Observing Dislocations

The textures in homeotropic lamellar phases of lecithin are studied in lecithin-water phases by polarizing microscopy and in dried phases by electron microscopy. In the former, we observe the La phase (the chains are liquid, the polar heads disordered)—the texture displays classical FriedeVs oily streaks, which we interpret as clusters of parallel dislocations whose core is split in two disclinations of opposite sign, with a transversal instability of the confocal domain type. In the latter case, the nature of the lamellar phase is less understood. However, the elementary defects (negative staining) are quenched from the La phase they are dislocations or Grandjean terraces, where the same transversal instability can occur. We also observed dislocations with an extended core these defects seem typical of the phase in the electron microscope. 

The regions of coalescence of the stripes were either without observable dislocations or had very few dislocations which originated from the LEO-GaN/Si02 interface, propagated vertically and terminated within one-third to one-half of the LEO-GaN Jhickness, as shown in FIGURE 4. Occasional 90° bending of fee dislocations occurred at fee 1101 planes, which effectively reduced the density of... 

A typical electron micrograph showing dislocation loops in molybdenite 87) is shown in Fig, 33. It is important to appreciate that the precise nature of the observed dislocation line (which has a thickness of the order of 100 A) depends, in turn, on the precise diffraction conditions that prevail. Thus, if the sample is titled with respect to the electron beam, we may, at some angles, observe two dark lines on either side of the dislocation, or a single dark line on one side, or no line at all. When no line is observed, the dislocation is said to be out of contrast, and the angle of tilt that produces such an effect yields, in conjunction with the selected area diffraction pattern, the direction of the Burgers vector of the dislocation. 

The observed dislocation slip systems do not differ from those of the other DO22 phases, and again twinning, which does not affect the order, is a major deformation mode at low and high temperatures. The number of independent deformation modes is smaller than prescribed by the Von Mises criterion, which contributes to the observed brittleness (see Sec. 2.3). The ductility at high temperatures results from... 

D. Maugis, G. Desalos-Andarelli, A. Heurtel, and R. Courtel, "Adhesion and Friction on A1 Thin Foils Related to Observed Dislocation Density, " ASLE Trans., 21, 1 (1978). ... 

TEM images have revealed the following. Irradiation at 100 °C (lluence 8.3 X 10 n/m ) did not induce observable dislocation loops and the defects produced are isolated point defects or small clusters. This means that irradiation-induced hardening in a sample irradiated at 100 °C, which had no clear dislocation loops, is due mainly to point defects, i.e., interstitials and vacancies. Probably Frenkel pairs are formed in this manner. At 470 °C with a fluence of 2.4 x lO, dislocation loops were also found by TEM observations... 

The criterion refers to the conditions which prevail when it first becomes possible for any threading dislocation to advance. The ability to detect the onset of relaxation depends on the resolution of the technique used to observe dislocation motion (Fritz (1987), Gourley et al. (1988)). X-ray diffraction methods detect average strain over an area on the order of a square millimeter, and a detectable change in average strain occurs only after extensive dislocation activity at thicknesses well in excess of the true critical thickness. Consequently, such methods are ineffective in efforts to actually observe the onset of dislocation glide in strained layers. 

Table 3.3. Experimentally Observed Dislocation Slip Systems in Of-AI2O3...

From the results given in Table 1, one can see the nature of the observed dislocations is changing for an apphed stress value within the interval from 560 to 1100 MPa. Obviously, these data are very scarce and more deformation tests have to be performed to further reduce this stress interval. 

These findings show that dislocation creep of such intermetallic alloys and of conventional disordered Ni-base or Fe-base alloys is controlled by the same mechanisms. Even the observed dislocation densities correspond to those in conventional disordered alloys, as has been shown (Rudy, 1986 Sauthoff, 1991b). This close similarity is surprising since the elementary atomic diffusion processes and the types and numbers of dislocation slip systems are quite different because of the different crystal structures. 

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