In the last decade, alloying of steel by copper became the subject of special attention since a new steel grade was designed in which high strength and ductility were attained due to precipitation of nanometer-sized copper-rich particles formed in the bcc Fe matrix. The mechanical properties of low carbon steels can be controlled by the structural state and improved by alloying. The decomposition in bcc Fe–Cu based alloys will be researched using statistical thermodynamics simulations with ab initio effective interactions. This approach, which includes an accurate first principles description of a disordered magnetic state as well as a vibrational free energy, allows for modeling of the decomposition in these alloys. This study is for one year with the emphasis on the effect of alloying by Al and Si on the decomposition in these alloys and will be conducted at Luleå University of Technology.