Grain boundaries triple junction

Solution. As shown in Fig. 15.17, for side grains and corner grains the number of triple junctions is one less than the number of neighboring grains, TV. For the side grains, the inclination of the boundary normal changes by 7r from one end to the other ... 

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. 

TEM micrograph of a Cu-Al203 nanocomposite. The Cu particles are elongated and located at grain boundaries and triple junctions. 

The diffusion coefficients of the process controlling superplasticity may be enhanced or retarded by the addition of impurities or solute atoms or by the addition of secondary phases, normally used as sintering aids, which distribute along the grain boundaries and triple-point junctions of the grains. 

However, when HRTEM was employed on the SiC samples, which showed a similar contrast variation across SiC grain boundaries in the SEM, the presence of residual intergranular films was not detected even at the triple junctions. Hence, Kleebe concluded that SEM imaging and Fresnel fringe TEM imaging alone do not enable a safe conclusion to be drawn about interface wetting in ceramic polycrystals. 

As we will discuss in chap. 10, the consideration of polycrystals demands an analysis not only of the grain boundaries that separate different grains, but also of the points at which several different boundaries intersect. The simplest model of a triple junction is of the type indicated schematically in fig. 9.49. For our present purposes, the basic idea is to determine a condition for the stability of such triple junctions which will at once determine both the structure and energetics of these triple points. 

Cavitation does not occur either at triple junctions or at grain boundaries. 

We are often preoccupied with grain boundaries, but triple junctions may be even more important in the sintering process since these may be open pipes for transporting material. Part of the reason for this bias is that triple junctions are very difficult to study and to model (see Section 14.9). 

For a grain structure to be in metastable equilibrium the surface tensions must balance at every junction between the GBs. It is theoretically possible to construct a three-dimensional polycrystal in which the boundary tension forces balance at all faces and junctions, but in a real random polycrystalline aggregate there are always going to be boundaries with a net curvature in one direction and thus curved triple junctions. Consequently, a random grain structure is inherently unstable and, on heating at high temperatures, the unbalanced forces will cause the boundaries to migrate toward their center of curvature. 

Compresave yield stress estimations from Grain boundary strength, <7 1, for sintered alumina (grain size D 1-5 pm) with grain boundary fracture touglmess [7] ss 1.5 MPa m, size of defects (e.g. pores at triple junctions)... 

After sintering, the additives are situated at the grain boundary (3-20 vol.%). The majority of the grain-boundary phase is concentrated in the triple junctions. The thickness of the grain boundary films between two grains is in the range of 0.8-1.5nm[41]. 

Fig. 7.12 TEM images of the fully dense nanosized spinel ceramics produced by sintering at 2 GPa and 795 °C. a and b Different areas revealing that some grain boundaries contain 1-nm amorphous phase regions, c An amorphous triple junction with the inset showing the fast Fourier transform of the main figure, d Image from a region containing nanotwins. Reproduced with permission from [49]. Copyright 2014, Elsevier...

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