Enhanced strength-ductility synergy in a new 2.2 GPa grade ultra-high strength stainless steel with balanced fracture toughness: Elucidating the role of duplex aging treatment

Abstract

Tuning thermal treatment schedule has been confirmed as an effective technical route for achieving optimum properties of steels. In the present work, the role of a unique duplex aging process on the enhanced strength-ductility synergy in a newly designed 2.2 GPa grade martensitic ultra-high strength stainless steel (UHSSS) was fully elucidated utilizing multi-scale characterization methods and various mechanical tests. Compared with the first aging-treated steel, the strength and ductility are synergistically enhanced after the second aging treatment due to increased volume fractions of Laves phase and reversed austenite as well as intensification of spinodal decomposition. Specifically, nanoprecipitates not only act as “obstacles” for dislocation movements, but also delay the deformation localization and necking instability, resulting in the dramatic increase of both strength (359 MPa for yield strength) and ductility (17% for reduction of area). The second aging-treated steel exhibited an impressive combination of mechanical properties: yield strength ∼ 1876 MPa, ultimate tensile strength ∼ 2259 MPa, elongation to fracture ∼ 11.5%, reduction of area ∼ 52%, and fracture toughness ∼ 43 MPa·m0.5 at ambient temperature. As revealed by the microstructural characterization, the aged steel is primarily strengthened by three distinct types of nanoprecipitates, including M2C, Laves phase, and α’Cr domains. The strengthening contributions of these precipitates were quantitatively estimated in terms of Orowan dislocation looping or cutting mechanisms. It is found that Laves phase contributed the maximum strength increment induced by precipitation hardening. An accessible pathway consisting of duplex aging treatment and co-precipitation of multiple nanoprecipitates is thus validated to exploit high-performance UHSSSs.