The evolution with temperature of Ni MBE (molecular beam epitaxy) ultra-thin films embedded between Au layers is described. Strong oscillations in the contrast, with a period of 4 atomic planes in the growth direction, appear on high-resolution electron microscopy (HREM) images. These oscillations are related to chemical modulations. For comparison, the thermal stability of solid solutions has been studied, where the same feature is found. The main conclusion is that anisotropic decomposition occurs only when the film or the solid solution is under an in-plane stress.