Nanostructured surfaces
Evolution of the structural-phase state and surface microhardness of stainless steel at ultrasonic impact treatment in different environments
Kukharyk S.S., Burmak A.P., Franchik N.V., Voloshko S.M.,
Sidorenko S.I.,Vasylyev M.A., Mordyuk B.N.

1Metal Physics Department, Igor Sikorsky National Technical University of Ukraine, 37 Peremohy ave., UA 03056 Kyiv, Ukraine
E-mail: burmak@kpm.kpi.ua
2 Department of Physical Fundamentals of Surface Engineering, Kurdyumov Institute for Metal Physics, 
NAS of Ukraine, 36 Academician Vernadsky blvd., UA-03142 Kyiv, Ukraine

Ultrasonic impact treatment of stainless steel AISI 321 was carried out at room temperature (in the argon environment – argon-UIT) and at cryogenic temperature (in liquid nitrogen – cryo-UIT) in the constrained conditions with application of the same mechanical energy to the treated specimens. The time dependencies of the surface hardness HV after the cryo-UIT and argon-UIT processes were of sigmoidal and parabolic shapes, respectively. The microstructural evolution in the surface layers of the deformed specimens was studied by XRD, OM, TEM and SAED analyses. The volume fractions of the deformation-induced α' (Vα') and ε (Vε) martensites were estimated using XRD approach and by magnetic and density measurements. Compared to the room temperature UIT in the argon environment (argon-UIT), the liquid nitrogen UIT (cryo-UIT) generates a higher density of deformation twins and stacking faults. In addition, a higher Vα' (~53%) and Vε (~3.5%) were observed after cryo-UIT in deeper surface layers (~200 μm). The argon-UIT process leads to the formation of either rectangular twin blocks or dislocation cells, which size ranged 200–500 nm. Conversely, after cryo-UIT (ē ≈ 0.95), a nanoscale grain structure of heterogeneous nature (α' and γ phases) was formed in the outmost surface layer simultaneously with the areas filled with networks of deformation twins and stacking faults. The minimum grain size of α'-martensite Dα' and austenite Dγ was respectively 25 and 45 nm, and twin thickness/spacing was of 60–120 nm. Both types of microstructure contribute to the material strength and result in higher hardness of the cryo-UIT processed specimens (~5–5.66 GPa) compared to that of the argon-UIT processed ones (~4.3 GPa). With the increase in Zener-Hollomon parameter lnZ, the grain/twin/spacing size is decreased while the Vα' and surface microhardness HV are increased.