Effects of trace-level compositional modifications on the high-temperature creep properties of additively manufactured Inconel 718

Abstract

Additively manufactured (AM) alloys such as laser powder bed fusion (LPBF) Inconel 718 often exhibit inferior high-temperature mechanical performance compared to their conventionally manufactured (CM) counterparts, in part due to impurity embrittlement. While compositional tailoring has been employed in CM alloys to mitigate impurity effects, this strategy remains largely unexplored in AM materials. In this study, a modified LPBF IN718 was developed by adding trace amounts of Zr, Hf, Mg, and Ca, to address impurity-related degradation (such as sulfur embrittlement) and the performance gap between CM and AM Ni-based superalloys. Ambient tensile tests and high-temperature (650 °C, 704 °C) creep tests were conducted to evaluate the modified alloy relative to standard LPBF IN718. While the modified alloy showed a slight reduction in tensile strength, it demonstrated improved ductility at room temperature and enhanced creep strength, ductility, and rupture life at elevated temperatures. Microstructural analysis using SEM, EDS, EBSD, STEM, and NanoSIMS revealed key differences between the alloys. In the standard alloy, sulfur segregated at alumina particles within grains and at grain boundaries (GB). In contrast, alumina was not detected in the modified alloy, which exhibited a higher fraction of carbides, likely influenced by the trace additions and contributed to the improved mechanical performance. Additionally, the formation of sulfo-carbides and sulfides likely stabilized sulfur, reducing its GB segregation and enhancing GB cohesion, thereby improving creep behavior.