Grain Boundary Character Distribution Research Articles

Grain Boundary Engineering of ECAPed OFHC Copper

Grain boundary character distributions (GBCD) of OFHC copper equal-channel angular pressing (ECAP) deformed and then annealed were analyzed by electron back scatter diffraction (EBSD). The experimental results showed that a combination of ECAP deformation and annealing treatments could significantly increase the fraction of low-Σ coincidence site lattice (CSL) boundaries (Σ≤29) and effectively interrupt the connectivity of random boundaries network in OFHC copper. An increase of low-Σ CSL boundaries from 45.27 to 71.06% was observed in as-received material after one pass ECAP strain followed by annealing at 350 °C for 48 h. The connectivity of random boundaries network was interrupted by high fraction of low-Σ CSL boundaries.

Measuring Grain Boundary Character Distributions in Ni-Base Alloy 725 Using High-Energy Diffraction Microscopy

We use high-energy X-ray diffraction microscopy (HEDM) to characterize the microstructure of Ni-base alloy 725. HEDM is a non-destructive technique capable of providing three-dimensional reconstructions of grain shapes and orientations in polycrystals. The present analysis yields the grain size distribution in alloy 725 as well as the grain boundary character distribution (GBCD) as a function of lattice misorientation and boundary plane normal orientation. We find that the GBCD of Ni-base alloy 725 is similar to that previously determined in pure Ni and other fcc-base metals. We find an elevated density of Σ9 and Σ3 grain boundaries. We also observe a preponderance of grain boundaries along low-index planes, with those along (1 1 1) planes being the most common, even after Σ3 twins have been excluded from the analysis.

Low temperature cross-rolling to modify grain boundary character distribution and its effect on sensitization of SS304

Sensitization of austenitic stainless steels is well-known in the temperature range of 400–800°C. This effect can be lowered by the introduction of low-Σ coincident site lattice (CSL) boundaries in the material. In present work, 304 stainless steel was thermo-mechanically processed to obtain microstructures containing varied fractions of low-Σ CSL boundaries. Processing consisted of multi-step cross rolling at −41°C followed by a short heat treatment at 1000°C. In order to understand the effect of grain boundary character distribution (GBCD) on sensitization behavior, firstly, heating condition corresponding to the maximum degree of sensitization (DOS) was found by comparing double loop electro-chemical potentiodynamic reactivation (DL-EPR) plots for all the samples, which were given sensitization heat treatment at 600°C for various time. Later, a relation between the GBCD and DOS was established by conducting DL-EPR test on all the processed samples. Low-temperature cross rolling was found to be effective in keeping the fraction of low angle grain boundaries (LAGBs) low and ensuring equi-axed grain structure. The sample, which was given low strain followed by short heat treatment, showed highest fraction of CSL boundaries among all other conditions. Grain size was found to decrease with increase in pre-strain. The results indicated that sensitization was inversely related to the fraction of CSL boundaries and grain size, while it was directly related to the fraction of random high angle grain boundaries (HAGBs). It was also found that sensitization was weakly related to the fraction of LAGBs indicating that these boundaries did not show “special” behavior, as they do in many other grain boundary related phenomenon.

Effect of Grain Boundary Character Distribution on the Adiabatic Shear Susceptibility

The adiabatic shear susceptibility of AISI321 stainless steels with different grain boundary character distributions (GBCDs) was investigated by means of split-Hopkinson pressure bar. The results indicate that the width of the adiabatic shear band of the specimen after thermomechanical processing (TMP) treatment is narrower. The comparison of the stress collapse time, the critical stress, and the adiabatic shear forming energy suggests that the TMP specimens have lower adiabatic shear susceptibility than that of the solution-treated samples under the same loading condition. GBCD and grain size affected the adiabatic shear susceptibility. The high-angle boundary network of the TMP specimens was interrupted or replaced by the special grain boundary, and smaller grain size hindered the adiabatic shearing.

Grain-boundary character distribution and correlations with electrical and optoelectronic properties of CuInSe2 thin films

Thin-film solar cells based on polycrystalline Cu(In,Ga)Se2 absorbers exhibit record conversion efficiencies of up to 22.6%. There is still a lack of a quantitative connection between the grain-boundary character distribution (GBCD) and the corresponding electrical and optoelectronic properties. The present work uses microstructural data from a CuInSe2 thin film acquired by electron backscatter diffraction (EBSD) to evaluate the GBCD. The most prominent features of the GBCD of CuInSe2 are Σ3 twin boundaries and the Σ9 and Σ27a symmetric tilt grain boundaries. Moreover, combining EBSD with electron-beam-induced current and cathodoluminescence (measurements on the same identical area) on a CuInSe2/Mo/glass stack provide the means to relate the grain-boundary character with the corresponding electrical and optoelectronic signals across the grain boundary. In part, determining this relationship is accomplished by means of correlation analysis using measurement data from more than 100 grain boundaries. However, the crystallographic, electrical and optoelectronic data showed no strong correlations, which is attributed to atomic reconstruction found in atomic planes adjacent to planar defects in polycrystalline CuInSe2 thin films and corresponding reductions of excess charge densities at these defects.

