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Oxide-dispersion-strengthened (ODS) steel is considered as a promising candidate structural material for nuclear applications. In this study, the microstructure and mechanical properties of Y4Zr3O12-added Fe–13.5Cr–2W ODS steels, containing high contents of C and N, prepared by mechanical alloying (MA) and two-step spark plasma sintering (SPS), were investigated. The results showed that pure Y4Zr3O12 powders, with a grain size of 3.5 nm, were well prepared with NH3·H2O addition by the sol-gel method in advance, in order to avoid the formation of some coarse or undesired oxides. W was completely dissolved into the matrix after 48 h of ball milling at 300 rpm, and the main elements were uniformly distributed on the surface of the milled powders. The unexpected face-centered cubic (FCC, γ)/body-centered cubic (BCC, α) dual-phase structure of the sintered specimens, could be explained by the unexpectedly high contents of C and N from the raw powder production process, fast-sintering characteristic of SPS, and inhibitory effect of W on the diffusion of C. The experimental results were approximately consistent with the simulation results from the Thermo Calc software. The temperature combination of 800 °C and 1100 °C during the SPS process, provided a relatively more homogeneous microstructure, while the combination of 750 °C and 1150 °C, provided the highest ultimate tensile strength (UTS), of 1038 MPa, with the highest uniform elongation (UE), of 6.2%. M23C6, Cr2O3, M2(C,N), and other precipitates, were mainly distributed at grain boundaries, especially at the triple junctions, which led to Cr depletion at grain boundaries.

Microstructure and tensile properties ranged from ambient temperature to 700 ℃ of the 12Cr ODS steel were investigated by scanning electron microscopy, electron backscatter diffraction technique, and tensile tests. Results show that microstructure is composed of elongated grains with axis parallel to rolling direction. Ultimate tensile strength and 0.2% offset yield strength gradually decrease with increasing test temperature. Uniform elongation and total elongation reduce with the temperature rising up to 500 ℃, whereas increase at higher temperatures due to activation of slip systems. Moreover, ductility in rolling direction is better than that in transverse direction. Fracture surfaces suggest that a large number of small and uniform dimples, and predominant cleavage-like facets present in rolling direction, and transverse direction, respectively.

Recrystallization behavior of a two-way cold rolled 12Cr ODS steel during annealing in the range of 1273–1473 K was systemically investigated by means of scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and hardness testing. Results show that original grains with axis were rotated to axis parallel to normal direction after ∼60% reduction in thickness. Recovery stage was obvious at 1273 K annealing taking about 100 min, but at higher temperatures it was shortly followed by recrystallization. ∼84% recrystallization was observed after annealing at 1473 K for 60 min, indicating that the new cold rolling process decreases the recrystallization temperature in comparison with the as-fabricated specimen completing recrystallization at 1673 K. In addition, the microtexture evolution was characterized as recrystallization proceeded. The intensity of {111} and {111} components decreased, while intensive {110} component was obtained. Dependence of the stored energy on the orientation of deformed grains is presumed for the reduction of {111} and {111} components, and oriented growth is probably responsible for the enhancement of {110} texture after recrystallization.

The anisotropy in creep properties and its microstructural origins in two oxide dispersion strengthened (ODS) steels (i.e., 12Cr-ODS-IMR and 12Cr-ODS-NIFS) were systematically investigated. Both ODS steels exhibited the anisotropic creep properties. The creep strength in longitudinal directions (LD) or extrusion directions (ED) specimens was better than that in transverse directions (TD) specimens. The microstructural characterizations revealed that both steels had elongated grains along the ED and relatively equiaxed grains in other orientations. Both steels had a streak-like distribution of precipitates along the ED that was composed of Ti-enriched precipitates and nano-scale oxide particles in local area. The fractography of the crept TD and LD specimens suggested that both steels fractured by the mixed ductile and brittle fracture modes. The propagation of cracks and intergranular decohesion along the boundaries of elongated grains were more severe in the TD specimens than in the LD specimens. The combined effects of elongated grain morphology with low grain shape aspect ratio and segregation of precipitates along the ED can explain the anisotropy in creep properties. The anisotropy in 12Cr-ODS-IMR steel was relatively higher than that in 12Cr-ODS-NIFS steel, and the additional cold rolling and lower final heat treatment temperature in 12Cr-ODS-IMR steel were responsible for it.

