Microstructural Evolution of INCONEL X-750 under Additive Manufacturing, Thermal Processing, and Irradiation Environments: A Comprehensive Review

Abstract INCONEL® X‑750 (UNS N07750) is a precipitation‑hardenable Ni–Cr superalloy strengthened primarily by γ′ (Ni₃[Al,Ti]) and, depending on thermal history, by secondary phases such as η‑Ni₃Ti and γ″/carbides. Its long‑standing roles in aerospace hot‑section hardware and nuclear reactor internals rely on a careful balance between high‑temperature strength/relaxation resistance, oxidation/corrosion tolerance, and SCC mitigation through microstructural control. Conventional wisdom—codified in AMS and vendor heat‑treat schedules—was developed for wrought materials; however, the recent adoption of laser powder bed fusion (PBF‑LB) and renewed scrutiny under irradiation challenge many assumptions. This review synthesizes 35+ sources to connect three domains often studied separately: (i) AM‑induced microsegregation and defect structures, (ii) irradiation‑driven γ′ disordering/dissolution and grain‑boundary phenomena, and (iii) advanced heat‑treatment pathways (solution, stabilization, aging, homogenization) from AMS and post‑AM strategies. We show how AM‑specific precipitate pathways (including anomalous γ′ formation during “solution” and η promotion) interact with irradiation temperature/dose windows to determine γ′ stability, hardness, and SCC susceptibility; we map design‑relevant process–structure–property linkages and propose tailored post‑AM thermal schedules and PWHT guidelines for welded and AM parts destined for high‑temperature or LWR environments. Keywords: INCONEL X‑750; UNS N07750; γ′; η‑Ni₃Ti; additive manufacturing; PBF‑LB; irradiation; SCC; AMS 5667/5668/5670; hydrogen water chemistry (HWC)