High-energy Grain Research Articles

Quantification of grain boundary connectivity for predicting intergranular corrosion resistance in BFe10-1-1 copper-nickel alloy

Abstract

Lower bound on grain boundary solubility in immiscible alloys

Immiscible alloys are postulated to exhibit increased solubility in non-crystalline regions and specifically in high energy grain boundary (GB) regions. A mean field model describing the lower bound of GB solubility is offered for highly immiscible dilute systems at low temperatures. The results of the model are compared to molecular dynamics simulations of mixing in dilute, highly immiscible, copper alloys with both lattice type and size mismatch. It is shown that solubility limit can be described using an effective nearest neighbor model of the type used to describe solubility in low mismatch, high solubility systems. It is demonstrated that the model can be generalized by using a single parameter set to describe all calculated systems. Variation of the solubility limit and model parameters with properties such as GB energy, temperature and pressure is presented and explained in terms of the simple model. This presents an opportunity for creating a simple transferable model which can allow reliable calibration for realistic systems. The model parameters are shown to offer a low temperature correction to the Fowler–Guggenheim adsorption isotherm.

Abnormal Grain Growth in AlScN Thin Films Induced by Complexion Formation at Crystallite Interfaces

Sputter deposited Al(1–x)ScxN thin films with a Sc content from x = 0 to 43 at% are investigated by electron microscopy in order to study and explain the formation and growth of abnormally oriented grains (AOG). It is found that the latter did not nucleate at the interface with the substrate, but at high energy grain boundaries, at which systematically higher Sc concentrations are detected. The AOGs are thus formed during the growth of c‐textured grains. They grow faster than those, and finally protrude from the c‐textured film surface, having at their end a pyramidal shape with three facets of a hexagonal wurtzite crystal: one (0001) and two (110) facets. Process conditions favoring less compact grain boundaries, and lower surface diffusion across grain boundaries are thought to promote nucleation of AOGs. Finally, a 4‐step growth mechanism explaining the nucleation from a Sc‐rich complexion and proliferation of AOGs with increasing film thickness is proposed.

Machine learning determination of atomic dynamics at grain boundaries

In polycrystalline materials, grain boundaries are sites of enhanced atomic motion, but the complexity of the atomic structures within a grain boundary network makes it difficult to link the structure and atomic dynamics. Here, we use a machine learning technique to establish a connection between local structure and dynamics of these materials. Following previous work on bulk glassy materials, we define a purely structural quantity (softness) that captures the propensity of an atom to rearrange. This approach correctly identifies crystalline regions, stacking faults, and twin boundaries as having low likelihood of atomic rearrangements while finding a large variability within high-energy grain boundaries. As has been found in glasses, the probability that atoms of a given softness will rearrange is nearly Arrhenius. This indicates a well-defined energy barrier as well as a well-defined prefactor for the Arrhenius form for atoms of a given softness. The decrease in the prefactor for low-softness atoms indicates that variations in entropy exhibit a dominant influence on the atomic dynamics in grain boundaries.

Forage type influences milk yield and ruminal responses to wheat adaptation in late-lactation dairy cows

