Boundary Misorientation Research Articles

A first principles study of zirconium grain boundaries

We present the results of first-principles calculations of selected structural and thermodynamic properties of a set of grain boundaries (GBs) in zirconium, spanning a range of misorientation angles and boundary planes. We performed plane-wave density functional theory calculations on low-Σ grain boundaries — five symmetric tilt GBs (STGBs) and three twist GBs; all with misorientation axes about [0 0 0 1] and in optimised microscopic configurations — to gain insight into the associated atomistic structures. We include in our analysis properties such as GB excess volume, interplanar spacing, volume per atom, electron density distribution, and a measure of GB width. We found the twist GBs to exhibit similar energetic and structural properties, whereas the STGBs showed substantially more variation. Our comprehensive analysis demonstrates how all five dimensions of GB space are crucial in determining properties such as the work of ideal separation and the length scale over which atoms are perturbed by the presence of the GB. Additionally, we compared our first-principles results to predictions from a widely used embedded-atom method (EAM) potential; we found the potential to perform generally well, particularly in predicting GB energy. We discuss the results in terms of representing the initial steps towards the development of a mechanistic understanding of the pellet-cladding interaction in nuclear fuel, which must involve encoding accurate materials behaviour at larger length and time scales. To this end, and so that our results can be useful for further investigations, we have published our data (including optimised structures, computed interface energetics and structural properties) to a public repository.

Orientation behavior in TRC-ZA21 magnesium alloy with fine grain size during nano-indentation process

The deformation behavior in magnesium alloy was greatly affected by grain orientation. It is crucial to analyze the relationship between deformation mechanism and grain orientation. In this study, the deformation behaviors in grains possessing different orientations for TRC-ZA21 alloy with fine grain size were investigated using the nano-indentation deformation coupling EBSD method. The effects of grain orientation and grain boundary on deformation behavior and propagation of deformation mechanism were discussed. The results illustrated that the irregular performance of the nano-mechanical properties could be attributed the inconsistency of grain orientation as well as the grain boundary misorientation angle. Grain orientation, followed by the criterion of Schmid factor, regulated the deformation mechanism of grains that were in direct contact with the nano-indenter. This signified the orientation dependence in plastic deformation. The deformation behavior in grains, induced by plastic deformation and indirectly in contact with the nano-indenter, was found to be influenced by the grain boundary. The orientation difference in adjacent grains determines the influence of grain boundary in plastic deformation, revealing the competitive and coordinative behavior associated with these deformation mechanisms. The c-axes angle between adjacent grains was the key factor to determine the influence of grain boundary on deformation propagation.

Insight into the impact of microstructure on crack initiation/propagation behavior in carbide-free bainitic steel during tensile deformation

The impact of microstructural characteristics on the crack initiation/propagation behavior of carbide-free bainitic steel was investigated during austempering at 320 and 280 °C. The primary mechanism for causing damage to the material was via of the formation of micro-voids and cracks. The results demonstrated that an uneven distribution of carbon content in the blocky martensite/retained austenite (M/RA) led to decreased carbon content at grain interiors. This caused splitting and decohesion of both blocky M/RA and M/RA-BF phase boundaries. During tests at 280 °C, the blocky M/RA content was decreased, and the microstructure refined. The selection and arrangement of bainite variants also affected the deformation compatibility of bainite ferrite (BF), M/RA and the density of high-angle boundaries (HAGBs). As the covariant boundary fraction of the CP (CP1) variant (V1/V2 – V6) in bainite was much higher than that measured in other covariant boundaries, the ability of covariant deformation between the two phases was greater, and the delayed initiation of micropores was observed. The effect of packet boundaries and block boundaries for hindering crack propagation was different, the most effective barrier for crack propagation was high-angle misorientation boundaries between packet boundaries.

Extended Shear Deformation of the Immiscible Cu-Nb Alloy Resulting in Nanostructuring and Oxygen Ingress with Enhancement in Mechanical Properties.

