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Session Chairperson: Diana Farkas, Dept. of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0237
8:30 am INVITED
MAKING LINKS BETWEEN GRAIN BOUNDARY CHARACTER DISTRIBUTIONS AND POLYCRYSTALLINE PROPERTIES: Alexander H. King, Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2275
Automated orientation analysis makes it possible to perform an "epidemiological" kind of materials research in which the occurrence of certain types of grain boundary is associated with certain types of polycrystalline property. What is left out of many studies of this kind, however, is the causal link that relates a certain grain boundary character distribution to a particular polycrystalline behavior. In this presentation, I will caution that focusing upon the grain boundary character in terms of "small-angle," "coincidence-related" and "general" categories can be misleading. I will provide a number of examples to show that these are not always the important (or desirable) features of the grain boundary character distribution, drawing my illustrations from work on high-Tc superconductors and metallic polycrystalline thin films. Finally, if time permits, I will comment upon the characterization of triple-junctions and show, once again, that currently popular types of characterization are seriously misleading. Acknowledgment: This work is supported by the National Science Foundation, under grant number DMR-9530314.
THE STRUCTURE OF DEFORMATION INDUCED HIGH ANGLE BOUNDARIES: D.A. Hughes, Materials and Engineering Sciences Center, Sandia National Labs., Livermore, CA 94550
Internal interfaces develop during deformation at the places where grains subdivide. Some of these interfaces develop into high angle boundaries that subsequently have a large effect on the materials properties including local crystallographic textures, flow strength and annealing behavior. The structure of very high angle boundaries (>30=B0) formed during deformation by dislocations and texture processes was examined using transmission electron microscopy and convergent beam diffraction. The large misorientation angles that develop by these processes are similar to those angles encountered in ordinary grain boundaries. These boundaries are characterized according to their angle/axis pair, boundary plane, tilt/twist character, thickness, sigma value and their association with trapped glide dislocations. In general these boundaries have a complex character and are not low sigma boundaries. The structure of these deformation induced boundaries is then compared to that of equilibrium grain boundaries such as those that form during recrystallization. This work supported by U.S. DOE under contract No. DE-AC04-94AL8500.
ANALYSIS OF TWINS AND STACKING DEFECTS IN THE CUBIC LAVES PHASE OF THE Hf-V-Nb ALLOY SYSTEM: D.E. Luzzi, D.P. Pope, A. Goldberg, G. Rao*, Dept. of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104-6272; *Present Address: Applied Materials Corporation, Santa Clara, CA
The brittle nature at low temperatures of Laves phase intermetallic compounds remains a major obstacle to the use of these complex-structured materials for their excellent high temperature properties. Deformation by twinning is seen as a possible mechanism by which to obtain acceptable levels of ductility and toughness at low temperatures. In this paper, the microstructures of a cubic HfV2+Nb Laves phase is studied before and after compressive deformation using conventional and high-resolution electron microscopy. The Laves phase occurs as precipitates within a matrix of a V-Nb bcc solid solution. In the underformed material, narrow stacking defects with thicknesses of from one to three times the (111) interplanar spacing are seen lying on the (111) crystallographic planes. The distribution of these defects is anisotropic on the mesoscopic scale with spacings ranging from approximately 2 nm to over 200 nm. The deformed material shows extensive twinning of the Laves phase precipitates. The twins occur in clusters and twin bands as small as 2 nm in width are seen. Analysis of the structure of the stacking defects and twins and comparisons between the mesoscopic distributions of stacking defects in the undeformed materials and twins in the deformed materials will be presented.
