Structures of Fe-Cr-Ni and Fe-Mn alloys subjected to severe plastic deformation under pressure have been studied using different methods. It is shown that in FCC gamma-Fe-18%Cr-10%Ni steel the nanocrystalline structure formed increases the amount of high-pressure HCP epsilon-phase. It is also shown that with the formation of nanocrystalline structure there is an increase of forward P-gamma ->epsilon and a decrease of reverse P-epsilon ->gamma martensitic transformation in FCC gamma-Fe-Mn alloys. It has been found that pressure increase to 20 GPa reduces the P-epsilon ->gamma martensitic transformation in nanocrystalline Fe-Mn alloys in FCC gamma-field by DAC in such a way that a 100% stabilization of high-pressure HCP epsilon-phase under normal conditions and a long endurance after pressure removal are possible.