Slip transfer at coherent twin boundaries in Ni3Al and ordered Cu3Au

Slip transfer at coherent twin boundaries in L1₂-ordered alloys has been investigated by analysis of plastically deformed Cu₃Au and Ni₃Al specimens in a transmission electron microscope. A comparison between the ex situ deformed material and previous in situ experiments is made. In ordered Cu₃Au, antiphase boundaries induced by slip transfer have been found at a coherent twin boundary (CTB), similar to the in situ deformed material. On the other hand, in Ni₃Al superlattice intrinsic stacking faults (SISFs) were detected at CTBs which have not been observed in in situ deformed material. A possible mechanism for SISF formation is discussed. The transfer of slip, and the associated creation of stacking faults at CTBs in both materials is described in terms of absorption of superdislocations in the boundary and the general criteria for slip transfer at grain boundaries.     

High-resolution electron microscopy of grain boundaries

Our ability to observe atomic-scale features of grain boundaries has tremendously improved during the past decade. In this paper we give, aided by a number of examples, a select overview, on progress in the field of grain boundary research directly related to the advent of modern high-resolution electron microscopy (HREM) instruments (point-to-point resolution better than 0.2 nm). Examples of grain boundary issues addressed by atomic structure observations of grain boundaries in oxides and metals will be given with emphasis on systematic investigations of the role of macroscopic and microscopic grain boundary parameters. Since comparisons between observed interface structures and atomistic computer modeling results are quite important, considerable efforts towards quantification have been undertaken recently by a number of authors. Most valuable insights have been obtained by the systematic examination of a range of grain boundary structures, using a combination of experimental observations and computer modeling results. In this manner HREM observations have been invaluable not only as a test of theoretical models, but also by exposing common atomic-scale features of high-angle grain boundaries. This has brought us closer to the goal of generating a general understanding of the interface structure and its connection to properties. Such studies have given valuable insights regarding the correlations between macroscopic grain boundary geometry, interfacial energy, and atomic relaxation modes.Work supported by the U.S. Department of Energy, Basic Energy Sciences, under contract W-31-109-Eng-38.     

[001] Twist boundaries in Cu3Au for T ∈ 0 K

Four different [001] twist boundaries in Cu₃Au spanning a wide range of the rotation angle are investigated for temperatures below the bulk order-disorder transition temperature. All the boundaries show almost identical behavior for the plane averaged stoichiometry and the order parameter at a given temperature. Within the accuracy of the calculations no segregation of either Cu or Au is observed to the boundary. At the boundary and below the transition temperature there is a layer which is more disordered than the rest of the crystal, and its width increases as the bulk order-disorder transition temperature is approached. This indicates wetting phenomena, which are in agreement with the experimental and theoretical results on other boundaries.     

High-resolution electron microscopy in structural studies of inorganic materials

A summary is given of the use of high-resolution electron microscopic imaging methods in structure determination, stressing their advantage over conventional diffraction methods, and the instrumental and specimen-dependent limitations on their applicability. Some examples of the use of this technique are given, both in systems where the unit-cell structure is known and only long-range order is lacking, and in cases of completely unknown phases.     

Engineering of YBa2Cu3O7–δ grain boundary Josephson junctions by Au nanocrystals

We prepared and investigated grain boundary Josephson junctions based on SrTiO₃ bicrystal substrates. During the deposition of YBa₂Cu₃O_7–δ (YBCO) gold nanocrystals forming from an intermediate gold layer can modify the crystalline structure and thus the properties of the YBCO grain boundaries. The variation of the film thickness of the Au seed layer changes the growth conditions of the YBCO film and the Au nanocrystals. The values of the characteristic I_CR_N product do not change whereas the values of the critical current I_C decrease.

     

Structure of grain boundaries in YBa2Cu3O7 thin-film step-edge junctions

The atomic structure of 90° [100] (or [010]) tilt grain boundaries in YBa₂Cu₃O₇ thin-film step-edge junctions and, for comparison, in the interface between a-axis and c-axis oriented YBa₂Cu₃O₇ grains is investigated by means of high-resolution transmission electron microscopy. For (100)(001)-type boundaries two different structures are found. In the first a (001) CuO₂ plane of one grain faces a (100) Y–Ba–O plane of the other grain, in the second a (001) BaO plane faces a (100) Cu–O plane. In the former structure an incomplete unit cell of YBa₂Cu₃O₇ terminates at the boundary and a smaller strain in the adjacent CuO₂ planes is detected in comparison with the latter. It is found that a combination of a partial dislocation with a 124 stacking fault is a way to accommodate the lattice mismatch between c and 3a of YBa₂Cu₃O₇ in the boundary. For a symmetric (103)(103)-type boundary a displacement of the Cu-atoms of the CuO₂ planes is found near the boundary plane. From this a redistribution of the oxygen atoms around the Y-atoms located right in the boundary plane is inferred. The possible effect of the boundary structure on the superconducting properties of YBa₂Cu₃O₇ films is discussed.     

