Study On Oxide Strengthened Dispersion Alloys For Generation Ⅳ Advanced Nuclear Systems

Nowadays one of the principal problems confronting mankind is the continuing strong growth in energy demand throughout the world, which must be reconciled with environmental and climate change concerns. Therefore nuclear fission energy is likely to continue playing an important role as part of the low-carbon energy mix, with recent shifts in national policies opening the way for high efficiency and lifetime extension of current plants. Supercritical water reactor (SCWR) has been considered as one of the candidates of Generation IV fission advanced nuclear systems due to its high economy, high thermal efficiency, and simple reactor design.The development of core structure materials is one of the key issues for the future application of SCWR, which will be operated in extremely harsh environments, including high neutron fluxes, large time-varying stresses, and aggressive corrosive surroundings. Oxide dispersion strengthened (ODS) ferritic steels have been considered as one of the most promising candidate materials for the SCWR fuel cladding owing to their excellent irradiation damage resistance, high temperature mechanical properties, and relatively good potential of corrosion resistence. These advantages are derived from their extraordinary micro structure that contains a high density of nano-sized oxide particles, which can normally act as not only the pinning points for effectively reducing both the dislocation mobility and the grain boundary velocity, but also stable sinks for trapping irradiation induced defects.It is well known that Cr content in ODS ferritic steel plays a very important role, the applications of ODS ferritic steels with low Cr content for the SCWR are limited owing to the insufficient corrosion resistance. However, too high Cr content can usually cause susceptibility to thermal-aging embrittlement of materials due to the formation of Cr-enriched phase segregations. Therefore, to obtain a type of ODS ferritic steel with adequate Cr content that can be quite qualified for the fuel cladding of the SCWR is of importance. In this study, four types of ODS ferritic steel samples with the Cr contents ranging from 12% to 18% (in wt.%) were fabricated by mechanical alloying (MA) and hot isostatic pressing (HIP). Subsequent hot working and heat treatment were conducted on the samples in order to optimize the mechanical properties and to reduce the residual stress resulted from the hot working, respectively. Microstructural characterization was performed on the ODS ferritic steels with different Cr contents by the means of optical microscope (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was revealed that the grain structures of different samples were typical of a bimodal character in size-level. A large amount of homogeneously distributed ultrafine oxide particles with a size ranging from several nanometer up to 50 nanometer were also observed in all these samples. Mechanical properties including Vickers micro-hardness, tensile properties, and Charpy impact toughness were also performed. The results indicated that the effect of Cr content on the mechanical properties of the ODS ferritic steels was not very significant. In addition, many other service properties with respect of the corrosion resistance to SCW environment at 600℃/25 MPa, thermal-aging stability after long-term being aged at 700 ℃, and irradiation resistance under dual-ion beams (1 MeV Kr+ and 15 keV He+) at 500℃with the maximum dose up to 8 dpa were also systemically investigated. On the basis of the experimental results, the sample with the Cr content of 16% exhibited a superior comprehensive service property as compared with the other samples.In addition to the fabrications of the ODS ferritic steels with different Cr contents, a new type Hastelloy XR-based ODS alloy applied for cladding of SCWR as well as very high temperature reactors (VHTR) was also fabricated by mechanical alloying (MA) and hot extrusion (HE). Compared with the conventional Hastelloy XR alloy, the yield strength (YS) and the ductility of the Hastelloy XR-based ODS alloy sample were significantly increased and decreased, respectively. Microstructural characteristion of the Hastelloy XR-based ODS alloy were observed and investigated using high-resolution transmission electron microscopy (HRTEM) with X-ray energy dispersive spectroscopy (EDS) and selected area electronic diffraction (SAD). It can be inferred that the loss of ductility of the XR-based ODS alloy was mainly attributed to the large-sized secondary phase precipitates extensively decorated along the grain boundaries, conterminous compounds and amorphous silicon oxide on the basis of the experimental analysis results.