The effects of different wheat adaptation strategies on ruminal fluid pH, dry matter intake (DMI) and energy-corrected milk (ECM) were measured in 28 late-lactation dairy cows. Cows were fed either perennial ryegrass (PRG) hay or alfalfa hay and had no previous wheat adaptation. Wheat was gradually substituted for forage in 3 even increments, over 6 or 11 d, until wheat made up 40% of DMI (∼8 kg of dry matter/cow per day). We found no differences in DMI between adaptation strategies (6 or 11 d) within forage type; however, cows fed alfalfa hay consumed more overall and produced more ECM. The rate of ruminal pH decline after feeding, as well as the decrease in mean, minimum, and maximum ruminal pH with every additional kilogram of wheat was greater for cows fed alfalfa hay. Cows fed alfalfa hay and on the 6-d adaptation strategy had the lowest mean and minimum ruminal fluid pH on 3 consecutive days and were the only treatment group to record pH values below 6.0. Despite ruminal pH declining to levels typically considered low, no other measured parameters indicated compromised fermentation or acidosis. Rather, cows fed alfalfa hay and adapted to wheat over 6 d had greater ECM yields than cows on the 11-d strategy. This was due to the 6-d adaptation strategy increasing the metabolizable energy intake in a shorter period than the 11-d strategy, as substituting wheat for alfalfa hay caused a substantial increase in the metabolizable energy concentration of the diet. We found no difference in ECM between adaptation strategies when PRG hay was fed, as there was no difference in metabolizable energy intake. The higher metabolizable energy concentration and lower intake of the PRG hay meant the increase in metabolizable energy intake with the substitution of wheat was less pronounced for cows consuming PRG hay compared with alfalfa hay. Neither forage type nor adaptation strategy affected time spent ruminating. The higher intakes likely contributed to the lower ruminal pH values from the alfalfa hay treatments. However, both forages allowed the rumen contents to resist the large declines in ruminal pH typically seen during rapid grain adaptation. Depending on the choice of base forage, rapid grain introduction may not result in poor adaptation. In situations where high-energy grains are substituted for a low-energy, high-fiber basal forage, rapid introduction could prove beneficial over gradual strategies.

From metallic glasses to nanocrystals: Molecular dynamics simulations on the crossover from glass-like to grain-boundary-mediated deformation behaviour

Nanocrystalline metals contain a large fraction of high-energy grain boundaries, which may be considered as glassy phases. Consequently, with decreasing grain size, a crossover in the deformation behaviour of nanocrystals to that of metallic glasses has been proposed. Here, we study this crossover using molecular dynamics simulations on bulk glasses, glass–crystal nanocomposites, and nanocrystals of Cu64Zr36 with varying crystalline volume fractions induced by long-time thermal annealing. We find that the grain boundary phase behaves like a metallic glass under constraint from the abutting crystallites. The transition from glass-like to grain-boundary-mediated plasticity can be classified into three regimes: (1) For low crystalline volume fractions, the system resembles a glass–crystal composite and plastic flow is localised in the amorphous phase; (2) with increasing crystalline volume fraction, clusters of crystallites become jammed and the mechanical response depends critically on the relaxation state of the glassy grain boundaries; (3) at grain sizes ≥10nm, the system is jammed completely, prohibiting pure grain-boundary plasticity and instead leading to co-deformation. We observe an inverse Hall–Petch effect only in the second regime when the grain boundary is not deeply relaxed. Experimental results with different grain boundary states are therefore not directly comparable in this regime.

Selective Grain Growth and Grain Boundary Character Distribution Relationships in Magnetostrictive Fe-Ga Thin Sheets

Grain growth and grain boundary character distribution relationships in Fe-Ga rolled sheet is investigated to study the influence of H2S gas content in argon on the development of selective grain growth through secondary recrystallization. Abnormal growth of (011) grains was predominant at the low content of H2S gas, while (113) grain growth was well developed at contents higher than 1.33% H2S. On the other hand, the development of (001) grains was challenging to produce because it is very sensitive to the anneal environment and has a relatively low fraction of high energy grain boundaries associated with misorientation angles that determine the mobility of boundaries.

Effects of rolling conditions on recrystallization microstructure and texture in magnetostrictive Fe-Ga-Al rolled sheets

The effects of different rolling conditions on the microstructure and texture of primary and secondary recrystallization in magnetostrictive Fe82Ga9Al9+0.1at%NbC alloy sheets were investigated. After the primary recrystallization annealing at 850 °C for 5 min, the as-rolled sheets prepared by warm-cold rolling with an intermediate annealing, can be fully recrystallized, and obtain the homogeneous matrix in which the fine dispersed NbC precipitate particles are distributed. The primary recrystallization textures of sheets with different rolling conditions consist mostly of strong {1 0 0} textures, γ-fiber textures, {4 1 1}〈1 4 8〉 texture and weak Goss texture. In the primary recrystallized sheets prepared by warm-cold rolling with an intermediate annealing, the high energy grain boundaries and ∑9 boundaries have the highest proportion. After high temperature annealing, the secondary recrystallizations of Goss grains in these sheets are more complete, and the size of abnormal grown Goss grain is up to several centimeters, which results in the strongest Goss texture. Correspondingly, the largest magnetostriction of 183 ppm is observed. The sample prepared by warm-cold rolling with an intermediate annealing, has homogeneous primary matrix, special texture components and grain boundary distribution, all of which provide a better surrounding for the abnormal growth of Goss grains. This work indicates that the control of rolling conditions of Fe-Ga-Al alloy sheets is necessary to achieve the strong Goss texture and obtain a possible high magnetostriction if other appropriate conditions (stress, domain structure) are achieved.

