Thermodynamic approximations for cooperative phenomena in solids are reviewed and a new approximation is proposed and developed. The method calculates the configurational free energy for Ising and Heisenberg model systems, from a series of quasi-chemical reactions, adopting a system renormalization solution strategy.Magnetic and suitable order-disorder transformations are properly represented as higher order transitions. An elastic energy term is introduced to take into account the atomic size and shape change effects in metallic alloys. With this method we can distinguish when an ordering transition becomes first order. The method is also suitable for a theoretical calculation of the magnetic free energy contribution to ferromagnetic alloys
