Strain hardening mediated by coherent nanoprecipitates in ultrahigh-strength steels

Strengthening from single lattice defect such as dislocations or nanoprecipitates generally leads to the so-called strength-ductility tradeoff, which becomes particularly pronounced at the strength level of above 2 GPa. Herein, we report a sustainable strain-hardening mechanism in ultrahigh-strength martensitic steels via manipulating interaction among different lattice defects. We show that fast precipitation of low-misfit B2-ordered Ni(Al, Fe) could efficiently prevent dense quench-in dislocations from recovery. During plastic deformation, the high cutting stress created by the ordered nanoprecipitates not only allows numerous retained dislocations to become mobile in planar mode, but also substantially expands the mean free path for dislocation movement in a heavily dislocated martensite. Simultaneously, the planar slips cause severe dislocation reactions with the pre-existing dislocations, which timely recover local cutting stress that has been weakened by cutting of the precipitates. This sort of timely established cutting stress minimizes simultaneously degree of slip concentration and magnitude of stored co-planar dislocations within planar slip bands while promoting pronounced band refinement as the main strain hardening mechanism, which gave rise to the simultaneous increment of the yield strength (2 GPa) and elongation to failure (9%). The current findings provide a possible means of simultaneously enhancing strength and ductility through tailoring the interplay among different types of lattice defects.