Synergistic alloying effects on nanoscale precipitation and mechanical properties of ultrahigh-strength steels strengthened by Ni₃Ti, Mo-enriched, and Cr-rich co-precipitates

The synergistic effects of Mo, Ti, and Cr on nanoscale precipitation and mechanical properties of maraging stainless steels were systematically studied using high-resolution scanning transmission electron microscopy, atom probe tomography (APT), thermodynamic and first-principles calculations, and mechanical tests. Our results reveal a notable precipitation pathway involving the co-precipitation of Ni₃Ti, Mo-enriched, and Cr-rich precipitates; their formations are not separated, but rather highly interacted. The APT results indicate that Mo partitions to the Ni₃Ti precipitate core in the early stage of precipitation, which doubles the number density of Ni₃Ti precipitates. Our calculations indicate that the Mo partitioning not only increases the chemical driving force, but also reduces the strain energy for nucleation, thereby accelerating Ni₃Ti precipitation. As the precipitation proceeds, Mo atoms are rejected from the Ni₃Ti precipitate core to the interface between the Ni₃Ti precipitates and matrix, which leads to the heterogeneous nucleation of Mo-enriched precipitates on the outer surface of the Ni₃Ti precipitates. This results in a substantial size refinement of Mo-enriched precipitates. In addition, the formation of Ni₃Ti precipitates consumes Ni from the matrix, which substantially inhibits the spinodal decomposition and refines the size of Cr-rich precipitates. The cooperative strengthening of Ni₃Ti, Mo-enriched, and Cr-rich co-precipitates leads to the development of new steels with a strength of 1.8 GPa; the contributions of these precipitates to the strengthening were quantitatively evaluated in terms of precipitate shearing and Orowan dislocation looping mechanisms.