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The corrosion behavior of GH3535 alloys in KCl–MgCl2 eutectic salts with or without magnesium (Mg) was investigated using the static immersion tests. The effect of purification treatment on the corrosion of GH3535 specimens was also evaluated by the different analysis. The KCl–MgCl2 salts purified with Mg and reducing service temperature can slow down the corrosion rate of GH3535 specimens. Infrared spectra results showed that purification treatment reduced the content of crystal water in the KCl–MgCl2 salts. The difference of hydrate content in KCl–MgCl2 salts and the diffusion rate of Cr in alloy are the main reasons for the change of GH3535 specimens corrosion rate. [ABSTRACT FROM AUTHOR]

The effect of metallic ions and carbides on the corrosion of the GH3535 alloy was studied in a eutectic (Li, Na, K) F melt at 700 °C for different immersion times (20, 40, and 60 h). The results indicate that the average ΔV value between the carbides and the γ matrix is approximately 65 mV, which leads to galvanic corrosion between them at grain boundaries in the molten fluoride salt. Metallic ions produced by the corrosion of the alloy, such as Cr3+, Mo3+, and Ni2+, accelerated its corrosion in molten fluoride salts, which caused an “autocatalytic zone” to form within the corrosion layer during the exposure process.

The effects of thermal exposure on tellurium (Te) diffusion behavior in MCrAlY coatings were investigated at 800 °C in Te vapor. The results showed that the diffusion depth of Te in MCrAlY coatings gradually increased with the exposure time, and the dense Cr 3 Te 4 layer formed on the coating surface in the initial stage was no longer continuous, especially under aging conditions with higher Te concentration. The high Cr content of the coating promoted the solid phase transition of the surface reaction product from Cr 3 Te 4 to Cr 7 Te 8 , and resulting in the formation of large α-Cr phase either within or beneath the reaction layer during long aging processes. As a result of this phase transition, excess Te atoms were released from Cr 3 Te 4 and continued to diffuse into the coating. Te did not exhibit obvious intergranular diffusion characteristics within the coating. When it encountered Y, it was captured by Y and formed a YTe phase. After aging for 3000 h, Te was distributed throughout the coating, and numerous voids were formed at the interface between the coating and the substrate. These two factors deteriorated the plasticity and adhesion of the coating. Results from molten salt corrosion tests indicated that the high content of Cr and Al in the coating, as well as high densities of grain boundaries providing diffusion pathways, reduced the corrosion resistance of the coating to molten salt. [Display omitted] • MCrAlY coating protects the alloy substrate from Te-induced intergranular cracking after 3000 h of aging in Te vapor. • Cr 3 Te 4 on the coating surface transforms to Cr 7 Te 8 phase during long-term aging. • Te primarily form the YTe phase in the inward Te diffusion region of MCrAlY coating. • MCrAlY coating shows poor corrosion resistance in fluoride molten salts due to the significant loss of Cr and Al elements. [ABSTRACT FROM AUTHOR]

The surface plastic deformation induced by shot-peening on the oxidation behavior for GH3535 alloy was investigated. The variation in microstructure and mechanical property of GH3535 with different shot peening intensities was compared, followed by exposure to air at 950 °C for 200 h. The structure and elemental distribution of the oxide film were analyzed by combining SEM, EBSD and TEM. The result shows that shot peening significantly improves the oxidation resistance of GH3535 alloy. This treatment promotes the formation of fine grains on the surface of the alloy, and the grain boundaries act as fast diffusion channels of Cr leading to the Cr effective diffusivity of shot peening samples increase, the Cr effective diffusivity value is 0.235, 4.8 × 10−3, 3.8 × 10−3, 3.7 × 10−3, respectively. It promotes the formation of protective scales for the continuous Cr 2 O 3 layer. Additionally, the effect of grain size on the diffusion of scale-forming elements (Cr, Si and Mn) in the GH3535 alloy was analyzed, with a particular focus on the diffusion of Cr and the synergistic effects of scale-forming element. [Display omitted] • Surface plastic deformation enhances the oxidation resistance of GH3535 alloy. • The grain refinement induced by shot peening promotes Cr outward diffusion. • The oxide scale structure of SP samples contains outer MnCr 2 O 4 and inner Cr 2 O 3. • The formation of SiO 2 about 300 nm under the Cr 2 O 3 layer. [ABSTRACT FROM AUTHOR]

