On the origin of thermal dependence of 3D printed Inconel 718: Roles of atom clustering

Despite the high interest in fabricating Inconel 718 by laser powder bed fusion (L-PBF) and the alloy's wide applicability at high temperatures in aerospace and energy applications, there is limited information on its thermal dependence. In this work, we studied the thermal dependence of Inconel 718 fabricated by L-PBF over a temperature range of 25 to 650 °C, and strain rates of 10 and 10 s. Serrated flow stress indicative of dynamic strain ageing (DSA) was observed at 250 and 450 °C at both strain rates. While previous studies reported that DSA was associated with the interactions of C and Nb with dislocations, direct examinations of solute cluster evolution using atom probe tomography (APT) in this study show strong evidence of Nb and Ti clusters at 250 °C, suggesting that both Nb and Ti clusters are responsible for the stress serrations at this temperature. At 450 °C, APT examinations revealed only weak evidence of Nb clusters but strong presence of Ti clusters, indicating that Ti might be most likely responsible for the DSA response at 450 °C. In addition, the yield strength decreased with temperature until 450 °C due to thermally-assisted softening, but the strength was regained with further temperature increase to 650 °C. APT and transmission electron microscopy (TEM) examinations reveal that the strengthening at 650 °C was imparted by nanoscale solute clustering (short-range ordering) of Al, Ti and Nb atoms, which had not developed into mature γ' and γ" precipitates. The insights revealed in this study provide a new perspective of the mechanisms underlying the thermal dependence of Inconel 718.