A combination of ion-accelerator irradiation, nanoindentation tests, and electron backscatter diffraction analysis was utilized to probe the crystal orientation dependence of irradiation-induced hardening in a Zr alloy. The nanohardness at the [0001] orientation exhibited the highest value, which gradually decreased when the orientation deviated from the [0001] axis to the [ 10 1 ¯ 0 ] and 2 1 ¯ 1 ¯ 0 axes, regardless of the irradiation dose. Crystal-orientation-dependent irradiation-induced hardening is further confirmed. The lowest hardening was observed at [0001] orientation, and the hardening effect intensified toward the [ 10 1 ¯ 0 ] and [ 2 1 ¯ 1 ¯ 0 ] orientations, reaching the maximum at a rotation angle of ~55° with respect to [0001].

In this study, the mechanical properties in a Zr-Nb alloy plate with bimodal basal texture microstructure have been evaluated by tensile, Vickers hardness and nano-indentation tests. Based on the results, the strength-hardness relationship is discussed with special attention paid to its textured microstructure. It is found that the correlation between strength and hardness varied greatly depending on whether the loading direction is parallel or perpendicular to the normal direction of the plate, demonstrating that the strength-hardness relationship is texture-dependent. In addition, the relation between strength and nano-hardness is obtained by subjecting specimens to cold rolling at various reduction rates. This allows for the evaluation of the strengthening effect using convenient and non-destructive nano-indentation tests instead of elaborate tensile tests, which may have particularly useful applications for nuclear engineering components when considering the irradiation strengthening occurred in these materials.

In order to clarify the instability of M23C6 in F82H steel under irradiation, both electron irradiation using a high voltage electron microscope (HVEM) and ion irradiation using an ion accelerator were performed. For the electron irradiation, in-situ observation under 2 MV electron irradiation and ex-situ high resolution electron microscopic (HREM) analysis were utilized to evaluate the response of M23C6 against irradiation. The temperature dependence of the irradiation induced instability of the carbide was first confirmed: 293 K < T < 573 K, by observation of lowering in contrast at the periphery of carbides, 698 K < T < 773 K, fragmentation at the interface between carbides and matrix, and at 773 K, formation and coarsening of new particles near the periphery of M23C6. HREM analysis showed the loss of the lattice fringe contrast at the pre-existing M23C6 precipitates at temperatures ranging from 473 to 773 K, indicating severe loss of crystallinity due to dissolution of the constituent atoms though irradiation- enhanced diffusion under the vacancy diffusion by the focused electron beam irradiation. For the ion irradiation, 10.5MeV-Fe3+ ion was applied to bombard the F82H steel at 673K to achieve the displacement damage of 20 dpa at the depth of 1.0 µm from surface. Cross-section TEM specimens were prepared by a focused ion beam technique. The shrinkage of carbide particles was observed especially near the irradiation surface. Besides, the lattice fringes at the periphery of carbide were observed in the irradiated M23C6 by the HREM analysis, which is different from that observed in the electron irradiation. It was clarified that the instability of M23C6 is dependent on the irradiation conditions, indicating that the flow rate of vacancy type defects might be the key factor to cause the dissolution of constituent atoms of carbide particles into matrix under irradiation.