Deformation processing of immiscible systems is observed to disrupt thermodynamic equilibrium, often resulting in nonequilibrium microstructures. The microstructural changes including nanostructuring, hierarchical distribution of phases, localized solute supersaturation, and oxygen ingress result from high-strain extended deformation, causing a significant change in mechanical properties. Because of the dynamic evolution of the material under large strain shear load, a detailed understanding of the transformation pathway has not been established. Additionally, the influence of these microstructural changes on mechanical properties is also not well characterized. Here, an immiscible Cu-4 at. % Nb alloy is subjected to a high-strain shear deformation (∼200); the deformation-induced changes in the morphology, crystal structure, and composition of Cu and Nb phases as a function of total strain are characterized using transmission electron microscopy and atom probe tomography. Furthermore, a multimodal experiment-guided computational approach is used to depict the initiation of deformation by an increase in misorientation boundaries by crystal plasticity-based grain misorientation modeling (strain ∼0.6). Then, co-deformation and nanolamination of Cu and Nb are envisaged by a finite element method-based computational fluid dynamic model with strain ranging from 10 to 200. Finally, the experimentally observed amorphization of the severely sheared supersaturated Cu–Nb–O phase was validated using the first principle-based simulation using density functional theory while highlighting the influence of oxygen ingress during deformation. Furthermore, the nanocrystalline microstructure shows a >2-fold increase in hardness and compressive yield strength of the alloy, elucidating the potential of deformation processing to obtain high-strength low-alloyed metals. Our approach presents a step-by-step evolution of a microstructure in an immiscible alloy undergoing severe shear deformation, which is broadly applicable to materials processing based on friction stir, extrusion, rolling, and surface shear deformation under wear and can be directly applied to understanding material behavior during these processes.

The initial corrosion behavior of Al–Cu–Li bicrystals: effect of misorientation and precipitates

Al–Cu–Li bicrystals with different misorientation gradients were prepared to study the effect of misorientation and precipitates on the initial corrosion behavior of the Al–Cu–Li alloy by scanning electron microscopy (SEM), transmission electron microscopy (TEM), immersion testing, and electrochemical testing. The results show that the initial corrosions were generated on the matrix rather than on the grain boundary for the as-quenched alloy. The effect of grain boundary misorientation on the corrosion rates of the as-quenched alloy is very limited. On the other hand, for the aged alloy, the initial corrosions were generated on the grain boundaries, which suffer more severe corrosion than the matrix. Meanwhile, the corrosion of high angle grain boundary (HAGB) is more severe than that of low angle grain boundary (LAGB) for the aged alloy. These results are associated with the high number density of the continuously distributed precipitates on grain boundaries.

Manipulate organic crystal morphology and charge transport

Organic electronics has become one of the fastest developing fields with exciting advances in discovering novel organic semiconductors, enhancing charge carrier mobility, and exploring high-performance electronic device applications. Nevertheless, the solution-induced growth of small molecular organic semiconductors provokes predominant crystal misorientation, thermal cracks and grain boundary, which poses a tough challenge to maneuver the crystal alignment towards device fabrication. In this article, we conduct an in-depth review of external force-based, additive-based, and binary solvent-based techniques to effectively control the organic semiconductor crystallization, thin-film morphology, and crystal orientations. We first introduce the recent progress and the various challenges of flexible electronics, and discuss how these issues are correlated to charge carrier mobility variations. Next, we discuss the miscellaneous external force alignment methods based on the categories of slot-die coating, blade coating, substrate patterning, and air flow. Furthermore, we investigate the additive-based methods for controlling crystal growth and orientations, including polymeric additives, small molecular additives, and nanostructured additives. Finally, we discuss the binary solvent-based methods to control the crystal morphology and dimensionality. By correlating the charge transport characteristics with the intrinsic merits of those unique methods, this article shares a useful insight into maximizing the performance of organic semiconductors from a new perspective.

Electron Backscatter Diffraction (EBSD) Analysis of Machinable Lead-Free Brass Alloys: Connecting Texture with Fracture

The current paper is related to the study of the microstructure and texture of two machinable lead-free brass alloys, namely CuZn42 (CW510L) and CuZn38As (CW511L), which were evaluated in the as-drawn and post heat treated condition. Electron backscatter diffraction (EBSD) was employed for the examination of the brass rods’ crystallographic properties in order to correlate the effect of post processing heat treatment on the evolution of phase structure and texture towards the interpretation of dynamic (impact) fracture properties. It is shown that α- and β-phase volume fractions, mean grain size, and grain boundary misorientation are the most influential factors altering the fracture resistance of single- and dual-phase brass alloy rods. The role of grain boundary engineering, through the formation of coincidence site lattice (CSL) boundaries and their evolution during thermomechanical processing, is of major importance for the design of the mechanical behaviour of new eco-friendly machinable brass alloys.

