Atomic structure of grain boundaries in YBa2Cu3O7-δ as observed by Z-contrast imaging

Determination of the atomic structure of grain boundaries is the key to fundamental understanding of the critical current density in polycrystalline superconductors. High-resolution images with incoherent characteristics, obtained using a high-angle annular detector on an atomic resolution scanning transmission electron microscope, are used to study the atomic arrangements of these technologically important boundaries. The incoherent Z-contrast images do not experience contrast reversals with defocus or sample thickness and display no Fresnel Fringe effects at boundaries. Observed rigid shifts of atomic columns at grain boundaries are independent of sample thickness and objective lens defocus. These characteristics allow unambiguous and intuitive interpretations of the generated images. We find the atomic structures at grain boundaries in YBa₂Cu₃O_7-δ are strongly influenced by the strong tendency of this compound to exist only as complete unit cells terminated at {001} and {100} planes. The weak-link behavior associated with high-angle grain boundaries may follow from this structure in which there is no clear connection between the {100} facets of adjacent grains. Symmetric grain boundaries where adjacent grains share a common boundary plane have also been observed in YBa₂Cu₃O_7-δ. In these boundaries partial structural coupling of the grains is maintained. There is evidence that these two boundary forms produce junctions with very different superconducting properties.     

Influence of the Order-Disorder Phase Transition on the Grain Structure of Iron–Palladium Alloy

Transmission diffraction electron microscopy and optical metallography are used to investigate the grain structure and the crystallographic parameters of grain boundaries in Pd₃Fe alloy with short- and long-range atomic orders having the superstructure L1₂. It has been found that ordering annealing of the Pd₃Fe alloy with the ordering temperature T _K lower than the recrystallization temperature is accompanied by changes in the grain structure. The average distance between the nearest boundaries and the average grain size decrease. The fraction of twinning boundaries for which the inverse density of coinciding lattice sites () is equal to 3 increases. This results in a decrease of the twinning boundary energy. A comparative analysis of changes in the grain structure of Ni₃Fe and Pd₃Fe alloys at the A1 L1₂ phase transition is performed. The mechanisms of changes of the grain structure during ordering annealing are discussed.     

The Electronic Structure of Pristine and Doped (100) Tilt Grain Boundaries in SrTiO3

To understand the electronic properties of doped grain boundaries, we reviewed the atomic scale techniques currently available to study the electronic structure at pristine SrTiO₃ grain boundaries. The knowledge gained from the pristine boundaries is used to interpret experimental and theoretical results from a Mn doped 5 SrTiO₃ grain boundaries. Mn atoms are shown to preferentially substitute at specific Ti sites at the grain boundary core. Furthermore, the formal oxidation state of the Mn atoms at the grain boundary core was found to be reduced compared to the Mn atoms substituting for Ti in the bulk. This change of valence did not, however, significantly affect the atomic structure of the grain boundary, as determined by Z-contrast imaging and electron energy-loss spectroscopy, which revealed similar fine-structure features at both the doped and pristine grain boundary. We conclude, therefore, that composition and atomic structure have different effects on the local electronic structure and should be treated separately in any segregation and electrical conductivity models for grain boundaries.     

Atomistic studies of the structure and composition of grain boundaries in Cu3Au and Ni3Al

In this paper we investigate the atomic structure and composition of grain boundaries in Cu₃Au (weakly ordered compound) and Ni₃Al (strongly ordered compound). Computer simulations employing both the molecular statics and Monte Carlo methods were performed and the Finnis-Sinclair type many-body central force potentials used. First, grain boundaries in stoichiometric alloys are studied with the goal to investigate the impact of ordering strength on the grain boundary structure and composition. In Cu₃Au grain boundaries may become compositionally disordered even at room temperature and the compositional disordering is associated with segregation of gold. In contrast, in Ni₃Al grain boundaries remain compositionally ordered up to very high temperatures. Secondly, the structures of grain boundaries and the effect of Ni and Al segregation in non-stoichiometric Ni₃Al are investigated. Nickel segregation leads to compositional disordering at grain boundaries, while aluminum segregation, which is strongly selective, leads to an ordered grain boundary structure with high Al content. The possible relationship between structural and compositional characteristics of grain boundaries and their mechanical properties, in particular the grain boundary brittleness and its alleviation by additional alloying, are then discussed in the light of the results of this study.     