Secondary recrystallisation behaviours of grain-orientated silicon steel produced by TSCR process

ABSTRACTGrain-oriented electrical steel is mainly used as the core material in electrical transformers, and its magnetic properties are closely related to the sharpness of Goss texture formed by secondary recrystallisation during high-temperature annealing. However, the mechanisms of abnormal Goss grain growth are still disputed in the literature. In this paper, grain-oriented silicon steel strips with AlN as main inhibitor were produced by thin slab casting and rolling process, and the evolution of microstructure and texture during high-temperature annealing was investigated. Moreover, the mechanisms of secondary recrystallisation were further discussed. The results show that secondary recrystallisation is actually a multi-stage process, and different mechanisms are at work at different stages. Coincident site lattice grain boundaries play an important role at the early stage of secondary recrystallisation, however, the role of high-energy grain boundaries and solid-state wetting should be more important at the later stage of secondary recrystallisation.

Cs diffusion in SiC high-energy grain boundaries

Cesium (Cs) is a radioactive fission product whose release is of concern for Tristructural-Isotropic fuel particles. In this work, Cs diffusion through high energy grain boundaries (HEGBs) of cubic-SiC is studied using an ab-initio based kinetic Monte Carlo (kMC) model. The HEGB environment was modeled as an amorphous SiC, and Cs defect energies were calculated using the density functional theory (DFT). From defect energies, it was suggested that the fastest diffusion mechanism is the diffusion of Cs interstitial in an amorphous SiC. The diffusion of Cs interstitial was simulated using a kMC model, based on the site and transition state energies sampled from the DFT. The Cs HEGB diffusion exhibited an Arrhenius type diffusion in the range of 1200–1600 °C. The comparison between HEGB results and the other studies suggests not only that the GB diffusion dominates the bulk diffusion but also that the HEGB is one of the fastest grain boundary paths for the Cs diffusion. The diffusion coefficients in HEGB are clearly a few orders of magnitude lower than the reported diffusion coefficients from in- and out-of-pile samples, suggesting that other contributions are responsible, such as radiation enhanced diffusion.

Microstructural evolution and solid state dewetting of epitaxial Al thin films on sapphire (α-Al2O3)

Solid state dewetting can be used for targeted patterning, but also causes degradation or failure of thin film devices. In this work the temperature-induced changes of a tetracrystalline model system with inhibited surface diffusion are studied. This is accomplished by growing Al thin films by molecular beam epitaxy on single crystalline (0001) oriented sapphire substrates. The as-deposited Al films form two orientation relationships (OR I and OR II) both subdivided in two twin-related growth variants leading to a tetracrystalline microstructure. Two processes evolve during annealing at 600 °C. Grain growth and texture evolution towards OR II occur in addition to the formation of drum-like voids in the Al film covered by a thin membrane. The surface oxide suppresses Al surface diffusion and in contrast to classical solid state dewetting interface and grain boundary diffusion dominate. High energy grain boundaries were identified as initial points of the void formation.

Stress corrosion crack initiation of alloy 600 in PWR primary water

Stress corrosion crack (SCC) initiation of three mill-annealed alloy 600 heats in simulated pressurized water reactor primary water has been investigated using constant load tests equipped with in-situ direct current potential drop (DCPD) measurement capabilities. SCC initiation times were greatly reduced by a small amount of cold work. Shallow intergranular attack and/or cracks were found on most high-energy grain boundaries intersecting the surface with only a small fraction evolving into larger cracks and intergranular SCC growth. Crack depth profiles were measured and related to DCPD-detected initiation response. Processes controlling the SCC initiation in mill-annealed alloy 600 are discussed.

