Effects of Co on Mechanical Properties and Precipitates in a Novel Secondary-Hardening Steel with Duplex Strengthening of MC and β-NiAl.

Synergistic strengthening of nano-scaled MC and β-NiAl has become a new route to develop ultra-high secondary-hardening steel. At present, the effect of Co on the synergistic precipitation behavior of duplex phases of MC and β-NiAl has been rarely reported. This paper revealed the effects of Co on the mechanical properties and duplex precipitates of MC and β-NiAl in a novel 2.5 GPa ultra-high strength secondary-hardening steel. The tensile tests indicated that a 10% Co-alloy steel achieved a much stronger secondary-hardening effects compared to a Co-free steel during aging process, especially in the early-aging state. Needle-shaped MC and spherical β-NiAl particles were observed in both Co-alloy and Co-free steels. However, the number density, and volume fraction of MC were significantly enhanced in the 10% Co-alloy steel. The Mo contents in MC carbide and α-Fe after aging treatment were both analyzed through experimental determination and thermodynamic calculation, and the results indicated that Co decreased the solubility of Mo in α-Fe, thus promoting the precipitation of Mo-rich carbides.