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.