Crystal plasticity finite element analysis of size effect on the formability of ultra-thin ferritic stainless steel sheet for fuel cell bipolar plate

The present study investigated the size effect on the formability of ultra-thin metallic bipolar plate for proton exchange membrane (PEM) fuel cell by crystal plasticity finite element method (CPFEM) in conjunction with the Marciniak-Kuczynski (MK) model. Mechanical behavior and crystallographic texture of a 0.08 mm-thick ferritic stainless steel (FSS) sheet were characterized by uniaxial tensile test and electron backscatter diffraction (EBSD) measurement. Nakazima formability test was carried out to measure the forming limit diagram (FLD) of ultra-thin FSS sheet. A hybrid cellular automata–Monte Carlo (CA–MC) model was developed to generate a ‘realistic’ representative volume element (RVE) that accurately reconstructed the measured texture and grain boundary misorientation distribution (GBMD). The predicted FLD by the CPFEM–MK model with the realistic RVE shows a good agreement with the experimental results. In order to explore the size effect on the formability, the forming limit analyses were performed using RVEs with various thickness-to-grain size ratios (t/d = 2∼10). The results reveal a significant degradation of the formability of the ultra-thin FSS sheet as t/d decreases. With decreasing number of grains through the thickness, the stress and strain heterogeneities in the surface grains are noticeably increased due to the less constraint by the subsurface grains, which plays an important role in the size effect. Furthermore, it is found that the surface strain hot spots in the material with low t/d can act as the geometrical imperfection to accelerate the failure, together with the increased stress triaxiality in the surface grains as the results of the localized strains and premature necking during the deformation. This attributes the decrease of forming limit strains to the early plastic flow instability in the ultra-thin sheet material with low thickness-to-grain size ratio.

Study of the Effect of Grain-Boundary Misorientation on Slip Transfer in Magnesium Alloy Using a Misorientation Distribution Map

The microstructure evolution of a Mg–Gd–Y alloy was studied using uniaxial tension combined with an electron backscatter diffraction technique. The results show that large amounts of slip transfer phenomena can be observed around the grain–boundary area after tension, and the activation of these slips depends largely on the misorientation of grain boundaries. The Mg–Gd–Y alloy shows almost randomized grain–boundary misorientation, but transferred slip traces were preferred at boundaries with misorientation around the [0001] axis between 0–30°. Theoretically, materials with a higher fraction of slip transfer at the grain–boundary area would improve the ductility. Upon comparing the two groups of magnesium alloy with different grain–boundary misorientation distributions, the one with more grain boundaries favored for slip transfer achieved higher elongation during a tension test. Therefore, in addition to weakening the texture, adjusting the misorientation of the grain boundaries appears to be a new method to improve the ductility of magnesium alloys.

Achieving Excellent Strength‐Ductility Balance and Lower Yield Ratio in a 690 MPa‐Grade Multiphase Construction Steel

In this article, thermomechanical controlled processing (TMCP) plus tempering (T) is adopted to acquire a novel 690 MPa‐grade high‐strength low‐carbon bainitic construction structural steel, which has an optimum balance of strength‐ductility and lower yield ratio (YR). The results show that the as‐rolled steel consists of bainitic ferrite, lath‐like bainite, and granular bainite. With an increase in the tempering temperature, a great deal of carbonitrides precipitates and maintains a small nano‐scaled size without coarsening. Their interaction with dislocations induces an increase in strength. When the tempering temperature further increases, some grain boundaries between adjacent bainitic laths gradually blur or even disappear, and the average width of the bainitic laths increases to ≈860 nm. The recovery of bainitic laths is the main reason for the decreased strength. Due to the synergistic effect of the small dot‐shaped martensite/austenite (M/A) constituents and grain boundaries misorientation distribution, maximum impact absorbed energy is achieved after tempered at 500 °C. Furthermore, the lower YR is achieved by appropriate control of strength difference between the soft bainitic ferrite and hard M/A constituents via tempering.