Corrosion of pure Cr and pure W in molten FLiNaK at 773 K was investigated to deeply understand the effect of the alloying elements on the corrosion mechanisms of reduced activation ferritic/martensitic (RAFM) steels. The molten salt blanket environment was simulated by hydrogen fluoride gas bubbling. Electrochemical polarization and weight change measurements were conducted to estimate corrosion rates. The oxidation reaction rate of pure Cr was extremely high (instantly reached 2.9 × 104 mm/year). The corrosion rate of W was lower than that of the RAFM steel, JLF-1 steel, and pure Fe by one order of magnitude. W was nobler than Ni, which is known as a corrosion-resistant metal to the molten fluoride salts and hydrogen fluoride. The compatibility of W coating with the first wall structure of blankets was evaluated with the local tritium breeding ratio (TBR). When a Pb neutron multiplier was used, thin W coating almost did not affect the local TBR.

Tensile properties and microstructure of a Zr–1.8Nb alloy subjected to 140-MeV C 4+ ion irradiation at 573 K up to 5.3 dpa have been investigated by using 0.18 mm-thick tensile specimens. Irradiation damage was introduced into the tensile specimens homogeneously over entire thickness using a variable energy degrader. Irradiation-induced strengthening and embrittlement (loss of ductility) were successfully evaluated. The yield strength of irradiated specimens (3.1- and 5.3-dpa) was 137% and 145% of unirradiated specimen, whereas the total elongation of those irradiated specimens was less than a half of unirradiated specimen. It appears that the rate of embrittlement is fairly faster than the rate of strengthening. Transmission electron microscopy (TEM) observation on the 3.1-dpa specimen revealed that the bcc-Nb/Zr precipitates originally contained in the alloy were stable in terms of crystal structure and size. The size and number density of dislocation loops were 7.2 nm and 1.2 × 10 21 /m 3 for the -loops, 15.4 nm and 6.5 × 10 20 /m 3 for the -type loops, respectively. The increase of yield strength expected from these dislocation loops was ∼64% of the actual irradiation-induced strengthening, indicating that the dislocation loops are the main contributors for the irradiation-induced strengthening.

Zirconium-based (Zr-based) alloys have been widely used as materials for the key components in light water reactors (LWRs), such as fuel claddings which suffer from waterside corrosion, hydrogen uptakes and strength loss at elevated temperature, especially during accident scenarios like the lost-of-coolant accident (LOCA). For the purpose of providing a safer, nuclear leakage resistant and economically viable LWRs, three general approaches have been proposed so far to develop the accident tolerant fuel (ATF) claddings: optimization of metallurgical composition and processing of Zr-based alloys, coatings on existing Zr-based alloys and replacement of current Zr-based alloys. In this manuscript, an attempt has been made to systematically present the historic development of Zr-based cladding, including the impacts of alloying elements on the material properties. Subsequently, the research investigations on coating layer on the surface of Zr-based claddings, mainly referring coating materials and fabrication methods, have been broadly reviewed. The last section of this review provides the introduction to alternative materials (Non-Zr) to Zr-based alloys for LWRs, such as advanced steels, Mo-based, and SiC-based materials.

The microstructure of the 12Cr-ODS steel was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) techniques. The results showed that the microstructure consisted of fine and elongated grains, high density of dislocations, and large concentration of nm-scale oxide particles, which were identified as five types of crystal structures by high resolution TEM (HRTEM) imaging, such as monoclinic and cubic Y 2 O 3 , hexagonal and orthorhombic Y 2 TiO 5 , and cubic Y 2 Ti 2 O 7 . The dominant {001} component and weaker parallel to normal direction texture were observed. Besides, the yield stress was experimentally measured and quantitatively estimated. The findings indicated that theoretical calculation was in accordance with the experimental measurements, and strengthening contributions from solid-solution atoms and grain boundaries were linearly additive, whereas those from dislocations and oxide dispersoids were averaged by root mean square summation.