Indentation-induced solid-state dewetting of thin Au(Fe) films

We studied the effect of local plastic deformation on the thermal stability and solid-state dewetting of thin homogeneous Au(Fe) films deposited on sapphire substrates. The films with ordered square arrays of indents produced by nanoindentation were annealed at the temperature of 700°C in a forming gas atmosphere. The behavior of the film in the region of shallow indents (reaching a depth up to one half of the film thickness) was very different from the one in the region of deep indents (with depths greater than one half of the film thickness). In the first case, the grain growth in indented and unperturbed regions of the film proceeded quite similarly, and nearly complete healing of the indents was observed. In the latter case, a recrystallization process in the vicinity of the indents resulted in the formation of small new grains with misorientation angles that were not present in the as-deposited film. The thermal grooving along the corresponding new high-energy grain boundaries caused an increase of the depth of the indents and the formation of the dewetting holes. The morphology of these holes and their size were different compared to the holes formed randomly in the unperturbed regions of the same films. In particular, the interaction between the individual indents of an array led to the preferential formation of holes at the periphery of the arrays. These findings shed a new light on the process of nucleation of the solid-state dewetting in thin films.

The three different types of paleokarstification and reservoir distribution of Leikoupo Formation, Middle Triassic in the northern Sichuan Basin, China

The Middle Triassic Leikoupo Formation in the northern Sichuan Basin is a new discovery sequence for strategy replacement in the gas exploration field. The carbonate paleokarst reservoirs of the Middle Triassic Leikoupo Formation in the northern Sichuan Basin provide a case study of gas exploration in carbonate paleokarst reservoirs. By conducting a comprehensive study of well drilling, drill cores, well loggings, and test data, combined with the analysis of geological background of the Sichuan Basin, it is considered that syndepositional, supergene, and burial paleokarstification can be identified in the Leikoupo Formation, and these types of karstification were vital for reservoirs formation. Syndepositional karstification controlled the formation and distribution of early reservoirs. This type of karstification was limited to high-energy grain shoals, and therefore its horizontal extent was limited by the distribution of this facies. During temporary episodes of relative sea level fall, carbonate shoals were exposed subaerially or within the fresh water lens, and significant selective dissolution occurs, forming moldic pores and intragranular dissolved pores. The reservoirs related to supergene karstification mainly occurred in the area of the buried hill containing carbonate rock at the top of the Leikoupo Formation, and are vertically limited within a depth of 70–90 m under the carbonate corrosion unconformity plane of the Leikoupo Formation. The reservoirs related to supergene karstification formed mainly in limestone breccia and vuggy limestone in surface karst zone, and the high-angle dissolved fracture and medium- and small-scale corrosion holes in vertical vadose zone, and medium and small corrosion holes, dissolved fractures, roof fracture-caves in horizontal karst zone. The karst monadnocks within karst slope are the most favorable microgeomorphic units for horizontal distribution of reservoirs. Burial karstification was related to dissolution of carbonate minerals by organic acids, and was the key factor in optimal transformation of reservoirs. Burial karstification corroded and expanded the pore system left after infilling of the earlier pore system, and mainly was to optimize the transformation the reservoir space redistribution of earlier pore system. The compact carbonate layers related to burial karstification only can be dissolved along the early cracks in the channel and formation cracked reservoirs, but development of this type of reservoir was limited. The reservoirs were influenced by syndepositional, supergene, and burial paleokarstification in the Leikoupo Formation, and carbonate paleokarst reservoirs were formed finally.

Visualizing Arsenate Reactions and Encapsulation in a Single Zero-Valent Iron Nanoparticle.

