Diffraction contrast dislocation loops

In accord with the optical examination, both specimens show under the TEM piogressivc zones of shock damage, reflecting the changing shock conditions in the depth of the sample. The laser-shocked specimen displays the largest variation in microstructures. Just below the excavated zone of this specimen, i.e., within a depth of -50 pm below the original preshock surface, we observe a spotty diffraction contrast (Fig. 1.11a). The spots represent tiny dislocation loops that may result from decomposition of CaCOs into CaO and CO2 [41,42]. 

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. 

Several different types of diffraction condition are used to characterise radiation damage. These are achieved by tilting the specimen with reference to the Kikuchi pattern. These include dynamical two-beam , bright-field kinematical and weak-beam conditions - see Jenkins and Kirk for a full description. Under dynamical two-beam conditions, small dislocation loops located close to foil surfaces exhibit black-white contrast (Fig. 9.3), and their symmetry can be used to determine the Burgers vectors and habit-planes. 

Contrast analysis on dislocation loops weak-bMm images of the same area imaged (a) with diffraction vector gr = (111) and (b) g = (200) close to the [011] pole, in the foil at 300 nm depth." ... 

Fig. 5.4 Diffraction contrast images of (a) stacking faults, (b) dislocations, and (c) a dislocation loop...

Diffraction contrast imaging is also widely used in analysis other defects such as dislocation loops, partial dislocations, strain field introduced by small precipitates, etc. In Eig. 5.4c, the Burgers vector for the dislocation loop always points to the center, so the areas perpendicular to the diffraction g show no contrast since g-b = 0. 

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