As a primary material of the thorium molten salt reactor (TMSR) that is a suitable candidate reactor of the Generation IV nuclear reactors, GH3535 superalloy was successfully welded. The effect of laser beam welding (LBW) on microstructure evolution of fusion zone (FZ) and heat affected zone (HAZ), such as element segregation, precipitate behavior and grain evolution, was investigated. The microhardness and tensile properties were tested and discussed. The results of microstructure evolution showed that a number of fine M6C-γ eutectic phases precipitated at solidification grain boundaries and interdendritic region in FZ. Compared to base metal zone (BMZ), the grain size of HAZ has no obvious change. While a few of M6C-γ eutectic phases were observed in partially melted zone (PMZ) of HAZ. The results of microhardness indicated that the hardness of FZ was higher than that of HAZ and BMZ. The results of tensile test showed that the ultimate tensile strength of joints at room temperature, 650 and 700 °C were 98%, 97% and 99% of that of BM, respectively. All the tensile specimens of joints failed in BMZ rather than in PMZ where M6C carbides had been transformed into M6C-γ eutectic phases.

•Ni-Cr binary alloy is used to study the diffusion of Te in nickel alloys.•The mechanism of Cr suppresses the diffusion of Te is discussed.•The process of Cr stopping the Te diffusion along grain boundaries of the alloys is analyzed.The embrittlement of Ni-based structural alloys caused by fission production Te is one of the major challenges for molten salt reactors. It has been reported that solution element Cr can prevent the situation of intergranular cracks caused by Te. However, there is no detailed mechanism explanation on this phenomenon. In this study, the effect of Cr on Te diffusion in Ni-Cr binary system was investigated by diffusion experiments at 800 °C for 100 h. Results show that Te reacts with the alloy mainly forming Ni3Te2, and strip shaped Cr3Te4 is only found on the surface of Ni-15%Cr alloy. According to the discussion of thermodynamic chemical reaction process, Cr3Te4 exhibits the best stability and preferential formation compound in Te/Ni–Cr system as its Gibbs free energy of formation is the lowest. With the increase of Cr content in the alloy, the diffusion depth of Te along grain boundaries significantly decreases. Moreover, the formation process of reaction product and diffusion process are described. The diffusion of Te can be suppressed by high content of Cr in Ni-Cr alloy due to the formation of Cr3Te4 and thus the grain boundary is protected from Te corroding.

•Grain boundary MC carbides are found to be instable above 750 °C.•Interface between MC carbides and the matrix exhibits a semi-coherent character.•MC carbides hold the cube-on-cube orientation relationship with the matrix.•The deviation angles from cube-on-cube OR become larger during thermal exposure.The evolution of grain boundary MC carbides during the thermal exposure has been investigated in the Ti-modified Hastelloy N alloy. MC carbides precipitate at the grain boundaries after exposed at 650 °C and 750 °C for 500 hours (h). The morphology of MC carbides at the grain boundaries was characterized as separated particles with a cube-on-cube orientation relationship with the matrix. The MC/Matrix interface exhibits a semi-coherent character, on which misfit dislocations are repeated in every sixth layer of (111)MC. With the further thermal exposure, the size of MC carbides remain stable at 650 °C. By contrast, M2C carbides undergo the initial coarsening and subsequent re-dissolution at 750 °C. At the same time, the deviation angles from the cube-cube orientation relationship (OR) between MC carbides and the matrix become larger at both 650 °C and 750 °C, which indicate the semi-coherent interfaces are transforming to the incoherent ones.

GH3535 alloy plates were welded by Gas Tungsten Arc Welding in order to study the evolution of microstructure and mechanical properties in the heat-affected zone (HAZ). Our results suggest that welding thermal cycles induced the morphology evolution of M6C carbides from block to eutectic near the fusion line in the HAZ. Electron backscatter diffraction (EBSD) results show that significant amounts of plastic strains occurred in the HAZ after welding. In addition, local coherent twin boundaries (Σ3) and dislocations were observed to decrease with the distance from the fusion line. Mechanical tests indicate that the hardness, yield strength and ultimate strength in HAZ are higher than those in base metal, and their values decrease with the distance from the fusion line. However, the elongation increases as the strengths decrease. The higher strength and lower elongation in the HAZ are mainly attributed to residual strains with the function of strain-hardening. Moreover, the change of Σ3 boundary which is in good agreement with that of elongation suggests a positive influence on the plastic deformation.

•Te atoms are homogeneously distributed along the surface plane direction in both grain lattice and grain boundaries (GBs) of Ni substrate after annealed at 1000 °C for 10 h.•The tellurium corrosion product at 1000 °C is Ni-Te solid solution with outward relaxation of the neighboring Ni atoms around Te.•The process of tellurium diffusing into nickel at high temperature is believed to be that the Ni-Te intermetallic compounds are first formed on the surface of Ni substrate then decomposed to tellurium and crystal Ni. Furthermore, tellurium diffuses into the Ni substrate and forms substitutional Ni-Te solid solution.Clarifying the atomic structure of tellurium corrosion product in nickel alloy will help understand the mechanism of Tellurium corrosion in Molten-salt reactor (MSR). Extended X-ray absorption fine structure (EXAFS) complemented by first-principles density functional theory (DFT) calculations was used to characterize the atomic local structure of tellurium in nickel at operando temperature of 1000 °C. The investigation indicated that Te atoms diffuse into the Ni substrate to form Ni-Te solid solution with outward relaxation of the neighboring Ni atoms around Te in both grain lattice and grain boundaries (GBs). Furthermore, we propose a process of tellurium diffusing into nickel at high temperature.

