Dislocations strain fields

Loss of crystallinity causes all diffraction contrast features in the TEM image to fade away. Moire fringes, lattice fringes, bend contours and the like will all lose contrast during irradiation [118]. Features that depend on orientation such as bend contours or dislocation strain field images will become smeared out, as directions in imperfect or very small crystals are less well defined (the reciprocal lattice spot increases in size). During irradiation, new contrast features -radiation artifacts - can appear temporarily and then fade with the rest. 

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. 

When the excitation error, s, is small, the presence of the dislocation is revealed in the image as a dark hne [Figs 7(c) and 7(d)]. These lines do not appear if g = (1 /do)(2 2 0) is used for the diffraction condition. This shows that the displacements of the dislocation strain field, which give rise to the contrast, are indeed perpendicular to the dislocation line, as expected for the Shockley partial dislocation of Fig. 4. The calculated images correspond closely to the experimental one [Fig. 7(e)], as illustrated also by the averaged intensity distributions of Figs 7(i)-7(k). 

The strain-hardening phenomenon is explained on the basis of dislocation-dislocation strain field interactions similar to those discussed in Section 7.3. The dislocation density in a metal increases with deformation or cold work because of dislocation multiplication or the formation of new dislocations, as noted previously. Consequently, the average distance of separation between dislocations decreases—the dislocations are positioned closer together. On the average, dislocation-dislocation strain interactions are repulsive. The net result is that the motion of a dislocation is hindered by the presence of other dislocations. As the dislocation density increases, this resistance to dislocation motion by other dislocations becomes more pronounced. Thus, the imposed stress necessary to deform a metal increases with increasing cold work. 

During plastic deformation, dislocation density increases, the average distance between adjacent dislocations decreases, and—because dislocation-dislocation strain field interactions, are, on average, repulsive—dislocation mobility becomes more restricted thus, the metal becomes harder and stronger. 

The higher chemical energy of the elastic strain field present around a dislocation. 

An important feature of the use of soft radiation is that both the extinction distance and the absorption distance are small, and therefore the X-rays penetrate only a veiy small distance into the crystal. We therefore examine only a small slice of the crystal close to the surface. The strain fields of dislocations deeper into the crystal do not contribute significantly to the image and, in a transmission experiment, overlapping of the images leads to an upper limit of about 10" cm on the dislocation density for individual defect imaging. With the Berg-Barrett technique this can be pushed to about 10 cm ... 

That a hollow core is formed by the creation of a free surface along a dislocation core implies that the curvature of the spiral step is reversed due to the strain field along the dislocation core. The effect of a strain field upon the advancement of a step was theoretically treated by Cabrera and Levine [14], [15],... 

Figure 5.11. Various step patterns appear because the advancing rate and the curvature of the spiral layers are affected by the strain field at dislocation cores.

Figure 5.13. Positive phase contrast photomicrograph of composite spiral in hematite, (0001). A bright contrast appears on the higher side of a step. By tracing the route from the lowest step to higher steps, the lowest step becomes the highest step after one turn, corresponding to Escher s staircase. Since the curvatures of the steps are reversed at the center of a group of dislocations, a depression appears due to the associated strain field (refer to Section 5.7). See also Fig. 5.11.

Since there is a strain field associated with a dislocation, etching originating from the outcrop at first forms a P-type etch pit. As etching proceeds further, a... 

Since dislocations are linear strain fields, if a crystal is treated in an appropriate atmosphere, impurity ions selectively precipitate along the dislocation. These can be detected by infra-red microscopes, and so the method was used to prove the presence of dislocations during the early period of dislocation studies. If a dislocation is decorated by metallic elements, the dislocations act as a resistance against etching, and only the portion apart from dislocations is etched, and decorated dislocations remain as protrusions. The resulting protrusions are etch hillocks. 

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