Martensitic transitions in nanocrystalline metastable materials

Microstructures of Fe-Cr-Ni and Fe-Mn alloys subjected to severe plastic deformation (SPD) under pressure have been studied by different methods. It is shown that in an austenitic steel (γ: face centred cubic structure, γ-Fe-18%Cr-10%Ni) subjected to SPD, the formation of a nanocrystalline structure results in an increase of the volume fraction of high-pressure hexagonal close-packed ɛ-phase. It is also shown that nanocrystals promote the forward (γ → ɛ) and retard the reverse (ɛ → γ) martensitic transformation in γ-Fe-Mn alloys. High diamond anvil cell pressures impede the (ɛ → γ) martensitic transformation in nanocrystalline Fe-Mn alloys and results in the presence of 100% high-pressure ɛ-phase, which under normal conditions remains stable after processing.