Precipitate evolution and strengthening behavior during aging process in a 2.5 GPa grade maraging steel

Development of precipitation strengthening steels with ultrahigh strength and high ductility requires thorough understanding of nanoscale precipitation mechanisms. In this study, atom probe tomography (APT), HRTEM and first-principles calculations were used to reveal an interesting co-precipitation mechanism of Ni3Ti and Mo-rich nanoparticles in a 2.5 GPa grade maraging steel. The Ni-Ti rich clusters preferentially nucleate from the supersaturated solid solution and grow into Ni3Ti with extension of aging time, meanwhile the rejection of Mo atoms leads to heterogeneous precipitation of Mo-rich nanoparticles adjacent to the Ni3Ti particles and finally forms a core-shell structure along with Ni3Ti phase. Calculations of interaction energy between alloying elements in different aging process exhibit that the preferential formation of Ni-Ti rich cluster is due to the low interaction energy between Ni and Ti atoms, however, the Ni-Ti cluster is only a transitional phase, and when stable Ni3Ti is formed, Mo atoms are rejected from Ni3Ti to form a core-shell structure along with Ni3Ti precipitates. Finally, four modified theoretical prediction models are introduced to describe the yield strength as a function of microstructure and precipitates characteristics of the experimental steel. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.