In-situ investigation of deformation behavior in additively manufactured FeCoCrNiMn high entropy alloy

Additively manufactured high entropy alloys (AMed HEAs) usually have many distinctive microstructures, which will have a great impact on the mechanical properties. Revealing the role of different microstructures in the deformation process can provide guidance for the microstructure design of AMed HEAs. This paper used in-situ electron backscattered diffraction and digital image correlation tests to investigate the deformation behaviors of FeCoCrNiMn HEAs fabricated by laser directed energy deposition. The as-built FeCoCrNiMn HEAs have two kinds of microstructures: equiaxed and columnar. The equiaxed microstructure has no texture, the grain size is 28 μm, and the grain boundary misorientation conforms to theoretical distribution, whereas the columnar microstructure is composed of <101> oriented and coarse grains, and has a large number of low angle grain boundaries. In the deformation processes, <101> oriented grains are more prone to rotational deformation than <001> and <111>, but the tensile strain produced by < 101> is lower than <001> and higher than <111>. And compared with the coarse grains, the fine grains tend to coordinate the deformation from surrounding grains through grain rotation, and the strain concentrations are more readily to emerge in fine-grain regions. The average grain boundary geometric compatibility factor (m') of equiaxed microstructure is 0.39, and that of columnar microstructure is 0.57, so the grain boundaries of equiaxed microstructure have a more effective role in impeding dislocations. In addition, the statistical result shows that the value of m' is related to the grain boundary misorientation.

Tuning ultra-high damping capacity of Mn–Cu alloy to fit wide-range service temperature

Mn–Cu-based damping alloy prepared by directional solidification is very promising for vibration and noise control. Aging treatment with and without application of magnetic field on the microstructure and damping performance of directionally solidified Mn–Cu alloy was studied by means of X-ray diffraction, differential scanning calorimetry, dynamic thermal mechanical analysis and scanning electron microscope. It is found that aging treatment makes the orientation shift from (200) to (111), and aging treatment under magnetic field helps Mn–Cu alloy keep high damping in the wide temperature range. The polymorphic microstructure characterised by fine twin bands with low-angle misorientation boundaries leads to a substantial increase of twin boundary internal friction up to several times higher than that of polycrystalline state.

Large-Scale Synthesis of Metal Additively-Manufactured Microstructures Using Markov Random Fields

A data-driven framework is developed and examined for creating spatially-varying crystallographic textures over component-scale Computer-Aided Design (CAD) models. Here, a set of three orthogonal 2D micrographs of an Additively-Manufactured (AM) specimen are first obtained experimentally through Electron Backscatter Diffraction (EBSD) and subsequently converted to a 3D representative unit cell using the Markov Random Field (MRF) technique. Features such as grain size, crystallographic orientation, and grain boundary misorientation distributions are used to validate the reconstructed 3D microstructure against input experimental EBSD images. The variations of microstructural features during a powder-based additive manufacturing process are subsequently modeled by merging patches from the 3D snapshot of AM microstructural unit cell in a part-scale geometry using a tensor-based optimization process. The optimization algorithm repeatedly pastes microstructural elements from the reconstructed MRF unit cell onto the geometrical CAD domain until it is entirely covered. Here, through a simple Graphical User Interface (GUI), the user specifies a tensor field over the volumetric CAD model, defining the local control over grain-scale, anisotropy, and crystal growth orientation. This new approach provides a workflow for reconstructing global maps of AM microstructures in real-time by embedding site-specific images based on known AM microstructural patterns seen in experimental characterization techniques. The numerical results are helpful specifically for the visualization of process–microstructure relationships in metal additive manufacturing techniques.

Design of pure aluminum laminates with heterostructures for extraordinary strength-ductility synergy

• Pure aluminum laminates with various heterostructures were engineered via extrusion, rolling and annealing. • The designed laminates exhibited an excellent strength-ductility synergy. • The evolution of GNDs was observed using in-situ tension. • The strain gradient across the hetero-interface was revealed using DIC technique. • Hetero-deformation induced (HDI) strengthening and hardening contributed to the superior mechanical properties. Heterogeneous metals and alloys are a new class of materials with superior mechanical properties. In this paper, we engineered sandwich-structured pure aluminum laminates composed of middle coarse-grained layer and outer fine-grained layer via extrusion, rolling and annealing. By controlling the post-annealing regimes, a larger degree of microstructure heterogeneities such as boundary spacing, misorientation and texture across the hetero-interface were obtained, which resulted in obvious mechanical differences. Tensile tests indicated that the 300 °C/30 min annealed laminates enabled a relatively high tensile ductility while simultaneously retaining a high strength, which was better than prediction by the rule-of-mixture. To explain the reasons behind it, the evolution of geometrically necessary dislocations and strain gradient at the hetero-interface zone were detected using in-situ tension and microscopic digital image correlation technique. It was found that with the increasing applied strain, a significant strain gradient was developed near the interface, which was accommodated by geometrically necessary dislocations, thereby contributing to higher hetero-deformation induced (HDI) strengthening and hardening.

