The Study of Defects in Metals Using High Resolution Transmission Electron Microscopy and Atomistic Calculations

1990 ◽  
Vol 183 ◽  
Author(s):  
M. J. Mills ◽  
M. S. Daw
Keyword(s):  
Grain Boundary ◽  
Good Description ◽  
Dislocation Core ◽  
Thin Foil ◽  
Core Structure ◽  
Embedded Atom ◽  
Pair Potentials ◽  
Transmission Electron ◽  
Atomistic Calculations ◽  
Stringent Test

AbstractThe coupling of HRTEM with atomistic calculations is described for the study of grain boundaries and dislocations in aluminum. HRTEM images of the Σ9 (221) [110] grain boundary are compared with molecular statics calculations using both the Embedded Atom Method (EAM) and two pair potentials. Comparison between observed and simulated images are shown to serve as a stringent test of the theoretical methods. Atomistic calculations can in turn provide threedimensional information about the defect structure. Using the EAM, it is also possible to account for the effects of thin foil geometries on the minimim energy configuration of defects. While these effects are found to be minimal for grain boundary structures, the influence of the thin-foil geometries on the core structure of the 60° dislocation in aluminum is discussed. These comparisons indicate that the EAM function provides a good description of grain boundary structures, but fails to reproduce the observed dislocation core structure due to a low predicted value of the intrinsic stacking fault energy (SFE) on the (111). In contrast, the pair potentials used in this study provide reasonable SFE values, but appear to be less accurate for the prediction of the Σ9 (221) [110] grain boundary structures.

Structural analysis of a Σ5 grain boundary in YBa2Cu3O7δ

1993 ◽  
Vol 51 ◽  
pp. 942-943
Author(s):  
J.-Y. Wang ◽  
Y. Zhu ◽  
A.H. King ◽  
M. Suenaga
Keyword(s):  
Grain Boundary ◽  
Lattice Mismatch ◽  
Weak Link ◽  
Coincidence Site Lattice ◽  
Large Angle ◽  
Dislocation Core ◽  
Superconducting Order Parameter ◽  
Outstanding Problem ◽  
Transmission Electron

One outstanding problem in YBa2Cu3O7−δ superconductors is the weak link behavior of grain boundaries, especially boundaries with a large-angle misorientation. Increasing evidence shows that lattice mismatch at the boundaries contributes to variations in oxygen and cation concentrations at the boundaries, while the strain field surrounding a dislocation core at the boundary suppresses the superconducting order parameter. Thus, understanding the structure of the grain boundary and the grain boundary dislocations (which describe the topology of the boundary) is essential in elucidating the superconducting characteristics of boundaries. Here, we discuss our study of the structure of a Σ5 grain boundary by transmission electron microscopy. The characterization of the structure of the boundary was based on the coincidence site lattice (CSL) model.Fig.l shows two-beam images of the grain boundary near the projection. An array of grain boundary dislocations, with spacings of about 30nm, is clearly visible in Fig. 1(a), but invisible in Fig. 1(b).

Structure of a Near-Coincidence Σ9 Tilt Grain Boundary in Aluminum

1989 ◽  
Vol 159 ◽  
Author(s):  
M. J. Mills ◽  
G. J. Thomas ◽  
M. S. Daw ◽  
F. Cosandey
Keyword(s):  
Grain Boundary ◽  
Interface Structure ◽  
Embedded Atom Method ◽  
Embedded Atom ◽  
Grain Boundary Plane ◽  
Transmission Electron ◽  
Symmetric Structures ◽  
Atomically Flat ◽  
Image Simulations

ABSTRACTA systematic study of the structure of tilt grain boundaries in aluminum has been initiated. High resolution transmission electron microscopy is being used to examine the interface structure of several bicrystals with <110> tilt axes. In this paper, we report the structure determination of a grain boundary close to the Σ9 (221) symmetric orientation. The grain boundary plane, which appears wavy at lower magnification, is actually composed of atomically flat microfacets. Two distinct, symmetric structures with (221) boundary planes have been identified within individual microfacets. These observations have been compared with structures calculated using the Embedded Atom Method. The semi-quantitative comparison between the observed and predicted grain boundary structures is accomplished using multislice image simulations based on the calculated structures. The results of these comparisons and the evaluation of the relative energies of the microfacets are discussed.

