7.5: Climb and Cross Slip

Climb and cross-slip

Climb and cross-slip are the two dominant processes by which dislocations become organised and annihilate each other by dislocation interactions.

Climb

Since the line and Burgers vector of an edge dislocation are perpendicular to each other, there is one plane in which the dislocation can slip. However, there is an alternative mechanism by which the dislocation can move to a different slip plane, known as climb. Climb is the mechanism of moving an edge dislocation from one slip plane to another through the incorporation of vacancies or atoms.
Climb can be either positive or negative. In positive climb, the dislocation acts as a vacancy sink, absorbing a vacancy to shift itself upwards relative to its initial position. In negative climb, the dislocation acts as a vacancy source. The vacancy at the bottom of the extra half plane is replaced by an atom, which causes the dislocation to shift downwards.

The following animation shows how positive dislocation climb occurs by the diffusion of vacancies around a crystal.

Cross-slip

Cross-slip is the movement of a screw dislocation from one allowable slip plane to another. Below is a video which explains how cross-slip work.

It should be noted that only perfect screw dislocations can cross-slip, as their line and Burgers vector are parallel to each other. Dislocations which have edge components can never cross slip.

Partial dislocations in an fcc system

Cross-slip becomes more complicated for an fcc metal. In an fcc metal, a perfect dislocation tends to dissociate into two partial dislocations and, therefore, cannot cross-slip when it dissociates. To understand this, consider the atomic packing on a closed-packed (111) plane in Figure 4:

Figure 4: Slip in a closed-packed (111) plane in an fcc lattice. (Source of image: G.E. Dieter, Mechanical Metallurgy (1988), p. 155)

The {111} planes are stacked on a sequence ABCABC…, and the Burgers vector $$b_{1}=\frac{a}{2}[10 \overline{1}]$$ defines one of the slip directions. However, the same shear displacement can be accomplished by the two-step path b2 + b3. According to Frank’s rule, the latter is more energetically favorable. Hence, the perfect dislocation is decomposed into two partials:

$\frac{a}{2}[10 \overline{1}] \rightarrow \frac{a}{6}[2 \overline{11}]+\frac{a}{6}[11 \overline{2}]$

Slip by this two-step process creates a stacking fault ABCACABC in the stacking sequence. The two partial dislocations, which are separated by the stacking fault, are collectively referred to as an extended dislocation. Since the extended dislocation has both edge and screw components, it defines a specific slip plane, in this case the {111} plane of the fault. Consequently, the two partial dislocations are constrained to move in this plane and cannot cross slip unless the partials recombine to form a perfect dislocation again (This recombination of two partials is referred to as constriction).

It should be noted that while a pair of dissociated partials does need to be forced back together into a single perfect (screw) dislocation in order to be able to cross-slip, this need not happen along the complete length of the dislocation at the same time. What usually happens is that a local constriction (to a short length of perfect (screw) dislocation) is formed, this small section cross-slips onto the new glide plane, where it again separates into two partials (different partials from the original pair). Figure 5 shows an example where it is energetically easier for constriction to take place along a certain length than the complete length becoming a perfect dislocation, and cross-slipping, at the same time.

Figure 5: Sequence of events envisaged during the cross-slip process. Four stages in the cross slip of a dissociation (a) by the formation of a constricted screw segment (b). The screw has dissociated in the cross-slip plane at (c) (Source of image: Hull and Bacon, 2011).

Below is an animation showing how cross-slip happens in an fcc crystal.

The influence of stacking-fault energy on the availability of cross-slip

It is important to note that cross-slip is more difficult in metals with a low stacking-fault energy (i.e. a wide stacking fault). This is because the partial dislocations, which are well-separated, cannot recombine to form a perfect dislocation to cross slip. For example, cross-slip is not observed in copper (which has a stacking-fault energy of 45 mJm-2, but is quite prevalent in aluminium (which has a stacking-fault energy of 166 mJm-2).

Stacking-fault energy is also particularly important at relatively low temperatures, since climb is then very difficult and cross-slip is virtually the only mechanism by which dislocations can do anything other than glide on a single slip plane (which is quite a severe limitation in terms of a region trying to undergo a general shape change, which requires independent slip systems)