Generalized correlation of nanocrystalline structure and mechanical properties formation in Fe-based alloys during severe plastic deformation.

The main result of the work is the scientific conception of formation in α-Fe and it’s alloys nano- and submicrostructures under severe plastic deformation by friction (SPDF) in gas atmosphere (argon, air, ammonia). This original new method allows one to refine grain structure down to nanometre scale together with modifying the surface by dopant element.  Minimum strain e necessary to produce 100 nm sized α-Fe grained structure was estimated to be about 10 with high strain rate (έ>10² s^-1) in dynamic recrystallization condition. It is concluded that grain refinement process and directional mass transfer of dissolved atomic nitrogen in iron influence by each other, facilitating the evolution of deformation-induced structure and resulting in improvement of nitriding efficiency. That is why SPDF with nitrogen diffusion provides for greatest extension of deformation-induced structure, which demonstrates both the smallest stabilised grain size as well as abnormally high amount of nitrogen in grain interior. Due to the grain refinement the amount of nitrogen was recorded to become higher by 10² times than that typical for conventionally grained a-Fe[N] solid solution obtained by nitriding without deformation. Finally stabilised grain size ensured by SPDF under diffusion flow of the dissolving dopant element (nitrogen) is found to be at least smaller by factor two compared to that induced by SPD with argon gas. Scale effect of grain structure on mechanical parameters such as Young’s modulus, nanohardness and plasticity characteristic d_A were detected by using nanoindentation technique.

In the work the important scientific and technical problem of nanocrystalline structure formation in bcc iron subjected to severe plastic deformation by friction (SPDF) in different gas atmosphere (argon, air, ammonia) was decided. The effect of the structural state changes on the features of diffusion and phases transformations in the indicated conditions, and also on the complex of mechanical properties (Young's modulus, hardness, plasticity characteristic) of material in the nanocrystalline state is generalized. It was found the necessary conditions (temperature, strain and strain rate) to obtain the nanocrystalline structure of bcc Fe alloys during SPD by friction.  It was found that SPDF combined with directional mass transfer of nitrogen results in increasing the nanohardness of sections consisted of submicro- and micrometer sized grains due to the solid solution hardening. High concentration of nitrogen in a–Fe[N]–solid solution does not influence on mechanical properties of nanocrocrystalline iron with grain sizes  d < 50 nm because deformation is predominantly controlled by grain boundary sliding.

These results are important both for the current basic research and for application in engineering practice.