High-throughput identification of the martensite start temperature in mixed-grain structures

Martensite is a typical microstructure of ultra-high-strength (UHS) steel, with the martensitic start temperature (Ms) being crucial for its mechanical properties. The prior austenite grain size (PAGS) significantly influences Ms, however, establishing a quantitative relationship is resource-intensive. Herein, a high-throughput strategy is proposed to efficiently determine Ms based on mixed-grain structures. The effect of PAGS on Ms in UHS martensitic steel is investigated by high-temperature laser confocal experiments. Additionally, the Gibbs free energy of the austenite phase in the Thermo-Calc thermodynamic database is modified with a correction value of -105 J/mol. Electron backscatter diffraction (EBSD) analysis together with phase-field simulations further confirm that a reduced PAGS leads to finer martensite effective grain sizes (MEGS) and an increased dislocation density. These findings provide a novel perspective for optimizing microstructure in UHS martensitic steel.