The Influence of Grain Boundary Inclination on the Structure and Energy of Σ3 Twin Boundaries in Copper

1991 ◽  
Vol 238 ◽  
Author(s):  
Ulrich Wolf ◽  
F. Ernst ◽  
T. Muschik ◽  
M. W. Finnis ◽  
H. F. Fischmeister
Keyword(s):  
Grain Boundary ◽  
Twin Boundary ◽  
Boundary Plane ◽  
Embedded Atom Method ◽  
Twin Boundaries ◽  
Atomic Structures ◽  
Embedded Atom ◽  
Partial Dislocations ◽  
Transmission Electron ◽  
Small Angle Boundary

ABSTRACTIn a combined theoretical and experimental study, the energies and structures of Σ3 [011] twin boundaries in Cu were investigated. The atomic structures and the grain boundary energies were calculated using the Embedded Atom Method (EAM). Grain boundary energies of welded Cu bicrystals of the same boundary orientations were also obtained by the thermal grooving technique. The atomic structure of the symmetric {211} incoherent twin boundary (SITB) was investigated by High Resolution Transmission Electron Microscopy (HRTEM). Calculated grain boundary energies γb plotted against the inclination angle Φ of the boundary plane relative to the {111} coherent twin boundary (CTB) plane show a mininmm for the CTB (Φ = 0°) and a second minimum at Φ = 82°. This dependence on the inclination is also confirmed by the measured energies. Common to all calculated boundary structures is a microface 11 ing into CTB and SITB segments with a symmetric orientation of the adjacent crystals. Additionally, strong relaxations occur for the grain boundaries near the second energy minimum. This relaxation can be interpreted as a sequence of stacking faults located almost perpendicular to the mean boundary plane. They are terminated by partial dislocations which form a small angle boundary. The most apparent feature of these structures is a bending of the {111} planes running across the boundary. The structural properties were confirmed by HRTEM.

Molecular dynamics simulations of grain boundary diffusion in Al using embedded atom method potentials

1995 ◽  
Vol 10 (7) ◽  
pp. 1589-1592 ◽  
Author(s):  
Chun-Li Liu ◽  
S.J. Plimpton
Keyword(s):  
Molecular Dynamics ◽  
Grain Boundary ◽  
Boundary Diffusion ◽  
Md Simulations ◽  
Embedded Atom Method ◽  
Melting Temperatures ◽  
Embedded Atom ◽  
Pair Potentials ◽  
Dynamics Simulations ◽  
First Time

Molecular dynamics (MD) simulations of diffusion in a Σ5(310) [001] Al tilt grain boundary were performed using for the first time three different potentials based on the embedded atom method (EAM). The EAM potentials that produce more accurate melting temperatures also yield activation energies in better agreement with experimental data. Compared to pair potentials, the EAM potentials also give more accurate results.

Atomic Structure of the (310) Twin in Niobium: Theoretical Predictions and Comparison with Experimental Observation

1992 ◽  
Vol 295 ◽  
Author(s):  
Geoffrey H. Campbell ◽  
S. M. Foiles ◽  
M. Rühle ◽  
W. E. King
Keyword(s):  
High Resolution ◽  
Grain Boundary ◽  
Atomic Structure ◽  
Mirror Symmetry ◽  
Interatomic Potentials ◽  
Pseudopotential Theory ◽  
Embedded Atom ◽  
Transmission Electron ◽  
Theoretical Predictions ◽  
Model Structures

AbstractHigh - resolution transmission electron microscopy (HREM) has been used to characterize the atomic structure of the symmetric 36.9° tilt grain boundary with [001] tilt axes forming a twin about (310) in Nb. The projected structure was imaged along two different directions in the plane of the boundary and was compared to model structures through high - resolution image simulation. The atomic structure of this Σ5 boundary was predicted with atomistic simulations using interatomic potentials derived from the Embedded Atom Method (EAM), Finnis-Sinclair (FS), and the Model Generalized Pseudopotential Theory (MGPT). The EAM and FS predicted structures with translations of the adjacent crystals which break mirror symmetry. The MGPT predicted one stable structure with mirror symmetry. The atomic structure of the (310) twin in Nb was found by HREM to be mirror symmetric. These findings indicate that the angular dependent interactions modeled in the MGPT are important for determining the grain boundary structures of bcc transition metals.

