A critical review of consolidation processes for ODS steels: Development as clad tube materials in Gen-IV nuclear reactors

Oxide Dispersion Strengthened (ODS) steels have been developed over the past decades as structural materials for fission reactors. In recent years, significant interest has grown in their use as cladding tubes in next-generation Generation-IV (Gen-IV) nuclear reactors. Cladding tubes serve to separate reactor fuel from the coolant and are subjected to extremely high temperatures, radiation doses, and biaxial stresses. As such, they must exhibit excellent high-temperature strength, radiation resistance, and creep resistance. To meet these demanding requirements, two primary grades, ferritic-martensitic (F/M) and ferritic ODS steels, are being extensively studied and developed for use as clad tube materials in Gen-IV nuclear reactors. ODS steels are mainly prepared through powder metallurgy (PM) routes, in which elemental powders or pre-alloyed powders are mechanically alloyed with nanosized yttria powders in a high-energy attritor or ball mill. The mechanically alloyed powders are consolidated through different routes, the main ones being hot extrusion (HE), hot isostatic pressing (HIP), and spark plasma sintering (SPS), with hot powder forging (HPF) being recently developed as an alternative consolidation route. The limitations associated with these conventional routes and opportunities with alternate consolidation routes are adequately discussed. The influence of various potential alloying elements on the microstructure and mechanical properties of ODS steels is widely reviewed here. With careful selection of compositions and processing routes, alloys with up to 1.5 wt.% yttria can be fabricated. Higher yttria contents lead to ODS steels with improved elevated temperature strength and creep resistance. Anisotropy in ODS steels with high Cr contents (>11 wt.%) is a concern, but it can be overcome by the application of the newly developed powder forging route; furthermore, ODS steels consolidated through the HPF route develop high density of the product. Finally, this review identifies potential candidate cladding materials for the upcoming Gen-IV nuclear reactors.