Effect of atomic displacement on the parameters of the grain boundary ensemble in nickel-based alloys with <Emphasis Type="Italic">L</Emphasis>1<Subscript>2</Subscript> superstructure

The effect complex stacking fault energy and mean-square atomic displacement have on the parameters of the grain boundary ensemble in nickel-based alloys with L1₂ superstructure is established experimentally. The higher the complex stacking fault energy, the lower the average number of special grain boundaries per parent grain. The lower the mean-square atomic displacement, the smaller the proportion of the Σ3 twins in the special grain boundary spectrum.     

Change in the crystallographic structure of grain boundaries in an order-disorder phase transition in Ni3Fe alloy

Electron diffraction microscopy and metallography are used to investigate the crystallographic structure of grain boundaries in Ni₃Fe alloy with short-range and long-range atomic order. It is found that the fraction of special boundaries in alloys with short-range and long-range order is 0.3–0.4. Heat treatment, which leads to ordering, causes a reorientation of some of the grains with boundaries of a general type, boundary migration, and also faceting of some of the boundaries of general type.Tomsk Engineering-Construction Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 3–10, July, 1992.     

Modeling of Surface Diffusion in hcp Zr and Ti

The statics and dynamics of vacancies and adatoms on different surface orientations in two hcp materials are studied by using static relaxation techniques and many-body potentials. Formation and migration energies and entropies as well as attempt frequencies are evaluated and used in the random walk approach to obtain correlation factors and diffusivities. It is found that the main features of surface diffusion are dominated by jumps on and between a few atomic layers, so that a consistent comparison between the two mechanisms is feasible. The activation energies and the diffusivities for different environments, namely, bulk Q _b, D _b, symmetric grain boundaries Q _gb, D _gb, and surfaces, Q _s, D _s, calculated using the same simulation technique and interatomic potentials, fulfil the expected relationships Q _s < Q _gb < Q _b and D _s > D _gb > D _b. It is also found that generally adatoms are faster surface diffusers than vacancies.     

稀土对镁合金应力腐蚀影响电子理论研究

建立了镁合金纯净晶界及其析出Mg₁₇Al₁₂相的晶界原子集团，应用实空间的递归方法计算了铝、稀土元素在晶界的偏聚能，晶界处铝、稀土原子间相互作用能和不同体系的费米能级.讨论了铝、稀土在晶界的偏聚行为，铝、稀土原子间的相互作用与有序化的关系及稀土对镁合金晶间应力腐蚀影响的物理本质.研究发现：铝、稀土原子偏聚于晶界；铝原子间相互排斥，在晶界区形成有序相Mg₁₇Al₁₂，稀土原子间互相吸引，形成原子团簇；稀土原子团吸引铝原子，使铝原子渗入稀土团簇中，形成稀土化合物.因此，稀土具有抑制铝在晶界形成导致应力腐蚀的阴极相Mg₁₇Al₁₂的作用，提高镁合金的晶间应力腐蚀抗力. 关键词： 电子理论 镁合金 应力腐蚀 稀土     

Positron lifetime as a function of grain size

Published data on positron annihilation lifetime in copper as a function of grain size have been analyzed to show that there is a linear relationship between the internal grain boundary surface area, per unit volume,S _v, and the positron lifetime, τ. The analysis indicates that grain boundaries are important in the trapping of positrons. It is suggested that the slope of the resulting straight line,dS _v/dτ, can be used to determine the annihilation rate of the grain boundaries.     

Point defect interactions with crystal interfaces

The structures of a small closed system of grain boundaries and the interactions of vacancies with these boundaries has been investigated using computer simulation techniques based on empirical interatomic potentials. The boundaries chosen are the {;111}; and {;112}; twins in both body centred cubic and face centred cubic metals, the potentials used being matched to the physical properties of iron and copper. Two stable structures arise for the {;112};_bcc twin so that effectively five boundaries have been studied. The structures and energies of these are extremely varied, the {;112};_fcc twin in particular being very broad. This influences the binding of vacancies to the boundaries and the migration of vacancies along the boundaries. Near the {;111};_bcc twin a split-vacancy consisting of a divacancy and an interstitial is the most stable configuration. This has a very high binding energy and an exceptionally high migration energy. Near the other boundaries the vacancy migration energies are less than in the bulk. The implications of the results are discussed.     

