Initiation, dislocations

The decomposition process can be significantly intensified by the mechanical activation of the material prior to chemical decomposition. Based on a thermodynamic analysis of the system, Akimov and Chernyak [452] showed that the mechanical activation initiates dislocations mostly on the surface of the grains, and that heterogeneities in the surface cause the predominant migration of iron and manganese to the grain boundaries. It is noted that this phenomenon is more pronounced for manganese than it is for iron. 

The initial dislocation density of the sample was estimated to be less than 10 m ". The orientation of the samples was chosen in order to align the torsion axis as close as possible with the c-axis ( 1°). The maximum resolved shear stress is then applied on the basal planes. The plastic deformation is accommodated by the glide of screw dislocations on the... 

The subsurface dislocation structures described above can be revealed by electron microscopy as shown in fig. 12.31. Such experiments are suggestive of the physical mechanisms involved in the onset of plasticity and provide the hope of quantifying the crystallography and size of the initial dislocation loops. 

We must, however, allow for the presence of some initial dislocations and this is most easily done by assuming a small initial value of Cp at = o. 

Silicon at 900 °C and germanium at 500 °C both show very pronounced yield points, when the initial dislocation density is low, and these correspond very closely in shape and position to those found for ice. Since a pronounced yield point is associated in the theory with a small value of the exponent m in the dislocation velocity relation (8.43), it is satisfying that direct observation of dislocation motion in these materials (Chaudhuri et al. 1962) gives m from 1-3 to i 9 in close similarity to the value m 1 found by Higashi Sakai (19616) for ice or the value between i 5 and 2 5 deduced from macroscopic experiments. These values are in... 

Plastic deformation results from the accumulated motion of numerous dislocations at the atomic scale. The dislocation density p is a parameter representing the average amount of accumulated plastic deformation or, in other words, the amount of deviation from the strictly geometrical lattice structure. The dislocation density p is defined as the total length of dislocation lines per cubic centimeter, and it is almost identical to the flow stress of a metal under hot forming or the internal stress Oi. Numerous dislocations are introduced by forming. For example, the initial dislocation density of a fully annealed structure po is about 10 (cm/cm ) but increases to lO (cm/cm ) or more after metal forming. 

The stress gradient at the crack tip is equal to 2 X 10 Pam the initial dislocation density is 2 X 10 The crack grows from 0.1 to 10 mm, see graph to the right below. 

Figure 4.51 indicates that the yielding in sapphire undergoing basal slip is a consequence of dislocation multiplication and is not due to the unpinning of a Cottrell-type atmosphere, where dislocation pinning results from impurities. The study of two types of sapphires, with different initial dislocation densities, was meant to point out the difference in their surface dislocation densities and the consequent differences in their yield phenomena. 

In various examples, it was demonstrated that epitaxy occurs when extensive differences in the lattice distances are evident. For example, it is also possible for epitaxy to initiate dislocations beyond interfaces. Thus the real distances between the host and guest crystals are larger than 15% (Meyer 1968). 

Grain boundaries are barriers for the movement of dislocations. As the crystal orientation in the neighbouring grain is different, a dislocation cannot simply enter it. The stress field of the dislocation may initiate dislocation movement in the neighbouring grain, but if the slip systems are less favourably oriented there, a larger stress is needed to move dislocations than in the first grain. 

Fig. 7 shows a stress-strain curve obtained at low temperature on a virgin sihcon crystal [54,55], The yield point usually found at medium temperatures is still present. This behavior is characteristic of a low initial dislocation density followed by a multiplication stage and an overshoot of the dislocation density after the multiplication stage. 

The microstructure obtained after room temperature deformation is complex. Nevertheless, several features can be evidenced. In some areas of the thin foil, the microstructure is not different from what is observed in an initially dislocation-free sample. It consists of undissociated dislocations with a/2<110> Burgers vectors elongated along the <123> direction. However, the more frequently observed... 

Critical resolved shear stress is the minimum resolved shear stress required to initiate dislocation motion (or slip) and depends on yield strength and orientation of slip components per Equation 7.4. 

The cross-slip of screw dislocations is important in the dislocation multiplication process, for a dislocation configuration similar to that shown in Figure 11 can be formed by cross-slip of a dislocation segment from one slip plane to a neighboring slip plane. By this process, an initial dislocation can give rise to many Frank-Read-type sources. Grain boundaries are also sources of dislocations. As discussed in the next section, grain boundaries can be considered as arrays of dislocations. 

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