Planar slip bands

In this study, the appearance and evolution sequence of planar slip bands, in addition to a dislocation cell structure with increasing e,, is identical to that observed in quasi-static studies of the effects of stress path changes on dislocation substructure development [27]. The substructure evolution in copper deformed quasi-statically is known to be influenced by changes in stress path [27]. Deforming a sample in tension at 90° orthogonal to the... 

Fig. 3.85 Slip bands in the intermediate region of the indented volume, a Slip band planarity and evidence of profuse pile-ups. The dislocations exhibiting paired lines in the boxed area are not dipoles but dissociated dislocations since the distance between partials is constant whether the dipole is imaged with the g or -g reflecting plane, b Intersecting slip bands, c The slight misalignment and differences in pile-up projected widths indicate that the slip bands are parallel to at least two crystallographically distinct planes [31]. With kind permission of Elsevier...

The photophysics of these six metallacycles has been studied in chloroform [71]. As expected for weakly interacting systems, the absorption spectra of the homonuclear species 9,10 and 9Zn, lOZn are very similar to those of the parent free-base and zinc-porphyrin chromophores in the Q-band region (Fig. 22), except for minor spectral shifts. A prominent difference between the planar and the slipped cofacial macrocycles is found in the Soret band region, in which a clear exciton splitting (of ca. 500 cm ) is present only for the latter compoimds (10 and lOZn). This result is as expected on the basis of the relative center-to-center distance in the two types of metallacycles (10.1 A in the slipped cofacial geometry as compared to 14.1 A in the planar one). The photophysics of the homo-dimers is very similar to that of the corresponding monomeric species. In particular, 9 and 10 exhibit the typical fluorescence of the free-base or zinc-porphyrin units (9 A. ,ax = 655, 716 nm, T = 5.7 ns 10 Amax = 656, 716 nm) and 9Zn and lOZn that of Zn-porphyrins (9Zn Amax = 608, 651 nm, t = 1.1 ns lOZn Amax = 600, 651 nm). The fife-times (9 and 10, 5.5 ns 9Zn and lOZn, 1.04 ns) are somewhat shortened (by 30-40%) with respect to the porphyrin components, as a consequence of the heavy-atom effect of the external ruthenium centers (see above for a detailed account of this phenomenon). 

The process of twinning gives a crystal in which the atom positions are reflected across the twin plane (Figure 10.12c). Twins also tend to occur in bands (Figure 10.12d). The distinction between slip and twinning is best determined by means of X-ray crystallography or transmission electron microscopy, as these techniques are able to reveal the crystallographic relations on each side of a planar boundary. 

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