Island Grains Research Articles

Multimaxima crystallographic fabrics (CPO) in warm, coarse-grained ice: New insights

Multimaxima crystallographic c-axis fabrics (CPOs) commonly observed in coarse-grained ice have been interpreted in several different ways but remain enigmatic. We review previous interpretations of these fabrics and present new data from Storglaciären, northern Sweden, for comparative purposes using both U-stage methods (c-axes only) and electron backscatter diffraction (EBSD) (both c-axis and a-axis orientations). Consistent with most previous studies, microstructures in our study indicate that all grains have been dynamically recrystallized with grain boundary migration as the dominant process. The CPO is characterized by grains oriented favorably for easy glide on the basal plane, given the local kinematics. When a representative sample is obtained, areas on the glacier margin undergoing non-coaxial deformation have a CPO generally consistent with simple shear. Multimaxima CPOs developed in the center of the glacier, which is associated with more coaxial deformation, likely represent a small circle girdle or partial girdle fabric associated with flow-parallel compression. In both locations, sampling bias, resulting from repeat measurements of single parent grains appearing several times as “island grains” in a 2D thin section, is likely responsible for the observed multiple maxima that, with bias removed, simplify to patterns expected for simple shear or coaxial deformation. An earlier study suggesting that multimaxima CPOs may result from twinning is not borne out from our data combining c-axes and a-axes that do not show the symmetry required of twinning.

Anomalous texture developments, new two-dimensional primary growth modeling, and fully [formula omitted]-textured electrical steels

Various anomalous texture developments have been investigated in C-free and AlN-free electrical sheets, and new two-dimensional (2D) primary growth modeling has been carried out to understand them. In one-stage cold-rolled sheets, the primary (001) texture pole intensity (TPI) increases with decreasing bulk sulfur content and sheet thickness. The high-index hkl and (001) TPIs increase concurrently with increasing annealing time. In other case, the (001) TPI increases to a maximum after which it rather decreases. In two-stage cold-rolled sheets showing the primary 001+(110) texture, the (001) TPI increases with decreasing sheet thickness and second-stage cold-rolling ratio (CRR). The final (001) grain size is determined by the combination between sheet thickness, bulk sulfur content, CRR, heating rate, and MnS precipitation reaction. New 2D primary growth modeling supports strongly that the higher surface energy grains as well as the lower surface energy ones can grow, although their growth is transient and peripheral. Several unique concepts implied in the 2D growth maps well explain the various anomalous texture developments. The (111) island grains in electrical sheets fully (001)-textured after final annealing are always consumed with further annealing, regardless of the grain size. However, the (110) island grains may grow or not, which depends on the grain size and the number of neighboring ones. Fully developed (110)[001] Goss or (001) texture can be obtained from electrical sheets of the same composition, depending on the processing variables. Fully (001)-textured electrical sheets of high magnetic properties were developed through the experimental results and the 2D growth modeling.

Persistence of abnormal grain growth in the presence of grain boundary complexion transitions: Thermodynamic analysis and phase field modeling

An anomalous feature in polycrystalline grain growth in metals and ceramics is the rapid abnormal growth of a few grains relative to the surrounding matrix grains. In certain systems, this phenomenon is reported to be associated with the presence of first order phase transitions in the grain boundaries often referred to as complexion transitions. The fundamental question of how the growth advantage of abnormally growing grains persists in this context is not yet well understood. We present a thermodynamically consistent mechanism by which abnormal grain growth may occur. The presence of two complexion transitioned boundaries at a triple junction provides a driving force for partial wetting at the triple junction leading to the formation of a quadruple junction. The subsequent dissociation of the quadruple junction into two partial wetting triple junctions favors the persistence of abnormal grain growth. We incorporate this dissociation process into a multi-order parameter phase field model and demonstrate the occurrence of abnormal grain growth without the need for mobility advantage or additional driving forces. The results from simulations of abnormal grain growth capture experimentally observed microstructural features such as peninsular and island grains. Through a systematic study we show that the initial fraction of complexion boundaries plays a significant role in abnormal grain growth and that maximum abnormal grain growth is observed when the fraction of complexion boundaries is less than about 2%.

