the formation of subgrains in the ferrite phase was observed after TMF tests. The plastic deformation of the austenite at high temperatures results in an unordered dislocation arrangement.

Effect of grain substructure refinement on tensile mechanical behavior of L21-strengthened Al-Cr-Fe-Ni-Ti-Mo high-entropy alloy

Generally, BCC-based high-entropy alloys containing L21-type precipitates show outstanding creep resistance at high temperatures. However, their weak grain boundary cohesion results in poor room-temperature ductility and limits their industrial applications. This study achieves room-temperature ductility in L21-strengthened Al10Cr13.3Fe59.5Ni11.2Ti4Mo2 alloy through grain substructure refinement with a suitable thermomechanical treatment. We found that the fine and homogeneously distributed L21 precipitates act as effective pinning sites for dislocation movement during hot-rolling, thereby forming a subgrain structure that is three orders of magnitude smaller than that of homogenized alloys. Further, short-term annealing after rolling relieves the internal strain energy by transforming the subgrain boundary structure with energetically favorable dislocation arrangement. Consequently, the L21-strengthened alloy with a refined substructure exhibits a greatly increased plastic strain (∼6%) owing to the reduced microcrack length, and detoured crack propagation around precipitates.

Arrangement and Decomposition of Grain Boundary Dislocations: Two-Mode Phase-Field Crystal Simulation

The grain-boundary dislocation arrangement and decomposition during constant-volume deformation of a nanoscale bi-crystal system in fcc-structured materials were studied by using the two-mode phase-field crystal (2PFC) method. The effects of different grain boundary misorientations (GBMs) and tensile deformation directions on the dislocation arrangement and decomposition are analyzed. In three different symmetrical tilt grain boundaries evaluated by PFC, the atomic density profile of grain boundaries changed periodically at equilibrium. The initial grain boundary dislocation arrangement of the three samples is almost the same when tensile deformation is applied to the samples in the x- or y- direction, and all are symmetrically arranged in a “bowknot ”structure. The stress at the grain boundary is concentrated with the increase of strain, and dislocation decomposition can effectively reduce the stress concentration. The time steps of dislocation decomposition at grain boundaries decreases with increasing strain rate. This work facilitates the application of PFC in the analysis of grain-boundary mechanics in an extended range of materials.

The Effect of Temperature and Phase Shift on the Thermomechanical Fatigue of Nickel-Based Superalloy

In this paper, the minimum temperature and phase shift effects on the thermo–mechanical fatigue (TMF) behavior of Inconel 713LC are investigated. TMF tests were performed under 0° (in-phase-IP) and +180° (out-of-phase-OP) phase shifts between mechanical strain and temperature. Cylindrical specimens were cycled at constant mechanical strain amplitude with a strain ratio of Rε = −1. Tests were performed with temperature ranges of 300–900 °C and 500–900 °C. The heating and cooling rate was 5 °C/s. Fatigue hardening/softening curves and fatigue life data were assessed. Results show that out-of-phase loading was less damaging than in-phase loading. Scanning electron microscopy (SEM) examination of metallographic sections indicated that the life-reducing damage mechanism was intergranular cavitation under in-phase loading. Transmission electron microscopy (TEM) revealed honeycomb structures for IP loading. The plastic strain localization into persistent slip bands was typical for OP loading. For out-of-phase loading, fatigue damage appeared to be dominant. The increase in the temperature range led to a significant decrease in fatigue life. The reduction of fatigue life was far more pronounced for out-of-phase loading. This can be ascribed to the accelerated crack propagation at high tensile stress under out-of-phase loading as well as the amount of accommodated plastic strain deformation. Based on the SEM scrutiny of metallographic sections and TEM observations of dislocation arrangement, the prevailing damage mechanisms were documented and the lifetime behavior was accordingly discussed.

Impact of Dislocation Density and Mobility on Yielding Behavior in Quenched Medium-carbon Martensitic Steel Tempered at Low Temperature

Middle-carbon martensite steels are vital materials for mechanical components and their mechanical properties have attracted significant interest. However, the decrease in the elastic limit of the as-quenched materials is one of the remaining puzzles. Herein, we quantitatively characterized the dislocation density and its structure in the as-quenched and tempered martensite steel by neutron diffraction line profile analysis and discussed their impact on the yield stress. The dislocation density in the as-quenched specimen was the highest at 9.7 × 1015 m−2, while it decreased with an increase in the tempering temperature. In addition, the component ratios of edge and screw dislocations decreased and increased, respectively, depending on the increase in the tempering temperature. The dislocation arrangement parameter (M) varied between the tempering temperatures of 220 and 290°C. Although there was a large difference between the yield stress obtained from the tensile test and that estimated from the dislocation density, the experimental results could be explained by correcting them with the inverse of M value as an index showing the effective dislocation density ratio.

