Atomic-scale characterization of multiple precipitating species in a precipitation-hardened martensitic stainless steel

Multiple precipitating species in a 2.2 GPa grade precipitation-hardened martensitic stainless steel with balanced ductility were characterized at atomic scale by atom probe tomography. The results indicated that the clustering of solute atoms was promoted with progressive aging treatments. (Cr, Mo)-rich carbide (M₂C) precipitated at the linear dislocations in the as-aged steels. Obvious segregation of Cr, Mo, and C at phase boundaries favored the precipitation of carbide and caused the formation of Cr-lean domains. Spinodal decomposition of martensitic matrix during aging led to the substantial precipitation of fine Cr-rich (α′_Cr) phase. Compared with the first aging treated samples, a synergistic enhancement of both strength and ductility of the secondary aging treated (SAT) samples was primarily ascribed to the enhanced precipitation of Cr-rich phase. Additionally, Ni-rich filmy reversed austenite precipitated at the lath boundary, which was beneficial to the ductility of SAT samples.