Abstract: Nanoparticle precipitation plays a pivotal role in determining the properties of structural materials, and understanding the mechanisms governing their formation and stability has been a central focus in materials science. In the context of advanced Fe-based alloys, key questions remain: What drives nanoparticle precipitation? How are core-shell structures formed? And what mechanisms underlie cooperative hardening? By introducing Al into Fe-Cu-Ni alloys, we demonstrate that Ni and Al co-cluster with Cu, forming BCC Cu(Ni,Al)-rich clusters that evolve into the thermally stable B2 phase, even after 100 h of annealing. Prolonged annealing leads to Cu diffusion into the precipitate core, resulting in a core-shell structure—a Cu-rich core surrounded by a B2-Ni(Al) shell—which correlates with a reduction in hardness. In contrast, FeCuNi alloys exhibit rapid precipitate nucleation due to the high diffusivity of Cu and Ni in the Fe matrix, but their core-shell structures show limited thermal and mechanical stability. The Cu(Ni,Al)-B2 precipitates, with their enhanced phase stability, achieve a higher number density and superior performance compared to FeCuNi alloys. Importantly, our findings reveal a strong correlation between the morphological evolution of precipitates with annealing time and their mechanical behavior, providing new insights into the design of high-performance structural materials.

Key words: Ferritic steel, Nucleation, Phase transformation, Transmission electron microscopy, Atom probe tomography