Recent X-ray and Mossbauer experiments have shown that mechanical alloying of iron with silicon in the Si-rich part of the phase diagram produces disilicides: α−FeSi2 at an early stage and β−FeSi2 later. In order to study the structure of the alloys in detail, density-functional calculations are conducted for β−FeSi2 with and without vacancies. The formation of a metastable α−FeSi2 instead of the equilibrium β−FeSi2 at low temperatures in many experiments is explained, and the conditions, under which the desired phase (metallic α or semiconducting β, or their combination) is formed, are specified.