Effect of aging time on precipitates and mechanical properties of the electron beam freeform fabricated Ni-Fe-Cr alloys reinforced by γ’ and TiC phases

Utilizing a synergistic multi-phase and multi-scale strengthening strategy, a novel Ni-Fe-Cr alloy reinforced by nano-scale Ni₃(Al, Ti)-γ’ and micron-scale TiC phases was designed and prepared by electron beam freeform fabrication with powder-cored wire. In the as-built sample, the long-strip-shaped TiC phases precipitated along the interdendritic regions with an area fraction of 6%, while the precipitation of the γ’ phase was inhibited. Tensile performance exhibited certain anisotropy, with elongation being higher in the vertical direction than in the horizontal direction. To tailor the strengthening phases and enhance mechanical properties, the as-built alloys were subjected to solution and aging treatments with different aging times. After heat treatment, the γ’ phases were adequately precipitated, with their size increasing to over 100 nm, enhancing their strengthening effect. A portion of TiC phases at the grain boundaries were transformed into Cr₂₃C₆ carbides through the TiC+γ → Cr₂₃C₆ + γ' reaction. Fracture mechanism analysis revealed that the Cr₂₃C₆ carbides with a continuous distribution at the grain boundaries significantly compromised the toughness of the alloys. Consequently, after heat treatment, the yield strength of the alloys significantly increased in both the horizontal and vertical directions. Elongation initially increased and then decreased in the horizontal direction, while it continued to decrease in the vertical direction, and the anisotropy in performance was still maintained.