Structural Materials For Nuclear Applications Research Articles

Effect of local chemical order on the irradiation-induced defect evolution in CrCoNi medium-entropy alloy

High- (and medium-) entropy alloys have emerged as potentially suitable structural materials for nuclear applications, particularly as they appear to show promising irradiation resistance. Recent studies have provided evidence of the presence of local chemical order (LCO) as a salient feature of these complex concentrated solid-solution alloys. However, the influence of such LCO on their irradiation response has remained uncertain thus far. In this work, we combine ion irradiation experiments with large-scale atomistic simulations to reveal that the presence of chemical short-range order, developed as an early stage of LCO, slows down the formation and evolution of point defects in the equiatomic medium-entropy alloy CrCoNi during irradiation. In particular, the irradiation-induced vacancies and interstitials exhibit a smaller difference in their mobility, arising from a stronger effect of LCO in localizing interstitial diffusion. This effect promotes their recombination as the LCO serves to tune the migration energy barriers of these point defects, thereby delaying the initiation of damage. These findings imply that local chemical ordering may provide a variable in the design space to enhance the resistance of multi-principal element alloys to irradiation damage.

HELIUM AND HYDROGEN EFFECTS IN STRUCTURAL MATERIALS FOR NUCLEAR APPLICATIONS

Displacement cascades produce a variety of defects under reactor conditions, but of particular concern is the simultaneous production of helium (He) and hydrogen (H), which enhances the degradation of structural materials. The overall majority of performed studies on helium and hydrogen interactions with materials were based on ion beam irradiation, which served as a convenient tool for the simulation of neutrons exposure over a variety of temperature and dose regimes due to the ability to widely vary and control the irradiation parameters. Experimental investigations of the hydrogen-defect interaction performed by thermal desorption spectroscopy, and the parameters of this interaction obtained by numerical simulations based on diffusion-trapping codes are debated. In this review, we also summarize previous studies on grain boundaries and nanoprecipitate effects on hydrogen transport in metals, as well as the role of hydrogen in the corrosion and cracking of steels. We discuss here issues of helium bubbles formation and some of the evidence for the synergistic effects of hydrogen and helium in the presence of displacement damage, and their influence on irradiation hardening and swelling. Particular attention was devoted to the features of hydrogen interaction with noble-gas bubbles, which were considered on the basis of most recent published data.

Effects of local chemical ordering on defect evolution in NiFe concentrated solid solution alloy

Single-phase concentrated solid solution alloys (SP-CSAs), including high-entropy alloys (HEAs), are promising candidates for structural materials in nuclear reactors. Recent studies have demonstrated that local chemical ordering (LCO) is an important structural feature in these alloys, which can significantly affect defect evolution. The presence of LCO leads to elemental rearrangement in CSAs and results in distinct interfaces separating the ordered and disordered phases. In this work, we study the role of LCO and order/disorder interface on defect evolution in NiFe CSAs. Our results indicate that the interface can act as defect sinks in the defect recombination stage. At the diffusion stage, defect evolution is strongly affected by LCO due to chemically biased diffusion in CSAs and preferential diffusion pathways of defect clusters. We further show that the degree of LCO and the dimensions of the LCO domains play significant roles in defect clustering and annihilation behavior. These results provide fundamental insight into the influence of LCO on defect evolution in CSAs and pave ways for designing highly irradiation-tolerant structural materials for nuclear applications.

Effect of ordered helium bubbles on deformation and fracture behavior of α-Zr

Radiation-induced helium bubbles are detrimental to the mechanical properties of metals, usually causing severe hardening and embrittlement. Hexagonal close-packed (HCP) α-Zr alloys are one of the primary structural materials for nuclear applications, however, the effect of helium bubbles on their deformation and fracture behaviors still remains unexplored. Here, we found that ordered helium bubbles prefer to align along the basal plane in HCP α-Zr. Micro-scale in situ tensile tests revealed that helium bubbles less than 8 nm in size can increase the critical resolved shear stress of the prismatic slip. However, once the helium bubbles are larger than 8 nm, a bubble-softening effect happens due to a decrease in number density of helium bubbles and an increase in porosity. Once the Schmid factor of basal slip is considerably higher than prismatic slip, bubble coalescence along the basal plane becomes the major failure mode in helium-irradiated α-Zr.

European standard on small punch testing of metallic materials

In the 1980s, studying the effect of neutron irradiation and temper embrittlement on structural materials for the fusion and fission programmes was a major challenge. In this context the development of small specimen test techniques began, allowing the characterization of structural materials for nuclear applications with small amounts of material. The small punch technique is of one these small specimen test approaches. It is widely used for the development and monitoring of structural materials, however there is currently no comprehensive international standard for small punch testing. An EN standard on small punch testing is currently being developed under the auspices of ECISS/TC101/WG1. Besides describing the apparatus, procedures, and specimens, it will include recommendations for the estimation of tensile, fracture and creep properties from small punch testing as well as machine readable formats for representing and transferring test data. This paper describes the current status of the standard and highlights some of the changes with regard to the current CWA 15672 (2007).

Comparison of Interfacial Strengthening in Creep Deformation and Radiation Damage Processes of Advanced Structural Materials for Nuclear Applications

The mechanisms for microstructural strengthening in creep deformation and radiation damage processes of advanced structural materials for nuclear applications are compared. During creep and irradiation, various defects are generated and move in the microstructure. Any microstructural features that can retard such defect movement may improve both creep and radiation damage resistance. Interfaces in the microstructure are important barriers for preventing defect motion. To achieve ultrahigh strength and enhanced radiation damage resistance, an extremely high density of interfaces has been designed in recently developed nanostructured materials. However, interface-mediated processes may govern the deformation of these materials, decreasing their creep properties. Methods for improving the creep resistance of nanostructured materials are reviewed and discussed.

The critical issues of SiC materials for future nuclear systems

SiC ceramic and SiC-based composites are promising structural materials for nuclear applications. This paper gives a viewpoint of the basic characterization and research-highlights of SiC-based composites with respect to applications in future nuclear systems. In particular, the following aspects are considered: 1) SiC fiber, 2) interphase, 3) SiC matrix, and 4) SiC joining. Furthermore, remaining challenges and specific technical issues of SiC-based composites for nuclear applications are discussed, and some suggestions to overcome these are proposed.