Transformations in the Grain Boundary Ensemble of M1 Copper Subjected to Equal-Channel Angular Pressing during Recrystallization Annealing

The grain structure of M1 copper subjected to equal-channel angular pressing (ECAP) and subsequent annealing at 593K for 1 h is studied by means of transmission electron microscopy and scanning electron microscopy with the diffraction of backscattered electrons. An increase in grain size and the formation of special boundaries (Σ3 twins both coherent and incoherent) are observed, along with the migration of high-energy Σ3 twins and common grain boundaries, the splitting of Σ9 special boundaries into Σ3 twins, and the splitting of common grain boundaries into Σ9 and Σ3 special boundaries. The local transformation of common grain boundaries into special boundaries also occurs. Particles of the Cu₂O phase are present on the migrating Σ3 twins and common grain boundaries.     

Change in grain-boundary ensemble upon the A1 → L12 phase transition in Ni3Mn alloy

The grain structure of Ni₃Mn alloy is investigated in the states with short-range and long-range atomic orders that arise at the A1 → L1₂ phase transition. It is found that in ordering annealing, new boundaries of a general type form, the fraction of special-type boundaries (including twin boundaries) is reduced, the share of high-energy special boundaries with Σ > 11 increases, and the alloy’s texture is transformed into a “gray” texture.     

Low-energy sputtering events at free surfaces near anti-phase and grain boundaries in Ni3Al

Atomic recoil events on free surfaces orthogonal to two different anti-phase boundaries (APBs) and two grain boundaries (GBs) in Ni₃Al are simulated using molecular dynamics methods. The threshold energy for sputtering, E _sp, and adatom creation, E _ad, are determined as a function of recoil direction. The study is relevant to FEG STEM (a scanning transmission electron microscope fitted with a field emission gun) experiments on preferential Al sputtering and/or enhancement of the Ni–Al ratio near boundaries. Surfaces intersected by {110} and {111} APBs have minimum E _sp of 6.5?eV for an Al atom on the Ni–Al mixed (M) surface, which is close to the value of 6.0?eV for a perfect M surface. High values of E _sp of an Al atom generally occur at a large angle to the surface normal and depend strongly on the detailed atomic configuration of the surface. The mean E _sp, averaged over all recoil directions, reveals that APBs have a small effect on the threshold sputtering. However, the results for E _ad imply that an electron beam could create more Al adatoms on surfaces intersected by APBs than on those without. The equilibrium, minimum energy structures for a (001) surface intersected by either Σ5[001](210) or Σ25[001](340) symmetric tilt grain boundaries are computed. E _sp for surface Al atoms near these GBs increases monotonically with increasing recoil angle to the surface normal, with a minimum value, which is only about 1?eV different from that obtained for a perfect surface. Temperature up to 300?K has no effect on this result. It is concluded that the experimental observations of preferential sputtering are due to effects beyond those for E _sp studied here. Possible reasons for this are discussed.     

Σ3 CSL boundary distributions in an austenitic stainless steel subjected to multidirectional forging followed by annealing

The effect of processing and annealing temperatures on the grain boundary characters in the ultrafine-grained structure of a 304-type austenitic stainless steel was studied. An S304H steel was subjected to multidirectional forging (MDF) at 500–800°C to total strains of ~4, followed by annealing at 800–1,000°C for 30 min. The MDF resulted in the formation of ultrafine-grained microstructures with mean grain sizes of 0.28–0.85 μm depending on the processing temperature. The annealing behaviour of the ultrafine-grained steel was characterized by the development of continuous post-dynamic recrystallization including a rapid recovery followed by a gradual grain growth. The post-dynamically recrystallized grain size depended on both the deformation temperature and the annealing temperature. The recrystallization kinetics was reduced with an increase in the temperature of the preceding deformation. The grain growth during post-dynamic recrystallization was accompanied by an increase in the fraction of Σ3ⁿ CSL boundaries, which was defined by a relative change in the grain size, i.e. a ratio of the annealed grain size to that evolved by preceding warm working (D/D₀). The fraction of Σ3ⁿ CSL boundaries sharply rose to approximately 0.5 in the range of D/D₀ from 1 to 5, which can be considered as early stage of continuous post-dynamic recrystallization. Then, the rate of increase in the fraction of Σ3ⁿ CSL boundaries slowed down significantly in the range of D/D₀ > 5. A fivefold increase in the grain size by annealing is a necessary condition to obtain approximately 50% Σ3ⁿ CSL boundaries in the recrystallized microstructure.     

