Grain Boundary Character Research Articles

High-resolution FIB-TEM-STEM structural characterization of grain boundaries in the high dielectric constant perovskite CaCu3Ti4O12

In this work, the grain boundaries composition of the polycrystalline CaCu3Ti4O12 (CCTO) was investigated. A Focused Ion Beam (FIB)/lift-out technique was used to prepare site-specific thin samples of the grain boundaries interface of CCTO ceramics. Scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectrometry (EDXS) and Electron Energy Loss Spectroscopy (EELS) systems were used to characterize the composition and nanostructure of the grain and grain boundaries region. It is known that during conventional sintering, discontinuous grain growth occurs and a Cu-rich phase appears at grain boundaries. This Cu-rich phase may affect the final dielectric properties of CCTO but its structure and chemical composition remained unknown. For the first time, this high-resolution FIB-TEM-STEM study of CCTO interfacial region highlights the composition of the phases segregated at grain boundaries namely CuO, Cu2O and the metastable phase Cu3TiO4.

Grain boundary wetting of different types of grain boundaries in the Cu–Ag system

Present work is dedicated to the investigation of grain boundary (GB) wetting phase transition on different types of grain boundaries in the Cu–Ag system. The character of GBs in the samples was determined by the electron backscattering diffraction. GB character distribution was based on the coincidence site lattice (CSL) model and on the values of the misorientation angle of GBs. Experimental results show that only the low angle GBs and the Σ3 CSL GBs do not reach complete wetting by the melt. Random high misorientation angle (HA) GBs reach complete wetting at 895 °C. Only Σ11, Σ5 and Σ13 CSL GBs have a higher wetting temperature than the HA GBs, so their GB energy should be lower than the energy of HA GBs.

Role of grain boundary character and its evolution on interfacial solute segregation behavior in nanocrystalline Ni-P

Interfacial solute segregation behavior of P in nanocrystalline (NC) Ni alloys is investigated at the atomic scale using cross-correlative PED-APT measurements and atomistic simulations. Inhomogeneous P-segregation in grain boundaries (GBs) of NC Ni is observed from both experimental measurements and simulation calculations. Interfacial excess (IE) of the solute within GBs is further studied as function of GB misorientation. Significant scatter is detected in the IE of boundaries with similar misorientation angles and even within special boundaries with identical Σ values. From atomistic simulations, a general trend of increasing IE with initial GB energy and volume is observed but with poor correlation, indicating that the segregation behavior cannot be captured with a single average measure of the entire GB character. Rather the results suggest that the extent of interfacial solute segregation correlates better with fraction of high energy atomic sites in the boundary. By comparing IE values with thermodynamic model predictions, it is shown that the interfacial segregation behavior is strongly influenced by the energy distribution of atomic sites in the GBs. However, in order to predict the extent of solute segregation, the evolution of the atomic energy landscape in the GB with continuous solute segregation must be considered.

Reflections on the Analysis of Interfaces and Grain Boundaries by Atom Probe Tomography.

Interfaces play critical roles in materials and are usually both structurally and compositionally complex microstructural features. The precise characterization of their nature in three-dimensions at the atomic scale is one of the grand challenges for microscopy and microanalysis, as this information is crucial to establish structure-property relationships. Atom probe tomography is well suited to analyzing the chemistry of interfaces at the nanoscale. However, optimizing such microanalysis of interfaces requires great care in the implementation across all aspects of the technique from specimen preparation to data analysis and ultimately the interpretation of this information. This article provides critical perspectives on key aspects pertaining to spatial resolution limits and the issues with the compositional analysis that can limit the quantification of interface measurements. Here, we use the example of grain boundaries in steels; however, the results are applicable for the characterization of grain boundaries and transformation interfaces in a very wide range of industrially relevant engineering materials.

The Effect of Magnetic Flux Focusing on the Current–Voltage Characteristics of YBa2Cu3O7- δ Grain Boundary Josephson Junctions

In order to obtain experimental evidence for the influence of flux focusing effect on the characteristics of planar grain boundary (GB) Josephson junctions, we measured the current–voltage characteristics and magnetic-field-dependent current–voltage steps of integrated YBa2Cu3O7- δ bicrystal GB Junctions with different junction widths. Under the self-magnetic field, the critical current of the GB junction is proportional to the square root of the junction width, which is different from the traditional linear relationship between the critical current and the junction width. When a vertical magnetic field is applied, the flux flow current–voltage step could be observed, and the step voltage increases monotonously with an increase in the junction width. When a magnetic field rotating in the bc plane is applied, the voltage value of the flux flow step decreases slowly as the magnetic field inclined angle increases when the field inclined angle is relatively large. Based on these results, we demonstrate the presence of the magnetic flux focusing effect on YBa2Cu3O7- δ GB Josephson junctions. The size of flux focusing in the GB is closely associated with the width of the junctions and the orientation of the magnetic field.

