Age-hardening mechanisms of heterogeneous-nanostructured SUS316LN stainless steel fabricated by heavy cold rolling

A stable SUS316LN austenitic stainless steel was heavily cold-rolled to 92% in reduction to have complicated heterogeneous nanostructure composed of “eye”-shaped twin domains, shear bands and conventional low-angle lamellae. The average boundary spacings in the twin domains and the low-angle lamellae were approximately 25 nm and 40 nm, respectively. The average width of the shear bands was about 40 nm. While the tensile strength along transverse direction was notably high to be 1.9 GPa, it was further raised up to 2.2 GPa by peak aging at 748 K. Nevertheless, any hardening mechanisms as spinodal decomposition, formation of G.P. zone, clustering, etc. could not be detected. The 3D-atom probe tomography analyses revealed segregation of the solute elements of Mo, Si and so on at twin and low-angle lamellar boundaries. A finite element calculation using the multiscale crystal plasticity simulation system indicated rise of strength due to impediment of dislocation motion by the grain-boundary segregation. The combined mechanisms of nano-lamellar structure and grain-boundary segregation, therefore, caused extremely high strengthening.