Effect of thermomechanical processing on grain boundary character distribution of Hastelloy X alloy

This study investigates the effect of thermomechanical processing on grain boundary character distribution (GBCD) in Hastelloy X alloy. The deformation ratio and annealing condition were adjusted to achieve optimum process performance. The experimental results showed that low prestrain provided appropriate driving force for coincidence site lattice (CSL) boundary regeneration through static recovery process. The short-time annealing treatment at high temperature was performed to obtain optimized GBCD and control grain growth. The fraction of special boundaries reached 72% and the random boundary network was significantly dispersive.

Grain boundary character distribution and its effect on corrosion of Ni–23Cr–16Mo superalloy

Grain boundary character distribution (GBCD) of the Hastelloy C2000 alloy (Ni–23Cr–16Mo) and the effect of coincidence site lattice (CSL) grain boundaries on corrosion resistance were examined by electron backscattered diffraction and electrochemical experiments. Various deformation followed by annealing was applied to optimise the GBCD of the alloy. After grain boundary engineering (GBE) treatment, the proportion of CSL boundaries increased from 37.7% to 62.4% and the corrosion current density of the specimens decreased in NaCl solution. The results indicated that GBE treatment is responsible for preferable corrosion resistance due to the increase of the fraction of special low energy grain boundaries with perfect grain boundary atom arrangement after thermomechanical process.

Evolution of microstructure and grain boundary character distribution of a tin bronze annealed at different temperatures

Specimens cut from a rolled tin bronze sheet were annealed at 400–800°C for 1h and evolution of their microstructures was then characterized in details by electron channeling contrast imaging and electron backscatter diffraction techniques. Particularly, statistics on special boundaries (SBs) with Σ≤29 and network connectivity of random high angle boundaries (HABs) in the annealed specimens were examined to probe optimization potentials of grain boundary character distribution (GBCD) for this material. Results show that the deformed microstructure in the as-received material begins to be recrystallized when the annealing temperature increase to 500°C and average grain sizes surge with further increasing temperatures. As a result of the recrystallization, a large number of annealing twins (with Σ3 misorientation) are produced, leading to remarkably increased fractions of SBs (fSBs). Thanks to preexisting dense low angle boundaries, the majority of SBs in the 500°C specimen with only partial recrystallization are Σ3ic (incoherent) boundaries, which effectively disrupt connectivity of random HABs network. Although the fSBs can be further increased (up to 72.5%) in specimens with full recrystallization (at higher temperatures), the Σ3ic boundaries would be replaced to some extent by Σ3c (coherent) boundaries which do not contribute directly to optimizing the GBCD. This work should be able to provide clear suggestions on applying the concept of grain boundary engineering to tin bronze alloys.

Mechanisms of Thermo-Mechanical Process on Grain Boundary Character Distribution of 316L Austenitic Stainless Steel

Grain boundary engineering (GBE) was carried out on 316L austenitic stainless steel with Thermo-mechanical processing (TMP), which was performed by unidirectional compression and subsequent annealing. The effect of TMP parameters including the strain and annealing time on grain boundary character distribution (GBCD) and the corresponding mechanism was investigated in the study. The results showed that high fraction of low-Σ coincident-site lattice (CSL) grain boundaries (about 55%) associating with interrupted network of random boundaries was obtained through TMP of 5% cold compression followed by annealing at 1000 °C for 45 min. The fraction of low-Σ boundaries increased with increasing the annealing time under all the experiment strain, but the mechanisms were different between the low and medium above levels of strain. Grains rotation and reaction of migratory boundaries might be the reasons of low-Σ boundaries growth in the strain of 5% and in the strain greater than or equal to 10%, respectively.

Microstructural Effects on the Oxidation Behavior of Alloy 800HT in Supercritical Water

Alloy 800HT is one promising candidate for use as a fuel cladding material in supercritical water-cooled rectors. In the present study, specific thermomechanical processing (TMP) was used to study the effects of grain size and grain boundary character distribution (GBCD) on the oxidation behavior of alloy 800HT in supercritical water (SCW). The processed samples were exposed to SCW at 600°C and 25 MPa for 100, 300, and 1000 h. The results showed that grain size and grain boundaries are important factors that affect the oxidation behavior of alloy 800HT in SCW. We also found that TMP improves the adhesion and integrity of the oxide scale.

