Dislocations mechanically induced

Since one desires only light reflections from etched facets, the entire surface of the sample should be covered with etch pits. This can be obtained by etching an abraded surface. Fig. 5 shows the results of the action of a preferential etch for germanium on an abraded and on a polished surface. The multitude of pits on a lapped surface raises an interesting question are these pits caused mainly by the multitude of Irregularities produced an the surface by abrasion or by the multitude of mechanically induced dislocations In an attempt to answer this... 

Usually, creep deformation of ice single crystals is associated to a steady-state creep regime, with a stress exponent equal to 2 when basal glide is activated . In the torsion experiments performed, the steady-state creep was not reached, but one would expect it to be achieved for larger strain when the immobilisation of the basal dislocations in the pile-ups is balanced by the dislocation multiplication induced by the double cross-slip mechanism. 

Factors other than dislocation abundance affect surface reactivity. As discussed previously, increasing the dislocation density by mechanically inducing deformations did not increase the dissolution rate in proportion to the number of defects (Holdren et al., 1988 Murphy, 1988 Casey et al., 1988a Blum et al., 1990). However, natural and induced dislocations may differ in reactivity. 

The lack of a strong correlation between dislocation densities and reaction rates has also been demonstrated experimentally, as shown in Table II. In these studies the dislocations were mechanically induced, creating dislocation densities which varied by several orders of magnitude. The upper limit for defect densities exceeded the 10 /cm threshold density proposed by Blum and Lasaga (83). However, the large range in dislocation densities produced only factor of two increases in the reaction rates. 

In the Sect. 8.2, one of the dislocation mechanisms was discussed-the pile-up concept-as the origin of brittle fracture due to the stress concentration occmring at the leading dislocation in the pile-up. However, in materials that is entirely brittle, as are most ceramics at RT and low temperatures, plastic deformation by dislocation motion does not occur or occurs to such a limited extent that cracks are sharp up to the atomic level. In order to understand the fracture behavior of ceramic materials, it is first necessary to understand the fracture mechanisms of materials that are entirely brittle. In such materials, the mechanism of fracture is associated with various flaws inherent or intentionally added to the ceramics. A fist of most of the flaws that may induce brittle failure by crack formation is given below, as indicated in Rg. 8.10 ... 

Figure 1. Synchrotron X-ray topograph of a PTS polymer crystal showing growth dislocations D, mechanically induced dislocations M and a planar twin boundary P (g 112, A= 1a (crystal plane (100))...

A difiiculty with this mechanism is the small nucleation rate predicted (1). Surfaces of a crystal with low vapor pressure have very few clusters and two-dimensional nucleation is almost impossible. Indeed, dislocation-free crystals can often remain in a metastable equilibrium with a supersaturated vapor for long periods of time. Nucleation can be induced by resorting to a vapor with a very large supersaturation, but this often has undesirable side effects. Instabilities in the interface shape result in a degradation of the quality and uniformity of crystalline material. 

Precipitates have important effects on the mechanical, electronic, and optical properties of solids. Precipitation hardening is an important process used to strengthen metal alloys. In this technique, precipitates are induced to form in the alloy matrix by carefully controlled heat treatment. These precipitates interfere with dislocation movement and have the effect of hardening the alloy significantly. 

The driving forces necessary to induce macroscopic fluxes were introduced in Chapter 3 and their connection to microscopic random walks and activated processes was discussed in Chapter 7. However, for diffusion to occur, it is necessary that kinetic mechanisms be available to permit atomic transitions between adjacent locations. These mechanisms are material-dependent. In this chapter, diffusion mechanisms in metallic and ionic crystals are addressed. In crystals that are free of line and planar defects, diffusion mechanisms often involve a point defect, which may be charged in the case of ionic crystals and will interact with electric fields. Additional diffusion mechanisms that occur in crystals with dislocations, free surfaces, and grain boundaries are treated in Chapter 9. 

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