Diffraction Effects from [111] Twist Boundaries in Gold
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DIFFRACTION EFFECTS FROM [111] TWIST BOUNDARIES
IN GOLD
I. MAJID, D. WANG AND P. D. BRISTOWE Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge, MA 02139
ABSTRACT The structural characteristics of (111] twist boundaries in gold are investigated using a combination of x-ray diffraction and computer modeling techniques. Comparison of the measured scattering effects with those generated from EAM computer models reveals that the [111] boundary displacement field is weak, rotational in form and centered on '0' lattice sites. Furthermore, the measured intensities of the strong '0' lattice reflections decrease smoothly with increasing boundary angle up to calculated' from the model. The effect of double positioning diffraction pattern and the structural analysis is discussed.
300, as on the
INTRODUCTION In a recent paper [1], the theoretical diffraction effects from a series of [111] twist boundaries in gold were calculated using an embedded atom model (EAM) to simulate the boundary displacements. In that study, the objective was to predict the strength and distribution of the kinematical scattering from individual [111] boundaries and also determine any additional effects introduced by a phenomenon known as double positioning. Double positioning is the coexistence, in the same crystal, of two distinct types of domains which are related by a simple twinning operation and differ in misorientation by 0
60 . Domains of doubly positioned (DP) boundaries are always found in (111) gold bicrystals prepared from epitaxially grown thin-films due to the presence of faults in the (111) stacking sequence and the formation of low-energy twin boundaries. It is clear that the presence of two distinct domain structures in a (111) bicrystal would complicate an x-ray diffraction study aimed at determining the atomic structure of the boundary. This is because, in general, each domain would be expected to diffract differently and the measurements would represent average intensities from both boundary types. In the calculations, however, it was found that a window of misorientation exists around 300 in which pairs of DP boundaries diffract with the same intensity and on the same reciprocal lattice section. Under these conditions, therefore, x-ray measurements could be made of the superimposed intensities and information derived concerning the structure (in projection) of each DP boundary. Outside this misorientation window, however, structure determination was predicted to be less reliable since the superimposed intensities were unequal and could not be separated. In the present paper, the calculated scattering from [111] twist boundaries is compared to some quantitative x-ray measurements on the same boundaries in gold. Effects due to double positioning are discussed and, because the amount of available data is limited, emphasis is placed on reproducing trends and obtaining consistency rather than attempting detailed structure determinations.
Mat. Res. Soc. Symp. Proc. Vol. 209. 01991 Mater
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