Crystallographic study of liquid-metal-embrittlement crack path

Compared to other intergranular degradation phenomena, liquid metal embrittlement (LME) is less understood. The main gap in knowledge has been the lack of fundamental information of how grain-boundary characteristics (e.g. grain boundary misorientation angle and grain boundary crystallographic plane) affects LME susceptibility. In this study, the recently developed trace analysis has been applied to the grain boundaries exhibiting LME cracking. Despite the presence of a diverse range of low- and high-index planes in the entire population of the grain boundaries, liquid metal selectively penetrated high-index random grain boundary planes such as (5 3 1), (5 1 1), and (7 3 3).

Relationship between mechanical properties and age-hardening behavior around grain boundaries of Al-Mg-Si alloy

Nanoindentation tests were conducted near the grain boundary (GB) of the Al-Mg-Si alloy, and the influence of GB character on the aging precipitation behavior and the mechanical properties was confirmed. After obtaining the GB characters by electron back scattered diffraction (EBSD) analysis, nanoindentation tests were carried out on under-aged, peak-aged, and over-aged samples. And then, the indentation areas were observed by back scattered electrons imaging (BSE) in order to identify indentation positions with respect to the GB. In this study, for the GB character, focusing on the rotation angle, the high-angle GB (HAGB) and the low-angle GB (LAGB) were selected. In addition, coincident site lattice GBs (CSL) were selected as the special GB. In the 180°C under-aged samples, the nano-hardness near GB is higher than that far from GB, while 180°C peak-aged samples, the nano-hardness is lower than that far from GB. Then the range near the GB where the hardness changes was larger at HAGB than at LAGB and CSL3. This suggests that the GB character affects the aging precipitation behavior and mechanical properties.

Microstructural Characterization of Molybdenum Grain Boundaries by Micropillar Compression Testing and Atom Probe Tomography

Abstract Molybdenum is part of many modern everyday items and, to some extent, also necessary for their production. Besides its high melting point of 2620 °C, molybdenum exhibits good electrical and thermal conductivity and a low coefficient of thermal expansion at the same time. However, pure molybdenum has low ductility at room temperature. The ductile-brittle transition which is typical of body-centred cubic metals occurs at around room temperature, depending on the processing state. Thus, components partly fracture along the grain boundaries due to brittle failure. The decisive reason for this are, inter alia, impurities which segregate at grain boundaries. In order to analyze individual grain boundaries, a load is applied in compression direction to micropillars in the size range of a few micrometres. This allows a targeted study of the mechanical properties, complemented by atom probe experiments analysing the chemical structure. The micrometre-scale analyses enable the determination and correlation of the mechanical with the chemical properties of a single grain boundary with the aim of better understanding the relationship between segregations and deformation behaviour in the grain boundary region.

3D nanostructural characterisation of grain boundaries in atom probe data utilising machine learning methods.

Boosting is a family of supervised learning algorithm that convert a set of weak learners into a single strong one. It is popular in the field of object tracking, where its main purpose is to extract the position, motion, and trajectory from various features of interest within a sequence of video frames. A scientific application explored in this study is to combine the boosting tracker and the Hough transformation, followed by principal component analysis, to extract the location and trace of grain boundaries within atom probe data. Before the implementation of this method, these information could only be extracted manually, which is time-consuming and error-prone. The effectiveness of this method is demonstrated on an experimental dataset obtained from a pure aluminum bi-crystal and validated on simulated data. The information gained from this method can be combined with crystallographic information directly contained within the data, to fully define the grain boundary character to its 5 degrees of freedom at near-atomic resolution in three dimensions. It also enables local atomic compositional and geometric information, i.e. curvature, to be extracted directly at the interface.

