Dislocation rosette

Fig. 6.2.1. Starlike dislocation rosette around pyramid impression on (100) face in LiF crystal, after etching. (After Shaskalska and Dobzhanski, 1962, from Yushkin, 1971)...

Fig. 6.2.3. Dislocation rosettes around Vickers pyramid impression (P = 2000 mN) on NaCl (111) face inside the crystal after polishing its surface. The crystal was previously irradiated with soft X-rays for 42 h.

Fig. 6.2.5. Dislocation rosettes around impressions on fluorite (111) face (a, b, c, d) and their profiles on wall of fracture (111) (a, b, c, d ) passing at different distances from the impression, P = 981 mN.

On the surface, dislocation rosettes appear along (110) directions, which indicates that they are traces of 111 planes. 

The plastic deformation patterns can be revealed by etch-pit and/or X-ray scattering studies of indentations in crystals. These show that the deformation around indentations (in crystals) consists of heterogeneous rosettes which are qualitatively different from the homogeneous deformation fields expected from the deformation of a continuum (Chaudhri, 2004). This is, of course, because plastic deformation itself is (a) an atomically heterogeneous process mediated by the motion of dislocations and (b) mesoscopically heterogeneous because dislocation motion occurs in bands of plastic shear (Figure 2.2). In other words, plastic deformation is discontinuous at not one, but two, levels of the states of aggregation in solids. It is by no means continuous. And, it is by no means time independent it is a flow process. 

In the unstrained material far from the center of an indentation, dislocations can move freely at much lower stresses than in the material near the center where the stress (and the deformation) is much larger. Thus, local plastic shear bands can form at the edges of the indenter, and do (Chaudhri, 2004). The lengths of these shear bands are often several times the size of an indentation. The leading dislocations in these bands move in virgin (undeformed) material, so they can move at lower stresses than the dislocations in the strain-hardened material near the center of an indentation.. The patterns they form are called rosettes. ... 

Silicon as whisker material is entering the area of engineering application, but the dominant area of interest is the use of silicon and doped silicon as semiconductor materials. The use of hardness measurement as a probe of the properties of specially prepared silicon crystals for semiconductor device use has been attempted several times but as some of the data presented below show, doping at the levels used in the device industry does not show up as change in hardness even through etch-pit rosette studies in the area around indentations show some dependence of dislocation movement on concentration and type of dopant. 

In order to reveal the dislocation etch pits a vigorous etchant has to be used 1 part 0.25 M K2Cr207 solution to 4 parts 40% HF. Etching times from 10 to 90 seconds are needed. At a 0.98 N load the rosette length is 9 pm at 1.96 N for a dopant concentration of 10 atoms cm" and, as the impurity level is raised to 12 x 10 , the rosette arms are 15 and 66 pm at the above loads, respectively. 

FIGURE 7.41 Histograms of the distances covered (a) by the leading screw dislocations, 4, and (b) by the edge dislocations, 4, in the indentation rosettes on the surface of NaCl single crystals in dry heptane and in moist air. The indenter load was 0.3 g force. (Redrawn from Shchukin, E Physical-chemical mechanics of solid surfaces, in Encyclopedia of Surface and Colloid Science, 2nd edn., Taylor Francis, New York, 2012, pp. 1-23.)... 

If a crystal can be deformed plastically, the appearance of slip bands and indentation rosettes (Fig. 24) on a surface Implies that the etch figures locate the emergence points of dislocations. Comparison of etch... 

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