Collective effects in grain boundary migration

In situ high-resolution transmission electron microscopy is used to study grain boundary structure and kinetics in bicrystalline Au films at elevated temperature. We report the first direct evidence for the existence of cooperative atomic motion in grain boundary migration. Certain nanoregions at grain boundaries, typically involving up to several hundred atoms, are found to switch back and forth between neighboring grains. Reversible structural fluctuations at temperatures near 0.5T(m) and above have been discovered in [110] and [001] tilt, as well as in general grain boundaries.     

The atomic structure of a Σ=5[001]/(310) grain-boundary in an Al-5% Mg alloy by high-resolution electron microscopy

A symmetrical tilt =5[001]/(310) grain-boundary and its surrounding matrix in an Al-5% Mg alloy, as prepared by cold rolling and annealing, have been studied by conventional transmission electron microscopy, high-resolution electron microscopy, and analytical electron microscopy. EDS measurements of the Mg concentration in the bulk and at grain boundaries indicate variations of Mg content ranging between 4 to 11 atomic percent. This variation is attributed to local ordering of Mg atoms in the alloy. HREM images show that the boundary runs parallel to the median of (310) and contains segments primarily composed of two types of structural unit. One unit contains seven atomic sites, while the other contains eight. In both types of unit, the grain-boundary coincident site lattice is continuous across the interface and exhibits periodicity in its core structure. The core structure, which is characterized by kite-like structural units containing seven atoms, conforms well with the structure of =5 grain boundaries in pure FCC materials. The presence of atomic-scale ordering of Mg atoms along [001] of the boundary core is associated with the existence of the structural unit possessing eight atomic sites.     

Grain Boundary Migration in Fe-3wt.%Si Alloys

The studies of three different laboratories on grain boundary migration in Fe-3wt.%Si alloys are presented. In all cases bicrystal techniques employing capillarity as driving force were used. [100] tilt boundaries were studied in the temperature range from 1223 K to 1373 K, and [110] tilt boundaries in the range from 1220 K to 1625 K. Proportionality between grain boundary velocity and driving force was confirmed. All data fulfil a linear relation between activation enthalpies and logarithms of the pre-exponential factors, corresponding to a compensation temperature of 1386 K where all boundaries theoretically should possess the same mobility. A considerably lower activation enthalpy was found in one case for an asymmetrical grain boundary compared to the symmetrical boundary of the same misorientation. High values of activation enthalpy of migration were found for special [100] boundaries compared to general ones although an opposite tendency was also observed for [100] boundaries.     

Chevron defect at the intersection of grain boundaries with free surfaces in Au

We have identified a new defect at the intersection between grain boundaries and surfaces in Au using atomic resolution transmission electron microscopy. At the junction line of 90 degrees <110> tilt grain boundaries of (110)-(001) orientation with the free surface, a small segment of the grain boundary, about 1 nm in length, dissociates into a triangular region with a chevronlike stacking disorder and a distorted hcp structure. The structure and stability of these defects are confirmed by atomistic simulations, and we point out the relationship with the one-dimensional incommensurate structure of the grain boundary.     

Finite-size effects on the structure of grain boundaries

We present a combined experimental and theoretical analysis of the structure of finite-sized Sigma 3 [112] grain boundaries in Au. High-resolution electron microscopy shows lattice translations at the grain boundary, with the magnitude of the translation varying along the finite-sized grain boundaries. The presence of this structural profile is explained using continuum elasticity theory and first-principles calculations as originating from a competition between elastic energy and the energy cost of forming continuous [111] planes across the boundary. This competition leads to a structural transition between offset-free and nontrivial grain boundary structures at a critical grain boundary size, in agreement with the experiments. We also provide a method to estimate the energy barrier of the gamma surface.     

Change of superdislocation splitting with strain in single crystals of the Ni3Fe ordered alloy

The method of transmission electron microscopy is used to study the dislocation structure in single crystals of the Ni ₃ Fe ordered alloy for different degrees of strain. The [1812], [011], [111] and [001] single crystal orientations are examined. The superdislocation splitting (r) is measured for different degrees of strain, distributions of r are constructed, and their average values 〈r〉 are estimated. The 〈r〉-ε curves are constructed. With the use of the experimental r values, the effective energy of antiphase boundaries ζ* is estimated. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 51–56, January, 2006.     

