Burgers partial dislocations

Figure 3.12 Partial dislocations in copper (a) a unit dislocation, Burgers vector bl (b) initially slip is easier in the direction represented by the Burgers vector of the partial dislocation b2 than bl (c) the result of the movement in (h) is to generate a stacking fault and (d) the combined effect of displacements by the two partial dislocations b2 and b3 is identical to that of the unit dislocation, but the partials are separated by a stacking fault.

Figure 3.17 Layer of the corundum (AI2O3) structure, projected down the c axis. The unit cell is marked. A unit dislocation, Burgers vector b, can be decomposed into four partial dislocations bl-b4.

Figure 3.18 Cadmium iodide, Cdl2, structure (a) perspective view of the structure as layers of Cdlg octahedra (b) one layer of the structure, with the lower I- anion (A) layer and the middle Cd2+ (c) layer shown complete and just three anions of the upper (B) layer indicated. The vector b represents the Burgers vector of one of three unit dislocations and bl and b2 represent the Burgers vectors of the two equivalent partial dislocations. The unit cell dimensions are a, b, and c.

The Burgers vector of unit dislocations in copper is equal to the line joining the closest atoms, written a/2[110]. (a) What is the length of this vector in nanometers This dislocation is often replaced by partial dislocations [211], (b) What is the length of this vector in nanometers (The copper structure is given in Supplementary Material SI and drawn in Fig. 3.10. a = 0.3610 nm.)... 

A partial dislocation (called a stair rod dislocation) is formed at the intersection of two stacking faults on different planes with different fault vectors R and R2 and its Burgers vector is bp = Ri—R2. An extended summary of the contrast from partial dislocations in fee metals has been given by Edington (1975). 

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... 

Figure 9.37. Schematic diagrams showing (a) a perfect edge dislocation in a structure consisting of alternating layers A and B, and (b) the climb dissociation of such a dislocation into two partial dislocations. As shown, the dissociation is due to the preferential precipitation of vacancies at A layers, or of interstitials at B layers. Burgers circuits are shown for the perfect dislocation (a) and the two par-tials (b). In BaTiOs, the A and B layers could correspond to BaO and Ti02 layers...

Another class of dislocations called partial dislocations is characterized by Burgers vectors corresponding to a translation of an atom from a lattice position to a twin-lattice position. Such dislocations are related to stacking faults in close-packed crystal structures and appear at the boundaries of stacking faults ending inside a crystal [5.49]. 

Dislocations can be imaged and their Burgers vectors determined by using transmission electron microscopy. This technique has shown that many dislocations have a Burgers vector that is less than the repeat distance of the structure. These are called partial dislocations. 

Because of the large Burgers vector involved the combined motion of partial dislocations may lead to slip. The background to this argument was presented in Chapter 12. Graphite is another hexagonal ceramic in which slip has been found to occur by the motion of partial dislocations. 

Fig. 3.57 Partial dislocations in an FCC structure a the split motion of atoms from site B to site C partial dislocation with Burgers vectors b2 and bs b extended dislocation composed of the two partials and the SF between them c schematic presentation of a SF [14]...

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. 

To indicate that the Burgers vector is related to partial dislocation, it is occasionally written as bp. Another parameter of importance is the grain size. For the following relations, grain size is taken into account in slip and twinning. Notice that these relations are similar, the difference being indicated by the subscripts for slip (S) and twinning (T) they are basically H-P equations ... 

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