Sessile dislocations

Two alternate core structures of the ordinary 1/2[110] dislocation, shown schematically in 1 gs. 2a amd b, respectively, were obtained using different starting configurations. The core shown in Fig. 2a is planar, spread into the (111) plame, while the core shown in Fig. 2b is non-plamar, spread concomitcmtly into the (111) amd (111) plames amd thus sessile. The sessile core is energetically favored since when a shear stress parallel to the [110] direction was applied in the (111) plane the planar core transformed into the non-plamar one. However, in a similar study emplo3dng EAM type potentials (Rao, et al. 1991) it was found that the plamar core configuration is favored (Simmons, et al. 1993 Rao, et al. 1995). 

Figure 3.15 Change of stacking across a dislocation loop in a face-centered cubic structure. The structure is that of a Frank sessile dislocation loop.

Since the (0 0 1) is not a slip plane, the product dislocation is immobile, or sessile. It provides an obstacle to the movement of other dislocations passing down the (1 1 1)and(1 1 1) planes. This particular case is known as the Lomer lock. 

As pointed out earlier, dislocations can interact strongly with each other and, thus, techniques to increase the dislocation density will also act to strengthen the material. As dislocations intersect, jogs are often formed and these act to pin dislocations. In addition, reactions can occur between dislocations to form sessile dislocations, which then act as barriers to the other glide dislocations. 

This reaction is correct, as may be seen by checking the components of the Burgers vectors and it is also energetically favorable. A consequence of the above reaction is the formation of a sessile dislocation, beyond which the trailing dislocations pile up. The Burgers vector of the newly-formed partial dislocation, i.e. 

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