Impact characteristic and crack propagation mechanisms of large-size molybdenum alloy single crystal and polycrystal

The impact characteristic and crack propagation mechanism for large-size Mo3Nb single crystal with a diameter of about 30 mm have been investigated and disclosed comprehensively by Charpy impact test. The fracture analysis shows that the single crystal crack propagates radially, resulting in a multi-step distribution on the fracture surface. The fracture of Mo3Nb polycrystal is composed of cleavage and quasi-cleavage, exhibiting a trans-granular fracture feature. The impact toughness of single crystal shows orientation dependence, which is attributed to the crack propagation path. Compared with the straight path, the waved path can delay the crack propagation and absorb higher impact energy. The impact toughness of Mo3Nb single crystals is lower than that of Mo3Nb polycrystal at room temperature because the single crystal lacks the coordinated deformation of grain boundary. The analysis of dislocation arrangement for Mo3Nb single crystal shows the different crack propagation paths is caused by the mobility of screw segments, which are controlled by the double cross-slip multiplication mechanism. However, the waved fracture morphology of Mo3Nb polycrystal is caused by the effect of grain boundaries instead of the cross-slip of screw dislocation.

Data-driven analysis of neutron diffraction line profiles: application to plastically deformed Ta

Non-destructive evaluation of plastically deformed metals, particularly diffraction line profile analysis (DLPA), is valuable both to estimate dislocation densities and arrangements and to validate microstructure-aware constitutive models. To date, the interpretation of whole line diffraction profiles relies on the use of semi-analytical models such as the extended convolutional multiple whole profile (eCMWP) method. This study introduces and validates two data-driven DLPA models to extract dislocation densities from experimentally gathered whole line diffraction profiles. Using two distinct virtual diffraction models accounting for both strain and instrument induced broadening, a database of virtual diffraction whole line profiles of Ta single crystals is generated using discrete dislocation dynamics. The databases are mined to create Gaussian process regression-based surrogate models, allowing dislocation densities to be extracted from experimental profiles. The method is validated against 11 experimentally gathered whole line diffraction profiles from plastically deformed Ta polycrystals. The newly proposed model predicts dislocation densities consistent with estimates from eCMWP. Advantageously, this data driven LPA model can distinguish broadening originating from the instrument and from the dislocation content even at low dislocation densities. Finally, the data-driven model is used to explore the effect of heterogeneous dislocation densities in microstructures containing grains, which may lead to more accurate data-driven predictions of dislocation density in plastically deformed polycrystals.

Semi-Automated, Object-Based Tomography of Dislocation Structures.

The characterization of the three-dimensional arrangement of dislocations is important for many analyses in materials science. Dislocation tomography in transmission electron microscopy is conventionally accomplished through intensity-based reconstruction algorithms. Although such methods work successfully, a disadvantage is that they require many images to be collected over a large tilt range. Here, we present an alternative, semi-automated object-based approach that reduces the data collection requirements by drawing on the prior knowledge that dislocations are line objects. Our approach consists of three steps: (1) initial extraction of dislocation line objects from the individual frames, (2) alignment and matching of these objects across the frames in the tilt series, and (3) tomographic reconstruction to determine the full three-dimensional configuration of the dislocations. Drawing on innovations in graph theory, we employ a node-line segment representation for the dislocation lines and a novel arc-length mapping scheme to relate the dislocations to each other across the images in the tilt series. We demonstrate the method for a dataset collected from a dislocation network imaged by diffraction-contrast scanning transmission electron microscopy. Based on these results and a detailed uncertainty analysis for the algorithm, we discuss opportunities for optimizing data collection and further automating the method.

The Role of Mandrel Rotation during CSET Processing Demonstrated on a 3003 Aluminium Alloy

Recently, the complex shearing of extruded tube (CSET) technique was proposed representing a new way of processing tubes from a bulk billet as a combination of extrusion with two passes of equal channel angular processing and possible mandrel rotation. In the present paper, the influence of mandrel rotation on the final microstructure and mechanical properties of the tube fabricated of a 3003 aluminium alloy was investigated. Electron back scatter diffraction (EBSD) revealed differences in grain size and misorientation angles between tubes processed with stationary and rotating mandrel. Kernel average misorientation maps obtained from EBSD experiments and ASTAR analysis performed in transmission electron microscopy proved differences in the dislocation density and arrangement. The tube processed using a stationary mandrel showed a recovered microstructure with elongated grains separated by low-angle grain boundaries into subgrains. The microstructure of the tube processed with a rotating mandrel was dynamically recrystallized with the grain size in submicrometer range. Vickers microhardness measurements revealed only a 40% HV increase in the sample prepared using stationary mandrel as compared with the initial billet. The mandrel rotation resulted in a much higher HV increase up to 200% as a result of substantial grain refinement.

