Grain boundaries dislocation structures

We are assuming an equal nucleation energy for all nucleation sites. In reality, the energy is less where there are breaks in structure such as grain boundaries, dislocations, etc. 

As well as low-angle grain boundaries, dislocation lines also serve as important sources and sinks for defects. The following parameters are convenient for describing the spatial configuration of dislocations a = radius of the core of a dislocation, = radius of influence of a dislocation upon defects. For a disordered dislocation structure it is only possible to approximate p as follows ... 

Numerous chemical reactions or micro-structural changes in solids take place through solid state diffusion, i.e. the movement and transport of atoms in solid phases. In crystalline solids, the diffusion takes place because of the presence of defects. Point defects, e.g. vacancies and interstitial ions, are responsible for lattice diffusion. Diffusion also takes place along line and surface defects which include grain boundaries, dislocations, inner and outer surfaces, etc. As diffusion along linear, planar and surface defects is generally faster than in the lattice, they are also termed high diffiisivity or easy diffusion paths. Another frequently used term is short circuit diffusion. 

Passive resorption is a solution-mediated process (extracellular liquid of the body) and predominantly initiates at potential stress accumulated regions in the structure. Porosities, grain boundaries, dislocations, cracks, irregularities, substitution ion sites, and atomic vacancies are known to be potential dissolution sites in ceramic apatites because of their higher sensitivity to acidic etching (Koerten and van der Meulen 1999 Ambard and Museninghoff 2006). 

It is well-known that the antiphase boundaries (APBs) in intermetallic compounds affect many material properties, such as the dislocation structures, the associated plastic deformation, and microstructures of many intermetallic compounds. APBs are two-dimensional defects, i.e. intercrystalline interfaces similar to grain boundaries. Therefore, the phenomenology of APBs has many parallels with that of grain boundaries. The structure and chemistry of APBs are discussed in Chapter 21 by Sun in this volume. 

Requirements on the extent of crystallinity in a film/coating is decided by the application involved. In uses where the bro Kl structural inhomogeneties such as grain boundaries, dislocations, stacking faults and other high energy sites in the material may have deleterious effects, amorphous coatings are preferred. Some typical examples where amorphous... 

Figure 1.13 The grain boundary and interface which can be formed between two crystals with the insertion of dislocations. In the grain boundary the two crystals are identical in lattice structure, but there is a difference in lattice parameters in the formation of the interface...

On cooling to room temperature after annealing, maraging steels transform completely to martensite. The as-annealed structure consists of packets of parallel lath-like martensite platelets arranged within a network of prior-austenite grain boundaries. The platelets have a high dislocation density but are not twinned. 

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