Correlation of orientation relationships and strain-induced martensitic transformation sequences in a gradient austenitic stainless steel

Orientation relationships (ORs) play a crucial role in the phase transformation of face-centered cubic austenite (γ-fcc) to body-centered cubic martensite (α′-bcc) upon plastic deformation. So far, ORs between γ-fcc and α′-bcc are reported frequently; however, the correlation to the transformation sequences is not clear. Here, the gradient structured austenitic stainless 304 steel prepared by ultrasonic nanocrystalline surface modification was examined to clarify the ORs that were involved in the strain-induced martensitic transformation (SIMT) behaviors. The results showed changes in the ORs from Nishiyama–Wassermann (N-W) to Kurdjumov–Sachs (K-S) and Pitsch with an increase in depth from the top-treated surface. The OR of α′-bcc formed directly on γ-fcc was identified as N-W, while those via the hexagonal closed packed e-plate (e-hcp) were the K-S and Pitsch as they were nucleated at the single and two e-plates intersections, respectively. Thus, the SIMT sequence was the direct γ → α′ with the presence of the N-W OR in the high strain depth, while it was the γ → e → α′ as the existence of K-S and Pitsch ORs in the low strain depth. The findings provide essential data for the in-depth and comprehensive study of the polymorphic martensitic transformation mechanisms in various steels and alloys.