Influences of Cu on microstructure and mechanical properties in Fe–Ni–Al ultra-strong maraging steels

Abstract

Although the influences of Cu on microstructure and mechanical properties have been extensively studied in low-alloy steels, the aging behavior of high-alloy maraging steels with Cu remains elusive. In this work, we systematically investigated the nanoprecipitation behavior and their effects on mechanical properties in Fe–Ni–Al ultra-strong maraging steels by introducing different amounts of Cu. Structural analyses with atom probe tomography (APT), transmission electron microscopy (TEM) and small angle neutron scattering (SANS) indicate that the addition of Cu promotes the precipitation of nanoscale B2–NiAl phases in the Fe–Ni–Al ultra-strong maraging steels. The primarily precipitated Cu-clusters can serve as the nucleation sites for the precipitation of B2–NiAl at the early stage of aging, and then are dissolved into the B2–NiAl nanoprecipitates, which increases the lattice misfit between the B2–NiAl and the matrix and thereby the growth rate of B2–NiAl. As a result, the addition of 0.6 wt% Cu significantly improves the ductility of the Fe–Ni–Al ultra-strong steel without sacrificing strength. With 0.6 wt% Cu added to the Fe–Ni–Al ultra-strong steel, the elongation increased from 3.2% to 8.7% while the strength remained 2.0 GPa, due to the high-density precipitation of Ni(Al, Cu) nanoparticles. The underlying mechanisms for the precipitation behavior and the precipitation strengthening effect were discussed in terms of lattice misfit, Cu segregation and stress field.