Microstructure of GaAs films grown by molecular-beam epitaxy at low temperature and delta doped with Sb was studied by transmission electron microscopy. The material contained 0.5 at. % excess of arsenic that precipitated during post growth anneals. The Sb delta doping was found to strongly affect the microstructure of precipitates (clusters) and their ripening rate upon annealing. Segregation of Sb impurity in the clusters was revealed. In contrast to the well known pure As clusters, the As-Sb clusters induced strong local deformations in the surrounding GaAs matrix. Until a threshold diameter of 7-8 nm the clusters and surrounding matrix were coherently strained. Larger clusters were associated with dislocation loops of interstitial type. The cluster-loop orientation relationships were determined. Relaxation of local strains by formation of the dislocation loops was studied both experimentally and theoretically.