Improving intergranular corrosion resistance in a nickel-free and manganese-bearing high-nitrogen austenitic stainless steel through grain boundary character distribution optimization

Grain boundary character distribution (GBCD) and the effect of GBCD optimization on the intergranular corrosion (IGC) of a cold rolled and subsequently annealed nickel-free and manganese-bearing high-nitrogen austenitic stainless steel were investigated. The results show that the fraction of low Σ coincidence site lattice boundaries increases from 47.3% for the solid solution treated specimen to 83.3% for the specimen cold-rolled by 7% and then annealed at 1423K for 10min. Σ3 boundaries of high fraction effectively interrupt the connectivity of non-corrosion-resisting boundaries (special boundaries like Σ9, Σ27, etc. and general high angle boundaries) network, thus improving the IGC resistance.

Control of grain boundary connectivity based on fractal analysis for improvement of intergranular corrosion resistance in SUS316L austenitic stainless steel

The connectivity of high energy random boundaries was investigated on the basis of the fractal analyses of grain boundary microstructures in SUS316L stainless steel, to prove the usefulness of a refined approach to grain boundary engineering (GBE) for more precise prediction and control of intergranular corrosion in polycrystalline materials. It was found that the maximum connectivity for random boundary network, termed the maximum random boundary connectivity (MRBC) had a fractal nature in the studied specimens of SUS316L stainless steel. The fractal dimension of MRBC tended to decrease with decreasing fraction of random boundaries, or in other words with increasing fraction of low-energy low-Σ coincidence site lattice (CSL) boundaries. The lower coefficient of variation of grain size distribution suggesting a more homogeneous grain structure, was found to result in the lower fractal dimension of MRBC for the specimens with a similar grain boundary character distribution (GBCD). The optimum grain boundary microstructure for enhanced intergranular corrosion resistance in the SUS316L stainless steel was discussed based on the results from the fractal analyses of MRBC for different grain boundary microstructures.

The most frequent interfaces in olivine aggregates: the GBCD and its importance for grain boundary related processes

Rocks consist of crystal grains separated by grain boundaries that impact the bulk rock properties. Recent studies on metals and ceramics showed that the grain boundary plane orientation is more significant for grain boundary properties than other characteristics such as the sigma value or disorientation (in the Earth’s science community more frequently termed misorientation). We determined the grain boundary character distribution (GBCD) of synthetic and natural polycrystalline olivine, the most abundant mineral of Earth’s upper mantle. We show that grain boundaries of olivine preferentially contain low index planes, in agreement with recent findings on other oxides (e.g. MgO, TiO2, Al2O3 etc.). Furthermore, we find evidence for a preferred orientation relationship of 90° disorientations about the [001] direction forming tilt and twist grain boundaries, as well as a preference for the 60° disorientation about the [100] axis. Our data indicate that the GBCD, which is an intrinsic property of any mineral aggregate, is fundamental for understanding and predicting grain boundary related processes.

Simulating effects of texture on grain boundary character distribution during grain growth

A modified Monte Carlo method has been used to simulate the impact of texture on grain boundary character distribution (GBCD) in this study. Polycrystals with random orientations and various texture intensities defined by fraction of Goss orientation initial microstructures were generated. Real grain orientations in Euler space were incorporated into our model and swap method were used to generate texture cluster to explore the effects on GBCD. It was found that the initial microstructure with texture cluster has a steadier GBCD than microstructure without texture. Distribution of grain boundary at triple junction was also analysed to evaluate materials susceptibility to crack propagation. Statistic shows that more special boundaries exist at triple junction with improved texture intensity. The significant increasing of Σ17b boundary was also found when strengthening texture intensity.

MICRO-CHARACTERIZATION OF DISSIMILAR METAL WELD JOINT FOR CONNECTING PIPE- NOZZLE TO SAFE-END IN GENERATION III NUCLEAR POWER PLANT

The dissimilar metal weld joint(DMWJ) in primary water system of pressurized water reactors(PWRs) has been proven to be a vulnerable component owing to its proneness to different type of flaws. Thus,maintaining integrity of such joint in case of defect presence is of great importance to the design and safe management of nuclear power plants(NPPs). For a reliable integrity analysis of DMWJ, it is essential to understand the microscopic characteristics in all regions of the joint. In this work, OM, TEM, SEM, durometer, AFM, MFM and SKPFM were utilized to investigate the microstructure, micro- hardness and the distribution of main elements, grain boundary characteristic and residual strain in the A508/52M/316 L DMWJ that used for connecting the pipe safeend and the nozzle of reactor pressure vessel in PWRs, and a comparative analysis about the microstructure and property along the radical direction of the DMWJ was obtained. The results showed that there was no region that differed from the other part of the weldment in terms of the microstructure and micro-hardness dramatically. A layer of fine grain resulting from unmelted filler metal was found in the backing weld part of the joint. The residual strain in the heat affected zone(HAZ) of 316 L was higher than that in other regions. Meanwhile, drastic variations in the microstructure, chemical composition distribution and grain boundary character distribution(GBCD) in both the 316L/52 Mw and the 52Mb/A508 interface regions were observed. The analyses using TEM and MFM test showed that a large number of chromium and molybdenum- rich precipitates particles distributed both along the grain boundaries and inside grains in the 316 L base metal, which were identified to be precipitates with complex elementary composition rather than the normal string delta ferrite in 316 L austenitic stainless steel. The SKPFM test result indicated that these precipitates were more prone to be corroded than the base metal. Therefore, further investigation about the cause of deformation and the impacts to the corrosion resistance, particularly the stress corrosion cracking(SCC) sensitivity of the precipitates needs to be carried out.

