Grown-in dislocations

To answer questions regarding dislocation multiplication in Mg-doped LiF single crystals, Vorthman and Duvall [19] describe soft-recovery experiments on <100)-oriented crystals shock loaded above the critical shear stress necessary for rapid precursor decay. Postshock analysis of the samples indicate that the dislocation density in recovered samples is not significantly greater than the preshock value. The predicted dislocation density (using precursor-decay analysis) is not observed. It is found, however, that the critical shear stress, above which the precursor amplitude decays rapidly, corresponds to the shear stress required to disturb grown-in dislocations which make up subgrain boundaries. 

The metal azides are, by common experience, brittle when subjected to mechanical stress, they shatter before appreciable plastic deformation takes place. This arises because, as with most inorganic materials, dislocation densities are low, grown-in dislocations are usually immobile, and slip can take place only on a limited number of planes. However, with the possible exception of diamond and certain borides and nitrides, few materials are ideally brittle, and some plastic deformation is possible, the amount depending upon the temperature and the rate of strain low temperatures and high rates of strain both favor brittle behavior. 

By using the serial sectioning technique, the tracer diffusion of l Au into single crystals was studied. Some effect of the dislocation density was detected. This was explained in terms of a model in which vacancy generation occurred at climbing grown-in dislocations. The values of the 2 effective diffusion coefficients for the dissociative mechanism were determined from the present, and literature, results. One coefficient took a value of about 3 x lO cm /s at 900 to llOOC, while the other could be described by ... 

Mineral grains formed at higher temperatures by crystallization from melts, by phase transformations, or by reactions, generally contain grown-in dislocations. The densities of such dislocations are typically low to moderate (i.e., below about lO cm ), unless special factors apply. Recrystallized grains are often dislocation-free. Most high temperature (HT) minerals have cooled over geologic times,... 

The grown-in dislocation density can be multiplied mechanical and thermal stresses. The usual densities are of the order of 10 cm cm although with care very low densities can be produced in metals (10 cm cm ), and in non-metals, such as silicon, dislocations can be eliminated altogether. ... 

In a very detailed EPR study, Pakulis and Jeffries (1981) and Pakulis (1983) investigated a large number of ultra-pure germanium crystals that were grown in vacuum or in a hydrogen or deuterium atmosphere. Several of the samples were dislocation free. The sensitivity of EPR was greatly... 

Fig. 12. Derivative curves of EPR in a highly dislocated As-doped germanium crystal grown in a H2 atmosphere. The magnetic field is oriented along the [100] direction. T= 2 K, /= 25.16 GHz. Note the sign reversal of the new lines as compared to the As-donor hyperfine structure. Dislocation density 2 x 104 cm 2. (Courtesy Pakulis and Jeffries, reprinted with permission from the American Physical Society, Pakulis, E.J., Jeffries, C D. Phys. Rev. Lett. (1981). 47, 1859.)...

The importance of the thermal conductivity of the crystal and the CRSS in determining the degree of difficulty of growing a specific material from the melt is understood in terms of the relationship between these parameters and the formation of dislocations in the crystal because of excess stress. Clearly, materials with lower values of the CRSS must be grown in systems with lower temperature gradients to prevent crystallographic slip. Low values of the conductivity make this difficult to achieve. 

The following subsections are concerned with the grown-in defects in synthetic quartz and the dislocation microstructures associated with the various stages of the creep and stress-strain curves. The microstructural evolution in natural quartz deformed under various conditions of water fugacity, and other experimental variables, will be considered in Section 9.5. 

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