A nanostructure-based mechanism is presented on the enrichment, separation, and immobilization of arsenic with nanoscale zero-valent iron (nZVI). The As-Fe reactions are studied with spherical aberration corrected scanning transmission electron microscopy (Cs-STEM). Near-atomic resolution (<1 nm3) electron tomography discovers a thin continuous layer (23 ± 3 Å) of elemental arsenic sandwiched between the iron oxide shell and the zerovalent iron core. This points to a unique mechanism of nanoencapsulation and proves that the outer layer, especially the Fe(0)-oxide interface, is the edge of the As-Fe reactions. Atomic-resolution imaging on the grain boundary provides strong evidence that arsenic atoms diffuse preferably along the nonequilibrium, high-energy, and defective polycrystalline grain boundary of iron oxides. Results also offer direct evidence on the surface sorption or surface complex formation of arsenate on ferric hydroxide (FeOOH). The core-shell structure and unique properties of nZVI clearly underline rapid separation, large capacity, and stability for the treatment of toxic heavy metals such as cadmium, chromium, arsenic, and uranium.

Evolution of Recrystallization Textures in Cold-Rolled Commercially Pure Aluminum

The evolution of recrystallization texture in commercially pure aluminum sheet cold-rolled by 90% reduction in thickness was measured by Juul Jensen et al. The cold-rolling texture consisted of the Goss {110}&lt;001&gt;, Brass {110}&lt;112&gt;, S {123}&lt;634&gt;, and copper {112}&lt;111&gt; components. When the cold-rolled aluminum sheet was annealed at temperatures between 253 and 341°C for times between 5 min and 20 h., the cube {001}&lt;100&gt; component evolved. The evolution of the cube texture cannot be explained by either the oriented nucleation theory by Burgers and Louwerse or the oriented growth theory by Barrett. The cube texture evolution originates from the Copper component by the strain-energy-release-maximization (SERM) theory by Lee. Once the Cube oriented, dislocation free nuclei evolve, they are in the best position to grow at the expense of neighboring deformed high energy grains of the Goss, Brass, S, and Copper orientations, and the volume fractions of the Goss, Brass, S, and copper components would decrease. However, the volume fraction of the Goss component increased a little at annealing temperatures of 253 and 278°C, at variance with expectation. Low stacking-fault-energy alloys with the brass {110}&lt;112&gt; rolling texture evolve the {236}&lt;385&gt; texture after recrystallization, whereas high stacking-fault-energy alloys with the brass rolling texture evolve the Goss texture after recrystallization by the SERM theory, resulting in the increase of the volume fraction of the Goss texture.

Resolving the Strength-Ductility Paradox through Severe Plastic Deformation of a Cast Al-7% Si Alloy

Ultrafine-grained (UFG) materials produced by severe plastic deformation (SPD) may show both enhanced ductility and strength and hence resolve the so-called strength-ductility paradox. To gain mechanistic insights into such resolution, the effect of high-pressure torsion (HPT) on the microstructure and mechanical behavior was studied using a cast Al-7 wt. % Si alloy. As expected, the grain size decreased while the fraction of high-angle grain boundaries and microhardness increased due to HPT processing. However, tensile testing at room temperature revealed a simultaneous increase in strength and ductility compared to the as-cast sample. The samples showing simultaneous increase in strength and ductility also showed an increased contribution from grain boundary sliding (GBS), even at room temperature, which is attributed to the existence of a high fraction of high-angle and high-energy grain boundaries. It is proposed that the occurrence of moderate GBS, providing ductility, in very small size grains provides Hall-Petch strengthening and this suggests a potential combination for simultaneously achieving high strength and high ductility in SPD-processed UFG materials.

Correlation between Primary and Secondary Recrystallization Texture Components in Low-temperature Reheated Grain-oriented Silicon Steel

Low-temperature slab-reheated grain-oriented silicon steel is characterized by a sharp {411}<148> primary recrystallization texture. To date, the influence of this texture on secondary recrystallization is not clear. Micro-textures in primary and secondary recrystallizcd sheets of low-temperature reheated grain-oriented silicon steel were examined using electron backscatter diffraction. By comparing the textures and microstructures of specific primary recrystallized grains neighboring secondary grains with those of other primary grains, the influences of primary recrystallization textures and microstructures on the orientations of secondary grains were investigated. Results show that for low-temperature reheated grain-oriented silicon steel, the primary recrystallization sheet comprises {411}<148>, {111}<112>, and {001}<120> texture components. During secondary recrystallization, the {111}<112> primary recrystallized grains were easily consumed by abnormally grown Goss, deviated Goss, Brass, or {210}<001> grains; the {411}<148> primary recrystallized grains were more resistant to being swallowed; and the {001}<120> grains were the most resistant to being consumed. For a particular primary grain, the distribution of its surrounding grain boundaries determined how easily it is consumed during secondary recrystallization. Primary grains surrounded by 20°–15° grain boundaries were consumed much earlier than those having grain boundaries above 15°, which is in accordance with high-energy grain boundary theory. In addition, special ∑9 boundaries between {111}<148> and Goss grains move more slowly than ∑9 boundaries between {111}<112> and Goss grains, which is attributed to the different positions of <110> rotation axis with respect to the normals of grain boundaries.

