Influence of Nano/Ultrafine-To-Micron Grain Ratio on the Strength-Ductility Synergy in a Heterostructured Fe–17Cr–6Ni Steel

This study introduces a novel heterostructure design strategy to achieve exceptional strength and ductility in Fe–17Cr–6Ni austenitic stainless steel. The alloy undergoes cryogenic rolling (35% reduction), followed by warm rolling at 250°C (80% total reduction), and short-term annealing. After annealing at 725°C for 15 min (HT3), a heterogeneous grain structure forms, consisting of approximately 50% nano/ultrafine grains (<1 μm) and 50% micron-sized grain (>1 μm). The HT3 specimen demonstrates a yield strength of 756.7 MPa, ultimate tensile strength of 1030.8 MPa, total elongation of 44.6%, a strength-ductility product of 46.0 GPa·%, and significant work hardening, with a yield-to-tensile gap of 265.5 MPa, outperforming specimens with other nano/ultrafine grains area fractions: HT1 (71%), HT2 (61%), HT4 (37%). Microstructural analysis reveals that the enhanced performance stems from the synergistic effects of heterodeformation-induced (HDI) hardening and transformation-induced plasticity. The HDI stress, generated from heterogeneous grain interactions, provides sustained hardening, while strain-induced martensitic transformation occurring preferentially at soft/hard interfaces further strengthens the material. Among the studied conditions, a balanced ratio of nano/ultrafine and micron grains is found to be crucial for improving work hardening and ductility, offering new insights into designing high-performance heterostructured metallic materials.