Hydrogen distribution in metallic polycrystals with deformation

To understand hydrogen embrittlement and predict probable damage sites based on high hydrogen segregation, it is important to identify hydrogen distribution mechanisms in metallic microstructure with deformation. Hydrogen segregation in metals is affected by various factors such as grain-size, the character of grain boundaries (GBs), loading direction and strain-rate. To this end, a computational framework consisting of non-local dislocation density-based crystal plasticity model coupled with slip-rate based hydrogen transport model is presented to study the role of (i) grain-size, (ii) loading direction, (iii) strain-rate and (iv) GB character on hydrogen distribution and segregation in the pre-charged metallic microstructure. The computational framework is capable of accounting for the change in local hydrogen concentration due to prevailing hydrostatic pressure, trapping by dislocations, GB energetics and local slip-rates in the metallic microstructure. The difficulty in dislocation motion at the inter-granular regions of polycrystal due to high cross-hardening offered by geometrically necessary dislocations (GNDs) is incorporated in the model as additional isotropic hardening, whereas the back stress due to the GND pileups is included as a kinematic term in the flow rule. In addition, GNDs act as hydrogen trap sites leading to increased hydrogen concentration in the inter-granular regions. The strain-rate factor initially provided by Krom & Bakker (1999) is modified in the present work to make it compatible with the dislocation density-based coupled framework, which is able to calculate trapped hydrogen in dislocations along the slip systems. Plastically deformed polycrystals of various grain-sizes, along different directions, with varying strain rates and containing Σ3[11¯0](111) and Σ5[001](210) GBs show that the gradients of hydrostatic pressure and GB character are major factors controlling hydrogen segregation in the microstructure.

Investigation on Grain Boundary Character Distribution During Dynamic Recrystallization of Austenitic Stainless Steel During Hot Deformation

Abstract The phenomenon of dynamic recrystallization (DRX) is central to the hot working and thermomechanical processing of austenitic stainless steel. This study aims to relate two distinct parameters that influence DRX: the content of interstitial atoms, such as nitrogen, and character of grain boundaries in the steel. 316LN austenitic stainless steel has been subjected to thermomechanical processing at 1,423 K through uniaxial compression at different strain rates. At these, the microstructure is fully recrystallized. The nature, character, and growth of DRX grains are studied using Electron Backscatter Diffraction (EBSD). EBSD is used to determine grain boundary character distribution and the Effective Grain Boundary Energy (EGBE) parameter. The evolution of EGBE and its relation to grain growth are analyzed. The concomitant development of special low-energy boundaries is correlated with the interstitial content in the steel, thus linking interstitial content to EGBE. The correlations so made are tested by examining the corrosion resistance of different boundaries formed at different deformation conditions.

A high-resolution characterization of the initiation of stress corrosion crack in Alloy 690 in simulated pressurized water reactor primary water

Stress corrosion crack initiation of alloy 690 in simulated pressurized water reactor primary water was studied through high-resolution characterization of grain boundaries at different stages of the initiation process. It was found that a compact layer of Cr2O3 initially forms over a migrated grain boundary, driven by the diffusion of Cr. After the surface Cr2O3 is breached by straining, oxygen diffusion results in formation of a mixture of NiO and Cr2O3 along the grain boundary. The crack nucleates along either the previous grain boundary or the heavily oxidized migration zone when the boundary strength falls below the local stress.

Effect of slip transmission at grain boundaries in Al bicrystals

The effect of slip transfer on the deformation mechanisms of Al bicrystals was explored using a rate-dependent dislocation-based crystal plasticity model. Three different types of grain boundaries (GBs) were included in the model by modifying the rate of dislocation accumulation near the GB in the Kocks-Mecking law, leading to fully-opaque (dislocation blocking), fully-transparent and partially-transparent GBs. In the latter, slip transmission is only allowed in pairs of slip systems (SS) in neighbour grains that are suitably oriented for slip transfer according to the Luster-Morris parameter. Modifications of the GB character led to important changes in the deformation mechanisms at the GB. In general, bicrystals with fully-opaque (dislocation blocking) boundaries showed an increase in the dislocation density near the GB, which was associated with an increase in the Von Mises stress. In contrast, the dislocation pile-ups and the stress concentration were less pronounced in the case of partially-transparent boundaries as the slip in one grain can progress into the next grain with some degree of continuity. No stress concentrations were found at these boundaries for fully-transparent boundaries, and there was continuity of strain across the boundary, which is not typical of most experimentally observed GBs (Hémery et al., 2018; Bieler et al., 2019). Simulations of ideal bicrystals oriented for favorable slip transfer on the most highly favored slip system in grains with high Schmid factors for slip transfer depends on the number of active SS in operation in the neighborhood indicating that most boundaries will lead to nearly opaque conditions while some boundaries will be transparent. Finally, the model was applied to a particular experimentally observed GB in which slip transfer was clearly operating indicating that the model predicted a nearly transparent GB.