Sensitization, desensitization, and carbide evolution of Alloy 800H made by laser powder bed fusion

Additively manufactured (AM) Alloy 800H made by laser powder bed fusion (L-PBF) exhibited a greater degree of sensitization than wrought 800H, leading to both intergranular and intercellular corrosion. Dislocation cellular boundaries showed mild Cr depletion as compared to high-angle grain boundaries (HAGBs). Boundary misorientation of < 5⁰ was found mitigating sensitization. Carbide growth in AM 800H was controlled by particle net growth at the early stage and Ostwald ripening at the later stage. Cellular structure was confirmed producing faster elemental diffusion than bulk diffusion and random dislocation structures by cold rolling, and leading more rapid carbide growth, chromium depletion, and desensitization on HAGBs. Sensitization along cellular boundaries is not the main cause of intercellular corrosion in the sensitized AM Alloy 800H. • Alloy 800H by additive manufacturing showed faster sensitization than wrought 800H, on both grain and cellular boundaries. • Dislocation cellular structures in AM 800H accelerated element diffusion and governed sensitization. • Rapid desensitization happened in AM 800H only on cellular boundaries, not grain boundaries. • Carbide nucleation was not observed on cellular boundaries with the angle of < 5⁰ in AM 800H. • Intercellular corrosion in the sensitized AM 800H is not mainly due to Cr depletion.

Significant improvement in CGHAZ toughness of HSLA steel via welding with trailing mechanical treatment

The Coarse-grained heat-affected zone (CGHAZ) is well known as the local brittle zone in the welding joints of HSLA steels. In the present work, compressive strain is applied to the specimen during the CGHAZ welding thermal simulation process to achieve microstructure refinement and modification for toughness improvement. The strain is applied to the specimen when the temperature of the simulated thermal cycle decreased to 1100 °C, with the purpose of achieving dynamic recrystallization in the CGHAZ. Parallel experiments without the compressive strain are also conducted as a reference. The microstructure of the CGHAZ with and without the compressive strain was observed by optical microscopy (OM) and scanning electron microscopy (SEM). Transmission electron microscopy (TEM) was used for observation of the matrix and microstructure of martensite-austenite (M-A) constituents. Electron back-scattered diffraction (EBSD) techniques were used to get the information of boundary misorientation distribution. The toughness of the specimens under each condition was assessed through instrumented Charpy impact test at −20 °C. Results showed that the compressive strain refined prior austenite grain (PAG) size and packet size, while the block width and lath width were almost unaffected. Refinement of the PAG also modified the microstructure of M-A constituents. The microstructure refinement and modification of M-A constituents improved crack initiation energy and crack propagation energy, resulting in significant promotion of toughness from 60.20 J to 163.10 J.

The impact of misorientation on the grain boundary energy in bi-crystal copper: an atomistic simulation study.

Atomistic simulations were performed to investigate the relationships among the misorientation, dislocation density, and grain boundary energy of twist and tilt bi-crystal grain boundaries. In this work, the grain boundary energies were calculated based on the embedded-atom method interatomic potential for Cu. The results show that the dislocation density of the grain boundary changes with the rotation angle, thereby affecting the grain boundary energy. Furthermore, the grain boundary energy of a grain boundary with no dislocations is greater than that of a grain boundary with dislocations, which results from the distribution of the atomic potential energy on the grain boundaries. Additionally, the grain boundary energy increases with the dislocation density of the grain boundary in the case of dislocations on the grain boundary. On this basis, a new relationship is proposed for the misorientation angle and grain boundary energy. We assume that when the driving force of dislocation nucleation breaks through the grain boundary energy barrier, the grain boundary energy declines.

