Influence of high-content cobalt on hierarchical martensitic microstructures and mechanical properties of stainless maraging steels

Through integrated experimental and first-principles approaches, the pivotal role of high-content Co in governing phase constituents and martensite is elucidated. Experimental observations reveal that the high Co content facilitates carbide dissolution and reduces the martensitic transformation temperature. First-principles calculations distinguish that the high-content Co intrinsically elevates the stacking fault energy and enhances the austenite thermodynamic stability. This together with Co-induced escalated shear moduli imposes a resistance to strain relief during martensitic transformation, kinetically promoting the burst transformation to refined martensitic microstructures enriched with high-angle twin-like V1-V2 variant boundaries. An optimal Co content of 9.7 wt.% featuring a balanced strength, toughness and alloying cost is unveiled, and novel opportunities to rationally engineer mechanical properties of multi-phase steels are provided through current control of phase fraction, morphological refinement, and martensite variant selection.