Strain Hardening Mediated by Coherent Nanoprecipitates in Ultrahigh-Strength Steels

Abstract

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 of different defects at the nano-scale. We show that fast precipitation of low-misfit B2-ordered Ni(Al, Fe) could efficiently prevent dense quench-in dislocations from recovery. During tensile deformation, the retained free dislocations under the strong restriction of the ordered nanoprecipitates not only act as numerous dislocation sources for planar slips, but also recover local cutting stress right after cutting of the precipitates owing to formation of immobile dislocation substructures. 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 refinement of the fine bands as the main strain hardening mechanism, which gave rise to the simultaneous increment of the yield strength (2 GPa) and ductility (9%). The current findings provide a possible means of simultaneously enhancing strength and ductility through tailoring the interplay among different types of lattice defects.