Polycrystalline microstructure

Microstructures are generally too complex for exact models. In a polycrystalline microstructure, grain-boundary tractions will be distributed with respect to an applied load. Microstructures of porous bodies include isolated pores as well as pores attached to grain boundaries and triple junctions. Nevertheless, there are several simple representative geometries that illustrate general coupled phenomena and serve as good models for subsets of more complex structures. 

Figure 19.13 demonstrates that for a given value of ip, AQc decreases as the dimensionality of the heterogeneous site decreases. However, the number of sites available for nucleation also decreases as the dimensionality decreases. Thus, the kinetic equations for nucleation theory must be used to predict which mechanism will dominate. To accomplish this, some assumptions about the polycrystalline microstructure must be made. Let ... 

The anisotropy fluctuations (AK) discussed above typically result from fluctuating orientations of the magnetic easy axis which varies from one grain to another in conventional polycrystalline microstructures, though Eq. (1) is not limited to this mechanism of anisotropy fluctuations. Hence, for conventional polycrystalline materials where 4 > Lcx, the parameter AK introduced into Eq. (1) can be well approximated by the magnetocrystalline anisotropy constant Kh However, as we discuss in the subsequent section, the approximation of AK by K is no longer applicable for small structural correlation lengths. 

As we have already belabored, the ideas presented above are predicated on the fundamental isotropy of the possible slip systems in the material of interest. This isotropy, in turn, derives from the polycrystalline microstructure in which it is... 

As is evidenced by fig. 9.38, there are a number of points of entry into a discussion of polycrystalline microstructures. In particular, fig. 9.38 shows that at the simplest level (frame (a)) we can consider bicrystal geometries. In keeping with... 

The first two terms have already been discussed in several other contexts (see eqns (2.46) and (10.45)) with the new twist that now we have a summation over all of the different interfaces. The third term, on the other hand, is a new feature and reflects the extra attention that must be given to triple points within the polycrystalline microstructure when attacked from the sharp interface perspective. 

Scales Above the Micron Scale. Much can be learned about the workings of a material with little more than an optical microscope and a well-polished sample. One of the first features of a material that will be evident upon inspection via optical means are the type of features shown in fig. 10.2 which reveals a polycrystalline microstructure. Of course, we well know that what we are seeing is evidence of the polycrystallinity of the material. The grain boundaries that separate different grains are clearly evident on the crystal surface. We can also see that depending upon the life history of the material, the grain size can vary considerably. 

As-cast Ti-3Al-2Si alloy has a typical polycrystalline microstructure of a solid solution of P-transformed Ti (a-Ti) with single particles of the secondary silicides (Fig. la). Additions of Zr promote the precipitation of the silicide (Fig. lb) and arising the first portion of eutectic (Fig.lc). Ti-3Al-2Si-5Zr and Ti-3Al-2Si-15Zr are two-phase alloys consisting of a-Ti and Ti5Si3 and a-Ti and (Ti,Zr)2Si phases, respectively (Table 1). 

In the last decade, many new oxide fibers with improved high-temperature performance have been commercialized. The keys to these improvements has been (1) the design of fiber microstructures to reduce the volume of amorphous phases and (2) the development of multiphase polycrystalline fibers. Eliminating amorphous phases prevents rapid, viscous deformation under load at high temperatures. Multiphase polycrystalline microstructures appear to inhibit creep, particularly at elevated temperatures. Examples of developmental fibers with improved high-temperature properties include polycrystalline AI2O3, YAG, and mullite filjers. 

Figure 2. The limitation of the free space available for dislocation or twin activity by the spatial size of the plastic zone in a single crystal (upper curve/upper x-axis), and by the grain size of polycrystalline microstructures (lower curve/lower x-axis) [6].

A remarkably large discrepancy between the hardness of plastically deformed single crystalline volumes and of polycrystalline microstructures with grains of the same size, indicates a significant contribution of grain-boundary deformation at the indentation site in polycrystalline alumina, even at room temperature and even for coarse microstructures. 

The polycrystalline microstructure is the microstructure we observe after sintering a powder compact to full density. In homogeneous polycrystalline materials, this microstructure is determined, in general, by the interfacial tension. Figure 3.1 illustrates an equilibrium state between three interfacial tensions. For this geometry, the sine law is satisfied so that... 

Smith, C. S., Some elementary principles of polycrystalline microstructure, Metall. Reviews, 9, 1-48, 1964. 

Grain — individual crystal within a polycrystalline microstructure. 

Fiber FP essentially has a metal-like polycrystalline microstructure having an average grain size of about 0.5 micron. 

The relation between conductivity, chemical and phase compositions, and the structure of separate phases of solid electrolytes is very important In many systems, the maximum in conductivity closely coincides with the existing F-phase (fluorite-phase) limit TEM image of Figure 13.3 shows the formation of a dense single-phase polycrystalline microstructure for the sintered ceramics [1]. X-ray diffraction and analysis of electron diffractograms of this sintered ceramic confirmed the fluorite structure. 

Fig. 3. Left the polycrystalline microstructure of BAM-0 forbids the regeneration of the oxidized surface. Right conversely, the migrations of inter-slabs species in BAM-L2 are favored by the lattice coherence, leading to phosphor s regeneration. Center summary of the effects of flux...

Ceramic abrasive with polycrystalline microstructure. High stock removal rates and long life with vitrified or resin bonds. 

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