Improving creep property of additively manufactured Inconel 718 through specifically-designed post heat treatments

The application of Inconel 718 (IN718) alloy parts at elevated-temperature under loading leads to creep failure. To date, the creep performance of additively manufactured IN718 is usually inferior to the conventionally fabricated counterparts. In this work, post heat treatment schemes were designed and applied to the LPBF-built IN718 to achieve enhanced creep performance. The results have revealed an effective heat treatment approach that can significantly improve the creep performance of LPBF IN718 to be comparable to conventional counterparts. Systematic microstructure investigations, as well as theoretical calculations, indicated that the improved creep properties originated from significant modifications on grain structures, Laves phase and secondary precipitates. Specifically, the change from columnar to equiaxed grain structure through post heat treatment reduced the grain boundary areas that were susceptible to cavitation. Meanwhile, the effective dissolution of grain boundary Laves phase reduced the number of sites for cavity nucleation and improved the creep lifetime. Moreover, the dissolution of Nb into the matrix promoted the formation of the main strengthening γ'' precipitates and thus enhanced the creep resistance. The findings in the current work provide the heat treatment strategy for the design of additively manufactured superalloys towards better creep performance.