Grain Scale Investigation of the Mechanical Anisotropic Behavior of Electron Beam Powder Bed Additively Manufactured Ti6Al4V Parts

Numerous factors, including variable grain structures and different inherent defects, impact the mechanical behavior of Ti6Al4V parts fabricated using metal Additive Manufacturing (AM) processes. This study focuses on an in-depth analysis of how different microstructural features, such as crystallographic texture, grain size, grain boundary misorientation angles, and inherent defects, as byproducts of the electron beam powder bed fusion (EB-PBF) AM process, impact its anisotropic mechanical behavior. Standard tensile testing, conducted on samples produced at different orientations relative to the build table, showed significant anisotropy in elastic-plastic constitutive characteristics. Furthermore, X-ray computed tomography (CT) and electron back-scattered diffraction (EBSD) analyses were conducted on as-built samples to assess the effects of inherent defects and microstructural anomalies on such behavior. The samples arranged vertically and parallel to build direction had an average porosity of 0.05%, while the horizontally built samples, which were perpendicular to the build direction, had an average porosity of 0.17%. Moreover, the vertical samples showed larger grain sizes, with an average of 6.6 µm, wider α lath sizes, a lower average misorientation angle, and subsequently lower strength values than the other two horizontal samples. Among the three strong preferred grain orientations of the α phases, &lt;1 1 2¯ 1&gt; and &lt;1 1 2¯ 0&gt; were dominant in the horizontally built samples, whereas the &lt;0 0 0 1&gt; orientation was dominant in vertically built samples. Finally, larger grain sizes and higher beta-phase volume ratios were observed in the areas located at distances further away from the build plate. This was possibly due to the change in thermal gradients, cooling rates, and some thermal annealing phenomena resultant from the elevated build chamber temperature.

Precipitation on grain boundaries in Al-Mg-Si alloys: The role of grain boundary misorientation

The application of Al-Mg-Si alloys is growing in fracture critical applications such as transportation. The formation of grain boundary precipitates during cooling after solution treatment is an important factor in controlling low energy intergranular fracture modes. A systematic study was conducted on the effects of grain boundary misorientation and cooling rate (between 3 and 1000 °C/s) on the size and density of grain boundary precipitates before and after artificial ageing. It was found that precipitates on high angle grain boundaries were ≈3 times larger than those formed on low angle boundaries. The grain boundary precipitate size decreased by a factor of 3 when the cooling rate increased from 3 to 80 °C/s with no precipitates observed at 1000 °C/s. However, even for a cooling rate of 1000 °C/s, grain boundary precipitation was observed during subsequent artificial ageing. The effects of grain boundary misorientation and cooling rate were rationalized by enhanced diffusion at grain boundaries.

The Microstructure and Mechanical Properties of Ferritic-Martensitic Steel EP-823 after High-Temperature Thermomechanical Treatment

The effect of high-temperature thermomechanical treatment (HTMT) with plastic deformation by rolling in austenitic region on the microstructure and mechanical properties of 12% chromium ferritic-martensitic steel EP-823 is investigated. The features of the grain and defect microstructure of steel are studied by Scanning Electron Microscopy with Electron Back-Scatter Diffraction (SEM EBSD) and Transmission Electron Microscopy (TEM). It is shown that HTMT leads to the formation of pancake structure with grains extended in the rolling direction and flattened in the rolling plane. The average sizes of martensitic packets and ferrite grains are approximately 1.5–2 times smaller compared to the corresponding values after traditional heat treatment (THT, which consists of normalization and tempering). The maximum grain size in the section parallel to the rolling plane increases up to more than 80 µm. HTMT leads to the formation of new sub-boundaries and a higher dislocation density. The fraction of low-angle misorientation boundaries reaches up to ≈68%, which exceeds the corresponding value after HTMT (55%). HTMT does not practically affect the carbide subsystem of steel. The mechanical properties are investigated by tensile tests in the temperature range 20–700 °C. It is shown that the values of the yield strength in this temperature range after HTMT increase relative to the corresponding values after THT. As a result of HTMT, the elongation decreases. A significant decrease is observed in the area of dynamic strain aging (DSA). The mechanisms of plastic deformation and strengthening of ferritic-martensitic steel under the high-temperature thermomechanical treatments are also discussed.