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Numerical simulations are used to study various dislocation interactions expected to occur when a strained epitaxial layer relaxes. Frank–Read sources located on parallel glide planes are shown to produce elaborate network patterns of dislocations in the unstrained substrate of the layer. These are strikingly similar to experimentally observed patterns in high-quality relaxed SiGe layers grown by low-temperature ultrahigh vacuum chemical vapor deposition. Frank–Read sources on intersecting glide planes, on the other hand, produce characteristic corner-pileup structures which have previously been attributed to a “modified” Frank–Read source mechanism. The blocking of threading arms by misfit dislocations is found to occur only very near the critical strain. A much more important mechanism preventing ideal layer relaxation appears to be the formation of immobile pairs of threading dislocations on parallel glide planes. © 1999 American Institute of Physics.