The present work studied the impact of SO42− ions on the corrosion behaviors of GH3535 weld joint in FLiNaK molten salt. The concentration of SO42− ions in the FLiNaK molten salt was controlled by adjusting the quantity of Na2SO4 added into the salt. Results indicate that the SO42− ions in the FLiNaK salt speed up the corrosion rate remarkably by promoting the dissolution of Cr from the alloy matrix into the salt. With the concentration of SO42− ions in the FLiNaK salt increases from 100 ppm to 1000 ppm, the weight losses and the Cr depletion layer depths of the corroded specimens increase linearly. Even in the case of the heavy corrosion attack caused by the SO42− ions, the corrosion performance is similar between the base zone and fusion zone in the GH3535 weld joint. It is demonstrated that the structural diversity caused by the welding process has little impact on the corrosion performances of GH3535 alloy in FLiNaK molten salt.

•Corrosion tests of weld joint of a Ni–Mo–Cr alloy were performed in a eutectic LiF–NaF–KF molten salt.•The joints exhibit Cr depletion and Fe enrichment in corrosion layer at 700 °C and 850 °C.•The thickness of the Cr depleted layer induced by the corrosion in the molten salt increases with the temperature.Corrosion tests of weld joint of a Ni–Mo–Cr alloy were performed in a eutectic LiF–NaF–KF molten salt for 400 h at different temperatures (550 °C, 700 °C, 850 °C). Results indicate that the joints exhibit Cr depletion and Fe enrichment in corrosion layer at 700 °C and 850 °C, that the structural differentiation caused by welding has little impact on the diffusion of the elements in the alloy. The thickness of the Cr depleted layer induced by the corrosion in the molten salt increases with the temperature. The grains of the alloy sample become coarser due to the annealing at higher temperature, which may also enhance the corrosion of the alloy in the salt.Corrosion tests of weld joint of a Ni–Mo–Cr alloy were performed in a eutectic LiF–NaF–KF molten salt for 400 h at different temperatures (550 °C, 700 °C, 850 °C). Results indicate that the joints exhibit Cr depletion and Fe enrichment in corrosion layer at 700 °C and 850 °C. The thickness of the Cr depleted layer induced by the corrosion in the molten salt increases with the temperature and the structural differentiation caused by welding has little impact on the diffusion of the elements in the alloy.

Ni–xW–6Cr alloys have been considered as one of the potential structural materials for molten salt techniques, whereas their microstructure and mechanical performance have not been sufficiently studied. In this study, the microstructure and tensile deformation behavior of Ni–(10–35 wt%)W–6Cr alloys have been systematically investigated. The phase diagram calculations indicated that the solubility limit of W is 34 wt% in Ni–xW–6Cr alloy. α-W phase is formed in the matrix while the W content exceeds such limit. The fracture of the Ni–(10–35 wt%)W–6Cr alloys at room temperature is in the transgranular ductile fracture mode. The tensile properties of alloys, except for the elongation of Ni–35 wt%W–6Cr alloy, are improved with the increase of W content, which can be explained by the larger lattice distortion, the lower stack fault energy and the higher length fraction of twin boundaries (Σ3 and Σ9 type) in the Ni–(10–35 wt%)W–6Cr alloys caused by the addition of more W. The reduced elongation of the Ni–35 wt%W–6Cr alloy is ascribed to the particles in α-W phase which act as the main nucleation sites for cracking.

The grain size refinement, enhancement of mechanical properties, and static recrystallization behavior of metallic nickel-silicon carbide nano-particle (Ni-3wt.%SiCNP) composites, milled for times ranging from 8 to 48 h have been examined. One set of Ni-SiCNP composite samples were annealed at 300 °C for 250 h, while the other set of samples were maintained at room temperature for control purposes (reference). The electron backscatter diffraction results indicate that the grain size of the annealed Ni-SiCNP composite was refined due to grain restructuring during static recrystallization. The x-ray diffraction results indicate that low-temperature annealing effectively reduced the density of dislocations; this can be explained by the dislocation pile-up model. Additionally, the tensile tests indicated that the annealed Ni-SiCNP composite had a significant increase in strength due to an increase of the Hall–Petch strengthening effect with a slight increase in the total elongation. The decrease of dislocation pile-up in the grain interiors and the increase in grain boundary sliding are assumed to be the main mechanisms at play. The relationship between the microstructural evolution and the variation of tensile properties is examined in this study.

