Effects of Ni and Al on microstructural evolution and mechanical properties in secondary hardening steels

To reduce the dependence on costly cobalt while preserving the superior strength-toughness synergy in secondary hardening steels, dual-precipitation-strengthened martensitic steels were developed through Ni and Al addition, which can promote the formation of both M2C carbides and NiAl particles. The fast nucleation of high-density NiAl particles effectively suppressed dislocation recovery during aging. This suppression inhibited the precipitation of coarse cementite and concurrently enhanced the nucleation efficiency of M2C carbides, resulting in their refined size, increased number density and volume fraction. While NiAl particles significantly contributed to strengthening through both dislocation and precipitation strengthening mechanisms, dislocations shearing NiAl precipitates led to particle fragmentation and microcrack initiation, ultimately promoting cleavage fracture and reducing impact toughness. Through the adjustment of NiAl particles, a favorable balance was achieved, with a yield strength of 1868 MPa and impact toughness of 26 J. These findings provide a viable pathway for designing cost-effective and high-performance secondary hardening steels with reduced Co content.