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.