Quantitative investigation on sink strength of nano-grain boundary for irradiation resistance

It has long been recognized that grain boundary (GB) is an effective sink to trap irradiation-induced defects. Introducing large densities of GBs becomes an effective strategy to enhance irradiation resistance. However, the nano-grained materials have poor stability under the extreme irradiation, and the effect of nano-GB characters on the irradiation sink strength is largely unknown. Here, the sink strength of nano-GB is quantitatively investigated in the nano-grained Cu and dilute Cu–W alloys with the average grain size ranging from ∼20 to ∼50 nm by ∼300 keV He ions irradiation at room temperature and 673 K. The irradiation induced void volume ratio, size and distribution are confirmed to strongly depend on the grain size and irradiation temperature. The nano-GBs with different characters, such as the misorientation and GB plane, have similar relative energy. The nano-GB sink strength is independent on the GB characters, deriving from the highly curved nano-GB plane having excess volume and energy. Combining with molecular dynamics simulations, we can conclude that nano-grain size is the most vital factor for the sink strength with respect to the GB characters. With the increase of temperature and the decrease of grain size, the stable nano-GBs exhibit a behavior of “ideal” defects sink due to their high volume ratio and the increased point defect recombination probability. Our work provides a fundamental understanding of the nano-GB sink efficiency and offer a guidance for designing nano-grained structural materials with optimum anti-irradiation performance for future fusion reactors.

Influence of crystal orientation and Berkovich tip rotation on the mechanical characterization of grain boundaries in molybdenum

In this study, nanoindentation experiments with a continuously measured stiffness are performed in the vicinity of grain boundaries in technically pure molybdenum. A significant change in hardness as a function of indentation depth is put in context with the corresponding deformation patterns in the indented crystals. The difference in hardness increase of 18% compared to 5% of two different grain boundaries with similar misorientation angle could be explained in this way. Five degrees of freedom have to be determined to fully describe a grain boundary. However, the influence of the rotation angle of a pyramidal indenter geometry around the loading axis as yet another loading parameter is assessed by backscattered electron micrographs and orientation deviation maps. Indentation experiments with different rotation angle at the same grain boundary confirm that a hardness increase close to the grain boundary is only apparent if a significant amount of plastic deformation is introduced towards the interface.

Correlation among grain boundary character, carbide precipitation and deformation in Alloy 690

ABSTRACTThe correlation among grain boundary character, carbide precipitation and deformation in the grain boundary engineering (GBE) treated Alloy 690 samples with and without pre-deformation aged at 715oC for 15 h was analysed by scanning electron microscopy and electron backscatter diffraction. The fraction of low Σ coincidence site lattice (CSL) grain boundary was enhanced by GBE treatment. The fraction of Σ3 grain boundary decreased, and most of Σ9 and Σ27 grain boundaries disappeared in the deformed GBE samples. After aging treatment, bigger carbide precipitated at coherent Σ3 grain boundary, however, most of plate-like carbide precipitated at incoherent Σ3 grain boundary disappeared in the pre-deformed GBE samples. The larger carbide precipitated on the random grain boundary in the 5% pre-deformed sample, while smaller carbide can be observed in the 15% pre-deformed sample. During the in situ tensile test of the aged GBE samples, grain boundary carbide migrated with the grain boundary migration. The slip bands go across Σ3 grain boundary directly, but cannot go across other grain boundaries. The high density of carbide plate precipitated near incoherent Σ3 and Σ9 grain boundaries can resist the evolution of slip bands. Compared to the Σ3 and Σ9 grain boundaries, Σ27 and random grain boundaries are more easily to form microcrack during deformation. The initiation of grain boundary microcrack not only related to the character of grain boundary but also related to the character of nearby grain boundaries. The phase interface of carbide and matrix is another region to initiate the microcrack.

Insight into void formation near grain boundary from phase-field simulations

Numerous point defects (vacancies and self-interstitials) generated during irradiation can diffuse and accumulate to form defect clusters inside grains or at grain boundaries (GBs). Void-denuded zone (VDZ) near the GB is related to irradiation condition, GB character and the kinetics of defects at the GB region. A phase-field model is developed to simulate void formation near different types of GBs. The sink term representing the annihilation of GB is modified by taking into account of the inclination angle for Σ GB and the defect mobility. The temperature-dependent material parameters of copper are derived from ab initio calculations. According to the numerical simulations, the boundaries of VDZs tend to move towards the interior grains with increasing the efficiency of GB sink term. In the neighboring region of the denuded zone, the number of voids increases rapidly and the average radius of void is higher than that far from the VDZs.

