Grain Boundary Structure Research Articles

Investigation of Nano-Scale Segregation in Nanostructured Ferritic Alloy 14YWT after Heavy Ion Irradiation.

Oxide-dispersion-strengthened (ODS) steels, which contain nano-scale Y-Ti-O particles, are being considered for high-temperature radiation environments of nuclear reactors. It is important to accurately characterize the structure of grain boundaries and understand the behavior of segregation at grain boundaries in ODS steels during irradiation. The effect of heavy ion irradiation at 700 °C on Nanostructured Ferritic Alloy 14YWT was investigated using Atom Probe Tomography. Enrichment of Cr occurs at the grain boundaries as well as at nano oxide particle surfaces in the unirradiated sample. The enrichment of Ti and Y at a grain boundary corresponds with Y-Ti-O nano oxide particles with larger size compared to those in the grain, and the Cr enrichment is particularly accentuated at these larger nano oxide particles. The segregation of W occurs at the grain boundaries that are without nano oxide particles. O is segregated at grain boundaries without oxide particles after irradiation. The segregation behavior of Cr, W, Ti, and Y at the grain boundary in the irradiated samples is similar to that in the unirradiated sample. The nano oxide particles embedded in the grain boundary are a primary reason for the increase in Cr segregation at the grain boundary.

Study on the Influence of Grain Size and Microstructure on the Mechanical Properties of Fe-6.5 wt%Si High Silicon Steel Prepared by CVD Method

As a soft magnetic material with excellent performance, silicon steel is widely used in motors and transformers, but its mechanical properties drop sharply when the silicon content is too high. Therefore, it is of great significance to study its influence on mechanisms to improve the product quality of silicon steel. In this paper, Fe-6.5 wt%Si silicon steel was prepared by vacuum tube furnace, combined with a metallographic experiment, and scanning electron microscope analysis to explore the influence of silicon infiltration temperature and time on grain and grain boundary size, and the tensile test of silicon infiltration 120 s at 1200 °C was obtained by the tensile test’s extension parameter. Given the difficulty in adjusting the size and structure of grains and grain boundaries in the test, this paper discusses the influence of different microstructures on the mechanical properties of silicon steel through tensile simulation. The tensile results show that grain refinement helps to improve the strength and elongation of silicon steel, and columnar grains can slightly increase their strength but greatly reduce the strain rate of silicon steel. This method can greatly reduce the research and development time of Fe-6.5 wt%Si silicon steel and can be used to improve the comprehensive performance of silicon steel.

Influence of decreased temperature on the plasticization effect in high-strength Al-Cu-Zr alloy

The influence of deformation temperature of tensile test on the mechanical properties of ultrafine grained (UFG) Al-Cu-Zr alloy in three different states: after high pressure torsion (HPT), subsequent low-temperature annealing as well as after annealing and a small additional deformation has been studied for the first time. The UFG structure was formed by HPT processing. The temperature dependences of the yield stress and elongation to failure under uniaxial tension in the temperature range of 77–293 K were obtained for all three states. The effect of increasing ductility from ∼3–5 to 11 % (the plasticization effect) at room temperature while maintaining a high strength of ∼465 MPa was observed after deformation-heat treatment (DHT), consisting of annealing and additional deformation. The plasticization effect decreases monotonically with decreasing temperature of tensile test and completely disappears at ∼223 K. In the temperature range 223–293 K, the temperature sensitivity of the yield stress and the value of activation energy (Q) of plastic flow differ in all three states, which is related to the changes in the grain boundary (GB) structure (dislocation density, size and shape of Al2Cu nanoprecipitates). In the temperature range of 77–223 K, the temperature sensitivity of the yield stress does not depend on the state of GBs. The plasticization effect at room temperature is preserved when the strain rate is changed from 10−4 to 10−3 s−1 and sharply decreases as strain rate is further increased. The strain rate sensitivity coefficient (m) for the UFG Al-1.47Cu-0.34Zr (wt%) alloy was determined in the states before and after DHT. The results and relevant mechanisms are analyzed by using the resulting m valuе and Q values as well as the observations of specific features of GB structure.