Stabilizing Zinc Anodes by a Uniform Nucleation Process with Cysteine Additive.

Zinc anodes in aqueous batteries face challenges such as dendrite growth and interfacial instability. This study investigates the use of cysteine as an electrolyte additive to address these issues. By establishing the correlation between the size of zinc nuclei and the surface tensions/contact angle at the electrolyte-anode interface, it is demonstrated that the addition of cysteine in the electrolyte alters the surface tensions/contact angle at the electrolyte-anode interface and the nucleation process of zinc. This alteration results in the formation of smaller and more dispersed nuclei, as opposed to the formation of larger island grains. This has a profound impact on the subsequent deposition growth process, enabling smooth and uniform zinc electrodeposition without the formation of dendrites. Additionally, cysteine molecules create a stable interface during zinc plating and stripping, effectively preventing corrosion from side reactions. The incorporation of cysteine in the electrolyte significantly enhances cycling stability and extends the lifespan of zinc anodes in aqueous batteries.

Ag/Ag2S–TiO2 composite film formed using dual phase Ag/Ag2O layer as a sulfurized layer for enhanced photocatalytic activity

The Ag/Ag2S–TiO2 composite films were synthesized through vulcanizing sputtering Ag/Ag2O dural-phase template capping layer on the TiO2 film. After hydrothermal vulcanization using thiourea as a sulfur precursor, the initially decorated Ag2O layer transformed into the Ag2S. The surface morphology of the continuous block structure of the Ag/Ag2S composite layer gradually shrinks to form irregular island grains dispersed on the surface of the composite film, with vulcanization reaction duration increased from one to 3 h. The light absorption ability of the Ag/Ag2S–TiO2 composite film was enhanced substantially compared to the TiO2 film because of the surface plasmon resonance effect from the Ag particles and the narrow energy band of the Ag2S. Among various surface morphologies of the Ag/Ag2S–TiO2 composite films, the irregular island Ag/Ag2S grains-decorated TiO2 composite film presents superior photoelectrochemical and photocatalytic degradation performances than the composite films consisting of continuous block and spider-web structure of the Ag/Ag2S decorated layers. The synergistic effects of surface plasmon resonance, Ag pathway between the semiconductors, and Z-scheme charge transfer mechanism explain the high-efficiency photocatalytic activity of the Ag/Ag2S–TiO2 composite films.

On the variability of grain boundary mobility in the isoconfigurational ensemble

Recent grain growth experiments have revealed that the same type of grain boundary can have very different mobilities depending on its local microstructure. In this work, we use molecular dynamics simulations to quantify uncertainty in the reduced mobility of curved grain boundaries for different types of boundary conditions and over a range of initial velocity seeds. We consider cylindrical island grains in Ni with a [001] tilt axis as a model system. Unexpectedly, we find large variation in the reduced mobility of curved grain boundaries depending on the imposed constraints, the initial velocity distribution and grain size. We perform a dynamic propensity analysis inspired by studies of glass forming liquids to analyze sources of variation in reduced mobility. Our work highlights the significant impact of initial velocity distributions on grain boundary motion which has not been analyzed in prior work.

Investigating the corrosion resistance of Cu-doped Ni-Mo low alloy steel through electrochemical tests

Corrosion resistance of Cu-doped Ni-Mo low alloy steel (Ni-Mo steel) in marine environments was systematically investigated through microstructure observation and galvanostatic polarization tests. Results show that solid solution Cu improved the corrosion thermodynamic stability of steel. The increasing Cu content promoted the formation of Cu-rich precipitates, martensite-austenite island, granular bainitic and high angle grain boundary, increasing the surface potential difference and the occurrence probability of local galvanic corrosion on steel surface. The addition of Cu improved the protectiveness of corrosion product film after galvanostatic polarization tests relying on the functions of CuFeO2 and CuO, whose formation was mainly related to the solid solution Cu in Ni-Mo steel.

Phase-Field Simulation on the Effect of Second-Phase Particles on Abnormal Growth of Goss Grains in Fe-3%Si Steels.

