Nanoscale interface engineering in a high Cr-Co martensitic steel

The pursuit of strong and ductile steels is an outstanding goal in almost all industries. We show here a high Cr-Co steel that can acquire an ultimate tensile strength (UTS) > 1.6 gigapascals with a uniform elongation ≥27% via simple thermal-mechanical treatments. A sequence of austenitizing, quenching, cryogenic-treating, and tempering steps is involved, leading to a hierarchical microstructure that consists of martensite and dispersed retained austenite (RA). The microstructures are further refined by nanoscale interfaces such as stacking faults (SFs), twin boundaries (TBs) and phase boundaries, giving rise to pronounced strengthening effects. Sustained strain hardening can be attributed to coordinated motion of dislocations in martensite and twinning/transformation induced plasticity (TWIP/TRIP) of RA and even deformation-induced amorphization throughout tensile period. The volume fraction, morphologies as well as distributions of both austenite and martensite are very delicate regarding the austenitizing temperatures, demonstrating the potential of further optimizing strength–ductility properties. This approach is technically straightforward and can be readily scaled up to industrial range.