Tri-functional co-nanoprecipitates enhanced cryogenic ductility by inducing structural heterogeneity and refining nano-twins in a low-stacking-fault-energy 17Mn steel

In this study, an innovative tri-functional co-nanoprecipitation strategy was employed to enhance the mechanical properties of a low stacking-fault energy (SFE) 17Mn steel for cryogenic applications. By combining severe cold deformation and subsequent annealing, a hierarchical structure emerged, featuring (Ti, Nb)C carbide (∼10 nm) and Cu-rich intermetallic (∼2 nm) in the austenitic matrix with heterogeneous grain size distributions. The co-precipitation (CP) sample exhibited superior performance compared to single-precipitation (SP) steel, with a yield strength of ∼1150 MPa, tensile elongation of ∼44.8%, and an impact toughness of ∼110 J at liquid nitrogen temperature (LNT), even surpassing the base-17Mn steel. The CP-17Mn samples displayed a higher density and thinner nano-twins at larger strains, leading to a rapid increase in geometrically necessary dislocations (GNDs). The detrimental martensitic transformation was effectively suppressed during both tensile and impact tests. The observed inverse strength-ductility and strength-toughness trade-off can be attributed to the tri-functional co-precipitates' roles: they provide disperse strengthening, induce structural heterogeneity, and act as effective barriers for twin thickening. The large-sized (Ti, Nb)C carbides facilitate grain refinement and pin boundary migration, while the smaller Cu-rich intermetallic inhibits the growth and thickening of nano-twins, preventing further dislocation movement due to their strong stress fields at the twin-precipitate interactions. This novel mechanism paves the way for developing higher-performance steels with fine and dense nano-twins at cryogenic conditions.