Strategies for strengthening-ductility and hierarchical co-precipitation in multicomponent nano-precipitated steels by Cu partitioning

We investigated the dependence of both the strength and ductility in Cu-bearing nano-precipitated steels on compositions, tempering and aging, where Fe-2Cu (wt%, Cu-steel) steel by Cu-rich precipitates (CRPs), Fe-2Cu-5Ni-3Mn-1.5Al steel (Cu/Ni steel) by co-precipitates of CRPs + NiAl, and Fe-2Cu-5Ni-3Mn-1.5Al-1Mo (Mo-steel) steel by co-precipitates of CRPs + NiAl + Mo2C precipitates were paid special attentions on. The strength and ductility of Cu-steel were sensitive to tempering temperatures rather than aging time. Tempering and aging in Cu/Ni steels remarkably increased the strength with seriously sacrificing of ductility because larger-sized blocky CRPs segregated along the grain boundaries where Cu partitioning along grain boundaries was partially dominate, resulting in brittle fracture in almost all of aged Cu/Ni steels. Mo-steel, however, showed superior strength and ductility than Cu/Ni steel because Mo addition reduced the possibility of blocky CRPs by decreasing Cu partitioning along grain boundaries, and promoted the formation of hierarchical co-precipitates where large sized coarsening co-precipitates contributed to the ductility and small-sized secondary co-precipitates had significant strengthening, thus resulting in the improvement of strengthening-ductility.