Polycrystal grain size

LEED beam is reduced to less than the sample grain size, individual grains in a polycrystal can be investigated. It is difficult to make low-energy electron beams this small, but in some materials large grain sizes can be achieved in polyctystals. 

Stresses can can be concentrated by various mechanisms. Perhaps the most simple of these is the one used by Zener (1946) to explain the grain size dependence of the yield stresses of polycrystals. This is the case of the shear crack which was studied by Inglis (1913). Consider a penny-shaped plane region in an elastic material of diameter, D, on which slip occurs freely and which has a radius of curvature, p at its edge. Then the shear stress concentration factor at its edge will be = (D/p)1/2.The shear stress needed to cause plastic shear is given by a proportionality constant, a times the elastic shear modulus,... 

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. 

Figure 16.5 Deformation mechanism map for Ag polycrystal a = applied stress, p = shear modulus, grain size = 32 pm, and strain rate = IGF8 s 1. The diffusional creep field is divided into two subfields the Coble creep field and the Nabarro-Herring creep field. From Ashby [20].

The first observation of superplasticity in a 3 mol% yttria-stabilized tetragonal zirconia polycrystal ceramic (YTZP) with a grain size of 0.4 pm was reported by Wakai et al in 1986 (Fig. 16.1). Since then, a large number... 

Dominguez-Rodriguez, A., Bravo-Leon, A., Ye, J.D., and Jimenez-Melendo, M., Grain size and temperature dependence of the threshold stress for superplastic deformation in yttria-stabilized zirconia polycrystals , Mater. Sci. Eng, 1998, A247, 97-101. 

Figure 38. Section of a polycrystal (bricklayer-model), (a) Grain size large compared to 4A (b) Grain size smaller than 4A. The white area refers to the space charge zones, the hatched to the electroneutral bulk and the dark to the interfacial core.151 (Reprinted from J. Maier, Thermodynamic aspects and morphology of nano-structured ion conductors. Aspects of nanoionics. Part I. Solid State Ionics 154-155, 291-301. Copyright 2002 with permission from Elsevier.)...

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... 

We discuss on the microscopic structure of our samples, in reference to the result of thermal conductivity measurements. A K. Collins et al. have reported that grain size of their polycrystal line sample could be estimated from the temperature dependence of low temperature thermal conductivity [6] with peak at around 200 K. They obtained values from several to 10 p m. The grain sizes in our samples are smaller than these value from SEM observations. Although the temperature of our measurement is higher the phonon scattering on the grain and pore boundaries may still be important. 

The strength of a ductile polycrystal usually increases with decreasing grain size. True or False ... 

Therefore, the development of new hard materials requires the preparation of phases with small microplastic deformability of the crystal lattice, and reduced grain sizes in the sintered polycrystals, and highly perfect microstructures. For example, sintered alumina ceramics which have a submicrometer microstructure and associate this small grain size with a macrohardness of 20-24 GPa (compared with 17-19 GPa in conventionally sintered corundum) were obtained only when new processing approaches became available, which could reduce the flaw population [1]. 

A difference appears only when, on increasing the load, the indentation size becomes much larger than the grains whereas in sapphire the deformability increases with the growing plastic zone, in the polycrystals this size effect is partly offset by the hindrance of dislocation activity due to the close spacing of the grain boundaries. 

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