A phase-field model was revised to study the abnormal growth of Goss grains during the annealing process in Fe-3%Si steels, in which the interaction between the second-phase particles and Goss grain boundaries (GBs) was considered. The results indicate that the abnormal growth of Goss grains occurs due to the different dissolvability of the particles at Goss GBs compared with the other GBs. Moreover, the degree of abnormal growth increases first and then decreases with an increasing particle content. Meanwhile, the size advantage of Goss grain can further promote the degree of abnormal growth. Two types of island grains were found according to the simulated results, which is consistent with the experimental observations. A proper GB dissolvability of particles is the key factor for the formation of isolated island grains, and a higher local particle density at GBs is the main reason for the appearance of serial island grains. These findings can provide guidance for the desired texture control in silicon steels.

Synthesis of a Gold-Inserted Iron Disilicide and Rutile Titanium Dioxide Heterojunction Photocatalyst via the Vapor-Liquid-Solid Method and Its Water-Splitting Reaction.

A solid-state Z-scheme system is constructed whereby rutile titania (TiO2) and beta-iron disilicide (β-FeSi2) were combined to act as oxygen (O2)- and hydrogen (H2)-evolution photocatalysts, respectively, connected by gold (Au). β-FeSi2 island grains with diameters in the 0.5-2 μm range were formed on the surface of Au-coated TiO2 powder by the co-sputtering method. On the surface of TiO2 powder, the Au-Si liquidus phase was obtained via a Au-Si eutectic reaction, which contributed to the selective deposition and crystallization of β-FeSi2 island grains onto Au. After the loading of the H2-evolution cocatalysts platinum and chromium oxide onto β-FeSi2, the system obtained catalyzed the evolution of H2 and O2 in a stoichiometric ratio from pure water under ultraviolet light irradiation. The transfer of photoexcited electrons in the conduction band (CB) of β-FeSi2 to Pt causes the reduction of protons to H2, and the photogeneration of holes in the valence band (VB) of TiO2 causes the oxidation of water to O2. In addition, the photogenerated holes in the VB of β-FeSi2 and the photoexcited electrons in the CB of TiO2 combined with each other in the Au layer, affording the completion of the overall photocatalytic water-splitting.

Strength–Ductility Matching Mechanism for Multi-Phase Microstructure Control of High-Ductility Ship Plate Steel

Generally, the development of ship plate steels is mainly concerned with the improvement of strength and toughness, such as F32 and F36. Due to the strength–ductility trade-off, it is difficult to combine excellent ductility with strength improvement, resulting in a poor deformation ability of the traditional ship plate steels during collision. In the present study, a series of high-ductility ship plate steels with property gradients were obtained by multi-phase microstructure control. The strength–ductility matching mechanism was analyzed. Meanwhile, the roles of M/A islands and lamellar pearlites in plastic deformation were also revealed. The results show that the microstructure of “quasi-polygonal ferrite + granular bainite + M/A islands + fewer lamellar pearlites” has the best strength–ductility match. The excellent ductility is mainly dependent on dispersive kernel average misorientation, recrystallized grains without distortion, and soft grains. In addition, the longer branch crack can effectively relieve the stress concentration at the tip of the main crack. Compared with lamellar pearlites, the dispersed M/A island grains have a higher strength contribution and more stable γ-fibers, which is beneficial to delay the appearance of internal micro-voids and micro-cracks. However, the lamellar pearlites can coordinate deformation only when the orientation of thinner lamellae exceeds two.

Different formation mechanisms of {210} orientation in η fiber texture of ultra-thin grain-oriented silicon steel using quasi-in-situ analysis method

