Influence of microstructural features and deformation-induced martensite on hardening of stainless steel by cryogenic ultrasonic impact treatment

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 of the surface layers of the deformed specimens was studied by X-ray diffraction (XRD), optical microscopy (OM), transmission electron microscopy (TEM) and selective area electron diffraction (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 generates a higher density of the 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 the 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.