Under Xe ion irradiation, the microstructural evolution of a nickel based alloy, Hastelloy N (US N10003), was studied. The intrinsic dislocations are decorated with irradiation induced interstitial loops and/or clusters. Moreover, the intrinsic dislocations density reduces as the irradiation damage increases. The disappearance of the intrinsic dislocations is ascribed to the dislocations climb to the free surface by the absorption of interstitials under the ion irradiation. Moreover, the in situ annealing experiment reveals that the small interstitial loops and/or clusters induced by the ion irradiation are stable below 600 °C.

Infiltration of molten FLiNaK salt into degassed nuclear graphite samples under inert gas pressure was studied. The weight gain of different grades (2020, 2114, IG-110, NBG-8, G1 and G2) of nuclear graphite during infiltration with different pressures was measured. Molten salt infiltration was compared with mercury intrusion porosimetry where it was found that mercury infiltration was a useful predictor of the threshold pressure and infiltration volume per gram graphite for molten salt infiltration. The distribution and morphology of salt in the graphite were observed by scanning electron microscopy, with very little difference between the molten salt content at the center and edge of samples for samples infiltrated at pressure higher than the threshold pressure. Increased molten salt infiltration with increased pressure resulted from the occupation of smaller pores and full occupation of the larger irregular pores. The similarity of weight gain between molten salt infiltration equilibrated at 20 and 100 h showed 20 h was adequate to obtain equilibrium.

•Te preferentially diffuses into the Ni-based alloy along grain boundaries at 800 °C.•Te can react with Cr to form cubic structure CrTe at both grain boundaries and intergranular carbide/matrix interfaces.•The CrTe is possible to induce intergranular cracking in the alloy.The corrosion behavior of a Ni–16Mo–7Cr–4Fe alloy was investigated in a tellurium (Te) vapor atmosphere at 800 °C. Te was identified via electron probe microanalysis at the grain boundary regions of the corroded alloy. The morphology, chemical composition, and crystalline structure of those areas were characterized in a transmission electron microscope. Chromium tellurides were observed at both grain boundaries and intergranular carbide–matrix interfaces. Based on the results, the mechanism of intergranular Te corrosion and its possible correlation with intergranular cracking is discussed.

•The alloy surface formation products are primarily Ni3Te2, CrTe and MoTe2.•The room temperature yield strength did not significantly change with Te content.•The ultimate tensile strength and elongation decreased with increasing of Te.•Te content determines cracks width but affects little its diffusion depth.•Te embrittle the grain boundaries, and further weaken the mechanical properties.Te was deposited on the surface of a Ni–16Mo–7Cr alloy by thermal evaporation at 700 °C, and the effect of Te on the intergranular cracking behavior and the tensile properties of the alloy was investigated. The results show that the reaction products formed on the surface of the alloy, the diffusion depth of Te in the alloy, and the yield strength of the alloy attacked by Te at room temperature are not changed remarkably with Te content increasing, whereas the ultimate tensile strength and elongation of the alloy is decreased distinctly. The primary surface reaction product are mainly composed of Ni3Te2, CrTe, and MoTe2, and the diffusion depth of Te in the alloys is about 50 μm. The intergranular embrittlement mechanism of the alloy induced by Te of is also discussed in this paper.