Investigating the Properties of Coil Tail in Ti–Nb–Mo Microalloyed Hot‐Rolled Strip

In the present study, the thermo‐mechanical controlled processing (TMCP) technology was applied to produce Ti–Nb–Mo microalloyed hot‐rolled steels. The strength at the tail of a strip coil is often found to be non‐uniform. The microstructural and the mechanical properties of the specimens in the outermost circle as well as in the third circle and the fifth circle from the outermost circle were analyzed in detail. The optical microscopy (OM) and the electron backscatter diffraction (EBSD) techniques are employed for the characterization of ferrite grains and grain boundaries. The precipitates were revealed by high‐resolution transmission electron microscopy (HR‐TEM) and energy dispersive spectroscopy (EDS). It is found that the precipitation strengthening induced by nano‐scale (Ti, Nb, Mo)C particles was the dominant factor for the disparity in properties in different positions of the coil tail. The volume fraction of nano‐scale (Ti, Nb, Mo)C particles increases from 0.220% in the outermost circle to 0.343% in the fifth circle. In addition, the lattice planes of (110)α and (100)(Ti, Nb, Mo)C followed a Baker‐Nutting orientation relationship. Nano‐scale (Ti, Nb, Mo)C particles mainly precipitated in ferrite grains during coiling and subsequent cooling. Furthermore, different cooling rates along the strip had inconspicuous influences on the grain size and lengths of ferrite grain boundaries.

Orientation-dependent grain boundary characteristics in tantalum upon the change of strain path

Although grain boundary (GB) exhibits crucial influence on grain subdivision and work hardening of tantalum (Ta), GB characteristics and the role it plays in deformation have not sufficiently investigated yet. By means of electron backscatter diffraction analysis and Vickers hardness measurements, we systematically analyzed microstructure and stored energy at GBs deformed under unidirectional rolling (UR) and 135° clock rolling (CR), respectively. Geometrical necessary dislocation densities of GB regions were calculated based on kernel average misorientation. GB deformation mechanisms were deduced from Schmid factor analysis. Results show that in UR-Ta {111}〈uvw〉 (〈111〉//normal direction) and {100} 〈uvw〉(〈100〉//normal direction) grains tend to be deformed in uniserial and multiple slipping, respectively, while only multiple slipping is activated in CR-Ta. Hence, in UR-Ta, uniserial slipping induces dense intragranular micro-shear bands (MSBs), 23°–45° relative to rolling direction, which penetrate through adjacent {111} grains, while in {100} grains multiple slipping introduces orientation-gradient cells instead of similar MSBs observed in {111} grains. Compared to orientation-dependent grain subdivisions in UR-Ta, the multiple slipping in CR-Ta generates only cell structures in both {111} and {100} grains. Interaction effects between {100}-{100},{100}-{111} and {111}-{111} grains are therefore varies with varying rolling paths.

Enhanced Sol-Gel Route to Obtain a Highly Transparent and Conductive Aluminum-Doped Zinc Oxide Thin Film.

The electrical and optical properties of sol–gel derived aluminum-doped zinc oxide thin films containing 2 at.% Al were investigated considering the modifying effects of (1) increasing the sol H2O content and (2) a thermal treatment procedure with a high-temperature approach followed by an additional heat-treatment step under a reducing atmosphere. According to the results obtained via the TG-DTA analysis, FT-IR spectroscopy, X-ray diffraction technique, and four-point probe resistivity measurements, it is argued that in the modified sample, the sol hydrolysis, decomposition of the deposited gel, and crystallization of grains result in grains of larger crystallite size in the range of 20 to 30 nm and a stronger c-axis preferred orientation with slightly less microstrain. The obtained morphology and grain-boundary characteristics result in improved conductivity considering the resistivity value below 6 mΩ·cm. A detailed investigation of the samples’ optical properties, in terms of analyzing their absorption and dispersion behaviors through UV-Vis-NIR spectroscopy, support our reasoning for the increase of the mobility, and to a lesser extent the concentration of charge carriers, while causing only a slight degradation of optical transmittance down to nearly 80%. Hence, an enhanced performance as a transparent conducting film is claimed for the modified sample by comparing the figure-of-merit values.