Influence of Al and Ti Content on γ′ Phase Precipitation and Cracking Sensitivity in NiCrFeAl_xTi_y Alloys Fabricated by Electron Beam Freeform Fabrication

Nickel-based superalloys with high γ′ content typically exhibit elevated cracking susceptibility during additive manufacturing. In this study, four Ni–Cr–Fe alloys with varying Al and Ti contents were fabricated by electron beam freeform fabrication (EBF³) using nickel-based powder-cored wire. The effects of Al and Ti contents on γ′ phase precipitation behavior and cracking sensitivity were systematically investigated, and the relationship between these two factors was established. Microstructural analyses revealed that the Al and Ti contents significantly influenced the γ′ precipitation temperature, which in turn affected its morphology, size, distribution, and volume fraction. As Al and Ti contents increased, both the size and volume fraction of γ′ precipitates increased. When the combined Al and Ti mass fraction reached 5 wt.%, the γ′ precipitate morphology transitioned from spherical to cubic. Crack characterization further showed that the observed cracks exhibited solidification-associated intergranular features, while local crack initiation was closely related to the extensive precipitation of γ′ phases in interdendritic regions, where the increased γ′/γ lattice misfit and γ′ volume fraction promoted transformation-induced shrinkage stress and local stress concentration during repeated thermal cycling. Once initiated, cracks preferentially propagated along high-angle grain boundaries under the combined effect of thermal residual stress and the solidification microstructure. These findings provide critical insights for the composition design of printable, higher γ′-content Ni-based superalloys tailored for additive manufacturing.