Structure and morphology of nanosized lead inclusions in aluminum grain boundaries

Grain boundary lead inclusions formed by ion implantation of mazed bicrystal aluminum films have been investigated by transmission electron microscopy. The vapor-grown bicrystal films contained mainly 90°(110) tilt boundaries with fixed misorientation but variable inclination, as well as some growth twins with 70.5°(110) symmetrical tilt boundaries and a few small-angle boundaries. It was found that the shape, size and orientation of the inclusions in the grain boundaries depend on the orientation of the aluminum grain boundary plane. Inclusions at 90°(110) tilt boundaries were invariably sharply faceted toward one aluminum grain and more rounded toward the other grain. The faceted side was a section of the cuboctahedral equilibrium shape of bulk lead inclusions in parallel topotaxy with the aluminum matrix. The rounded side, where the aluminum grain was rotated by 90° with respect to the lead lattice, approximated a spherical cap. At specific low-energy segments of the grain boundary where a (100) plane in grain 1 meets an (011) plane in grain 2, only two of several possible shapes were observed. One of these was preferred in as-implanted samples while both types were found after melting and re-solidification of the lend inclusions. The observations are discussed in terms of a modified Wulff construction.     

Computer simulation of vacancy migration in a fcc tilt boundary

Vacancy jumps in a bicrystal model of Σ = 5 (36.9°) [001] tilt boundary in aluminum have been observed at temperatures between 700 and 800 K by means of molecular dynamics simulation. Preliminary results indicated considerable structure dependence and yielded an activation energy for vacancy migration of 0.38 eV. In contrast to a previous study of vacancy migration in a bcc tilt boundary, thermal activation of a boundary interstitial by Frenkel pair production was not observed.     

In situ scanning electron microscopy of graphene growth on polycrystalline Ni substrate

Scanning electron microscopy (SEM) is shown to be capable of imaging a monolayer of graphene, and is employed to observe in situ the graphene growth process by segregation of bulk-dissolved carbon on a polycrystalline nickel surface. Because of a wide field of view, SEM could easily track the rapid graphene growth induced by carbon segregation. Monolayer graphene extended on (111)- and (011)-oriented nickel grains, but was excluded from the (001) grains. This is due to the difference in carbon-nickel binding energy among these crystalline faces. This work proves the usefulness of in situ SEM imaging for the investigation of large area graphene growth.     

Microstructures of grain boundaries in (001)-oriented strontium bismuth tantalate thin films grown by pulsed laser deposition

(001)-oriented strontium bismuth tantalate thin films have been grown on Pt/TiO₂/SiO₂/Si (100) substrates by pulsed laser deposition. The room-temperature current–electric field dependence of the films has been investigated, which revealed a space-charge-limited conduction mechanism. The microstructures of grain boundaries and structural defects in these films were also examined by transmission electron microscopy and high-resolution transmission electron microscopy, respectively. The grains of the films deposited at 550 °C exhibited polyhedral morphologies, and the average grain size was about 50 nm in length and 35 nm in width. At a small misorientation angle (8.2°) tilt boundary, a regular array of edge dislocations with about 3-nm periodic distance was observed, and localized strain contrast near the dislocation cores was also observed. The Burgers vector b of the edge dislocation was determined to be [110]. At a high misorientation angle (39.0°) tilt grain boundary lattice strain contrast associated with the distortion of lattice planes was observed, and the mismatching lattice images occurred at about 2 nm along the boundary. The relationship between microstructural defects at grain boundaries and leakage currents of these films is also discussed. Received: 8 September 2000 / Accepted: 18 December 2000 / Published online: 28 February 2001     

Combined high-resolution electron microscopy and computer modeling of a Σ=5 (210) [001] tilt boundary in AuCu3

Combined high-resolution electron microscopy, computer modeling, and image simulations are used to study the structure of the =5 (210) [001] tilt boundary in the ordered compound AuCu₃. On the experimental side, we have prepared thin samples cut in a bulk bicrystal containing an interface close to the above (_tilt 37°). The high-resolution images obtained at 400 kV along the [001] tilt axis show that among possible configurations, the boundary adopted predominantly a symmetrical one. Theoretically, using the standard geometrical approach, we derived possible atomic configurations of the boundary and we identified some of the defects allowing for the coexistence between these variants. Finally, we performed energy minimizations using these geometrical models and an n-body potential adapted to this compound. In agreement with the experiment, one of the two possible symmetrical variants is associated to the lowest excess energy. Moreover, the computations show that large variations affect the spacing of (210) atomic planes near the boundary.     