•Ultra-fined Ni–SiCNP composites prepared by a powder metallurgy route.•Dispersed SiCNP kept good thermal stability after high temperature aging.•Milling time effect on the microstructure and tensile property were investigated.•Dispersion SiCNP strengthen nickel based alloys was confirmed.The microstructural evolution and tensile properties of nickel-silicon carbide nano-particles (Ni-3wt.%SiCNP) composites with milling time ranged from 8 to 48 h have been examined. High temperature aged and reference samples were investigated to clarify their high temperature stability. The TEM characterizations showed that the coarsening of dispersed SiCNP occurred with increasing milling time, whereas their sizes kept stable under different aging temperatures. The EBSD results indicated that the mean grain size decreased and increased respectively with increasing milling time and aging temperature. Two different recrystallization mechanisms, boundary bowing out and subgrain growth, were assumed to be responsible for the grain growth. The tensile test results showed that both the yield and tensile strengths of reference and aged samples increased as a function of the milling time. As milling time extended, the total elongations gradually decreased in the case of reference samples, but slightly decreased firstly and then increased in the case of the aged ones. The relationship between the microstructural evolution and the variations of tensile properties has been discussed in this study.

Bulk metallic nickel–silicon carbide nano-particle (Ni–SiCNP) composites, with milling time ranged from 8 to 48 h, were prepared in a planetary ball mill and sintered using a spark plasma sintering (SPS) furnace. The microstructure of the Ni–SiCNP composites was characterized by transmission electron microscopy (TEM) and their mechanical properties were investigated by tensile measurements. The TEM results showed well-dispersed SiCNP particles, either within the matrix, between twins or along grain boundaries (GB), as well as the presence of stacking faults and twin structures, characteristics of materials with low stacking fault energy. Dislocation lines were also observed to interact with the SiCNP which were plastically nondeformable. A synergistic relationship existed between Hall–Petch strengthening and dispersion strengthening mechanisms, which was shown to greatly influence the mechanical properties of the Ni–SiCNP composites. Both the maximum yield and tensile strengths were found in the Ni–SiCNP composite with a milling time of 48 h, whereas the increased rate of strengths drastically decreased in material milled above 8 h due to the significant SiCNP agglomeration. The ball milling process resulted in the formation of nano-scale, ultra-fine grained (UFG) Ni–SiCNP composites when the milling time was extended for longer periods, greatly strengthening these materials. The sharp decrease in elongation percentages, however, should be comprehensively considered before irreversible inelastic deformation.

The effect of long-term thermal exposure on the grain boundary carbides and the tensile behavior of two kinds of Ni–Mo–Cr superalloys with different silicon contents (0 and 0.46 wt%) was investigated. Experimental results showed granular M2C carbides formed at the grain boundaries after exposure for 100 h for the non-silicon alloy. Furthermore, these fine granular M2C carbides will transform into plate-like M6C carbides as exposure time increases. For the Si-containing alloys, only the granular M6C carbides formed at the grain boundaries during the whole exposure time. The coarsening of the grain boundary carbides occurred in both alloys with increasing exposure time. In addition, the coarsening kinetics of the grain boundary carbides for the non-silicon alloy is faster than that of the standard alloy. The tensile properties of both alloys are improved after exposure for 100 h due to the formation of nano-sized grain boundary carbides. The grain boundary carbides are coarsened more seriously for non-silicon alloys than that of Si-containing alloys, resulting in a more significant decrease in the tensile strength and elongation for the former case. Silicon additions can effectively inhibit the severe coarsening of the grain boundary carbides and thus avoid the obvious deterioration of the tensile properties after a long-term thermal exposure.

