Development of a high-strength Fe–12Mn maraging steel via designing lath interfacial and intragranular nanostructures

A Heusler Fe₂TiSi-phase strengthened 12Mn maraging steel Fe–12Mn–5Co–1Ti–1Mo–1Si was extensively studied using various experimental (EBSD, TEM and uniaxial tensile) techniques. The results indicate that the fine and dispersed Fe₂TiSi particles, precipitating within the martensite, were the primary strengthening phase when aged at 450 °C regardless of whether the alloy is cold rolled or solid-solution treated. Furthermore, a soft reverted austenite nano-layer was observed at the lath interface, imparting significant toughness to the martensite matrix. By carefully optimizing the aging process parameters, the 12Mn alloy achieved a high tensile strength of ∼1700 MPa with a moderate elongation of ∼7% at break. When aged at 400 °C, unexpected precipitation of α-Mn particles within the martensite structure was observed, displacing the Fe₂TiSi phase as the dominant strengthening phase. Simultaneously, elemental Mn segregation was found to occur at the lath interface, instead of the formation of reverted austenite. This Mn segregation layer was identified as the main factor contributing to brittle fracture behavior. These findings illustrate that synergistic improvement of strength and plasticity can be realized via an optimal combination of precipitation strengthening and interfacial layer toughening mechanisms.