Dislocation dipole

Although this sometimes occurs through the operation of Frank-Read sources it is not generally observed. What does generally occur is similar, but more complex. The process is called multiple-cross-glide, and was proposed by Koehler (1952). Its importance was hrst demonstrated experimentally by Johnston and Gilman (1959). In addition to its existence, they showed that the process produces copious dislocation dipoles which are responsible for deformation-hardening. 

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 9.2S. Glide dislocations and dislocation dipoles within a twin lamella in calcite compressed normal to (lOTO) to 10 percent strain at 2S°C with 1 GPa confining pressure. (From Barber and Wenk 1979.)...

Figure 2. The activation energy for the annihilation of non-jogged screw-dislocation dipoles in copper by cross slip versus inverse dipole height. From [18] (where the alternative ordinate axis is also explained).

For screw-dislocation dipoles with dipole height just above the critical dipole height for annihilation by spontaneous cross slip the activation energy is so low that the annihilation process can be modeled by molecular dynamics (Vegge et al. [20]). This allowed us to determine the preexponential for cross slip P (with dimension m s 1) in the equation... 

In 2.2 we derived an activation energy of 3eV for cross slip of a single non-jogged screw dislocation in copper - a magnitude which is prohibitively high for cross slip at room temperature. We also considered the substantial reduction of the activation for cross slip for sufficiently narrow screw-dislocation dipoles. 

Figure 3. The postulated linear relation between inverse dipole height and the activation energy for cross-slip annihilation of jogged screw-dislocation dipoles in copper - based on two points the point at 0.45 nm 1 where annihilation becomes spontaneous (activation energy zero) and the point corresponding to an islolated screw dislocation (at 0 nm"1). An alternative absciss axis in terms of applied stress is added. From [22].

Brown s statistical theory [30] of annihilation of screw dislocation dipoles by thermally activated jog migration determines the PSB nanostructure and the saturation stress. The statistical theory is compatible with the nanotheory and the required activation energies are available for both cross-slip and jog motion in copper, as described in 2.2 and 2.3. What remains is to combine and quantify the above theories of thermally activated fatigue hardening, PSB nucleation, cyclic saturation and PSB surface damage to test their quantitative predictions against experimental data. 

X. Tang, and K.P.D. Lagerof, and A.H. Heuer, Determination of pipe diffusion coefficients in undoped and magnesia doped sapphire (a-A1203) a study based on annihilation of dislocation dipoles, J. Am. Ceram. Soc. 86, 560-565 (2003). 

Figure 12.18 Reprinted from Bontinck, W. and Amelinckx, S. (1957) Observation of helicoidal dislocation lines in fluorite crystals, Phil. Mag. 2, 1. With permission from Taylor and Francis, http //www.tandf.co.uk/journals Figure 12.19 Reprinted from Phillips, D.S., Plekta, B. J., Heuer, A.H., and Mitchell, T.E. (1982) An improved model of break-up of dislocation dipoles into loops application to sapphire (a-A Os), Acta Metall. 30,491. Copyright 1982, with permission Elsevier.

Figure 9.16 Formation of a faulted dipole in sapphire, (a) Original perfect dislocation dipole (b) Dipole with each partial undergoing conservative climb dissociation (c) Faulted dipole formed by the annihilation of the inner partials.

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