Characterization of Recrystallization and Second-Phase Particles in Solution-Treated Additively Manufactured Alloy 718

Recrystallization and second-phase particles were characterized in solution-treated additively manufactured (AM) alloy 718 to understand its unusual recrystallization behaviors with respect to alloy 718 manufactured by wrought process. Three types of solution treatment temperatures, 1255 K (subsolvus: SUB), 1333 K (supersolvus: SUP), and 1473 K (ultra supersolvus: USS) with various holding times were tested. The AM alloy showed much slower recrystallization kinetics in the SUB and SUP conditions and retarded grain growth in the USS condition with respect to the wrought alloy. Microchemical and microstructural analyses have revealed the existence of a few 10-nm-sized oxygen-containing particles, enriched in C, N, Ti, Nb, Mo, and O in the extreme case, enriched in Al and O in the opposite extreme case and in between them in many cases. Coarse carbide phases and delta-Ni3Nb phase precipitates were also observed in the tested conditions. The Al- and O-rich particles were found to show the highest stability against particle coarsening during the solution treatments among all the types of particles observed. The slow recrystallization kinetics in the AM alloy was assumed to be due to the existence of the oxygen-containing fine particles which may retard strain-induced boundary migration (SIBM) which is likely the main recrystallization mechanism in the AM alloy 718.