Pre-martensitic state of the FCC lattice and the formation of an intermediate phase during the γ → ε transformation in CrMnN austenitic stainless steel

High-nitrogen nickel-free austenitic steels are unique materials, as random filling of atomic positions in the crystal lattice with elements of different sizes (Fe, Cr, Mn and N) contributes to its significant distortion. As a result, functional characteristics change, including elongation at low temperatures due to various transformations in single-phase solid solutions. The CrMnN austenitic stainless steel was studied after both quenching from 1373 K in water and tensile tests in the temperature range of 293 – 77 K by X-ray method. Synchronous changes in the plastic elongation, microstrains of the crystal lattice and the root-mean-square displacements of atoms from their equilibrium positions were observed with lowering the test temperatures. The maximum values of the above parameters were recorded at 253 K. At this temperature, the following patterns were revealed by transmission electron microscopy. The structural mechanisms of stress relaxation were twinning and the martensitic transformation with the formation of a crystal lattice different from the cubic and hexagonal ones, but characterized by the interplanar distances partially coinciding with both of them. The new phase is regularly oriented with respect to the austenite, hence it is coherent. It was assumed that the new phase was an intermediate one in the γ → ε (FCC → HCP) transformation at 253 K.