We present the results of a combined study of the defect structure and residual stress in a diamond layer, grown by PECVD on a polycrystalline copper substrate with a titanium interlayer. For the defect studies, both continuous and pulsed positron beam techniques were applied. X-ray diffraction techniques were used for both the stress determination in the diamond layer and for a phase analysis of the complete composite structure. The layer was found to contain a significant fraction of vacancy clusters and single vacancy type defects, situated within the individual grains. The presence of the larger defects can be correlated to a compressive stress in the layer.