Using high-permeability grain-oriented silicon steel as starting material, an ultra-thin grain-oriented silicon steel was prepared by the primary recrystallization method, which was mainly composed of strong η fiber (<100>//RD) texture including major Goss ({110}<001>) component, a certain amount of {210}<001> component and a few Cube ({100}<001>) grains. In this study, the formation of η fiber texture, especially the {210}<001> component, was analyzed by a quasi-in-situ analysis method. The results show that the {210}<001> texture exists in two forms. The first type, characterized by some {210}<001> grains distributed in Goss grain colony, is common in the product. The texture originates from exact Goss initial grain, and the {210}<001> nuclei are mainly observed in shear bands of {111}<112> rolling texture. The {210}<001> nuclei are mainly distributed in the surface and subsurface layers and are formed by roller friction. The second type is a sharp {210}<001> texture together with a few scattered Cube grains. After rolling, large {210}<001> initial grain rotates toward {112}<241> deformation texture, which is swallowed by transition bands composed of many {210}<001> nuclei and a few Cube nuclei during annealing. This is because the transition bands act as principal sites for nucleation. A few Cube grain clusters originating from {210}<001> initial island grains are observed, and the Cube nuclei are formed by a strong rotation effect of initial grain boundaries. Due to the high-angle boundaries with Goss grains, the {210}<001> grains grow abnormally at higher temperature, and the magnetic properties deteriorate because of large deviation angle of <001> axis from the rolling direction.

Effect of hatch spacing and laser power on microstructure, texture, and thermomechanical properties of laser powder bed fusion (L-PBF) additively manufactured NiTi

This study systematically evaluates the effects of laser powder bed fusion additive manufacturing (L-PBF-AM) parameters (hatch spacing and laser power) on the thermomechanical behavior and microstructure of Ni50.8Ti49.2 shape memory alloy. The samples were fabricated with hatch spacings from 40 to 240 µm and laser powers of 50 and 100 W at a constant scanning speed of 125 mm/s, resulting in parts with volumetric energy density levels from 55 to 666 J/mm3 and two sets of linear energy densities of 0.4 and 0.8 J/mm. The results showed a reduced melt pool size and discontinuity of scan tracks with decreased laser power. Additionally, the porosity level was increased with larger hatch spacing and lower laser power. More notably, the transformation temperatures increased, and the critical stress, recoverable strain, and functional stability of samples improved with lower hatch spacing, where the recovery ratio of up to 90% was observed, regardless of the employed laser power. This study also discussed the relationship between the fabrication process and texture formation in the L-PBF-AM process. The advantage of L-PBF-AM was revealed in tailoring the microstructure from highly textured samples in [111] or [001] direction when hatch spacing lower than laser beam focused was employed, to the appearance of equiaxed solidification front with island grains and random orientations.

Stress sensitivity of high-temperature microstructures in ice, with potential applications to quartz

Quartz and ice both exhibit distinctive microstructures when deformed at low stress and high homologous temperature, known as grain-boundary migration (GBM) microstructures. These are difficult to reproduce experimentally in silicate minerals, and no correlation has been established between quantifiable aspects of the microstructure and deformational conditions. We carried out direct shear experiments to investigate the effect of stress and temperature (T) on GBM microstructures in ice, which is crystallographically analogous to quartz. Differential stress was 0.7–6.0 MPa, confining pressure 4–9 MPa, and T −3° to −25 °C. There is a clear transition at ∼ −12 °C from granular microstructures produced by rotation recrystallization at high stress and low T, to GBM microstructures at low stress and high T, accompanied by an increase in strength of the crystallographic preferred orientation (CPO). Samples with GBM microstructure show lobate grain-boundaries and “island grains” where several distinct and separate areas have the same crystallographic orientation, representing lobes of a single grain isolated on the cut surfaces. The size of lobes and island grains define an array with respect to stress with a slope of 0.9, similar to but offset from the slope of published stress/grain-size data. This suggests the possibility of determining paleostress from GBM microstructures in both ice and quartz.

Complete Goss Secondary Recrystallization by Control of the Grain Size and Texture of Primary Recrystallization in Grain-Oriented Silicon Steel.

Matrix microstructure and texture controlling is an important way to optimize Goss ({110}<001>) abnormal grain growth (AGG) in high magnetic induction grain-oriented silicon (Hi-B) steel during primary recrystallization. In the present work, a matrix with homogeneous grain size and favorable texture components was obtained through two-stage normalized annealing followed by primary recrystallization. Furthermore, secondary recrystallization was performed for sharp Goss orientation by slow heating and purified annealing. It was found that plenty of island grains, which occurred and disappeared gradually, accompanied the process of AGG. Through analyzing the evolution of microstructure and texture, we realized that the formation of island grains was related to the large-size grains in matrix, and the elimination of that was attributed to the special grain boundaries which satisfied both coincident site lattice (CSL) and high-energy (HE) models. It was essential to control grain size and favorable orientations in matrix comprehensively for the high-efficient abnormal growing of sharp Goss orientation, through which excellent magnetic properties could be obtained simultaneously.

