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.