Ag diffusion in SiC high-energy grain boundaries: Kinetic Monte Carlo study with first-principle calculations

The diffusion of silver (Ag) impurities in high energy grain boundaries (HEGBs) of cubic (3C) silicon carbide (SiC) is studied using an ab initio based kinetic Monte Carlo (kMC) model. This study assesses the hypothesis that the HEGB diffusion is responsible for Ag release in Tristructural-Isotropic (TRISO) fuel particles, and provides a specific example to increase understanding of impurity diffusion in highly disordered grain boundaries. The HEGB environment was modeled by an amorphous SiC (a-SiC), which approximately represents the local environments of HEGBs. The structure and stability of Ag defects were calculated using ab initio methods based on Density Functional Theory (DFT). The defect energetics suggested that the fastest diffusion takes place via an interstitial mechanism in a-SiC and that is what is modeled. Interstitial sites were identified by a gridded search and transition states were computed using the climbing image nudged elastic band method. The formation energy of Ag interstitials (relative to bulk Ag) and the Kinetic Resolved Activation (EKRA) energies between them were well approximated with Gaussian distributions that were then sampled in the kMC. The diffusion of Ag was simulated with the effective medium model using kMC. Ag diffusion coefficients were estimated for the temperature range of 1200–1600°C. Ag in a HEGB is predicted to exhibit an Arrhenius type diffusion and with a diffusion prefactor and effective activation energy of (2.73±1.09)×10−10m2s−1 and 2.79±0.18eV, respectively. The comparison between HEGB results to other theoretical studies suggested not only that GB diffusion is predominant over bulk diffusion, but also that the HEGB is one of fastest grain boundary paths for Ag diffusion in SiC. The Ag diffusion coefficient in the HEGB shows a good agreement with ion-implantation measurements, but is 2–3 orders of magnitude lower than the reported diffusion coefficient values extracted from integral release measurements on in- and out-of- pile samples. The discrepancy between GB diffusion and integral release measurements suggests that other contributions are responsible for fast release of Ag in some experiments and we propose that these contributions may arise from radiation enhanced diffusion.

Comparison of 20 nm silver nanoparticles synthesized with and without a gold core: Structure, dissolution in cell culture media, and biological impact on macrophages.

Widespread use of silver nanoparticles raises questions of environmental and biological impact. Many synthesis approaches are used to produce pure silver and silver-shell gold-core particles optimized for specific applications. Since both nanoparticles and silver dissolved from the particles may impact the biological response, it is important to understand the physicochemical characteristics along with the biological impact of nanoparticles produced by different processes. The authors have examined the structure, dissolution, and impact of particle exposure to macrophage cells of two 20 nm silver particles synthesized in different ways, which have different internal structures. The structures were examined by electron microscopy and dissolution measured in Rosewell Park Memorial Institute media with 10% fetal bovine serum. Cytotoxicity and oxidative stress were used to measure biological impact on RAW 264.7 macrophage cells. The particles were polycrystalline, but 20 nm particles grown on gold seed particles had smaller crystallite size with many high-energy grain boundaries and defects, and an apparent higher solubility than 20 nm pure silver particles. Greater oxidative stress and cytotoxicity were observed for 20 nm particles containing the Au core than for 20 nm pure silver particles. A simple dissolution model described the time variation of particle size and dissolved silver for particle loadings larger than 9 μg/ml for the 24-h period characteristic of many in-vitro studies.