In-situ suppression of Laves phases in additively manufactured Inconel 718 via Co alloying: first-principles calculations, influencing mechanisms, and performance

Abstract

The chain-like Laves phase that ubiquitously precipitates in the additively manufactured nickel-based superalloys is well-documented to degrade the mechanical performance of the as-built parts. However, methods that suppress the Laves phase during additive manufacturing remain to be investigated. Here, in this study, a novel method was proposed to in-situ suppress the Nb segregation and Laves phase formation in the additively manufactured Inconel 718 via Co alloying. Thin-walled parts made of Inconel 718 and 5 wt% Co alloyed Inconel 718 were fabricated, and their microstructure, hardness as well as tensile properties were investigated. First-principles calculations demonstrated that the added Co contributed to the solid solution of Nb in the Inconel 718 γ matrix, leading to the increased Nb content in the matrix of 5 wt% Co alloyed Inconel 718 part, which brought two key benefits. On one hand, the increased Nb content in the matrix directly led to the Nb content reduction in the interdendritic liquid phase, thereby shortening the growing period of the Laves phase. On the other hand, the increased Nb content in the matrix promoted the formation of the precipitation-hardening phase γ'' (Ni₃Nb), while also contributing to the nucleation and grain refinement of the γ phase. As a result, the ultimate tensile strength, yield strength, and ductility of the 5 wt% Co alloyed Inconel 718 part increased by 11.7 %, 12.7 %, and 41.4 %, respectively. The smaller, more dispersed Laves phases with lower area fraction are considered the main reason for the improvement in material properties. By elucidating how Co alloying influences the Nb–matrix interactions in the γ phase of Inconel 718, this work provides fundamental insights and atomic-level guidance for mitigating Nb segregation and Laves phase formation in the additively manufactured nickel-based superalloys.