The significant role of bimodal lamellar heterostructure for Lüders deformation and TRIP effect in 18Cr–8Ni austenitic stainless steel

Abstract

A series of 18Cr–8Ni ASSs with homogeneous or heterogeneous structures were prepared to reveal the unique deformation mechanism of hetero-deformation-induced (HDI) hardening in lamellar heterostructure. By adjusting the annealing temperature, the nano-grained (NG) and coarse-grained (CG) unimodal homogeneous structures were obtained by annealing at 600 °C and 1000 °C, respectively, while the nano-grained/ultrafine-grained (NG/UFG) and ultrafine-grained/fine-grained (UFG/FG) bimodal heterogeneous structures were achieved by annealing at 700 °C and 800 °C. Compared to the conventional bimodal homogeneous structure (UFG/FG), the hetero-zone boundaries (HBs) of bimodal lamellar heterostructure (NG/UFG) can not only provide extra HDI hardening effect, but also coordinate the inhomogeneous deformation for Lüders deformation. The HDI hardening and Lüders deformation greatly facilitate the transformation induced plasticity (TRIP) effect to achieve. This is due to the lower austenite stability in soft region (coarser grain), leading to the formation of deformation-induced martensite (DIM) under lower stress, on the other hand, the Lüders deformation achieved a dynamic balance between both sides of hetero-boundary, avoiding the stress concentration and promoting the TRIP effect. Consequently, the DIM content of the NG/UFG specimen is considerably higher than that of the UFG/FG specimen (78 vol% to 27 vol%).