Nanostructured surfaces

The mechanical behavior of the tempering surface nanocrystalline structure obtained by mechanical-pulse treatment
O.V. Maksymiv, V.I. Kyryliv, H.M. Nykyforchyn
Karpenko Physico-Mechanical Institute of NAS of Ukraine, Lviv
Naukova str., 5, Lviv, 79060, Ukraine.
E-mail: kyryliv@ipm.lviv.ua

Nowadays the great interest of scientists is attracted to the nanocrystalline structures (NCS) and developing technologies of its forming. One of the main tasks up-to-date material science is analysis physical and mechanical properties of nanomaterials for optimum relationship of their strength and plasticity. The technology of mechanical-pulse treatment (MPT) was developed in Karpenko Physico-Mechanical Institute for obtaining surface NCS. It is based on using energy of high-speed friction which generates severe plastic deformation and formats gradient surface NCS. The grain size of created NCS on the top-surface was 40 nm for 65G (0.65C-1Mn) steel and 60 nm for 40Kh (0.4C) steel.
The effect of temperature of tempering on physical and mechanical properties of surface NCS on the studied steels was showed. The tests were carried out on the plane specimens 2 mm thick with both sides strengthening. It was established that tempering leads to changing of grain size of surface NCS and, consequently, theirs properties. The peculiarity of MPT is presence of compressive deformation and unidirectional shear deformation in the direction of strengthening tool velocity vector. All these conditions form non-equilibrium fragmented grains elongated in the direction of shear thermoplastic deformation in the surface layers. The MPT strengthened specimens were underwent tempering in the temperature range 100 ‑ 500°С. It was revealed that surface grain size is reduced at the tempering temperature below 300°С with subsequent theirs growth in the temperature range 300-500°С. Such behavior is unexpected for both steels. Insignificant rise the microhardness and microstresses in the lattice of the surface layer was accompanied with a decrease of its grain size.
It was showed that the best combination of strength and plasticity of surface NCS was reached after tempering at 500°С for both steels.