Clarifying the mitigation effect of proton irradiation on the intergranular oxidation of 316L stainless steel in high temperature water

The role of proton irradiation in the intergranular oxidation of 316L stainless steel after long-term immersion in simulated PWR primary water was clarified by comparing the microstructural and microchemical features of oxide between the irradiated grain boundary (GB) and its un-irradiated counterpart. As such, the interference from difference in GB structure can be ruled out. Surprisingly, the results reveal that the intergranular oxide penetration in the irradiated region is shallower than that in the un-irradiated region. It is found that Si segregated at the irradiated GB diffuses outwards preferentially and gets oxidized due to its high diffusivity and affinity to oxygen. The Si-enriched oxide in the intergranular oxide tip of irradiated GB can act as a temporary diffusion barrier for oxygen, although it tends to dissolve near the sample surface. Meanwhile, the efficiency of elements (especially Cr) transport along irradiated GB to the oxidation front is promoted mainly due to the vacancies created by preferential diffusion of Si, resulting in higher Cr content at the intergranular oxide tip. The combination of Si and Cr enrichments in the intergranular oxide tip can enhance the resistance to oxidation and eventually leads to a lower oxidation rate for the irradiated GB. For the first time, our study determines that irradiation can enhance the intergranular oxidation resistance of stainless steel after long-term immersion in simulated PWR primary water when the sample is not stressed.

Structure characterization of interior invisible single grain boundary by nanorobot pick-and-place grain in bulk ZnO under SEM

• Invisible grain boundary is first creatively characterized using nanorobot in SEM. • Irregular shape of invisible grain boundary can be exhibited in plane X-Y. • Three-dimensional virtual reconstruction is utilized to verify irregular structure. In order to independently characterize invisible structure, a new approach is first creatively proposed to express interior invisible single grain boundary using nanorobot pick-and-place single grain in SEM. Picking of a single grain handled by single probe and then rolling-over placing manipulated by dual-probes can be sequentially conducted by nanorobot. The upper and lower invisible surface interface layer profile of single grain can be extracted by SEM imaging in plane X-Y. Experimental results demonstrate that interior invisible single grain boundary characterized by nanorobot seems to be reasonably feasible. The upper and lower surface structures of single grain in plane X-Y can also be virtually modeled by 3D software. This method can further provide a unique characterization approach of interior invisible structure of single grain boundary.

In-situ mapping characterization of structure and electrical property of grain boundary in polycrystalline ZnO using nanorobot in SEM

Different from the existing methods of individual characterization of structure or electrical property, a unique approach for in-situ mapping characterization of single grain boundary using nanorobot in SEM is proposed to simultaneously realize the characterization of structure and electrical property in micro-area polycrystalline ZnO. Z-direction height differences constructed by multiple adjacent grains relative to a single target grain can be individually probed by the nanorobot single end tip and correspondingly virtually reconstructed. Electrical impedances complex planes of grain boundaries in-situ detected by nanorobot dual probes can be fitted by equivalent circuits. Experimental results illustrate that in-situ mapping characterization of structure and electrical property of single grain boundary using nanorobot in SEM can be verified. A single grain boundary layer, composed of adjacent multi-grains, seems to be nonlinear and unequal height along the Z direction. Z-direction relative height differences probed by the nanorobot can actually be regarded as the Z-direction projected lengths of surrounding grain boundary interfaces of single target grain. A three dimensional stereogram of single target grain boundary can be virtually reconstructed based on 3D software. It is deduced that schottky barrier existing in grain boundary interface exhibits larger height difference, basically proportional to the thickness of grain boundary in plane X–Y. Furtherly, it can promote an advance in in-situ mapping characterization of structure-property of single/multi micro area in other bulk inorganic polycrystalline bulk materials.

Hydrogen-Induced Dislocation Nucleation and Plastic Deformation of 〈001〉 and 〈11¯0〉 Grain Boundaries in Nickel.

The grain boundary (GB) plays a crucial role in dominating hydrogen-induced plastic deformation and intergranular failure in polycrystal metals. In the present study, molecular dynamics simulations were employed to study the effects of hydrogen segregation on dislocation plasticity of a series of symmetrical tilt grain boundaries (STGBs) with various hydrogen concentrations. Our study shows that hydrogen both enhances and reduces dislocation nucleation events from STGBs, depending on different GB structures. Specifically, for STGBs, hydrogen does not affect the mode of heterogeneous dislocation nucleation (HDN), but facilitates nucleation events as a consequence of hydrogen disordering the GB structure. Conversely, hydrogen retards dislocation nucleation due to the fact that hydrogen segregation disrupts the transformation of boundary structure such as Σ9 () STGB. These results are helpful for deepening our understanding of GB-mediated hydrogen embrittlement (HE) mechanisms.

