We predict the decomposition peculiarities of the heavy isoelectronic impurities doped AlxGa1-xNyAs1-y alloys. The AlN and GaAs precipitates in the high GaAs and AlN content Al xGa1-xNyAs1-y epitaxial layers, respectively, should be formed. The free energy of the alloys is expressed as a sum of the free energies of the constituent compounds, strain and elastic energies, and configurational entropy term. The regular solution model is used for describing the free energies of the compounds and strain energy. The interaction parameters between the compounds are estimated by the valence force field model. The stiffness coefficients of the wurtzite AlAs and GaAs as well as the interaction parameter between AlN and AlAs are calculated.