Abstract: Structural components usually require inexpensive materials with ultrahigh strength, large ductility, and low density, which are still lacking and being searched. We hereby develop a partially recrystallized (Rexed) low-density steel having unprecedentedly ultrahigh specific yield strength and large ductility at the same time. The former results from both dislocation hardening in unRexed grains and the significant precipitation hardening of κ-carbides having the sizes of several, tens of and hundreds of nanometers in the Rexed, unRexed grains and at boundaries, respectively. Particularly, the κ-carbides within Rexed grains can grow toward the critical sizes for the maximum cutting-through precipitation hardening during the optimal ageing process, which can be guided by ab initio calculations; consequently, the strength difference between the Rexed and unRexed grains is minimized so that they can deform more compatibly. In addition, the presence of ductile precipitation-free zones close to the discrete intergranular κ-carbides reduces the tendency to the brittle intergranular cracking. A nearly half Rexed microstructure may provide the possibly largest quantity of Rexed/unRexed grain boundaries for the maximum hetero-deformation-induced strain-hardening, contributing to large ductility. Therefore, we propose that such a hierarchical precipitation in a nearly half Rexed matrix may be a new microstructural strategy for manufacturing ultrastrong lightweight alloys.

Key words: Low-density steel, Mechanical property, κ-carbide, Ab initio calculation, Hetero-deformation-induced strain-hardening