Nucleation site, dislocation

The Stonybrook group under Dudley has studied the behaviour of ice bicrystals and has shown that under certain conditions, grain boundaries can act as somces of dislocations. Grain boundary facets have been shown to act as dislocation nucleation sites and grain boundaries themselves have been observed to act as barriers to dislocation motion. 

All of the above discussion is strictly applicable only to homogeneous gas phase reactions. Usually the above considerations do apply reasonably well to non-polar liquids and nonpolar solutions, although normal Z values may be an order of magnitude less than for gas reactions. Reactions in solids are often much more complex, since they are usually heterogeneous, involve catalytic effects, reactions at preferential sites (dislocations, etc), and nucleation phenomena. These complicated processes are quite beyond the scope of the present article. For some description of these phenomena, and further references, the reader should consult Refs 9, 10 11... 

Up to this point we have dealt with random nucleation processes in a homogeneous phase. However, in solids with many structural imperfections, it is very likely that nonrandom, heterogeneous nucleation takes place. The basic idea of this mode of nucleation is that the energy of the imperfection is brought into the energy balance of the critical nucleus. Let us demonstrate the basic idea with a dislocation line as the preferred nucleation site. We assume that a cylindrical precipitate (p) forms along the dislocation line and, in the spirit of Eqn. (6.2), we obtain per unit length of the nucleus... 

G denotes the shear modulus and a is the specific interfacial energy. In the sense of Eqn. (6.8), we can use Eqn. (12.5) to calculate the activation energy for the nucleation of martensite. Normally, AGtr >RT, which implies that martensite nucleation is unlikely to be induced by thermal fluctuations. We conclude that the nucleation is heterogeneous and dislocation arrays are the nucleation sites. 

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. 

From the discussion above, we can conclude that further increases in the SiN deposition time will result in more effective dislocation reduction up to a point. Fiowever, much thicker GaN overlayers or modified growth conditions are needed for coalescence as the SiN deposition time is increased. When SiN was deposited more than 6.5 min, we could not get a coalesced surface even at 10 pm regrowth under the current growth conditions employed. The possible reasons might be the unoptimized lateral overgrowth rate when islands with (1101) prismatic planes were formed as well as the larger separation between nucleation sites. 

Fig. 6.81 A schematic illustration of Zener s idea, explaining how a crack of atomic dimensions can nucleate at dislocation sites here, the growth of a crack is initiated by the coalescence of two or three dislocations...

Sources of Nucleation Sites on Surfaces, Steps and Dislocations... 

The occurrence of defects in the crystal lattice can act as nucleating sites for step growth and in particular dislocations with a screw component of the Burgers vector normal to that of the crystal face (Figure 2.12). The most important type of dislocation is a screw dislocation that causes a discontinuity in the crystal surface called a screw dislocation, which is a hne defect in the crystal surface (Figure 2.12). The height displacement brought about by this slip creates a step... 

Dislocations are one-dimensional defects. They are largely responsible for the plastic behaviour of solids. Two of their properties are particularly important in connection with solid state reactions 1. They can act as sites of repeatable growth within a crystal. 2. They can serve as fast diffusion paths. They also act as preferential nucleation sites for the formation... 

During plastic deformation, existing dislocations serve as nucleation sites for new dislocations to form hence, the dislocation density of the material increases significantly. Whereas the dislocation density (in units of dislocation distance per unit volume mm/mm or mm ) of pure metallic crystals is on the order of 10 mm , the density may reach 10 mm in heavily deformed metals. It should be noted that line defects may not always be detrimental. As we will see in Chapter 3, the interactions among neighboring dislocations are responsible for work hardening of metals. 

The explanation appears to lie in the accumulation of lattice vacancies at the nucleation sites of the helium bubbles. The average energy of a neutron in the fast reactor is above 100 keV, greatly in excess of the 25 eV or so which is required to displace an atom from a lattice site. Neutron collisions, followed by multiple cascade processes, therefore lead to a high density of vacancies and interstitials. The interstitials have a higher mobility than the vacancies and tend to be more rapidly absorbed at grain boundaries and dislocations, where they lose their identity. The surplus vacancies are then available for the formation of voids at the nucleation centers of the helium produced by the (n, a) reactions. 

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