Suppression of Prior Austenite Grain Boundary Fracture by Grain Boundary Segregation of Mo in Quenched Fe–10%Mn–0.1%C Alloy

The Fe–10%Mn–0.1%C alloy exhibits an ultra-fine microstructure containing α’-martensite (bcc), ε-martensite (hcp), and retained austenite (fcc). However, Mn causes grain boundary embrittlement and intergranular fracture at the prior austenite grain boundaries at low temperatures. In this study, <0.9 mass% Mo was added to Fe–10%Mn–0.1%C alloys to suppress intergranular fracture and improve their toughness in their as-quenched states. Electron backscatter and neutron diffraction revealed that Mo addition displayed little influence on the prior austenite grain size, martensitic block width, and phase fractions. Scanning transmission electron microscopy–energy-dispersive X-ray spectroscopy indicated that Mo segregated at the prior austenite grain boundaries at approximately 3 mass% after austenitizing 0.9%Mo steel at 1173 K, whereas Mn segregation remained almost unchanged. Thermodynamic calculations using the Hillert–Ohtani model confirmed that Mo exhibited a stronger segregation tendency than that of Mn. Charpy impact tests indicated that the ductile-to-brittle transition temperature (DBTT) decreased from 275 K for the Mo-free steel to 220 K for the 0.9%Mo steel. Fractographic observations revealed that intergranular fracture, which was dominant in the Mo-free steel at 77 K, was significantly suppressed by Mo addition. In the 0.9%Mo steel, intragranular fracture occurred with a characteristic size corresponding to the block size of α’-martensite. Therefore, Mo segregation effectively strengthens the prior austenite grain boundaries and suppresses intergranular fracture within Fe–10%Mn–0.1%C steel, leading to an improved low-temperature toughness and a reduced DBTT.