Full crystallographic orientation (&amp;lt;i&amp;gt;c&amp;lt;/i&amp;gt; and &amp;lt;i&amp;gt;a&amp;lt;/i&amp;gt; axes) of warm, coarse-grained ice in a shear-dominated setting: a case study, Storglaciären, Sweden

Abstract. Microstructures provide key insights into understanding the mechanical behavior of ice. Crystallographic preferred orientation (CPO) develops during plastic deformation as ice deforms dominantly by dislocation glide on the basal plane, modified and often intensified by dynamic recrystallization. CPO patterns in fine-grained ice have been relatively well characterized and understood in experiments and nature, whereas CPO patterns in “warm” (T&gt;-10∘C), coarse-grained, natural ice remain enigmatic. Previous microstructural studies of coarse-grained ice have been limited to c-axis orientations using light optical measurements. We present the first study of a axes as well as c axes in such ice by application of cryo-electron backscatter diffraction (EBSD) and do so in a shear-dominated setting. We have done this by developing a new sample preparation technique of constructing composite sections, to allow us to use EBSD to obtain a representative, bulk CPO on coarse-grained ice. We draw attention to the well-known issue of interlocking grains of complex shape and suggest that a grain sampling bias of large, branching crystals that appear multiple times as island grains in thin sections may result in the typical multimaxima CPOs previously identified in warm, coarse-grained ice that has been subjected to prolonged shear. CPOs combined from multiple samples of highly sheared ice from Storglaciären provide a more comprehensive picture of the microstructure and yield a pronounced cluster of c axes sub-normal to the shear plane and elongate or split in a plane normal to the shear direction as well as a concomitant girdle of a axes parallel to the shear plane with a maximum perpendicular to the shear direction. This pattern compares well with patterns produced by subsampling datasets from ice sheared in laboratory experiments at high homologous temperatures up to strains of ∼1.5. Shear strains in the margin of Storglaciären are much higher than those in experimental work. At much lower natural strain rates, dynamic recrystallization, particularly grain boundary migration, may have been more effective so that the CPO represents a small, final fraction of the shear history. A key result of this study is that multimaxima CPOs in coarse-grained ice reported in previous work may be due to limited sample sizes and a sampling bias related to the presence of island grains of a single host that appear several times in a thin section.

Secondary Recrystallization Behavior in Fe-3%Si Grain-oriented Silicon Steel Produced by Twin-roll Casting and Simplified Secondary Annealing

Grain-oriented silicon steels were produced by the shortest processing route involving twin-roll strip casting, two-stage cold rolling with intermediate annealing, and simulated continuous annealing. The secondary recrystallization behavior of grain-oriented silicon steels under different inhibition conditions was in-situ observed by combining the confocal laser scanning microscopy (CLSM) and electron backscattered diffraction (EBSD) techniques. The results revealed that the optimal temperature of secondary recrystallization showed a proportional relationship with the Zenner pinning force. In the case of weak pinning force, the abnormal grain growth occurred quickly at ~1050 °C. The corresponding growth rates were in the range of 60–1400 μm/min and decreased gradually as the secondary recrystallization proceeded. In the case of strong pinning force, the incubation time and onset temperature of the secondary recrystallization was significantly increased, but the total time of the secondary recrystallization was obviously shortened from 685 s to 479 s, and the final magnetic induction of B8 was increased from 1.7 T to 1.85 T. After the secondary annealing, some island grains and coarse primary grains were retained. The formation of island grain was related to the low migration of grain boundaries. The findings of coarse γ- grains indicated that the primary grain size also played a crucial role during secondary recrystallization, apart from the primary recrystallized texture, which attracted more attention previously.

