A direct result of dislocation intersection is a contribution of strain hardening, that is, the increase in flow stress with increasing strain. In reality, dislocations need to intersect forest dislocations (dense existing dislocation networks) in order for the plastic deformation to continue. These dislocation intersection phenomena could

be quite complex. Here, we discuss only the cases of intersection between straight edge and/or screw dislocations. The intersection of dislocations may create two types of sharp breaks, only a few atoms wide — (a) Jog. A jog is a sharp break on the dislocations moving it out of the slip plane. (b) Kink. It is a sharp break in the dislo­cation line but lies in the same slip plane.

To understand the dislocations intersection event better, let us consider first the case of two edge dislocations with their Burgers vectors perpendicular to each other, as shown in Figure 4.19a. An edge dislocation AB (with Burgers vector b1) is gliding on the slip plane PAB. The edge dislocation CD (Burgers vector b2) lies on its slip plane PCD. The dislocation AB cuts through the dislocation CD and creates a sharp break JJ) on dislocation CD. The sharp break produced on dislocation CD is called a jog, as depicted in Figure 4.19b. Hence, the jog has a Burgers vector of b2, but with a length (or height) of b1. Hence, the strain energy of the jog (Ej) would be aGb^. If b1« b2 = b, we can write Ej = aGb2b1 = aGb3. However, the jog is a very small dislocation seg­ment, no long-range elastic strain energy is possible. The jog energy mostly consists of the core energy. So, instead of a = 1, a with a value of 0.1-0.2 is more appropriate.

Now another example of two orthogonal edge dislocations with parallel Burgers vectors is discussed (Figure 4.20a). Dislocation XY is moving toward dislocation WV

In this case, jogs SS’ and QQ are created on dislocations XY and WV, respectively, as illustrated in Figure 4.20b. But these are not real jogs as they are on the same slip plane as the dislocation lines. They are called kinks as we have already defined. These kinks are generally not stable and disappear as the dislocations glide.

The intersection of a screw dislocation with an edge dislocation creates a jog with an edge orientation on the edge dislocation and a kink with an edge orientation on the screw dislocation (right-handed). Intersection between two like screw disloca­tions would produce jogs of edge orientation on both the dislocations involved. Here, all jogs have height in the order of atomic spacing. These are called elemen­tary or unit jogs. There are many cases where jogs with height of more than one atom spacing have been found. They are called superjogs. However, the discussion on superjogs is outside the scope of this book.

Jogs produced by the intersection of edge dislocations are of edge orientations and they lie on the original slip planes of the dislocations. They can glide with the edge dislocations on the stepped surface instead of a single slip plane. Hence, jogs found on such edge dislocations do not hinder their motion. However, jogs pro­duced by the intersection of screw dislocations are of edge orientation. As we know, edge dislocations can only glide in the plane that contains both the disloca­tion line and its Burgers vector. The screw dislocation can slip (i. e., move conserva­tively) with its jog if it glides on the same plane. However, if the screw dislocation tries to move to a different plane (MNN’O) as illustrated in Figure 4.21, it can take its jog with it only via nonconservative motion such as dislocation climb. As the dislocation climb process requires higher temperatures (i. e., thermally activated), the movement of jogged screw dislocations becomes temperature dependent. That is why at lower temperatures, the movement of screw dislocations is sluggish com­pared to edge dislocations as their motion is impeded by the existing jogs. At high stress regimes, the movement of jogs would leave behind a trail of vacancies or interstitials based on the dislocation sign and the direction of its movement. If the jog leaves behind vacancies, it is called vacancy jog, and if the jog goes in the oppo­site direction producing a trail of interstitials, it is called interstitial jog.

4.2.7