The mechanical strength of metals depends on their resistance against various microscopic
deformation processes. In ductile metals, the most important process is shearing of the crystal
lattice by dislocations. One of the fundamental aspects of dislocation motion is cross-slip of
screw dislocations, the process by which they change their glide plane. In Face-Centered
Cubic (FCC) metals, cross-slip is supposed to play a role in dislocation structuring, work
hardening, recovery, fatigue, etc. Most prior studies on cross-slip in FCC metals focused on
pure metals. There have been few studies of solute effects on cross-slip, which are important
for engineering alloys. Here, the effects of substitutional solutes are studied using atomistic
simulations and statistical modeling.

In the first part of the thesis, the mechanism and energy of cross-slip of short (40 Burgers
vectors long) dislocations in Ni-Al, Al-Mg and Cu-Ni alloys are determined using atomistic
calculations. These calculations are carried out with real random alloys and with "average"
alloys, where the real atom types are replaced by a single average type. By comparison, it
is shown that cross-slip is controlled by fluctuations in the solute concentration, i.e. the
activation energy for cross-slip is a distributed variable with a large variance around the mean
value. The latter changes only little with concentration. Most importantly, activation energies
that are significantly lower than the mean value can be observed in random alloys. A linear
correlation between the activation energy and the energy difference between the state of the
dislocation before and after cross-slip is observed. An analytical, parameter-free model of
this energy difference is developed, which takes random changes in solute-dislocation and
solute-solute binding energies into account. Thus, it is possible to predict the distribution of
activation energies for nucleation of cross-slip.

In the second part, cross-slip of long (10^2-10^3 Burgers vectors) dislocations is studied using a
random walk model. Cross-slip is seen as a discrete process, where one Burgers vector long
subsegments of the dislocation cross-slip one after another. Associated with each step is a
random energy due to random changes in solute binding energies, as well as a deterministic
energy change due to constriction formation and stress effects. The random walk model
allows the calculation of the activation energy distribution for arbitrary dislocation lengths
and stresses. Cross-slip of long dislocations is unlikely at zero stress, due to increasing
frequency of high activation energies with increasing length. However, an external stress
eliminates these high barriers. Cross-slip then becomes a weakest-link problem. Like in the
case of short dislocations, activation energies that are significantly lower than average-alloy
estimates can be observed in real random alloys.