Twin band

Along a different line of research on shock compression of solids, namely, recovery experiments, great progress was also being made. Shock-induced recovery-type chemical reactions in encapsulated samples were first reported by Riabinin in 1956. Shock-induced metallographic transformation and the observation of twin bands in iron were first reported by Smith in 1958. Another major breakthrough was the shock-induced synthesis of diamond in 1961 by DeCarli and Jamieson. 

Many different etchants have been developed for the evaluation of different types of crystal defects such as flow-pattern defects [115], stacking faults [74, 192, 193], dislocations [69, 72-74, 194], dislocation network [72, 193, 195], oxide precipitates [196], swirl patterns [74, 75, 148], striations [74, 197], hillock defects [198], epitaxial defects [192], epitaxial alignment [199], grain boundary [69, 72, 200], twin band [69], diamond saw damage [201], pn junction [202-204], metallic precipitates [205], and damaged layer of mechanically polished surface [206],... 

Occasionally, annealing twins appear under the microscope as in Fig. 2-23(a), with one part of a grain (B) twinned with respect to the other part (.4). The two parts are in contact on the composition plane (111) which makes a straight-line trace on the plane of polish. More common, however, is the kind shown in Fig. 2-23(b). The grain shown consists of three parts two parts (Ai and A 2) of identical orientation separated by a third part (B) which is twinned with respect to Ai and A2. B is known as a twin band. 

Figure 2-24 illustrates the structure of an FCC twin band. The plane of the main drawing is (ITO), the (111) twin plane is perpendicular to this plane, and the [111] twin axis lies in it. Open circles represent atoms in the plane of the drawing and filled circles those in the layers immediately above or below. The reflection symmetry across the twin plane is suggested by the dashed lines connecting several pairs of atoms. 

In this terminology, the symbols themselves are imaged in the mirror C, the twin plane. At the left of Fig. 2-24 the positional symbols A, B, C are attached to various (111) planes to show the change in stacking which occurs at the boundaries of the twin band. Parenthetically, it should be remarked that twin bands visible under the light microscope are thousands of times thicker than the one shown in this drawing. 

Fig. 2-24 Twin band in FCC lattice. Plane of main drawing is (lIO).

In BCC structures, the twin plane is (112) and the twinning shear is in the direction [llT]. The only common example of such twins is in a-iron (ferrite) deformed by impact, where they occur as extremely narrow twin bands called Neumann bands. It should be noted that, in cubic lattices, both 112 and 111 reflection twinning produce the same orientation relationship however, they dilfer in the interatomic distances produced, and an FCC lattice can twin by reflection on 111 with less distortion than on 112, while for the same reason 112 is the preferred plane for BCC lattices. 

In HCP metals, the twin plane is normally (10T2). The twinning shear is not well understood in a gross sense, it takes place in the direction [2Tl] for metals with cja ratios less than -s/s (Be, Ti, Mg) and in the reverse direction [2lT] for metals with cja larger than yjh (Zn, Cd), but the direction of motion of individual atoms during shear is not definitely known. Figure 2-23(c) illustrates the usual form of a twin band in HCP metals, and it will be noted that the composition plane, although probably parallel or nearly parallel to the twin plane, is not quite flat but often exhibits appreciable curvature. 

Twins, in general, can form on different planes in the same crystal. For example, there are four 111 planes of different orientation on which twinning can take place in an FCC crystal. Accordingly, in the microstructure of recrystallized copper, for example, one often sees twin bands running in more than one direction in the same grain. 

Because of the efficacy of intrathecal cannabinoids in various pain models, it is not surprising that moderate levels of CBi receptor are found in the regions of the spinal cord associated with analgesia. In particular, the superficial layers of the dorsal horn, the dorsolateral funiculus, and lamina X all have moderate levels of CBi receptor (Farquhar-Smith et al. 2000). Cannabinoids inhibit glutamate release from afferents in lamina I of the dorsal horn in a CBi receptor-dependent fashion (Jennings et al. 2001 Morisset and Urban 2001). Providing anatomical support for these functional studies, CBi receptors are found in the dorsal horn in a characteristic twin band corresponding to lamina I and the inner portion of lamina II (Farquhar-Smith et al. 2000). 

Twinning facilitates relaxation of stress by plastic deformation but interaction of twin bands with grain boundaries may initiate cracks. Plasticity of Crl8Re and Cr35Re alloys is sufficient to impeach the formation of such cracks even after 71% deformation at room temperature. 

Examine region 2 (2000-1500cm 1). Here you will find C=0 stretch, usually the most intense band in the spectrum C=C and C=N stretches, less intense and sharper N=0 stretch (from N02) intense and sharp and with a twin band in region 3 N-H bending vibrations - do not confuse with C=0. 

Although the process of twinning is physically distinct from that of slip—where there is no rotation of the lattice—it is often convenient 16, 17) to regard the creation of a twin as being the result of the alignment of partial dislocations. Often, twin boundaries occur in pairs within a crystal, so that reference is frequently made to twin bands or twin lamellae, which are the regions bounded by the pair of twin composition planes. Such a situation prevails in the case of graphite (see Fig. 13 and later). 

Fig. 13. Illustration of twin band in graphite, consisting of two twin or tilt boundaries at AB and CD. 10...

Fig. 29. Twin bands on graphite (the bright striations arise because there are two tilt boundaries—compare Fig. 13). 400 x. [157)...

Figure 10.12 (a) Slip in a rod, characterised by diagonal planes across which atoms in the crystal have sheared because of an applied load (b) shp band, in which slip planes are aggregated into narrow regions (c) mechanical twin planes, across which the atoms in the crystal are reflected because of an apphed load (d) twin band, in which twin planes are aggregated. Note slip and twimiing are both caused by stress and are difficult to distinguish in macroscopic samples... 

Any one of these mechanisms may apply in specific instances of dealloying. For example, twin bands in brass, visible in the completely or incompletely dezincified layer, constituted early evidence for a volume diffusion mechanism of zinc transport from the bulk alloy to the surface [26]. In the gold-copper alloy system, copper corrodes preferentially, without dissolution of gold, leaving a porous residue of gold-copper alloy or pure gold. 

Fig. 4.37 Schematic hypothetical illustration (no change in shape is shown) a before deformation b after deformation by slip only c after deformation by slip and twinning. Note the twin bands in some crystallites [7]...

Monosialoganglioside haematoside, Gm, a precursor of more complex gangliosides, has been isolated from human liver and characterized. The haematoside gave twin bands on t.l.c. but each band had similar base and sugar compositions although there were striking differences in the fatty acid contents. 

A microscopist should recognize a paper as rag, mechanical coniferous, chemical nonconiferous, or as combinations of these by studying the paper surface. He/she should recognize such surface features on small particles as scales on wool, crossover marks on silk, striations on viscose rayon, twin bands or calcite, melt and crystal patterns on micrometeorites, lamellar steps on mica, the fibrous structure of incinerated wood sawdust, etc., which help identify that substance. 

---