We have investigated Al0.12Ga0.88N layers with and without a 1 µm GaN buffer, grown on the c-face of α – Al2O3 substrate with an intermediate AlN nucleation layer grown by LP-MOVPE. We used spatially resolved cathodoluminescence spectroscopy at a temperature of 8K to investigate the strain and the homogeneity of composition that can be determined from the energy of the luminescence peak. The larger thermal expansion coefficient of the sapphire in comparison to the nitrides leads to a biaxial compressive strain of the upper GaN layer when cooling down from growth temperature. For AlGaN layers directly grown on the nucleation layer this cannot be confirmed. The layer stays relaxed and fluctuations in the aluminium composition of 0.4% can be observed. When growing an intermediate GaN buffer, the AlGaN layer gets tensilely strained. This strain is of elastic nature and microcracks can be observed preferentially at the edges due to the smaller lattice constant of AlGaN in comparison to GaN. Even detaching of the AlGaN layers grown on the buffer can be observed. In the regions without cracks the layers are quite homogeneous. A deformation potential of (19±4)eV was estimated for Al0.12Ga0.88N.