Abstract: One of the critical issues in the additive manufacturing of Hastelloy-X (HX) superalloy is solidification cracking, which imposes higher demands on alloy composition design. While rare-earth yttrium (Y) has been recognized for improving the service performance of superalloys, its effect on cracking susceptibility remains controversial. In this study, HX superalloy with and without Y modification was fabricated using directed energy deposition (DED). Advanced characterization techniques were employed to analyze the microstructural difference, specifically the precipitation and segregation behavior at the crack-associated grain boundaries (GB) were quantified using atom probe tomography (APT). The results reveal that the introduced Y predominantly exists as Y₂O₃ or segregates at the GB during the DED process. According to the results of density functional theory (DFT) calculations, the segregated Y atoms tend to occupy the sites that would otherwise be preferentially occupied by Mo atoms, leading to a higher concentration of free Mo atoms at the GB. This redistribution modifies the local thermodynamic environment, thereby promoting additional solidification cracking. These findings provide new theoretical insights into the role of Y in HX superalloys and offer a valuable basis for designing crack-resistant, rare-earth-modified superalloys tailored for additive manufacturing applications.

Key words: Solidification cracking, Rare earth, Atom probe tomography, Hastelloy-X superalloy, Directed energy deposition