Superior strength and ductility in a low density duplex steel studied by in situ neutron diffraction

A low density, high strength medium-Mn steel was processed to produce a bimodal duplex microstructure consisting of coarse grained γ-austenite with fine δ-ferrite grains decorating the γ-austenite boundaries. Using a combination of ex situ analysis and in situ neutron diffraction, the deformation mechanisms and lattice strains within each phase were identified for specimens undergoing uniaxial tension during room and elevated temperature loading up to 473 K. The coarse-grained high stacking fault energy γ-austenite deformed by dislocation glide, providing work hardening and ductility. Simultaneously, the fine grained δ-ferrite produced an elevated yield strength by strengthening the steel via a composite reinforcing mechanism. Neutron diffraction reveals that the yield strength reduction at elevated temperatures is due to a reduction in the δ-ferrite strength. The resulting combination 1200 MPa ultimate strength and 0.3 ductility achieved in this microstructurally engineered bimodal duplex steel exceeds that of typical hot worked medium-Mn steels.