According to the common phase diagrams, iron and magnesium are almost immiscible at ambient pressure Massalski (1986). In the liquid phase, the solubility of Mg in Fe is of the order of 0.025 atomic percent (at.%). The maximum solid solubility of Fe in Mg is 0.00041 at.% and the Fe content in Mg at the eutectic point is less than 0.008 at.% (Haitani et al., 2003). Below 1273 K the solubility of Mg in α-Fe is below the detection limit and about 0.25 at.% Mg can be solved in δ−Fe at the monotectic temperature. The immiscibility of Fe and Mg at ambient conditions is in line with the well-known Hume-Rothery rules, according to which more than 15% atomic size difference between alloy constituents hinders solid solution formation (Massalski, 1996). In spite of the negligible solubility of Mg in Fe, several Fe-rich metastable Fe-Mg solid solution have been synthesized. According to the pioneering work by Hightower et al. (Hightower et al., 1997), mechanical alloying produced Fe-Mg substitutional solid solutions with up to 20 at.% Mg and having the body centered cubic (bcc) crystallographic phase of α-Fe. Later, using the similar alloying procedure, Dorofeev et al. (Dorofeev et al., 2004; Yelsukov et al., 2005) found that about 5 − 7 at.% Mg in α-Fe forms supersaturated solid solution. These authors suggested that the driving force for the formation of Fe-Mg solid solutions is associated with the excess energy of coherent interfaces in the Fe-Mg nanocomposite, which facilitates incorporation of Mg into α-Fe. Indeed, based on semiempirical thermodynamic calculations, Yelsukov et al. (Yelsukov et al., 2005) obtained 6 kJ/mol for the enthalpy of formation for Fe-Mg solid solutions, compared to 20 kJ/mol calculated for the corresponding Fe-Mg nanocomposites. In addition to the mechanical alloying techniques, pressure was also found to facilitate the solid solution formation between Fe and Mg. Dubrovinskaia et al. (Dubrovinskaia et al., 2004) reported that at pressures around 20 GPa and temperatures up to 2273 K, the solubility of Mg in bcc Fe was increased to 4 at.%. They found that the lattice parameter of the bcc Fe-Mg alloy increased approximately by 0.6 % per at.%Mg. Furthermore, recent experimental measurements in combination with theoretical simulations demonstrated that at the megabar pressure range more than 10 at.% Mg could be dissolved in liquid Fe, which then could be quenched to ambient conditions (Dubrovinskaia et al., 2005). The mechanism behind the 4