Inverse temperature-dependent toughness and exceptional cryogenic damage tolerance in a plain bcc steel

Steels with the body-centered cubic ( bcc ) structure suffer low-temperature brittleness due to an inherent ductile-to-brittle transition that inhibits plastic deformation. Strategies to improve the cryogenic toughness generally involve stabilizing a face-centered cubic ( fcc ) phase to prevent this transition; however, this involves alloying with high concentrations of nickel, cobalt, and chromium, which are expensive and unsustainable due to their high environmental impact, energy-intensive extraction processes, and limited global reserves. Here, we engineered a low-carbon, micro-alloyed steel to possess a dual-phase, ultrafine-grained ferrite/martensite lamellar microstructure. This structure confers an unusual inverse-temperature dependence of impact toughness across a broad temperature range (383 K to 77 K) and exceptional resistance to fracture under both impact and quasi-static loading conditions at cryogenic temperatures (77 K). These properties are achieved through a combination of extrinsic toughening from delamination and crack bridging, as well as intrinsic toughening by interface dislocation-mediated plastic deformation within ferrite and activation of multiscale substructure sliding in martensite. This microstructural design strategy offers a pathway to engineer plain bcc steels with exceptional cryogenic damage tolerance without the addition of expensive and critical elements.