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We report on the alloy design strategies, precipitation mechanism and mechanical properties of an ultra-high strength steel hardened by co-precipitation of nanoscale NiAl and Cu particles. The steel, developed through a computational-aided alloy design approach, exhibits a tensile strength of ∼1.9 GPa, an elongation of ∼10% and a reduction in area of ∼40%. Atom probe tomography (APT) reveals an interesting type of co-precipitation mechanism of NiAl and Cu nanoparticles, in which the NiAl particles first come out of the supersaturated solid solution and the rejection of Cu solutes leads to the heterogeneous precipitation of Cu particles adjacent to the NiAl particles. The observed precipitation sequence of “supersaturated solid solution → NiAl → NiAl + Cu” is substantially different from the one previously reported in Cu-strengthened steels, which involves the process of “supersaturated solid solution → Cu → …