Decoration of Dislocations

This is one mechanism by which dislocations can become decorated (Section 3.7.3). A classic example is the decoration of dislocations in potassium chloride,... 

Figure 15-2. Decoration of dislocations in olivine by internal oxidation. Precipitates are Fe304 [by courtesy of D. Kohlstedt, University of Minnesota].

It was at that time that Franck and Seitz proposed mechanisms for the multiplication and generation of vacancies by intersection of dislocations88 explaining the observed softness of crystals and providing models that were subsequently verified by the technique of decoration of dislocations.89... 

Fig, 4. TEM micrograph of elongated He filled channels in 6-ZrT alloys aged for 836 days. The channels arise from He decoration of dislocations and further ejection of matter from the core region. 

Figure 3.1 Electron micrograph showing a dislocation in silver, imaged as a dark line. The small triangular features that decorate the dislocation are stacking faults formed by the aggregation of point defects. [From W. Sigle, M. L. Jenkins, and J. L. Hutchison, Phil. Mag. Lett., 57 267 (1988). Reproduced by permission of Taylor and Francis, http //www.informa world.com.]...

Since dislocations are linear strain fields, if a crystal is treated in an appropriate atmosphere, impurity ions selectively precipitate along the dislocation. These can be detected by infra-red microscopes, and so the method was used to prove the presence of dislocations during the early period of dislocation studies. If a dislocation is decorated by metallic elements, the dislocations act as a resistance against etching, and only the portion apart from dislocations is etched, and decorated dislocations remain as protrusions. The resulting protrusions are etch hillocks. 

For the detection of dislocations by electroetching as well as by chemical etching, it is frequently necessary to "decorate " the dislocations by means of one or more impurities in the base metal. The impurity atoms interact with the dislocations. This idea was first put forward by Wyon and Laeombe (11) in the case of the Al. The same approach has also been extended to Fe (12), Si-Fe alloys (13), Zn (14), Cu (15), and others. 

Fig. 1. (a) Dark-field optical micrograph of an extensive dislocation network in halite revealed by the decoration of the dislocations with colloidal silver [16]. (b) Optical phase image of growth steps, one unit cell high (0.79 nm) on the prism surface of a beryl crystal. The inner step joins a pair of opposite-handed screw dislocations, which are imaged as dots. The c-axis is almost parallel to the straight steps [17]. 

Fig. 8. TEM micrographs of dislocations in hematite intergrowths (a). Magnetite-hematite intergrowth with zonal dislocation structures [249]. (b) Exsolution of ilmenite platelets in hematite. Ilmenite is decorated with a network of misfit dislocations to account for lattice strain caused by differences in lattice...

The dislocation theory of plastic deformation was confirmed by direct observation of dislocations during the 1950 s, first by decorating them with impurity atoms and later directly by transmission electron microscopy, and for the first time a basic understanding of the mechanical properties of materials emerged. 

Vacancies and impurity atoms may also attract to form complexes. One expects large substitutional atoms and interstitial impurity atoms to be attracted to vacancies because of reduction in total strain energy similar to the attraction of impurity atoms to dislocations to form atmospheres. Segregation about dislocations may reach the stage where a distinct second phase forms decorating the dislocation. Dislocations in MgO decorated by MgFe204 precipitates are shown in Figure 22. 

Figure 3.14. Optical micrograph of a dislocation source in silicon, decorated with copper...

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