The effect of Mo addition on microstructures of β-quenched and subsequently annealed Zr-1.2Nb alloys is systematically investigated. Differential scanning calorimetry, electron backscatter diffraction, transmission electron microscopy, X-ray energy dispersive spectrometry techniques and microhardness tests are jointly utilized to characterize the microstructure and to evaluate the mechanical property. Martensitic microstructure, a mixture of lath martensites and lens martensites, is observed in the β-quenched specimens. Width of lath martensite reduces and fraction of lens martensite increases as the Mo content increases. This suggests that Mo addition retards slip deformation and enhances twinning deformation during the martensitic transformation, and that segregation of Mo atoms is also responsible for the lath width reduction. After annealing at 873 K, the martensitic structures are maintained, accompanying with the presence of numerous precipitates, which is attributed to the fact that the β-quenched microstructure is strain-relieved instead of being recrystallized. The distribution of the misorientation angle between the neighboring grains in both β-quenched and subsequently α-annealed specimens is predominantly around 60° regardless of the Mo content. Microhardness increases with increasing Mo content both in the β-quenched and the annealed specimens, and it drops greatly after annealing at a fixed composition. Keywords: Zr-Nb alloy, Zr-Nb-Mo, Martensite, Mo addition, Twinning, Precipitates

Hardness behavior and deformation of the microstructure beneath the indentation in 12Cr-ODS steel after exposure to heavy ion irradiation at 573 K were investigated with combined applications of nano-indentation tests, EBSD analysis, and TEM observation. The inhomogeneous nano-indentation hardness behavior was firstly investigated in both non-irradiated and irradiated specimens and compared to those at room temperature (RT) irradiation. Subsequently, TEM observations were performed to understand the deformation behaviors beneath the indentation tested at typical low-indexed orientations. Obvious irradiation hardening was confirmed in both the normal direction (ND) and rolling direction (RD) specimens, and the hardening effect intensified with an increase in irradiation dose. Meanwhile, various anisotropic hardness behaviors were observed such as a lower hardness value and a weaker hardening effect in the RD specimen compared to the ND specimen, and a lower hardness value but a greater hardening effect in [001] orientation relative to [111] orientation. Furthermore, results from 573 K irradiation showed similar anisotropic hardness behaviors with those obtained from RT irradiation, despite the fact that irradiation at 573 K exhibited a weakened irradiation hardening effect in this steel. TEM observation of the deformation microstructure beneath the indentation reveals that (i) the depth of the deformation affected zone varied with orientation, ranging from ~6-9 times that of the indent depth, and (ii) distinct dislocation-grain boundary (GB) interactions beneath the indentations was confirmed to exhibit dependence on the geometric relation between GB and indenter impression direction. Correlation between the anisotropic hardness behavior and microstructural features is discussed based on Schmid factor analysis, comparison of geometric favorability between indenter and possible slip planes, and TEM observation of deformation microstructure. The results conclude that the anisotropic hardness behaviors in the present 12Cr-ODS steel are ascribed to the combined effect of crystal orientation and the non-uniform grain morphology with the existence of the elongated grains having low aspect ratio in the ND plane and nano-layered grains in the RD plane.

Dissimilar-metals bonding between V–4Cr–4Ti and Hastelloy X alloy was investigated by explosive welding, which is essential to connect coolant pipes of Flibe blanket and external components such as tritium extractor and heat exchanger in fusion reactor. The nano-indentation hardness and corresponding microstructure at the interface are evaluated. After the explosive welding, a characterization of the typical wavy interface indicates good weldability. Discontinuous vortex regions appear at the interface and reveal obvious hardening up to 806 HV. Microstructural analysis indicates that the interlayer consists of Ni37.1V21Cr20.5Fe15.4Mo4.4Ti1.0 Co0.6 with a fcc lattice structure and a high density of dislocations, and no intermetallic forms. Based on the compositions, the formation of solid solution was predicted by the entropy and enthalpy of the multi-component alloy system, despite the phase diagrams of binary alloys of the constituent elements indicating the formation of intermetallics. Study of hardening mechanism at the interlayer indicates that obvious hardening is mainly attributed to solid-solution hardening and dislocation hardening.