DISSOCIATION MECHANISMS FOR EXTRINSIC GRAIN BOUNDARY DISLOCATIONS: S.G. Song, J.S. Vetrano, S.M. Bruemmer, Pacific Northwest National Laboratory, Richland, WA 99352
The plastic deformation of grain boundaries (GBs) is central to the understanding of a wide range of materials behavior including superplasticity and creep. In analogy to the bulk deformation carried out microscopically by lattice dislocations, GB deformation cannot occur without the involvement of grain boundary dislocations (GBDs). The present investigation examines the stability of extrinsic GBDs in Al alloys. It is found that the dissociation of extrinsic GBDs not only is a function of temperature but also of other variables such as solute content and GB structures. Given a boundary structure, the Burgers vectors of the secondary GBDs, resulting from the dissociation of extrinsic GBDs in general GBs, can be predicted based on the DSC-lattice. The visibility of the secondary dislocations varies with the magnitude of their Burgers vectors. The GBD stability of different boundary misorientations is compared so that common properties of grain boundary plastic behavior can be drawn. Work supported by the Materials Division, Office of Basic Energy Sciences, U.S. Department of Energy under Contract DE-AC06-76-RLO 1830.
EFFECT OF THE MAGNITUDE OF GRAIN BOUNDARY DISLOCATION BURGERS VECTORS O DISSOCIATION OF EXTRINSIC GRAIN BOUNDARY DISLOCATIONS: S.G. Song, J.S. Vetrano, S.M. Brummer, Structural Materials Interfaces, Pacific Northwest National Laboratory, Richland, WA 99352
The concept of Burgers vectors of secondary grain boundary dislocations (SGBDs), resulting from the dissociation of extrinsic grain boundary dislocations (EGBDs), in general grain boundaries can be extrapolated from those of GBDs in CSL boundaries. Increasing CSL boundary index results in the decrease in magnitude of the basis vectors of the corresponding DSC lattice on which the Burgers vectors of the SGBDs are determined. There exists a critical size of the elementary Burgers vectors of the dissociated grain boundary dislocations. Below this value, an EGBD disappears eventually after the SGBDs resulting from the dissociation spread to a sufficient distance that is dependent on temperature and instrument parameters. On the other hand, the resulting SGBDs are always visible if their Burgers vectors are above the critical size.
10:30 am BREAK
10:50 am INVITED
CONTROL OF METAL PRECIPITATE MORPHOLOGY BY SOLID STATE REACTIONS: Monika Backhaus-Ricoult, CECM-CNRS, 15 Rue G. Urbain, F 94 407 Vitry sur Seine
Internal reduction of transition metal doped mixed oxides yields formation of fine dispersion of metal particles within the oxide matrix. Selection of special reaction parameters like oxygen partial pressure gradient, reaction temperature, chemical composition of the mixed oxide and the type of matrix oxide and dopant allows to control the microstructure of the reduced scale, the morphology of the metal precipitates and the fine-structure of the metal-ceramic interfaces down to an atomic scale. Experimental results for different mixed oxides containing magnesia, alumina or zirconia will be reported. Growth and thermodynamical equilibrium shapes of the precipitates will be presented. Precipitate morphology, relative orientation relationship and interface fine structure will be interpreted in terms of the diffusion field (which allows the metal precipitation), the chemical reaction at the precipitate interface, the interface energy and its anisotropy and the mechanical response of the system to solid state reaction related stresses.
INTERFACIAL DEBONDING IN MULTI-LAYER THIN FILM SYSTEMS: X.H. Liu, C.F. Shih, Division of Engineering, Brown University, Providence, RI 02912
To improve the reliability of electronic devices, it is important to understand the interfacial debonding of multi-layer thin film systems. The interfacial fracture energy can be measured experimentally using a sandwich four-point bending specimen. This interfacial fracture energy includes both the intrinsic debonding energy of the metal/ceramic interface and the plastic dissipation in the metal layers. The plastic dissipation makes a substantial contribution to the interfacial fracture energy. In contrast to the plastic dissipation, which depends on the layer thickness, the intrinsic debonding energy is a material property of the interface and is important in the evaluation of interface adhesion and design of multi-layers. A micromechanical model is used to investigate interfacial debonding. From the model the intrinsic debonding energy can be obtained using the measured interfacial fracture energy. The effects of interfacial adhesion and metal layer properties on the interfacial fracture energy are discussed.
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