The Accommodation of Lattice Mismatch on the (111) Interphase Boundary Plane in FCC Metals
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THE ACCOMMODATION OF LATTICE MISMATCH ON THE (111) INTERPHASE BOUNDARY PLANE IN FCC METALS P. GUMBSCH and H.F. FISCHMEISTER Max-Planck-Institut fiir Metallforschung, Institut fur Werkstoffwissenschaft, 7000 Stuttgart, FRG ABSTRACT Using the embedded atom method we atomistically model the accommodation of the lattice nismatch and study the properties of the misfit dislocations in parallel oriented bicrystals. We calculate and analyze in detail the excess interfacial energies on the (100) and (111) boundary planes. The Ag/Ni system is chosen as a model system for metal/metal interfaces with a large lattice mismatch. Among the possible boundary planes in parallel oriented fcc bicrystals, the ones with terminating {111} planes arc energetically most favourable and are most often observed in experiments. This can be explained by a detailed analysis of the elastic strain fields in the interfaces, which correspond to networks of misfit dislocations. While the misfit dislocations on the (100) and (011) planes have a 1 [01i] Burgers vector, those on (111) can dissociate into misfit partials. The elastic strains connected with the misfit partials are, of course, much smaller than those for other types of misfit dislocations. The misfit partials form a triangular network within the boundary plane. INTRODUCTION Interfaces in solids play a major role in the processing and performance of most engineering materials. Understanding the influence of the interfaces on macroscopic material properties generally requires detailed knowledge of the interfacial structures and of the properties of the interfaces themselves. For example, the surprising mechanical properties of heterophase multilayers (e.g. [1]) can be directly related to the existence of the interphase boundaries. Therefore, the structure of interfaces between dissimilar materials has attracted much interest in recent years. Experiments like epitaxy or sphere-on-plate sintering, which indicate low energy configurations, show that the parallelity of all lattice directions is expecially favoured. Such boundaries are said to be coherent if they separate two crystals with equal interatomic spacings in the direction parallel to the interface. Semicoherent interfaces are formed between crystals with unequal interatomic spacings. They are characterized by networks of misfit dislocations separating nearly coherent regions. The geometry of such misfit dislocation structures can be described by O-lattice theory [2]. The excess interfacial energy of semicoherent heterophase boundaries contains a chemical component which arises from the chemical inhomogeneity in the interface region, and a structural component due to the distortions associated with misfit dislocations. In cohcrent Mat. Res. Soc. Symp. Proc. Vol. 209. @1991 Materials Research Society
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interfaces, the chemical component is the only contribution. The structural component has been calculated by van der Merwe [3] for a model system of two semiinfinite elastic continua with unidirectional misfit. The interfacial interact
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