Mechanical alloying and severe plastic deformation of nanocrystalline high-nitrogen stainless steels

High-nitrogen, nickel-free nanocrystalline stainless steels with Fe-(18–20)Cr-1N and Fe-(23–25)Cr-(10–11)Mn-1N (wt %) compositions were obtained by mechanical alloying (MA) in a high-energy planetary ball mill in an argon atmosphere. As a source of nitrogen, we used chromium and manganese nitrides that enter the composition of the powder mixture with pure metal components. Comparative studies of the evolution of the structure during MA using X-ray diffraction, Mössbauer spectroscopy, and transmission and scanning electron microscopy have demonstrated that, in an Fe-Cr-N system, a ferrite bcc structure is preserved for up to 120 h of MA. In an Fe-Cr-Mn-N system, where nitrogen was supplied by chromium nitrides, complete austenization occurred after 60 h of the MA. The maximum acceleration of the formation kinetics of a high-nitrogen austenite was obtained in an Fe-Cr-Mn-N system, where nitrogen was obtained from a manganese nitride. The mechanisms of solid-phase reactions in powder mixtures during MA are discussed under both the deformation-induced dissolution of nitrides in a metallic matrix and the stability of a nitrogen-supersaturated ferrite with respect to α → γ transformation.