Abstract: The present study revealed the intrinsic mechanism of post-deformation (relaxation after deformation) ferrite transformation and validated that manipulating grain size and volume fraction of ferrite enhances strength-ductility synergy in a low-carbon martensitic steel. In-situ neutron diffraction and microscopic investigations uncovered that austenite to ferrite transformation preferentially occurs at austenite grain boundaries during relaxation due to localized dislocation concentration. According to the in-situ neutron diffraction measurements, the retained dislocation density was obviously higher than the level before deformation during relaxation at 755 °C. Conversely, dislocation density could fully decrease to the level prior to deformation during relaxation at 765 °C. Thermodynamic calculations demonstrated that high chemical driving force with sufficient dislocations effectively enhances nucleation and coalescence of similarly oriented grains. Meanwhile, the stored dislocations during relaxation govern the types of transformation behaviors. Therefore, distinct transformation behaviors allow precise tuning of ferrite microstructural features: grain size and volume fraction. This strategy, leveraging the heterogeneity in grain-boundary transformation by holding various relaxation times, increases the mechanical properties of low-carbon martensitic steel. These findings provide valuable microstructure design concepts for overcoming the strength-ductility trade-off in high-strength martensitic steels.

Key words: Post-deformation transformation, Nucleation rate, Dislocation density, Gibbs energy, In-situ neutron diffraction