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.     

Mechanical properties of twist grain boundaries in Cu

In a previous paper we studied the Young's and shear modulus of a high-angle twist grain boundary (5) in Cu, using the EAM, and related it to the uniaxial strain derivatives of single crystals. In this paper, we discuss elastic properties of ten additional twist grain boundaries, from 8.8–43.6°. The monolayer Young's modulus at each boundary was calculated and found to be 20–50% higher than the bulk value for all eleven boundaries for both csl and type1 structures. The monolayer shear modulus at each boundary was calculated and found to be 93–98% lower than the bulk value for six grain boundaries with csl structure and found to decrease with increasing twist angle. The critical shear stress was also calculated for eleven boundaries with csl structure and found to roughly decrease with increasing twist angle.     

High-resolution electron microscopy observations of continuous precipitates with Pitsch-Schrader orientation relationship in an Mg–Al based alloy and interpretation with the O-lattice theory

High-resolution electron microscopy was applied to analyze the continuous precipitated particles of the γ-Mg₁₇Al₁₂ phase with Pitsch-Schrader OR in the heat-treated AZ91 alloy at 473 K for 8 h. The existence of a continuous precipitated particle with the Pitsch-Schrader OR including the selection of the habit plane and the growth direction in Mg–Al system is rationalized by the constrained coincidence site lattice/constrained complete pattern shift lattice (CCSL/CDSCL) model and the O-lattice theory.     

Investigation of changes in the order parameters near antiphase boundaries in the Cu<Subscript>3</Subscript>Au alloy

Investigation of the specific features of order-disorder structural phase transitions in ordering alloys in the presence of antiphase boundaries of conservative and nonconservative types has been performed in a computer experiment by the example of the model Cu₃Au alloy. The distributions of the long-range and short-range order parameters in the planes parallel to antiphase boundaries have been obtained by the Monte Carlo method for different temperatures. The mechanisms of disordering near conservative and nonconservative boundaries in ordering alloys are revealed. It is shown that the effect of diffusion of superstructural parameters is significant near nonconservative antiphase boundaries in comparison with conservative boundaries. Obviously, nonconservative boundaries can make a relatively large contribution to the effect of positive temperature dependence of the yield strength of ordering alloys with the L1₂ superstructure.     

Microstructure and transport properties of various 90° grain boundaries in YBa2Cu3O7 thin films

The microstructure and transport properties of various 90° grain boundaries in (103) oriented YBa₂Cu₃O₇(YBCO) thin films grown epitaxially in situ by 90° off-axis sputtering are compared. The (103) films grown on (101) LaAlO₃ and (101) SrTiO₃ substrates have specific sets of 90° grain boundaries in both principal in-plane directions: 90° [010] twist boundaries along the [101] direction, and 90° [010] symmetrical tilt boundaries and 90° [010] basal-plane-faced tilt boundaries along the (301) direction. No weak-link behavior is observed across some of these boundaries by transport critical current density and normalized magnetic field dependence of J _c measurements along both those in-plane directions. High-resolution transmission electron microscopy reveals variations in the structure and microfaceting of the 90° boundaries, which may contribute to the absence of weak-link behavior. These results have important implications for understanding the behavior of step-edge Josephson junctions.     

Transmission electron microscopy investigation of domains and boundaries in bulk La2/3Ca1/3MnO3

The microstructure of domains and boundaries in a bulk orthorhombic La_2/3Ca_1/3MnO₃ material is analysed by applying symmetry group theory. Group theory predicts that the 120° and 90° orientation domains occur in bulk La_{1? x} Ca _x MnO₃ (LCMO) when the cubic LCMO transforms into the orthorhombic LCMO by means of a structural phase transition. The orientation domains are observed and characterized by means of electron diffraction and high-resolution electron microscopy. Antiphase boundaries due to the loss of translation operations have displacement vectors of 1/2[010]o, 1/2[101]o and 1/2[111]0, according to the group theory. Diffraction contrast technique reveals that the displacive vector of most antiphase boundaries is 1/2[111]o, which is also confirmed by means of high-resolution electron microscopy.     