Silicon carbide nanoparticle-reinforced nickel-based composites (Ni–SiCNP), with a SiCNP content ranged from 1 to 3.5 wt%, were prepared using mechanical alloying and spark plasma sintering. In addition, unreinforced pure nickel samples were also prepared for comparative purposes. To characterize the microstructural properties of both the unreinforced pure nickel and the Ni–SiCNP composites transmission electron microscopy (TEM) was used, while their mechanical behavior was investigated using the Vickers pyramid method for hardness measurements and a universal tensile testing machine for tensile tests. TEM results showed an array of dislocation lines decorated in the sintered pure nickel sample, whereas, for the Ni–SiCNP composites, the presence of nano-dispersed SiCNP and twinning crystals was observed. These homogeneously distributed SiCNP were found located either within the matrix, between twins or on grain boundaries. For the Ni–SiCNP composites, coerced coarsening of the SiCNP assembly occurred with increasing SiCNP content. Furthermore, the grain sizes of the Ni–SiCNP composites were much finer than that of the unreinforced pure nickel, which was considered to be due to the composite ball milling process. In all cases, the Ni–SiCNP composites showed higher strengths and hardness values than the unreinforced pure nickel, likely due to a combination of dispersion strengthening (Orowan effects) and particle strengthening (Hall–Petch effects). For the Ni–SiCNP composites, the strength increased initially and then decreased as a function of SiCNP content, whereas their elongation percentages decreased linearly. Compared to all materials tested, the Ni–SiCNP composite containing 1.5% SiC was found more superior considering both their strength and plastic properties.

The irradiation-induced damage on the fine-grained isotropic nuclear graphite, IG-110, was investigated by 3-MeV proton irradiation at room temperature. The irradiation effects were characterized using scanning electron microscopy, transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), and nano-indentation. The surface morphology showed a fragmented shape after irradiation, indicating that the surface microstructure of the graphite was damaged by proton bombardment. The TEM images revealed clear and convincing evidence for the increase in defect clusters (probably interstitial clusters), basal plane bending, and basal plane dislocations, which might be the main reason for property changes. Raman studies indicated a rapid increase in the interstitial and vacancy defects, and decrease of in-plane “crystallite size”. The XRD results indicated a slight increase in the interlayer spacing and decrease in crystallite size. The enhancement in the hardness and modulus can be attributed to the pinning of basal plane dislocations by lattice defects produced by proton irradiation.

Pyrolytic carbon (PyC) coatings were deposited on IG-110 nuclear graphite by thermal decomposition of methane at ∼1830 °C. The PyC coatings are anisotropic and airtight enough to protect IG-110 nuclear graphite against the permeation of molten fluoride salts and the diffusion of gases. The investigations indicate that the sealing nuclear graphite with PyC coating is a promising method for its application in Molten Salt Reactor (MSR).

The interaction between nuclear graphite and molten fluoride salts (46.5 mol % LiF/11.5 mol % NaF/42 mol % KF) is investigated by synchrotron X-ray diffraction and C K-edge X-ray absorption near-edge structure (XANES). It is found that there are a large number of H atoms in IG-110 nuclear graphite, which is attributed to the residual C–H bond after the graphitization process of petroleum coke and pitch binder. The elastic recoil detection analysis indicates that H atoms are uniformly distributed in IG-110 nuclear graphite, in excellent agreement with the XANES results. The XANES results indicate that the immersion in molten fluoride salts at 500 °C led to H atoms in nuclear graphite partly substituted by the fluorine from fluoride salts to form C–F bond. The implications of these findings are discussed.

•Effect of Cr3+ on the corrosion of SiC in LiF-NaF-KF molten salt was investigated.•Cr3+ in LiF-NaF-KF molten salt can aggravate the corrosion of SiC.•The thickness of corroded region in salt with Cr3+ is much larger than that in pure salt.•The corroded region is composed of Cr2C3, Cr7C3, Cr2O3, and graphitic carbon.•Corrosion induced by Cr3+ should be attributed to reactions of Cr3+ with SiC.Effects of Cr3+ on the corrosion of SiC in LiF–NaF–KF molten salt were investigated. Results reveal that Cr3+ can drive the corrosion of SiC. Thickness of the corroded region induced by Cr3+ is greater than 12 μm for 400 h, while the corroded region induced by pure salt is ∼2.5 μm. Corrosion induced by Cr3+ should be attributed to the reactions of Cr3+ with SiC. Cr3+ reacts with SiC to form Cr3C2, Cr7C3, and graphitic carbon in the corroded region of SiC, which results in the dissolution of element Si from SiC into the salt.