Thermal aging experiments on a 9Cr-ODS steel were carried out at 700 °C with a period from 100 up to 10000 h, and the effects on mechanical properties and microstructural evolutions were investigated. It was found that hardness and tensile properties exhibited negligible degradation by thermal aging. The mixed ductile and brittle fracture modes in the tensile specimens remained almost no change. Although a slight recovery of grains/subgrain structures and dislocations took place, the homogeneously dispersed nano-size oxides with an ultrahigh density in matrix were not affected during the thermal aging. However, a significant coarsening of carbides around grain/subgrain boundaries appeared. Benefited from the extreme stability of nano-size oxides, the 9Cr-ODS steel possesses the excellent thermal stability of mechanical properties exposed to the elevated temperature of 700 °C up to 10000 h.

The anisotropic hardening response in a 12Cr-ODS ferritic steel was investigated before and after irradiation with 2.8 MeV Fe2+ at room temperature up to displacement damages of 0.5 and 15 dpa. For post irradiation examination, techniques of nano-indentation and orientation imaging microscopy were jointly applied to link crystal orientation with nano-hardness. Results showed that the averaged hardness of normal direction-transverse direction (TD-ND) specimen is less than that of rolling direction-transverse direction (RD-TD) specimen irrespective of the dose of irradiation damage. However, the amount of irradiation-induced hardening is observed to be weaker in TD-ND specimen relative to RD-TD specimen. These anisotropic phenomena are considered to be mainly attributed to the elongated grain structure with a very high grain aspect ratio of the present steel, in which smaller-sized grains are exhibited in TD-ND plane nevertheless fairly coarse grains are in RD-TD plane. In addition, the orientation dependent hardness and irradiation-induced hardening were confirmed. Specifically, the hardness of [001]-oriented grain is lower than that of [111]-oriented grain with and without irradiation. With the presence of irradiation, it was found that the extent of hardening is more obvious at [001]-oriented grain than [111]-oriented grain, which is attributed to a more activated primary {110} slip beneath the indenter when tested at [100]-oriented grain compared with [111]-oriented grain.

Two types of oxide dispersion strengthening (ODS) steels (i.e. 12Cr-ODS-NIFS and 12Cr-ODS-IMR) with similar chemical compositions were produced through different fabrication processing. A systematic comparison was performed to investigate the effects of different fabrication processing on microstructure and mechanical properties of ODS steels. Compared to 12Cr-ODS-NIFS, the 12Cr-ODS-IMR steel exhibited significantly enhanced mechanical properties, such as higher Vickers hardness, tensile strength and creep strength. To understand the microstructural reasons for the excellent mechanical properties of ODS steels, systematic microstructure characterizations were carried out through a combined study of X-ray diffraction, electron backscattered diffraction, and transmission electron microscopy. It was found that both steels had similar grain structures comprising fine and heavily deformed grains smaller than ~1 μm and elongated coarse grains larger than ~10 μm, both of which possessed similar precipitates including coarse TiC x O 1− x particles and homogenously distributed Y 2 TiO 5 nanoparticles with diameters of several nanometers. 12Cr-ODS-IMR had a finer grain size, larger aspect ratio of grain length to width, and higher density of dislocations, which should be derived from the additional cold rolling and responsible for its better mechanical properties compared with 12Cr-ODS-NIFS.

Mo-modified Zr–1.2Nb alloys were prepared and the effects of Mo addition on precipitation in Zr–Nb alloys were elucidated. It was found that an increase of Mo content decreased α→(α+β) phase transformation temperature in Zr–Nb alloys. A new kind of precipitate, β-(Nb, Mo, Zr), was identified in the Mo-containing alloys. These particles are formed because Mo atoms replace Nb atoms in the β-(Nb, Zr) phase. Further, it was found that Mo retarded the growth of the precipitates but increased their number density.