An overview of the number of unusual phase transformations taking place in nanocrystalline pearlitic steels in conditions of the severe deformation, i.e. combination of high pressure and strong shear strains will be given.
Strain induced cementite dissolution is a well-documented phenomenon taking place at cold plastic deformation of pearlitic steels. Recently new results which can shed an additional light on mechanisms of this process were obtained thanks to 3D AP and HRTEM investigations of pearlitic steel following the high pressure torsion (HPT). It was shown that the process of cementite decomposition starts with depleting of carbides with carbon, which indicates that deviation of chemical composition of cementite from stoichiometric one is the main reason for thermodynamic destabilisation of cementite at plastic deformation. Important results were obtained concerning the distribution of released carbon atoms in ferrite. It was experimentally confirmed that carbon segregates to dislocations and grain boundaries of nanocrystalline ferrite.
Another unusual phase transformation taking place in nanocrystalline peartitic steel during room temperature HPT is a stress induced \alfa\-->γ\
transformation, something which never occurs at conventional deformation of coarse grained iron and carbon steels. This was concluded to have occurred due to a reverse martensitic transformation. Atomistic mechanism and thermodynamic of the transformation, as well as issues related with stability of reverted austenite will be discussed.