  Understanding alloy phase transformations during continuous cooling is
important for post-processing design and optimization. In this work,
continuous-cooling-transformation (CCT) diagrams of Inconel 718 alloys
manufactured by laser powder bed fusion (LPBF) and suction casting are
developed under different homogenization conditions. Unlike the available CCT
diagrams in the reported studies, no gamma double prime and gamma prime
precipitates can be observed. NbC and delta are determined to be the
precipitates after cooling from the gamma matrix. Importantly, homogenization
time and manufacturing methods are found to affect the Nb homogeneity in the
matrix near NbC particles and thus significantly influence the precipitation
process of the delta phase, which has a high content in Nb. In the alloys with
high Nb homogeneity, the nucleation process mainly contributes to the
precipitation, whereas in the alloys with low Nb homogeneity, the precipitation
is primarily associated with the growth process. Subgrains are found to form
after cooling at 0.1 K/s and can cause the highest hardness in samples. This
work provides a new viewpoint on the study of processing-structure-property
relationships during cooling in Inconel 718 and is beneficial to the
development of alloy post-processing strategies.