On the dislocation mechanisms of dynamic strain ageing in fatigued plain carbon steels

Abstract The fatigue life of plain ferritic carbon steels fatigued in stress-controlled (plastic-strain-controlled) tests exhibits a maximum (minimum) at a temperature of approx. 300 °C as a result of dynamic strain ageing (DSA). In the regime of DSA, an enhanced cyclic hardening related to an increased dislocation density is observed, and the dislocation arrangement changes from a dislocation cell structure (at intermediate stress amplitudes) at room temperature to a dense edge di-/multipolar bundle/wall/vein structure in the temperature range of DSA and back to a dislocation cell/ subgrain structure at higher temperatures. Related effects occur also in α-iron containing some carbon when fatigued at room temperature at a very low strain rate. This behaviour is considered to be typical of body-centred cubic materials. In the present study, the dislocation mechanisms responsible for this behaviour are discussed in detail, and a dislocation glide model is developed which can explain both the enhanced dislocation density and the change of the dislocation arrangement in the regime of DSA. The model is based on the different mobilities of mobile edge and less mobile screw dislocations in body-centred cubic metals and on the strong tendency towards multiple slip leading to a dislocation cell structure at lower temperatures. In the regime of DSA, it is proposed that the edge dislocations are hindered more strongly in their glide motion by the solute carbon atoms than the screw dislocations so that screws and edges are then more or less equally mobile. Taking into account that screw dislocations of opposite sign annihilate mutually by cross slip, this explains the observed dislocation arrangement of dense edge di-/multipolar bundles/ walls/veins of high dislocation density which, in turn, leads to an enhanced cyclic hardening rate. The dislocation sub-grain structure observed at higher temperatures reflects the increasing tendency for the transformation from dislocation cell to subgrain structures, as the temperature is increased.

Experimental study on dressing CBN grinding wheel with orderly arrangement of abrasive sintered Roller

In order to improve the CBN grinding wheel in the grinding process of wear problem, adopt the method of soldering preparation and orderly arrangement of sintered diamond wheel on the nap, analyzes the roller surface grinding grain of dislocation in the matrix, and the phyllotaxis configuration mode of the grinding wheel dressing effect, based on the workpiece roundness, after dressing grinding wheel grinding workpiece surface quality and the microhardness of the data analysis, choose the optimal configuration. The results show that the workpiece roundness of matrix arrangement is slightly better than that of dislocation and leaf order arrangement. The surface roughness and 3D surface topography of the workpiece under leaf sequence arrangement are better than those under matrix and dislocation arrangement. The surface microhardness of the workpiece has no obvious difference under the three arrangement modes. Comprehensive analysis shows that the dressing effect of the roller under the arrangement of abrasive grain leaf sequence is the best.

Prediction of vacancy formation energies at tungsten grain boundaries from local structure via machine learning method

Grain boundary (GB) plays a crucial role in the mechanical properties and irradiation resistance of nuclear materials. It is thus essential to understand and predict the defect properties near GBs. Here, we present a framework for predicting vacancy formation energy (EVf) near GBs in tungsten (W) by machine learning (ML) technique. The EVf values of 4496 atomic sites near 46 types of [001] symmetry tilt GB (STGB) in W are calculated as database and eight appropriate variables are selected to characterizing the surrounding atomic configuration and location of atomic sites. Via the support vector machine with the radial basis kernel function (RBF-SVM), the good predicted results of cross validation (CV) and generalized verification prove the suitability and effectiveness of the selected variables and RBF-SVM method. Beside, due to their big differences in dislocation arrangement and atomic configuration, the STGBs need to be divided into three types, high angle, low angle-I and low angle-II STGBs, for adopting the Separate CV, and their predicted accuracies were found to have big improvements. Because the present method adopts geometrical factors, such as spatial size characteristic, density and location, as descriptors for the ML analysis, it is robust and general to other materials such as α-Fe, and beneficial to predict and understand the vacancy formation near interfaces.

Dislocation structures in a Ti–6Al–4V alloy subjected to cyclic tensile deformation

The present work investigates dislocation structures developed within both the α and β phase of a Ti–6Al–4V alloy undergoing cyclic tensile straining. The cyclic loading-unloading was performed using the stress levels of 830 and 580 MPa (R∼0.7) at a strain rate of 2 × 10-3 s-1 to 15125 cycles, as part of a synchrotron in-situ experiment. The macroscopic yield stress was about 950 MPa. The dislocation arrangement, type and density were studied post-mortem employing transmission electron microscopy, together with the automatic determination of crystal lattice orientations via precession nanobeam diffraction. The deformation took place exclusively through dislocation slip and no presence of deformation twins was detected within either of the studied phases. The α phase dislocation structure displayed some similarity to that reported for monotonic deformation and no evidence of the formation of regular planar dislocation networks, proposed in the literature, has been found. The prevalent deformation modes were prismatic and basal glide, based on the experimentally determined Burgers vectors together with the global Schmid factor and reported critical resolved shear stress values. The <a>-type dislocations observed after deformation generally possessed a large screw component, with most of them being of a pure screw type. There were also some <c+a> dislocations present within the α phase and these typically displayed a mixed character. Dislocation Burgers vectors present after straining within the β phase frequently deviated from the Schmid factor rule and these dislocations were often wavy and largely possessed a significant screw component.

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.