Dislocations networks

Figure 6.5. Dislocation networks and deformation twins in 4340 steel shock loaded to 15 GPa.

Figure 11.1. Scanning tunnelling microscope image of a periodic array of Fe islands nucleated on the regular dislocation network of a Cu bilayer deposited on a platinum (111) face (after Urune...

Another reaction mechanism, which is conveniently mentioned under this heading, is due to Hill [479] who suggested that ions (atoms or molecules) frorh the product may move through the dislocation network of the reactant and activate potential nuclei, particularly in the vicinity of the reaction interface. Thus a reaction zone, within which potential nucleusforming sites are activated, is developed in front of an advancing interface. With appropriate assumptions, this reaction model provides an alternative explanation of the exponential rate law, eqn. (8), which in Sect. 3.2 was discussed with reference to chain reactions. 

In one dimensional diffusion experiments (e.g., starting with a thin film source of A on a B crystal surface) one often finds an exponential decrease in the A concentration at the far tail of the concentration profile. This behavior has been attributed to pipe diffusion along dislocation lines running perpendicular to the surface. Models have been introduced which assume a (constant) pipe radius, rp, inside which Dl = p-D, b and p denoting here bulk and dislocation respectively. P values of 103 have been obtained in this way. It is difficult to assess the validity of these observations. The model considerably simplifies the real situation. During diffusion annealing, the structure of the dislocation networks is likely to change because of self-stress (see Chapter 14) and chemical interactions. 

Figure 3-5. Co304 spinel precipitate in a CoO matrix due to cooling. Dislocation network in the matrix stems from the misfit between CoO and Co304 [T. Pfeiffer (1990), unpublished].

Figure 3-6. a) Small-angle lilt boundary wiih edge dislocations, b) Small-angle twist boundary formation of a screw dislocation network. 

Here, G denotes the shear modulus, and f(c/r) is a function of the ratio c/r in which c and r are the spheroidal semiaxes of the precipitate. For spheres, f(c/r= 1) = 1 = /max. For discs as well as for rods, /< 1. In principle, shear stress energies and energies arising from misfit dislocation networks also have to be added. They influence AG by additional energy terms. 

Boundaries between solids transmit shear stress, particularly if they are coherent or semicoherent. Therefore, the strain energy density near boundaries changes over the course of solid state reactions. Misfit dislocation networks connected with moving boundaries also change with time. They alter the transport properties at and near the interface. Even if we neglect all this, boundaries between heterogeneous phases are sites of a discontinuous structural change, which may occur cooperatively or by individual thermally activated steps. 

Climbing Dislocations as Sinks for Excess Quenched-in Vacancies. Dislocations are generally the most important vacancy sources that act to maintain the vacancy concentration in thermal equilibrium as the temperature of a crystal changes. In the following, we analyze the rate at which the usual dislocation network in a... 

Consider a segment of dislocation in the dislocation network of a crystal pinned at its ends and lying in its slip plane as in Fig. 11.9. Suppose that an oscillating shear stress of the form... 

Templates — Dislocation Networks and Ordered Domains in Biphases. 260... 

Fig. 7.9 TEM micrograph taken from the fatigue crack tip (R = 0.15, T = 1400°C, and vc = 0.1 Hz) showing the formation of dislocation networks in the alumina matrix reinforced with SiC whiskers. The dislocation network is pinned by the SiC whiskers.

The most obvious microstructural characteristics of recovery are probably subgrain boundaries consisting of arrays of parallel dislocations or dislocation networks. 

Figure 9.16. BF images (g = lOU) showing the dislocation microstructure in wet synthetic quartz deformed at 475°C and subsequently annealed at atmospheric pressure at 600°C for 2 hours. Note the bubbles in (a) and the dislocation networks in (b). This microstructure should be compared with that shown in Figure 9.IS.

If a surface is sputtered at sufficiently low temperature to avoid bulk diffusion, atoms of the species preferentially sputtered can reach the surface by displacement mixing and radiation-induced segregation. This leads to a so-called altered layer, which has a composition different from that of the bulk and a thickness close to the penetration depth of the projectiles. Since surface diffusion has a much lower activation barrier than bulk diffusion, annealing a sputtered alloy surface first leads to a local equilibrium between the surface and the immediate subsurface layers, which still belong to the altered layer. Only after the onset of bulk diffusion is reached, usually around 60 - 70% of the melting temperature, the altered layer equilibrates with the bulk and true equilibrium segregation is observed [45]. For alloys of atoms with different size the existence and dissolution of an altered layer can be studied by STM because of the development of a misfit dislocation network between the altered layer and the bulk [46] (Fig. 7). 

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