Ex-Situ Time Sequential Observation on Island and Peninsular Grains in Abnormally Growing Goss Grains in Fe–3%Si Steel

Evolution of abnormally growing Goss grains in Fe–3%Si steel was observed time sequentially by electron backscattering diffraction (EBSD). For example, some specimens were heated at 1080 °C for 5 min, then cooled, polished and observed by EBSD, whose procedure was repeated for 6 times. After each heat treatment at 1080 °C, the abnormal grain growth morphology of Goss grains in Fe–3%Si steel could be observed by polishing away a thin layer of the oxidized surface. Goss grains grew abnormally in a highly irregular way. As a result, some matrix grains were isolated at the growth front, which produced island grains. The irregular growth often resulted in incomplete isolation, which produced peninsular grains. Numerous matrix grains were isolated by the impingement of abnormally growing Goss grains. Once matrix grains became isolated, island or peninsular grains, they shrank much faster than before. If the grain boundary mobility of island and peninsular grains is not low enough, they would shrink in an accelerated way because their local curvature of grain boundary becomes larger as they shrink. Therefore, the high shrinking rate of island and peninsular grains indicates that island and peninsular grains are formed not because of their low grain boundary mobility but because of solid-state wetting.

Abnormal growth of columnar grains and formation of Σ3 grain boundaries in non-oriented electrical steels

The remarkably abnormal growth behavior of columnar grains and the formation of Σ3 grain boundaries (GBs) were studied via plastic deformation and annealing based on an electron backscattering diffraction (EBSD) investigation. Driven by the stored energy and GB mobility, coarse austenite columnar grains were formed by the abnormal growth of fine equiaxed grains with higher grain orientation spread (GOS) values in the reducing atmosphere of hydrogen. Moreover, extraordinarily coarse columnar grains with high GOS values and non-{1 1 1} texture were formed by microstructure inheritance during γ → α transformation. In addition, slender ferritic columnar or island grains with Σ3 GBs and low GOS values were generally obtained, and the formation mechanism was strictly associated with the Kurdjumov-Sachs (K-S) relationship, which was induced during variant selection driven by the GB mobility and surface effect in the reducing atmosphere of hydrogen.

Time Sequential Observation of Abnormally Growing Goss Grains in Fe-3%Si Steel

Evolution of abnormally growing Goss grains in Fe-3%Si steel was observed time sequentially by electron backscattering diffraction (EBSD). For example, some specimens were heated at 1080oC for 5 min, then cooled, polished and observed by EBSD, whose procedure was repeated for 6 times. Highly irregular growth of Goss grains produced island and peninsular matrix grains. Once matrix grains became island or peninsular grains, they shrank much faster than before. This means that these grains were not formed by their low grain boundary mobility but formed by solid-state wetting. Also, grains trapped by impingement of abnormally-growing Goss grains shrank much faster.

Phase Field Modelling of Abnormal Grain Growth

Heterogeneous grain structures may develop due to abnormal grain growth during processing of polycrystalline materials ranging from metals and alloys to ceramics. The phenomenon must be controlled in practical applications where typically homogeneous grain structures are desired. Recent advances in experimental and computational techniques have, thus, stimulated the need to revisit the underlying growth mechanisms. Here, phase field modelling is used to systematically evaluate conditions for initiation of abnormal grain growth. Grain boundaries are classified into two classes, i.e., high- and low-mobility boundaries. Three different approaches are considered for having high- and low-mobility boundaries: (i) critical threshold angle of grain boundary disorientation above which boundaries are highly mobile, (ii) two grain types A and B with the A–B boundaries being highly mobile, and (iii) three grain types, A, B and C with the A–B boundaries being fast. For these different scenarios, 2D simulations have been performed to quantify the effect of variations in the mobility ratio, threshold angle and fractions of grain types, respectively, on the potential onset of abnormal grain growth and the degree of heterogeneity in the resulting grain structures. The required mobility ratios to observe abnormal grain growth are quantified as a function of the fraction of high-mobility boundaries. The scenario with three grain types (A, B, C) has been identified as one that promotes strongly irregular abnormal grains including island grains, as observed experimentally.