Probing the impact of grain size distribution on the deformation behavior in fine-grained austenitic stainless steel: A critical analysis of unimodal structure versus bimodal structure

Abstract

This study investigated the effect of grain size distribution on the deformation behavior in fine-grained (FG) austenitic stainless steels (γ-SSs). Commercial Cr–Mn–N γ-SS was selected as the coarse-grained (CG) specimen, which was cold rolled and then annealed at 1000 °C for 60 s and 700 °C for 3600 s to obtain FG specimens with similar grain sizes but different grain size distributions. The CG specimens annealed at 1000 °C for 60 s and 700 °C for 3600 s were referred to as unimodal-FG and bimodal-FG specimens, respectively. The tensile properties revealed that the yield strength was enhanced from 383 MPa to 685 MPa when the grain size was refined from CG (10.7 μm) to FG (∼1.8 μm). Moreover, the bimodal-FG specimen exhibited a higher total elongation of 48% than 36.5% for the unimodal-FG specimen. Transmission electron microscopy, electron backscatter diffraction and X-ray diffraction were used to identify the microstructural characteristics during tension. The results indicated that the main deformation mechanisms of γ-SSs specimens during tension were dislocation generation and accumulation at the initial stage, deformation twinning at the intermediate stage, and deformation-induced martensite (DIM) at the final stage. Compared with the unimodal-CG counterpart, grain refinement in the unimodal-FG specimens restricted the formation of DIM due to the increased austenite stability. A newly discovered phenomenon was that the bimodal-FG specimens promoted the formation of geometrically necessary dislocation (GND) and DIM compared to the unimodal-FG, which was due to the uneven deformation in grains located on hetero-boundaries (HBs). Moreover, the bimodal-FG specimen was preferentially formed DIM on HBs, especially in the smaller grains side rather than inside larger grains due to the higher stress concentration in smaller grains.