Three atomic shell cluster of SiC is treated as a set of chemical bonds with tetrahedral coordination. Chemical bonds energies are determined in tight binding approximation taking into account second neighbors interaction and relaxation of atomic positions. Correlations in behavior of Al and N atoms in Si-C-Al-N system determine the quasibinary character (SiC)_1-x(AlN)_x alloys. Inhomogeneous regions in (SiC)_1-x(AlN)_x system were evaluated using the condition of mixing free energy minimum. We assumed that doping does not change the vibration spectra of the crystal.