Microstructure polycrystal

Capillary forces induce morphological evolution of an interface toward uniform diffusion potential—which is also a condition for constant mean curvature for isotropic free surfaces (Chapter 14). If a microstructure has many internal interfaces, such as one with fine precipitates or a fine grain size, capillary forces drive mass between or across interfaces and cause coarsening (Chapter 15). Capillary-driven processes can occur simultaneously in systems containing both free surfaces and internal interfaces, such as a porous polycrystal. 

With increasing zvalue the density of P-SiAlON decreases linearly, similarly lowering Young s modulus, strength, thermal conductivity, hardness and fracture toughness.30 On the other hand, for low zvalues (z< 1) the hardness and fracture toughness increase and when z > 1 they decrease. It has been also depicted that glass-free microstructures could be obtained in P-SiAlON polycrystals with substitutional level z> 2.31... 

Gleiter, H., in Deformation of Polycrystals Mechanism and Microstructure (N. Hansen, A. Horsewell, T. Leffers, and H. Lilholt, Eds.), 15, Riso National Laboratory, Rackilde, Denmark (1981). 

Often, it is not possible to obtain single crystals that are large enough to be worked with in a convenient manner. In those cases, physical properties must be measured on poly crystalline samples. There is always discrepancy, or disagreement, between measured physical properties of single crystals and polycrystals due to microstructural effects. Hence, physical properties measured from polycrystalline samples are sometimes considered less reliable from a reproducibility standpoint. 

Because of their strong chemical bonds, bulk ceramics are most efficiently fabricated by means of densification of powders. The fabrication process involves two main stages (1) consolidation of the powder to form a porous, shaped article (the green body), also referred to as forming, and (2) heating of the shaped powder form to produce a dense article, referred to as firing or sintering. The final product commonly consists of a relatively dense polycrystal with some residual porosity (Fig. 1). The microstructure, which... 

Both columnar and equiaxed microstructures are obtained from polycrystal growth. Columnar micro structure is composed of columnar grains of preferred orientation that grow from the first nucleated location on a substrate surface. The microstructure is attributed to high supersaturation and low temperature and hence more limited diffusion. A columnar microstructure is suitable for high-temperature structural applications, such as thermal barrier coatings. Figure 6.6 shows the typical columnar CVD SiC microstructure in C/SiC composites. 

Our intent in this section was not to give an exhaustive account of the nature of grain boundaries in real polycrystals. Rather, it was meant as a gentle reminder that despite the importance of bicrystals, they are but a first step in understanding the deeper question of the nature of grain boundaries in real microstructures. 

Microstructure in Single Phase Polycrystals. We begin with a discussion of the simplest microstructure (at least to characterize geometrically) in which the grain size is the dominant microstructural parameter. We are all used to the idea that the technologies that surround us are constrained by various codes which attempt to standardize products and the processes used to realize them. Such codes exist for everything ranging from skyscrapers to microwave ovens. It is... 

The microstructure described above may be seen as the most homogeneous limit of those we will consider here. A complementary set of information concerning microstructures of the type featured above can be obtained by mapping the orientations of the various grains making up the polycrystal. The basic idea is that one may determine the preponderance of different crystal orientations. The new technique of orientation imaging microscopy now allows for the determination of such orientational information with high spatial resolution. An example of the type of results that are obtained via this technique is shown in fig. 10.3. 

In the example given above, the formation of a material microstructure was seen to take place as a result of the deposition of atoms on a substrate. Another, equally important, route to solid microstructures is via the solidification process. During the solidification process, the baseline microstructure, which will have a significant impact on both the material s properties as well as its subsequent microstructural evolution, is created as the liquid is superseded by a solid. The nature of the microstructure in the resulting solid can be quite diverse, ranging from featureless equiaxed polycrystals, to microstructures riddled with dendrites. 

BUN 82] BUNGE H.J., Texture and structure of polycrystals , in SNYDER R.L., FIALA J., BUNGE H.J., Defect and microstructure analysis by diffraction, lUCr Monographs on crystallography, no. 10, Oxford University Press, p. 264-317,1999. 

Figure 1.3 (a) Schematic of a polycrystalline sample. A polycrystal is made up of many grains separated from one another by regions of disorder known as grain boundaries. (h) typical microstructure as seen through an optical microscope. 

A polycrystal is much more than many tiny crystals bonded together. The interface between the crystals, or the grain boundaries which separate and bond the grains, are complex and interactive interfaces. The whole set of a given material s properties (mechanical, chemical and especially electrical and magnetic) depend strongly on the nature of the microstructure. 

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