First-principles investigation on electronic structures and energetic characteristics of γ/γ tilt grain boundaries in γ-TiAl intermetallic

The energetic and electronic characteristics of γ/γ tilt grain boundaries (GBs) in γ-TiAl intermetallic were comprehensively analyzed using first-principles methods. The interfacial bonding energy, strength, and features of GBs were quantitatively and qualitatively described by the formation energy, excess free volume (EFV), electron localization function (ELF), bonding charge density (Δρ), and density of states (DOS). The formation energies and EFV of Σ3, Σ11, and Σ5 GBs are relatively low in sequence among different GBs. The electronic structure analyses manifested that the smaller the local lattice strain, the stronger the Ti-d-Al-p bonding, the weaker Al-p-Al-p bonding nearby the GB interface, and the lower the GB formation energy. Further, the interface structure of Σ11 GBs in Widmanstätten structure and that of Σ5 GBs in feathery structure were firstly characterized by TEM. Based on the analyses, the interfacial energetic mechanism of metastable transformations, and possible alloying effects were discussed. This work provides an insight into not only the atomic and electronic basis of GBs but also the understanding of the mechanism of metastable transformations and interface engineering in γ-TiAl alloy.

Grain boundary structural transformation induced by co-segregation of aliovalent dopants

Impurity doping is a conventional but one of the most effective ways to control the functional properties of materials. In insulating materials, the dopant solubility limit is considerably low in general, and the dopants often segregate to grain boundaries (GBs) in polycrystals, which significantly alter their entire properties. However, detailed mechanisms on how dopant atoms form structures at GBs and change their properties remain a matter of conjecture. Here, we show GB structural transformation in α-Al2O3 induced by co-segregation of Ca and Si aliovalent dopants using atomic-resolution scanning transmission electron microscopy combined with density functional theory calculations. To accommodate large-sized Ca ions at the GB core, the pristine GB atomic structure is transformed into a new GB structure with larger free volumes. Moreover, the Si and Ca dopants form a chemically ordered structure, and the charge compensation is achieved within the narrow GB core region rather than forming broader space charge layers. Our findings give an insight into GB engineering by utilizing aliovalent co-segregation.

Controlling the Formation of Conductive Pathways in Memristive Devices.

Resistive random-access memories are promising candidates for novel computer architectures such as in-memory computing, multilevel data storage, and neuromorphics. Their working principle is based on electrically stimulated materials changes that allow access to two (digital), multiple (multilevel), or quasi-continuous (analog) resistive states. However, the stochastic nature of forming and switching the conductive pathway involves complex atomistic defect configurations resulting in considerable variability. This paper reveals that the intricate interplay of 0D and 2D defects can be engineered to achieve reproducible and controlled low-voltage formation of conducting filaments.The author find that the orientation of grain boundaries in polycrystalline HfOx is directly related to the required forming voltage of the conducting filaments, unravelling a neglected origin of variability. Based on the realistic atomic structure of grain boundaries obtained from ultra-high resolution imaging combined with first-principles calculations including local strain, this paper shows how oxygen vacancy segregation energies and the associated electronic states in the vicinity of the Fermi level govern the formation of conductive pathways in memristive devices. These findings are applicable to non-amorphous valence change filamentary type memristive device. The results demonstrate that a fundamental atomistic understanding of defect chemistry is pivotal to design memristors as key element of future electronics.