Low-temperature resistance anomaly at SrTiO3 grain boundaries: evidence for an interface-induced phase transition

Variable temperature transport between 1.4 and 300 K, structural imaging, and theoretical calculations were used to characterize the properties of electrically active 24 degrees and 36.8 degrees [001] tilt SrTiO3 grain boundaries with 0.1 at. % niobium doping. An anomaly in boundary resistance and capacitance characteristics typical of a positive temperature coefficient effect is observed. This behavior is indicative of interface-induced dipole ordering. The detailed atomic structures of these grain boundaries were determined from a comparison of ab initio calculations and Z-contrast TEM images. The number of excess electrons at the boundaries determined experimentally and theoretically agrees and is associated with the boundary structural units.     

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.     

Atomic structure and electronic properties of the ∑37(610) and ∑29(520) [001] tilt grain boundaries in Ge

400 kV high resolution electron microscopy (HREM), deep level transient spectroscopy (DLTS) and steady state electrical measurements have been applied to 37(610) and 29(520) [001] tilt grain boundaries (GBs) in germanium bicrystals. The atomic boundary structures were revealed by experimental HREM images taken under different defocus conditions. Later, structure models were refined by means of a trial-and-error method applying alternatively the image simulation and the molecular static calculation of relaxed structures. The structures were shown to be consistent with the modified structural unit model. Although the structures are different for the two GBs studied, DLTS data and steady state measurements were found to be quite similar for both GBs. Thus, the results point to the extrinsic origin of localized deep states at the GBs. The analysis of DLTS spectra indicates the impurity segregation at the boundary, e.g., the formation of vacancy-type oxygen complexes of a donor-like state at E _c-0.21 eV, which results in the fluctuation of the potential barrier. Defects in the GBs—like facets, atomic steps and secondary grain boundary dislocations—which are characteristic of both boundaries can act as nuclei to the impurity segregation.Presented at the Workshop on High-Voltage and High-Resolution Electron Microscopy, February 21–24, 1994, Stuttgart, Germany.     

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.     

Nucleation of dynamic recrystallization and variant selection in copper bicrystals

Orientation-controlled copper bicrystals containing [001] symmetrical tilt boundaries aligned parallel to the loading axis were deformed in tension at 923?K and a strain rate of 4.2?×?10^?4?s^?1. The nucleation of dynamic recrystallization (DRX) was investigated along the grain boundary. For this purpose, both optical and orientation imaging microscopy methods were used. After grain-boundary migration (GBM) and bulging, nuclei appeared behind the most deeply indented grain boundary regions. The critical strain for nucleation was about one-quarter to one-half of the peak strain and depended on the misorientation angle. All the nuclei were twin-related (Σ3) to the matrices. Furthermore, all the primary twin traces were parallel to those of the inactive slip planes of the parent single crystals. Crystallographic analysis revealed the important role of the direction of GBM on twinning-plane variant selection. The characteristics of grain boundary nucleation depended sensitively on grain boundary character and on grain boundary mobility. The observed DRX nucleation mechanism is discussed in relation to the occurrence of GBM and twinning.     

Evolution of the dislocation structure and the stage of work-hardening of Ni3Fe alloy with [001] orientation

Using transmission diffraction electron microscopy, the dislocation structure of work-hardened iron nickel alloy with the compression axis in [001] orientation is studied over a wide interval of gradual strain. A connection is established between the change of the flow curve and the type of substructure formed. The quantitative characteristics of the observed structural types are determined and their behavior at different stages of the strain curve is traced.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 18–24, February, 1988.     

New Structures and Atomistic Analysis of the Polymorphism for the ∑ = 5 (210) [001] Tilt Boundary

Using interatomic potentials derived from the second moment approximation of the tight-binding scheme, we study the relative stabilities for the different structures of the = 5 (210) [001] tilt boundary in Cu and Ag. Relatively to the numerous previous studies, we obtain two new structures characterised by a larger periodicity along the tilt axis. Depending on the metal and on subtle features of the potentials, we show a very significant evolution of the relative stabilities. Using a local analysis on the different sites of the grain boundary, we link the stability of a given structure to peculiar features of the interatomic potential and to physical properties of the metal.