A transmission electron microscopy and atom-probe tomography study of martensite morphology and composition in a dual-phase steel

Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom-probe tomography (APT) are utilized to study systematically the morphology and solute distributions in martensite formed after intercritical annealing in a dual-phase steel. The SEM observations demonstrate that the samples annealed at 760 °C for 5 min, and either water-quenched or air-cooled, contain martensite with a volume fraction of ~0.17 or ~0.10, respectively, in the vicinity of former ferrite/ferrite grain boundaries. APT measurements reveal that the levels of C and Mn in martensite are higher in the air-cooled sample than in the water-quenched sample. Combined TEM and APT analyses show that the water-quenched sample forms typical lath martensite with a high density of dislocations. In contrast, the air-cooled sample contains martensite with complex substructures, including fine twins, a mixture of laths and twins, and carbide precipitates, as well as a small portion of retained austenite that is highly enriched in C and Mn. The formation mechanisms of different martensites are discussed based on the experimental observations and thermodynamic calculations. The results provide insights into the phase transformations and corresponding microstructures formed during intercritical annealing, which are followed by either water quenching or air cooling at a moderate rate.