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

Abstract

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.