Sc在Al-Zn-Mg-Cu超高强铝合金中作用机理的电子理论研究

通过晶胞平移获得Al-Zn-Mg-Cu合金中α-Al,Al₃Sc及η相原子集团模型,采用自编软件建立α-Al/液态Al界面、α-Al/Al₃Sc界面原子团模型.用递归法计算合金中各组织的态密度、结合能、费米能级,合金元素Sc与空位相互作用能等电子参数.依据电子参数解释合金晶粒细化、腐蚀的物理本质.研究表明： Al₃Sc从液态金属析出时释放的能量比α-Al从液态金属析出时所释放的能量少,可先于α-Al从液态金属中析出;且α-Al 关键词： 电子结构 腐蚀 超高强Al合金     

The effect of Na on the electronic properties of Cu(In,Ga)Se2 thin films: A local‐probe study

We investigated the effect of Na incorporation on the electronic properties of polycrystalline CuIn_0.7Ga_0.3Se₂ thin films using scanning tunneling microscopy and spectroscopy. The tunneling spectra indicate a reduced in‐gap density of states at grain boundaries and reveal a downward band‐bending in Na‐rich grain boundaries with respect to the adjacent grains, in agreement with our conductive atomic force microscopy data. It thus appears that Na passivates deep‐level defects at grain boundaries and induces a downward band‐bending there. Moreover, we provide evidence that Na passivates mainly Cu vacancy related defects. We suggest that the grain‐boundary passivation, which reduces the recombination rate of photogenerated carriers, is at least of major importance in the well known Na‐induced improvement in the efficiency of the corresponding solar cells. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Grain structure of Ni3Fe alloy

The grain structure of aN ₃Fe polycrystal is studied by means of optical metallography taking into account the type of grain boundaries. Grains with random boundaries having special boundaries on the inside are revealed. Grains are classified by the presence and position of special boundaries. The energy of different types of grain boundaries is estimated. Tomsk State University of Architecture and Building. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 34–42, November 1999.     

Influence of the Grain Boundaries on the Heat Transfer in Laser Ceramics

Grain boundaries play a key role in determining several key properties of polycrystalline laser ceramics. Heat transfer measurements at low temperature constitute a good tool to probe grain boundaries. We review the results of heat transfer measurements in polycrystalline Y₃Al₅O₁₂ garnets as well as Y₂O₃ and Lu₂O₃ sesquioxide materials obtained by self-energy-driven sintering of nano-particles. The average phonon mean free path in Y₃Al₅O₁₂ was found to be significantly larger than the average grain size and to scale with temperature as T ⁻² at low temperature. Existing models describing the interaction between phonons and grain boundaries are reviewed. Correct temperature dependence of the mean free path and order of magnitude of scattering rates were found by assuming the existence of a grain boundary layer having acoustic properties different from those of the bulk. A different temperature dependence of phonon mean free path was found for the sesquioxides and was ascribed to the stronger elastic anisotropy of these materials. The thermal resistance associated to the grain boundaries of laser ceramics was found to be lower than in other dense polycrystalline ceramic materials reported in the literature.     

Effect of atomic ordering on the role of grain boundaries in the plastic deformation of Ni<Subscript>3</Subscript>Fe alloy

The structure of dislocations and the defect structure of grain boundaries and their parameters in Ni₃Fe alloy with short-range order (SRO) and long–range order (LRO) at different stages of plastic deformation are studied by means of transmission diffraction electron microscopy using thin foils and replicas. It is found that atomic ordering reduces the Σ3 twins plasticizing effect, increases the density of grain boundary defects, slows their annihilation during deformation, and intensifies the microstrains at the triple junctions of grain boundaries.     

Temperature and orientation dependences of the grain-boundary diffusion coefficient of antimony in copper bicrystals

An investigation is made of the diffusion of antimony through the bulk and along grain boundaries in copper bicrystals containing a symmetric 〈100〉 misorientation boundary with misorientation angles from 20 to 37.2°. The bicrystals are grown by the method of horizontal zone recrystallization. The temperature range for these studies is 480–580 °C, where the solubility of Sb in Cu is about 6 atomic % and practically temperature-independent. The concentration profiles are obtained by x-ray spectral microanalysis, and the grain-boundary diffusion parameters are computed by the method of Whipple and Suzuoka. The orientation dependence of the triple product P=sδD _b (where s is the segregation coefficient, δ the width of the grain boundary, and D _b the grain-boundary diffusion coefficient) is nonmonotonic, with a maximum for the special ∑5 misorientation boundary (36.9°). The effective activation energy for grain-boundary diffusion ranges from ∼70 kJ/mol for ∑5 to140 kJ/mol for general boundaries. Fiz. Tverd. Tela (St. Petersburg) 39, 1153–1157 (July 1997)