Observation of strong electron dephasing in highly disordered Cu93Ge4Au3 thin films

We report the observation of strong electron dephasing in a series of disordered Cu93Ge4Au3 thin films. A very short electron dephasing time possessing very weak temperature dependence around 6 K, followed by an upturn with further decrease in temperature below 4 K, is found. The upturn is progressively more pronounced in more disordered samples. Moreover, a lnT-dependent, but high-magnetic-field-insensitive, resistance rise persisting from above 10 K down to 30 mK is observed in the films. These results suggest a nonmagnetic dephasing process which is stronger than any known mechanism and may originate from the coupling of conduction electrons to dynamic defects.     

High-resolution electron microscopy of individual metallofullerene molecules on the dipole orientations in peapods

Electron microscopy with atomic sensitivity enables us to obtain a direct image of the intra-molecular structure of metallofullerenes encapsulated inside single-walled carbon nanotubes. By a comparison of high-resolution images with a simulation to extract the relative atom positions for encaged metal atoms in each molecule, the distribution of the molecular orientations and interactions between adjacent molecules in metallofullerene peapods have been statistically analyzed. The results are suggestive of strong interactions between fullerene–fullerene and fullerene–tube in peapods at room temperature. Received: 10 October 2002 / Accepted: 25 October 2002 / Published online: 10 March 2003 RID="*" ID="*"Corresponding author. Fax: +81-298/61-6310, E-mail: suenaga-kazu@aist.go.jp     

AlSb/GaAs(001)失配位错的高分辨电子显微学研究

用200kV六硼化镧光源的高分辨透射电子显微镜观察了AlSb/GaAs(001)外延薄膜的失配位错,结合解卷处理方法把[110]高分辨电子显微像转换为试样的结构投影图,其分辨率接近电子显微镜的信息极限.根据赝弱相位物体近似像衬理论,通过分析AlSb薄膜完整区解卷像的衬度随试样厚度的变化,确定了哑铃原子对中Al和Sb原子的位置.在此基础上构建出失配位错的结构模型,再结合模拟像与实验像的匹配,确定了AlAs型界面以及Lomer和60°两类失配全位错的核心结构.     

Observation of structural units at symmetric [001] tilt boundaries in SrTiO3

Incoherent Z-contrast imaging in the scanning transmission electron microscope allows atom column positions to be deduced directly from the experimental image, including locations where the column separation is less than the resolution limit. Maximum entropy analysis applied to the incoherent image locates the high-Z columns to an accuracy of ±0.2 Å. Oxygen coordination at the boundary plane can be deduced by high spatial resolution electron energy loss spectroscopy, and approximate column positions determined by simple bond-valence sum calculations. Observations of 25° (=85), 36° (=5) and 67° (=13) [001] symmetric tilt grain boundaries in SrTiO₃ bicrystals show that half columns are a ubiquitous feature of grain boundary structural units. The observed structural units can be combined to produce structural models for symmetric tilt boundaries over a 0–90° range. The =17 (410), =5 (310), and =5 (210) are found to be favored boundaries and the structures of all the other tilt boundaries are comprised of these units combined with =1 (100) and =1 (110) structural units. All the proposed boundary models show continuity of grain boundary structure over the entire misorientation range. The =17 (410) structural unit is asymmetric which induces microfacetting on all boundaries less than the =5, 36.87° misorientation.Work Supported by the U.S. Department of Energy under contract DE-Ac05-84OR21400.     

Transmission electron microscopy studies of femtosecond laser induced modifications in quartz

Cross-sectional transmission electron microscopy investigations of femtosecond laser induced sub-micrometer structural modifications inside crystalline quartz were carried out. Modifications from single laser shots and from lines built of overlapping shots were imaged. Both single laser shot modifications and line structures show an amorphous core surrounded by a disturbed crystalline structure. A strong strain field surrounding the central, irradiated, core is responsible for an increase of the refractive index. Finite element method calculations of the strain field show maxima on both sides of the irradiated core, which are in good agreement with optical measurements of the refractive-index change. Received: 29 September 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +49-3641/947792, E-mail: tatiana.gorelik@rz.uni-jena.de     

Effect of addition of La2O3 in the grain boundaries of a material based on BaTiO3

Results of an impedance study of capacitors of a material based on BaTiO₃ with variable concentrations of La are presented. It is observed that variation in tanδ is related to variation in covalency of the added La at the grain boundaries. The tanδ is temperature dependent. The discussion brings out that added La controls the electron density at the grain boundaries.