•The influence of Fe ion on molten FLiNaK salts corrosion was investigated.•Cr-, Mo-depletion layers and Fe-rich layer formed in the alloy surface.•Cr diffused faster than Fe in the alloy matrix during the corrosion process.•The valence state of Cr and Fe in the alloy remained unchanged after corrosion.•Carbides in the alloy were also corroded by FLiNaK.The effect of Fe ion impurity on the corrosion behavior of Hastelloy N (UNS N10003) alloy in molten FLiNaK salts at 850 °C has been investigated by combined synchrotron radiation and other characterization techniques. Results showed that Mo and Cr were depleted from the alloy surface, where Fe-rich layer formed. The corrosion process was mainly controlled by the redox reaction between Fe ion and Cr, and no new compound with high valence state formed in the alloy surface. The loss of Mo and Cr in M12C carbide occurred due to the presence of a concentration-gradient between M12C carbide and matrix.

•The tensile behavior of GH3535 alloy at elevated temperature was studied.•The yield strength anomaly was observed in the temperature range from 550 to 800 °C.•The formation of M12C improves the grain boundary strength to a certain extent.•Inhomogeneous deformation at 650 °C results in the ductility loss of the alloy.•The interaction between solute atoms and dislocations results in the PLC effect.The tensile behavior of GH3535 alloy has been investigated at strain rates of 8.33 × 10−5/s−1–8.33 × 10−3/s−1, in the temperature range of 25–800 °C. The results showed that the ultimate tensile strength was decreased with increasing temperature and increased with rising strain rate, whereas the yield strength kept almost a constant value at the temperature range from 550 to 800 °C in all strain rates test. The formation of M12C carbides at the grain boundary during the tension process played an important role in increasing the yield strength of the alloy at elevated temperatures. But inhomogeneous deformation at 650 °C resulted in the minimum ductility of the alloy. Additionally, various types of serrations were noticed on the stress-strain curves for the alloy tested in the temperature range of 500–800 °C. Normal Portevin-Le Chatelier (PLC) effect and positive strain rate sensitivity were observed in this alloy. Type A and A + B serrations were presented to stress-strain curves at temperatures below 650 °C, whereas type C serration was noticed when the temperature rose above 650 °C. The analysis suggested that the interactions between substitutional solutes migration and mobile dislocations were the main reason for the serrated flow behavior in this alloy.

Ni-26W-6Cr based superalloy is considered a potential structure material for the molten salt reactors due to its high strength and good compatibility with the fluoride salt. In the present work, the temperature dependence of the tensile behavior of the alloy was studied by tensile tests in the temperature range of 25–850 °C. This alloy exhibited a good ductility at RT and 450 °C, a ductility minimum from 650 to 750 °C and an intermediate ductility at 850 °C. TEM and EBSD characterization was performed on specimens tested at three typical temperature points (RT, 650 °C and 850 °C) to determine the deformation and fracture mechanisms accounting for the intermediate temperature embrittlement. At RT, the grain boundaries can accommodate enough dislocations to provide compatibility of the sliding between adjacent grains, then M6C carbides act as crack origins and cause the fracture. In case of 650 °C, the grain boundaries cannot withstand the local stress even if only a small number of dislocation pile-ups exist. The premature cracks at grain boundaries impede the development of plastic deformation from single slips to multiple ones and cause the low ductility. If tested at 850 °C, the fracture process is retarded by the dynamic recovery and local dynamic recrystallization at crack tips.

The hot ductility behavior of Ni–16Mo–7Cr alloys (named GH3535) exposed at 700 °C for different durations has been investigated by means of tensile test. It was found that the alloy exhibited a constant low ductility within the first 10 h exposure, and then showed an increasing ductility with the exposure time until 1000 h. After that, the ductility of the alloy decreased gradually with the increasing exposure time up to 10000 h. Detailed microstructural investigations using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) have shown that the change in the ductility of the alloy with the exposure time could be attributed to the precipitation of M12C carbide at the grain boundary. Such precipitates with size of 200 nm, which are formed during the thermal exposure within 1000 h, can significantly restrain the grain boundary sliding and crack initiation, resulting in the high ductility of the alloy. Further exposure will cause the coarsening of the carbides, making them as the source of grain boundary cracks, hence decreases the ductility of 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.

•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.

The diffusion behavior of tellurium into pure nickel is investigated at different annealing temperatures. The purpose is to understand the diffusion mechanism of Te by studying the influence of Te on the surface morphology and grain boundary of nickel. The results show that the surface morphology, reaction product and penetration of Te are greatly dependent on annealing temperature. The depth of Te penetration increases with the elevated temperature from 500 °C to 1000 °C, and the surface reaction products is NiTe0.67 (Ni3Te2) or mixture of a spot of NiTe0.69. Te diffuses into nickel predominantly along the grain boundary at low temperature (below 900 °C), while the diffusion mechanism of Te turns to lattice diffusion above 1000 °C.