Electron microscopy of transformation dislocations at interphase boundaries
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INTRODUCTION
M A N Y questions concerning the fine structure of interphase boundaries (IB's), and, particularly, the nature of deformation singularities visible along these IB's, can now be answered with confidence because high-resolution transmission electron microscopy (HRTEM) has begun to resolve interatomic spacings. Indeed, HRTEM techniques provide experimental tests of theoretical approaches to the atomic structures of interfaces, as well as new ways to determine if the concepts of continuum elasticity are still reasonable at scales less than 1 nm. In order to approach the problems posed by the fine structure of IB's and their discrete or continuous descriptions, a few results concerning grain boundary structures will be briefly recalled. Some properties of low-energy IB's, as well as HRTEM observations of various defects linked to IB ledges, will then be presented. An analysis of the displacement fields around some of these defects is also detailed using the Somig|iana dislocation (SD) concept, t1-4~ Finally, the experimental and computed positions of the atomic rows are compared in the vicinity of three interfacial ledges; among those, one interacts with a stacking fault of a crystal. II. STRUCTURAL UNITS AND TRANSFORMATION DISLOCATION-STRUCTURAL UNITS OF IB'S Grain boundary structures as imaged in HRTEM appear to vary considerably with the orientation relationships of the crystals and the local interface orientation.15-sl The best-known structures are those of parR. BONNET, Director of Research of CNRS, and M. LOUBRADOU, Teaching Associate, are with the Institut National Polytechnique de Grenoble, 38402 Saint Martin D'Hrres Cedex, France. A. CATANA, Staff Research Engineer, is with IBM at Ziirich, Switzerland. P. STADELMANN, Professor, Institut of Electron Microscopy, is with the Ecole Polytechnique Frdrrale de Lausanne, Switzerland. This paper is based on a presentation made in the symposium "The Role of Ledges in Phase Transformations" presented as part of the 1989 Fall Meeting of TMS-MSD, October 1-5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL. METALLURGICAL TRANSACTIONS A
ticular twin boundary facets in Si o r G e , [9-131 because the larger unit cell parameters make HRTEM observations easier. For pure metals, e.g., Mo and A n , [9'14-16] good experimental images of the atomic structures are now available due to recent improvements in high-resolution electron microscope performances. Particular grain boundary structures have been derived from computations minimizing the free energy of a finite number of atoms, l~7-z~ These computations and the HRTEM observations indicate that when the relative orientation of the crystals is exactly a twin orientation, each facet has its own well-defined atomic pattern, the structural unit (SU) that repeats periodically along the facet. Any angular misorientation from the twin orientation is accommodated by grain boundary facets along which the same or different S U ' s [161
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