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Microstructure of Ceramic Materials UNITED STATES DEPARTMENT OF COMMERCE • Luther H. Hodges, Secretary NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director Microstructure of Ceramic Materials Proceedings of a Symposium April 27-28, 1963 Held under the auspices of the Ceramic Educational Council of the American Ceramic Society, with the cooperation of the National Bureau of Standards, and under the sponsorship of the Edward Orton Junior Ceramic Foundation and the Office of Naval Research. The Symposium took place at the 65th Annual Meeting of the American Ceramic Society in Pittsburgh. National Bureau of Standards Miscellaneous Publication 257 Issued April 6, 1964 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C., 20402 Price 31.75 (Buckram) Library of Congress Catalog Card Number: 64-60020 Contents troduction Geometry of Microstructures Lawrence H. Van Vlack Department of Chemical and Metallurgical Engineering University of Michigan Ann Arbor, Michigan Experimental Techniques for Microstructur e Observation Van Derek Frechette Alfred University Alfred, New York The Effect of Heat Treatment on Microstructure Joseph E. Burke Research Laboratory General Electric Company Schenectady, New York Correlation of Mechanical Properties with Microstructure Robert J. Stokes Research Center Minneapolis-Honeywell Regulator Co. Hopkins, Minnesota Microstructure of Magnetic Ceramics A. L. Stuijts Philips Research Labs. Eindhoven Netherlands Microstructure of Porcelain Sten T. Lundin Royal Institute of Technology Stockholm Sweden iii . Introduction In all materials, the physical and even the chemical properties of a polycrystal- line body are not exactly the same as those of a single crystal of the same materials. Many materials are anisotropic - their properties depend upon the orientation of the measuring system with respect to the crystallographic axes - and the polycrystalline properties are some form of an average over the crystal directions. Thus the di- electric constant, elastic constants, index of refraction, magnetic susceptibility, and many other "bulk" properties depend to some extent on the microstructure of the specimen Beyond this, some properties depend on the motion of various entities through the material-transport of atoms and ions in diffusion, transport of phonons in ther- mal conduction and electrons and ions in electrical conduction, motion of dislocations and other defects in plastic deformation and of domain walls in ferromagnetic and ferroelectric switching, and even the propagation of cracks in fracture. In these transport processes, the grain boundaries between the crystals in a polycrystalline body behave differently from the bulk material, and their presence markedly affects the resulting properties. Diffusion is usually faster at grain boundaries, especial- ly at low temperatures, so that diffusion is enhanced in polycrystalline bodies. Electrons and phonons are scattered by grain boundaries, so that electrical and ther- mal conduction tends to be lower in the polycrystals . The movements of dislocations across grain boundaries are impeded, so that plastic deformation is inhibited by their presence, and polycrystalline bodies tend to be stiffer and less ductile than the corresponding single crystals. Finally, the presence of grain boundaries not only modifies the behavior, but sometimes even introduces new elements. Thus in brittle fracture the grain bounda- ries provide sources of cracks, making polycrystals weaker in general than single crystals. The presence of strain and of impurities at grain boundaries raises the local free energy, so that chemical effects, such as etching rates, are enhanced. It is clear, then, that a knowledge of the microstructure of a polycrystalline body is essential in any attempt to study and control its properties. This is par- ticularly important in the field of ceramics, where the overwhelmingly important form is the polycrystalline body. In order to review the problems involved in specifying and studying microstructure in ceramics and the factors involved in the interaction between microstructure and physical properties of ceramics, this Symposium on Microstructure of Ceramic Materials was held. The papers presented are published in this volume. Primary responsibility for their technical content must rest, of course, with the individual authors and their organizations. In the first two Chapters, Prof. Van Vlack reviews the geometry of microstruc- tures and how they can be specified and Prof. Frechette describes the principal experimental techniques by which observations of microstructures are made. In Chapter 3, Dr. Burke then describes the factors controlling the development of the microstructure during heat treatment of the ceramic, and their relation to the processing variables of time and temperature. In the next two Chapters, Dr. Stokes discusses the influence of microstructure on the mechanical behavior, and Dr. Stuijts describes the influence on the ferro- magnetic properties of ferrites. In the last Chapter, Prof. Lundin examines in detail the microstructure of one material, porcelain, and its ramifications. Ivan B. Cutler, Chairman University of Utah Joseph E. Burke Willis E. Moody, Co-Chairman General Electric Company Georgia Institute of Technology William D. Kingery Alan D. Franklin, Editor Massachusetts Institute of National Bureau of Standards Technology iv Geometry of Microstructures Lawrence H. Van Vlack 1. Introduction The internal structure that a material possesses on a microscopic scale is gen- erally called mic ro s true ture . This presentation is concerned with the geometry of such microstructures as encountered in ceramic materials. A study of microstructures does not involve the study of atomic coordination as it exists in various crystalline and amorphous phases. However, it does involve the phase and grain relationships at the lower end of the electron microscopic range, and extends through the size spectrum up to and well into the "hand lens" range. Examples at the lower end of the mic ro struc tural size range include the nuclei which start the crystallization of glass and therefore introduce a heterogeneity of structure. Micro- structures at the coarse end of the size range are found in ceramic products which can be illustrated by abrasive grinding wheels. These products contain a distribution of specific sizes and shapes of abrasive phases that are bonded with a silicate or similar material and include closely controlled porosity. The geometric variations which are encountered in microstructures include (1) size, {?.) shape, and (3) the preferred orientation of constituent grains-'- (Fig. 1). In addition, when more than one phase is present, there can be the added variables: (li) amount of phases, and (5) the distribution of phases among each other (Fig. 2). Item above is most closely related to the chemistry of the ceramic product because the amount of phases depends directly upon the composition. The other structural variables are less closely related to the composition and depend more specifically upon factors of processing and service history. Each of the preceding five micro- structural variations involves grain boundaries and the consequent crystal structural discontinuities. Many ceramic materials possess porosity. From a micro struc tural point of view, pores can be considered as an additional phase of zero composition. Of course this "phase" is a very important feature in the microstructure, because the pores markedly affect the micro struc tural dependent properties. As will be pointed out in later presentations, the microstructures and therefore the consequent properties of the ceramic are not static in behavior, for they may be altered by external factors such as mechanical forces, thermal conditions, chemical environments, and electric or magnetic fields. Therefore, a microstructure may be varied by processing factors and servicing conditions. This presentation will first consider the characteristics of internal boundaries, then single phase microstructures, and finally polyphase microstructures. 2. Internal Boundaries The most general characteristic of a microstructure is the presence of its internal boundaries which separate the grains and phases within the material. Whether these internal boundaries are between the disoriented grains of one phase, between grains of different phases, or between electrical and magnetic domains of one grain, they represent a specific change in the internal structure of the ceramic. Internal boundaries are thus an important feature in a ceramic and have significant effects on the properties of the material. It is possible to characterize boundaries as grain boundaries, domain boundaries, phase boundaries, or surfaces. However, such characterization is usually unnecessary for a general discussion, because each of the above boundaries may be considered to be a surface or zone of crystalline mismatch. Boundary discontinuities in a microstructure x The term, grain, as used in the discussion of microstructure denotes a single crystalline volume. This is in contrast to grog grains which are usually 1-10 mm in size and contain numerous small crystals. 1 . represent locations of higher atomic energies; therefore a "driving force" exists which tends to reduce the boundary areas with consequential boundary movements. 2.1. Boundary Structure The atomic structure of a grain or phase boundary must be inferred because it is currently impossible to view the involved atoms directly. However, several conclusions
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