The real-time stress evolution has been investigated during molecular-beam epitaxial growth of GaAs1-xSbx/GaAs metamorphic buffer. These real-time data were obtained using an in situ multibeam optical sensor measurement and has been combined with detailed analysis of data obtained from x-ray diffraction, transmission electron microscopy, and atomic force microscopy. We compare the strain relaxation of two different compositions of GaAs1-xSbx, and correlated the development of dislocation structure and morphology. Several distinct stages of the strain relaxation were observed during growth, which can be separated in three main regimes: pseudomorphic growth, fast strain relaxation, and saturation. Transmission electron microscopy data show that GaAs0.5Sb0.5 buffer layers have a larger fraction of pure-edge dislocations that arise during the earliest stages of growth. This could have a significant influence in the fabrication of buffer layers, since pure edges are favored over the threading dislocations. The strain relaxation profile for each film was modeled using a modified model of Dodson and Tsao [Phys. Rev. B 38, 12383 (1988)] that takes into account the elastic interactions of misfit dislocations. The model results agree with the experimental data and show that interaction of misfit dislocations is responsible for the large residual stress. In addition, following the description developed by Dodson and Tsao [Phys. Rev. B 38, 12383 (1988)] for the rate of dislocation multiplication, we were able to determine the line density of threading dislocations from the experimental data. This has a potential application in the design of metamorphic buffer layers because our observations are - made in real time on individual growth, without the need of external characterization to measure the dislocation density.