Prismatic loop

Small prismatic loops viewed edge-on may give rise to a characteristic contrast of two symmetrical lobes, particularly when g is normal to the plane of the loop, as discussed in Section S.7.2. The contrast and orientation of the (arrowed) dislocation loop viewed edge-on in Figure 9.11 (a, b) are consistent with its being a prismatic loop of b = [0001]. 

Doukhan and Doukhan (1986) suggested that the climb dissociation is due to the precipitation of point defects on the prismatic loops when the specimens are cooled. The equilibrium concentration and the mobility of the point defects are both expected to be very high at 1,400°C, thus favoring deformation by dislocation climb. 

Often, in quenched crystals which have been subsequently annealed so as to favor climb processes, dislocations in the form of long spirals have been observed. These are known as helical dislocations and a convincing explanation for their occurrence has been given by Amelinckx et al. (19). In essence, the helix consists of a screw dislocation parallel to the axis of the helix and a set of prismatic loops similar to that shown in Fig. 15. 

Climb, deformation twinning, and prismatic loops are also important in the multiplication of dislocations (6, 7). 

Fig. 7.57 Transmission electron micrograph showing long dipoles, elongated prismatic loops, and screw dislocations [27], With kind permission of John Wiley and Sons - ...

To estimate believe that loop of misfit dislocation is the equatorial location on the spheroidal precipitate Rp = the self-energy prismatic loop (Kolesnikova et al., 2007)... 

The important fact to note at this point is that, since metadislocation loops have [0 01] Burgers vectors, they are pure edge loops, also referred to as prismatic loops [3], that is, they possess only edge-type and no screw type segments. 

As an indenter creates an indentation it causes at least three types of finite deformation. It punches material downwards creating approximately circular prismatic dislocation loops. At the surface of the material it pushes material sideways. It causes shear on the planes of maximum shear stress under itself. Therefore, the overall pattern of deformation is very complex, and is reflected... 

The HTTR is an experimental helium-cooled 30 MW(t) reactor. The HTTR is not designed for electrical power production, but its high temperature process heat capability makes it worthy of inclusion here. Construction started in March 1991 [47] and first criticality is expected in 1998 [48]. The prismatic graphite core of the HTTR is contained in a steel pressure vessel 13.3 m in height and 5.5 m in diameter. The reactor outlet coolant temperature is 850°C under normal rated operation and 950°C under high temperature test operation. The HTTR has a primary helium coolant loop with an intermediate helium-helium heat exchanger and a pressurized water cooler in parallel. The reactor is thus capable of providing... 

Still another force will be present if a dislocation is curved. In such cases, the dislocation can reduce the energy of the system by moving to decrease its length. An effective force therefore tends to induce this type of motion. Consider, for example, the simple case of a circular prismatic dislocation loop of radius, R. The energy of such a loop is... 

Figure 11.13 Annealing prismatic dislocation loop taken as a circular array of vacancy...

Figure 11.15 Formation of prismatic dislocation by vacancy precipitation and collapse, (a) Excess vacancies dispersed in crystal, (b) Precipitation of excess vacancies, (c) Collapse of vacancy precipitate to form dislocation loop.

Solution. The helical dislocation may be regarded as equivalent to a stack of circular prismatic edge dislocation loops of radius a as illustrated in Fig, 11.16. This may be confirmed by realizing that the stack of loops can be converted into a helix in a conservative fashion by cutting each loop at its intersection with AB and then sliding... 

Figure 11.16 (a) A stack of four prismatic edge dislocation loops perpendicular to AB... 

Figure 23.4 Prismatic dislocation punching at spherical precipitate, (a) A dislocation dipole loop is generated in the interface. One side expands into the matrix while the other remains in the interface, (b) Segments of the loop in the matrix glide downward to form additional loop length in the interface, (c) The loop in (b), which is partially in the interface and partially in the matrix, pinches together at its lowest point and splits into two loops, with one remaining in the interface and the other gliding into the matrix. From Porter and Easterling [4],...

Figure 5.5. Schematic diagrams showing (a) a random distribution of vacancies, (b) condensation of a cluster onto a single plane, and (c) the collapse of the planes to form a prismatic dislocation loop.

Figure 5.IS. Schematic diagram showing (a) a prismatic dislocation loop lying in the plane of the foil and (b) its image when g b = 0.

Ashby and Brown (1963b) extended the analysis to cover the type of strain field found around platelike precipitates whose mismatch with the surrounding crystal is appreciable only in a direction normal to the plate. The analysis is also applicable to the strain normal to the plane of a prismatic dislocation loop of Burgers vector b. When the plane of the loop or precipitate is more or less normal to the foil, the images are similar to that... 

Figure 5.24. Variation of the 20-percent image width with g, b, and 0 for a prismatic dislocation loop (or a platelike inclusion) in an isotropic crystal matrix for several values of / ,. Thickness of foil = s = 0 r" = 10/g,...

Figure 9.8. BF image (g = lOTl) showing prismatic dislocation loops (b = <1120 and strain-free bubbles in wet synthetic quartz after heating at 550°C for 2 hours. (From McLaren et al. 1989.)...

Thus, the main loop (A, D) is a prismatic edge-dislocation loop (the Burgers vector is normal to the plane of the loop) and can expand in its plane only by climb. Segments of the loop have dissociated into pairs of partial dislocations (B, C and E, F), presumably by the reaction... 

In the present analyses, prismatic dislocation loops distributed on different slip planes are used as agents for dislocation generation. For copper, sources length of about 0.60 p,m are used. It is worthy to mention that the boundary conditions of the computational cell sides are different in FE and DD parts of the code. In DD, periodic boundary condition for the representative volume element RVE is used to ensure both the continuity of the dislocation curves and the conservation of dislocation flux across the boundaries, by that we take into account the periodicity of single crystals in an infinite media. In FE analysis however, the sides are constrained to move only in the z direction so that a imiaxial strain consistent with the shock experiment is achieved. In order for the boundary conditions in FE and DD to be consistent, periodic FE bormdary condition is implemented as well. The result of this implementation is discussed in the next section. 

Fig. 15. Representation of formation of a prismatic dislocation loop. The vacancies have aggregated on a close-packed plane, and the disc so formed has collapsed to produce an edge dislocation loop.

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