Electrical resistivity of polycrystalline graphene: effect of grain-boundary-induced strain fields

We have revealed the decisive role of grain-boundary-induced strain fields in electron scattering in polycrystalline graphene. To this end, we have formulated the model based on Boltzmann transport theory which properly takes into account the microscopic structure of grain boundaries (GB) as a repeated sequence of heptagon–pentagon pairs. We show that at naturally low GB charges the strain field scattering dominates and leads to physically reasonable and, what is important, experimentally observable values of the electrical resistivity. It ranges from 0.1 to 10 kOmegaupmu m for different types of symmetric GBs with a size of 1 upmum and has a strong dependence on misorientation angle. For low-angle highly charged GBs, two scattering mechanisms may compete. The resistivity increases markedly with decreasing GB size and reaches values of 60 kOmegaupmum and more. It is also very sensitive to the presence of irregularities modeled by embedding of partial disclination dipoles. With significant distortion, we found an increase in resistance by more than an order of magnitude, which is directly related to the destruction of diffraction on the GB. Our findings may be of interest both in the interpretation of experimental data and in the design of electronic devices based on poly- and nanocrystalline graphene.

Revealing the role of Σ3{112} Si grain boundary local structures in impurity segregation

The interfacial structure of a silicon grain boundary (Si-GB) plays a decisive role on its chemical functionalization and has implications in diverse physical–chemical properties of the material. Therefore, the GB interface is particularly relevant when the material is employed in high performance technological applications. Here, we studied from first principles the role of GB interface by providing an atomistic understanding of two different Σ3{112} Si-GB models. These models are (1×1) and (1×2) Σ3{112} Si-GBs, which lead to different structural reconstruction. Starting from these two models, we have shown that geometry optimization has an important role on the structural reconstruction of the GB interface and, therefore, on its properties. For this reason, we discussed different methodologies to define an optimal relaxation protocol. The influence of the local structures in (1×1) and (1×2) models has also been investigated in the presence of vacancies where different light impurities of different valency (C, N, H, O) can segregate. We studied how local structures in (1×1) and (1×2) models are modified by the presence of vacancies and impurities. These structural modifications have been correlated with the changes of the energetics and electronic properties of the GBs. The behavior of (1×1) and (1×2) models was demonstrated to be significantly different. The interaction with vacancies and the segregation of C, N, H, and O are significantly different depending on the type of local structures present in Σ3{112} Si-GB.

Dramatic Enhancement of Thermoelectric Performance in PbTe by Unconventional Grain Shrinking in the Sintering Process

Nanostructure engineering is a key strategy for tailoring properties in the fields of batteries, solar cells, thermoelectrics, and so on. Limited by grain coarsening, however, the nanostructure effect gradually degrades during the materials' manufacturing and in-service period. Herein, a strategy of cleavage-fracture for grain shrinking is developed in a Pb0.98 Sb0.02 Te sample during sintering, and the grain size remains stable after repeated tests. Moreover, the initial grain boundary is filled by fractured slender grains and enriched by dislocations, evolving into a hierarchical grain-boundary structure. The lattice thermal conductivity (klat ) is greatly reduced to approach the amorphous limit. As a result, a record-high ZT value of ≈1.9 is obtained at 815 K in the n-type Pb0.98 Sb0.02 Te sample and a decent efficiency of 6.7% in thermoelectric device. This strategy for grain shrinking will shed light on the application of nanostructure engineering under high temperature and extreme conditions in other material systems.

Comparative study on the initial oxidation behavior of conventional and nanocrystalline MCrAlY coatings - effect of microstructure evolution and dynamic mechanisms

Two MCrAlY coatings with the same nominal composition but different microstructures are prepared by magnetron sputtering (MS) and arc ion plating (AIP). Their oxidation at 1000°C led to very different results. The relatively coarse grained AIP coating formed a duplex-layer scale, consisting of external θ-Al2O3 and internal α-Al2O3, with small amounts of NiCr2O4. Severe surface undulation was accompanied by the development of numerous cracks and voids. Aluminum depletion transformed the AIP coating from β to γ, causing volume shrinkage, promoting scale spallation and leading to premature failure. In contrast, the nanocrystalline MS coating was less degraded due to its columnar crystal structure and high-density of grain boundaries which provided rapid Al diffusion. Oxidation of this coating was characterized by a faster θ-Al2O3 to α-Al2O3 transformation and formation of an exclusive α-Al2O3 scale, protecting the surface.

