Precipitation mechanism and mechanical properties of an ultra-high strength steel hardened by nanoscale NiAl and Cu particles

Abstract 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 → Cu + NiAl". The modulation of the precipitation sequence is attributed not only to the relatively high Ni/Cu and Al/Cu ratios but also the synergistic combination of Ni, Al, Mn and Cu additions in the steel. In addition, APT also reveals the precipitation of a small amount of nanoscale Fe₃(Mo, W)₃C- and NbC-type carbides. The combination of the strengthening effects from the nanoscale NiAl particles, Cu particles and carbides contributes significantly to the overall ultra-high strength of the steel.