Design of a 2.7 GPa ultra-high-strength high Co–Ni secondary hardening steel by two-step nano-size precipitation tailoring

High Co–Ni secondary hardening steels are valuable in the field of aviation materials. With the increasing demand of lightweight, further enhancing its strength has received widespread attention. In this study, instead of using only M2C carbides as the reinforcements in the traditional high Co–Ni secondary hardening steels, nano-size NiAl phases were introduced to provide additional strength. To obtain the collaborative precipitation of these two reinforcements, systematically thermodynamic calculation, including the driving force, coarsening rate, etc., was made to quantitatively tailor the composition and the two-step aging parameters. The calculation results showed that adding 1 % Al into the standard composition of M54 is a plan with high potential and a second aging process at 450 °C should be added to promote NiAl precipitation. Based on the experimental validation, the target microstructure formed as the design. With the dual-phase strengthening of NiAl particles and M2C carbides, the yield and tensile strength are greater than 2.3 and 2.7 GPa, respectively, which increases the strength of M54 steel by >30 %, retaining sufficient elongation and relatively low cost. This new designed alloy increased the strength–ductility balance of high Co–Ni secondary hardening steels and aviation maraging steels from ∼12.5 to 13.5 GPa%.