Determining Atomic Structure of Grain Boundaries and Heterostructure Interfaces at Solid-State Electrode/Electrolyte Heterostructures in Lithium Based Oxides

Determining Atomic Structure of Grain Boundaries and Heterostructure Interfaces at Solid-State Electrode/Electrolyte Heterostructures in Lithium Based Oxides

Grain boundary modification of multi-main-phase Nd0.7(La, Ce)0.3-Fe-B magnet towards 2 T coercivity

Effective grain boundary (GB) reconstruction process has been proposed by introducing Pr-Fe-Al-Ga intergranular additive in the B-lean Nd-La-Ce-Fe-B multi main phase (MMP) magnets. A coercivity of ~2.0 T was yielded in the Nd-Fe-B sintered magnet with 24 wt% Nd substituted by La-Ce. Detailed structural analysis revealed that the RE6Fe13(Ga, Al)1 phase was apparently formed in the triple junction area, triggered by the conducive intergranular composition environment provided by the B-lean composition and the Pr-Fe-Al-Ga addition. Simultaneously, continuous nonferromagnetic GB phases were generated to decouple the intergranular exchange, accompanied by the suppressed fraction of REFe2 and REOx phases. Elemental distribution analysis demonstrated that Pr preferred to enter La/Ce-rich grains to form a Pr-rich shell that enhanced the magnetocrystalline anisotropy field of La/Ce-rich grain surface. Overall, the Pr-rich magnetic hardening shells and the reconstruction GB structure jointly gave rise to the restrained reversion of magnetic domains, thus an extraordinary coercivity was achieved in the Nd-La-Ce-Fe-B magnet. These phenomena may provide a new insight for the development of high-figure-of-merit Nd-La-Ce-Fe-B permanent magnets.

Exploring the Atomic Scale Structure and Properties of Grain Boundary in SrTiO3 by Electron Beam Imaging and Spectroscopy

Exploring the Atomic Scale Structure and Properties of Grain Boundary in SrTiO3 by Electron Beam Imaging and Spectroscopy

Probing Grain-boundary Structure and Electrostatic Characteristics in a SrTiO3 Bi-crystal by 4D-STEM

Probing Grain-boundary Structure and Electrostatic Characteristics in a SrTiO3 Bi-crystal by 4D-STEM

Carrier Separation Enhanced by High Angle Twist Grain Boundaries in Cesium Lead Bromide Perovskites.

Grain boundaries (GBs) have a profound impact on mechanical, chemical, and physical properties of polycrystalline materials. Comprehension of atomic and electronic structures of different GBs in materials can help to understand their impact on materials' properties. Here, with aberration-corrected scanning transmission electron microscopy (STEM), the atomic structure of a 90° twist GB s in CsPbBr3 is determined, and its impact on electron-hole pair separation is predicted. The 90° twist GB has a coherent interface and the same chemical composition as the bulk except for the lattice twist. Density functional theory (DFT) calculation results indicate that the twist GB has an electronic structure similar to that of the bulk CsPbBr3. An electronic potential at the GBs enhances the separation of photogenerated carriers and promotes the motion of electrons across the GBs. These results extend the understanding of atomic and electronic structure of GBs in halide perovskites and propose a potential strategy to eliminate the influence of GBs by GB engineering.

Microstructure, grain boundary evolution and anisotropic Fe segregation in (0001) textured Ti thin films

The structure and chemistry of grain boundaries (GBs) are crucial in determining polycrystalline materials’ properties. Faceting and solute segregation to minimize the GB energy is a commonly observed phenomenon. In this paper, a deposition process to obtain pure tilt GBs in titanium (Ti) thin films is presented. By increasing the power density, a transition from polycrystalline film growth to a maze bicrystalline Ti film on SrTiO3 (001) substrate is triggered. All the GBs in the bicrystalline thin film are characterized to be Σ13 [0001] coincident site lattice (CSL) boundaries. The GB planes are seen to distinctly facet into symmetric {7¯520} and {134¯0} and asymmetric {101¯0} // {112¯0} segments of 20–50 nm length. Additionally, EDS reveals preferential segregation of iron (Fe) in every symmetric {7¯520} segment. Both the faceting and the segregation are explained by a difference in the CSL density between the facet planes. Furthermore, in the GB plane containing Fe segregation, atom probe tomography is used to experimentally determine the GB excess solute to be 1.25 atoms/nm2. In summary, the study reveals for the first time a methodology to obtain bicrystalline Ti thin films with strong faceting and an anisotropy in Fe segregation behaviour within the neighbouring GB facets.