Tilt Grain Boundaries In Metals Research Articles

Structural Vacancy Model of Grain Boundaries

A structural vacancy model of tilt grain boundaries in metals has been developed. For construction of the stable structure of the boundary, the initial pattern was chosen according to the CSL model. The introduction of additional atoms and vacancies into the boundary region and shifting of atoms by the interatomic forces stabilize its structure. The criterion of a stable structure is the grain-boundary energy. The comparison of two main approaches to the stabilization of the grain structure demonstrated that changing the number of atoms at the boundary is more energetically advantageous than the relative shift of grains. The stability of the structure obtained has been studied under the shear stress. In the model developed, atomic structures obtained with pair and many-body potentials have been compared. The comparative analysis has shown that the grain-boundary structure does not depend on the choice of potential; atomic positions differ by less than 0.1 A, which is 2.5% of the lattice parameter. The atomic structure is in agreement with experimental images of grain boundaries.

Structure imaging of grain boundaries in F.C.C. metals

One can now extract information about the projected atomic structure of tilt grain boundaries in metals from zone axes orientations which are not the most open structures as in previous lower voltage microscopic studies of a few years ago. This has been made possible in part by the development of intermediate voltage microscopes capable of a point resolution exceeding 2Å level For these studies it was also necessary to develop techniques for fabricating tilt boundary structures from epitaxially grown Au thin films placed in contact. Analysis of grain boundary micrographs further required that Fourier image filtering and enhancement be applied to the original micrographs. Subsequent computer modelling of the dynamical diffraction processes and images was also necessary to insure the proper image interpretation.An example of a ∼80Å thick thin film containing a symmetrical tilt boundary corresponding to a Σ = 17/[001]28.1° (410) is displayed in Fig. 1. There is a [100] tilt axis common to both grains and the {200} crossed sets lattice planes on each side of the interface require a resolution level of 2.03Å which is well beyond the {111} planar spacings of 2.3Å of the most open [110] orientation in Au. Here the atom columns are designated by either open circles or crosses corresponding to a two layer repeat. The boundary dislocation cores are designated by structural units of capped trigonal prisms or the same with one cap modifying to contain a rhombohedron unit.

FIRST-PRINCIPLES CALCULATIONS OF THE ELECTRONIC STRUCTURE OF GRAIN BOUNDARIES

FIRST-PRINCIPLES CALCULATIONS OF THE ELECTRONIC STRUCTURE OF GRAIN BOUNDARIES

High resolution microscopy of a Σ = 17 grain boundary in gold

Because of the resolution limits of most microscopes, the imaging of the atomic structure of tilt grain boundaries in metals has been restricted to the most open structures with relatively large projected lattice parameters. In the case of Au the boundaries have been observed along a common [110] axis to both grains of bicrystalline specimens where coincidence cases have been studied such as the: Σ= 11, (113); Σ= 3, (111) and (112) and the ∑ = 19, (331) as well as other low and high angle boundary structures. Here the lattice periodicities visualized correspond to the {111} planes of 2.3Å. For the case of the b.c.c. lattice a Mo Σ = 41, (910) boundary was viewed along a common [001] were the {110} lattice planes of 2.2Å were visualized in a 200kV microscope. Even more recently the same author studied a boundary between the ∑ = 41 and ∑ = 25 reference orientations in Mo in a 400kV microscope, but apparently showed the same resolution as the previously mentioned study. Consequently, only a few studies of boundaries in metals have approached the 2Å resolution limit and recently images were obtained from a ∑ = 5, (310) symmetrical boundary in Au. In this latter study a relatively noisy image was obtained due to the reduced contrast transfer function of a 300kV microscope in the region of the second phase contrast zone. In the case of a 400kV microscope it is expected that a considerable improvement in information transfer would occur. It is therefore the intent of this study to present high resolution results for a tilt boundary in Au near a ∑ = 17, (530) orientation taken at 400kV which has been image processed to enhance the visualization of structural detail.