Relaxation of compositionally graded SiGe films during growth has recently been shown to occur through the multiplication of dislocations by the so‐called modified Frank–Read mechanism. Here, we show that the same mechanism is observed, during ex situ annealing of highly metastable, compositionally uniform layers. However, unlike graded layers, threading dislocations penetrate the upper layers of the film after relaxation. We explain the occurrence of the modified Frank–Read mechanism in ungraded layers by the lack of dislocation nucleation sites. The threading dislocations result from the fact that, in compositionally uniform layers, all dislocations tend to be located at (or very near) the plane of the interface, generating numerous pinning sites.