Grain Boundary Dislocation Structure and Motion in an Aluminum Σ=3 [011] Bicrystal

1996 ◽  
Vol 466 ◽  
Author(s):  
D. L. Medlin ◽  
S. M. Foiles ◽  
C. B. Carter
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
High Resolution ◽  
Grain Boundary ◽  
Twin Boundary ◽  
Defect Structure ◽  
Boundary Dislocation ◽  
Structure And Motion ◽  
Transmission Electron ◽  
Atomistic Calculations

ABSTRACTHigh-resolution transmission electron microscopy (HRTEM) observations are presented of a/3[111] grain-boundary dislocations in an aluminum Σ=3[011] bicrystal. These dislocations are present on both (111) (coherent twin) and (211) (incoherent twin) facets of the bicrystal boundary. The dislocations on the coherent twin facet migrate by a climb process that increases the thickness of the twinned material. These dislocations originate on a Σ=3 (211) incoherent twin boundary where they are closely spaced and dissociated in a wide core configuration. Atomistic calculations of the defect structure and interaction of multiple a/3[111] grain boundary dislocations are discussed.

Atomistic Simulation of Dislocation Motion as Determined by Core Structure

1994 ◽  
Vol 350 ◽  
Author(s):  
Kevin Ternes ◽  
Diana Farkas ◽  
Zhao-Yang Xie
Keyword(s):  
Atomistic Simulation ◽  
Dislocation Core ◽  
Dislocation Motion ◽  
Core Structure ◽  
Planar Structure ◽  
Interatomic Potentials ◽  
Atom Type ◽  
The Core ◽  
Embedded Atom ◽  
Structure And Motion

AbstractTwo different interatomic potentials of the embedded atom type were used to study the relationships between dislocation core structure and mobility. Core structures were computed for a variety of dislocations in B2 NiAl. Several non-planar cores were studied as they reacted to applied stress and moved. The results show that in some cases, the dislocation core transforms to a planar structure before the dislocation glides, whereas in some other cases the core retains the non-planar structure at stresses sufficient to sustain glide. The effects of stoichiometry deviations on the core structure and motion were also studied.

Structurally-induced elastic anomalies in a superlattice of (001) twist grain boundaries

1989 ◽  
Vol 4 (6) ◽  
pp. 1427-1443 ◽  
Author(s):  
D. Wolf ◽  
J. F. Lutsko
Keyword(s):  
Grain Boundary ◽  
Elastic Constants ◽  
Mean Field ◽  
Three Dimensional ◽  
Mean Field Model ◽  
Jones Potential ◽  
Embedded Atom ◽  
Atomistic Calculations ◽  
Elastic Anomalies ◽  
Strained Layer Superlattices

It is suggested that the “supermodulus effect” observed for composition-modulated strained-layer superlattices may arise from the presence of the structurally disordered solid interfaces and not necessarily from electronic-structure effects. The latter are excluded by investigating the elastic properties of a so-called grain-boundary superlattice in which chemically identical materials are joined to form a three-dimensional superlattice. Both an embedded-atom-method and Lennard-Jones potential are employed in our zero-temperature atomistic calculations of the elastic constants and moduli of such a superlattice. They yield qualitatively similar results which, for large modulation wavelengths, can be represented by a mean-field model in which the interfacial regions are characterized by a set of effective elastic constants which are different from those of the bulk regions. The appearance of a maximum in the biaxial modulus and a minimum in the shear modulus is shown to arise from the interaction between interfaces. It is also shown that such extreme anomalies appear only in the moduli but not in the elastic constants of the grain-boundary superlattice.

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