Grain boundary character distribution of CuNiSi and FeNi alloys processed by severe plastic deformation

In this work the Grain Boundary Character Distribution (GBCD) in general and the relative proportion of low-Σ CSL (Coincidence Site Lattice) grain boundaries are determined through EBSD in Cu-2.5Ni-0.6Si (wt.%) and Fe-36Ni (wt.%) alloys after processing by high-pressure torsion, equal- channel angular pressing and accumulative roll bonding.

Grain boundary character distribution derived from three-dimensional microstructure reconstruction

Manual serial sectioning which includes consecutive steps of sample preparation and Electron Back Scattering Diffraction (EBSD) measurement was employed to extract the twodimensional (2D) sections of a pure nickel sample and to reconstruct the three-dimensional (3D) microstructure. A general alignment algorithm based on the minimization of misorientation between two adjacent sections has been developed to accurately align the sections. By employing this alignment algorithm, any in-plane (translational) and rotational misalignment as well as the planparallelity can be corrected. Surface triangulation technique was used to reconstruct the grain boundary surfaces. The Grain Boundary Character Distribution (GBCD) was derived from reconstructed grain boundaries. The results show that a smoother grain boundary plane can be obtained after precise translational and rotational alignment and correction of planparallelity.The relative grain boundary energy was computed as a function of the five grain boundary parameters based on equilibrium at triple lines. The results show that the grain boundary planes carrying a Σ3 type misorientation are dominantly parallel to the {111} crystal plane, which indicates the presence of coherent twin boundaries. It was observed that coherent Σ3 type boundaries exhibit the minimum relative grain boundary energy, which is approximately 57% smaller than the average of all Σ3 boundaries, including also incoherent twin boundaries.

Grain boundary engineering in a thermo-mechanically processed Nb-stabilized austenitic stainless steel

Three different thermo-mechanical strategies—annealing, strain recrystallization and strain annealing—were applied to a Nb-stabilized 304H austenitic stainless steel in order to study their effects on grain boundary character distribution (GBCD). An Electron Backscatter Diffraction (EBSD) analysis revealed specific combinations of cold reduction-temperature-time that favor annealing twinning. A uniform increase in microstructural size and special boundaries (particularly for Σ3, Σ9 and Σ27 boundaries) was achieved under strain annealing conditions (low cold reductions) and long times at high temperatures (≥ 990°C). These conditions provide a high fraction of special boundaries (about 80%), which replace the random grain boundary network and thus optimize the GBCD. The profuse presence of Σ3n boundaries is attributed to the geometric interaction of twin-related variants during grain boundary migration. In addition to all this, precipitation takes place at the temperature range where optimum GBCD is achieved. The significance of precipitation in the different strategies was also tackled.

Control of grain boundary character distribution and its effects on the deformation of Fe–6.5 wt.% Si

We demonstrate control of the grain boundary character distribution (GBCD) in Fe–6.5wt.% Si using directional recrystallization. This processing leads to an elongated grain structure through directional grain boundary migration, producing a GBCD with a high frequency of both low angle and low Σ coincidence site lattice boundaries. A thickness reduction during cold rolling of ∼30% can be obtained in the resulting material.

Recent Developments in Material Microstructure: a Theory of Coarsening

ABSTRACTCellular networks are ubiquitous in nature. Most engineered materials are polycrystalline microstructures composed of a myriad of small grains separated by grain boundaries, thus comprising cellular networks. The recently discovered grain boundary character distribution (GBCD) is an empirical distribution of the relative length (in 2D) or area (in 3D) of interface with a given lattice misorientation and normal. During the coarsening, or growth, process, an initially random grain boundary arrangement reaches a steady state that is strongly correlated to the interfacial energy density. In simulation, if the given energy density depends only on lattice misorientation, then the steady state GBCD and the energy are related by a Boltzmann distribution. This is among the simplest non-random distributions, corresponding to independent trials with respect to the energy. Why does such simplicity emerge from such complexity? Here we describe an entropy based theory which suggests that the evolution of the GBCD satisfies a Fokker-Planck Equation, an equation whose stationary state is a Boltzmann distribution.