In this study, MCrAlY coatings were prepared by arc ion plating (AIP) on a Ni-16Mo-7Cr alloy to investigate its role of inhibiting tellurium (Te) diffusion into the alloy. The results show that the depth of Te diffusion in the MCrAlY coating is significantly lower than that in alloy. The ultrafine grain structure of the coating and its higher Cr content promote the formation of a dense Cr 3 Te 4 layer on the surface of coating during the Te diffusion, which contributes to prevent inward diffusion of Te and thus improve the resistance to intergranular cracking of the alloy induced by Te. • Effect of MCrAlY coating on the resistance to Te diffusion in Te vapor was investigated. • MCrAlY coating can significantly decrease the diffusion depth of Te into the coating. • A reaction layer composed of Cr 3 Te 4 forms on the surface of coating. • A dense of Cr 3 Te 4 layer can impede the Te diffusion on the grain boundary of the coating. [ABSTRACT FROM AUTHOR]

In recent years, the development of next-generation nuclear reactors with enhanced requirements for the safe and reliable production of nuclear energy has been attracting increasing attention. The thorium molten salt reactor (TMSR) has been regarded as the most promising prospective next-generation nuclear reactor because of its high security, desirable online refueling properties, minimization of nuclear waste, nuclear non-proliferation, etc. The structural materials for molten salt reactors should exhibit high temperature resistance and good corrosion and neutron irradiation resistance. The Ni-Cr-Mo-based superalloy GH3535 is the preferred material for TMSR applications because of its superior corrosion resistance and good mechanical properties. The Mo content of GH3535 is 15%~18% (mass fraction), which leads to the precipitation of a large amount of Mo-enriched M6C carbides in the matrix. Numerous studies have shown that the precipitation of these carbides directly affects the grain size and mechanical properties of GH3535 alloy. In this study, the effects of heat treatment on the grain size, carbide distribution, and mechanical properties of GH3535 alloy were investigated by cold-rolled-pipe tests. To provide the experimental and theoretical basis for applying heat treatment to control the properties of GH3535 alloy, the thermodynamic and kinetics characteristics of GH3535 were calculated using the JMatPro simulation software. The influence of heat treatments on the size and homogeneity of grains, the precipitation character of carbides, and the mechanical properties of the alloy were investigated. The results show that the equilibrium precipitate of the GH3535 at temperatures between 900℃ and 1500℃ is a Mo-rich carbide of M6C type and that the initial precipitation temperature of this M6C-type carbide is in the liquid-solid phase range. The grains grow slowly when the solution temperature is less than 1200℃. When the solution temperature is increased to 1230℃, the grains grow quickly to an average size of 160 μm; the grains are homogeneous when the temperature is maintained at 1180℃ for 10 min. Tensile tests show that a higher solution temperature decreases the strength and increases the elongation. The tensile fracture mechanism of GH3535 alloy is microporous aggregation.

Highlights • Triple-layer scales occur in both the fine-grained and coarse-grained specimens. • Fine-grained specimens possess the thicker outer layer and the discrete inner layer. • Fine-grained specimens possess the shallower Te permeation along grain boundaries. • Fine-grained specimens possess the shallower Cr 3 Te 4 existence along grain boundaries. • Fine-grained specimens provide more paths for the outward diffusion of Cr. Abstract The effects of grain size on the tellurium corrosion behaviors have been investigated systematically in GH3535 alloy. After the exposure to Te vapor, the triple-layer scales, which consist of Ni 3 Te 2 , Cr 3 Te 4 , MnTe, M 6 C carbides and Fe-riched γ phase, occur in both the fine-grained and coarse-grained specimens. The thicker outer layer, the more discrete inner layer, the shallower Te permeation and Cr 3 Te 4 existence along grain boundaries can be observed in the fine-grained specimens. By comparing the Gibbs free energies and calculating the effective diffusion coefficients, the tellurization mechanism and the role of grain size were discussed. [ABSTRACT FROM AUTHOR]

The GH3535 alloy was irradiated with 7 MeV Xe 26+ ions to a dose of 10 dpa at room temperature (RT) and 650 °C, and subsequently examined using Transmission Electron Microscopy (TEM) and nanoindentation. High numbers of nano-sized black dots, identified as dislocation loops were observed in both irradiated samples. The dislocation loops detected at the high temperature irradiated sample (size/number density: 9.5 nm/1.9 × 10 21 m −3 ) were found to be larger in size but less in amount as compared to that of the case of RT irradiation (6.9 nm/18.7 × 10 21 m −3 ). In addition, the large-sized Mo-Cr rich precipitates (16.4 nm/3.7 × 10 21 m −3 ) were observed in the sample irradiated at 650 °C. Moreover, the Xe bubbles, with smaller size (2.9 nm) but higher number density (77.8 × 10 21 m −3 ) among the irradiated induced defects, were also detected in the case of high temperature irradiated sample via the diffusion and aggregation of Xe atoms. Nanoindentaion measurements showed a hardening phenomenon for the irradiated sample, and the hardness increment is higher in the case of high temperature irradiated sample. Dispersed barrier-hardening (DBH) model was applied to predict the hardening produced from the irradiation induced defects. The yield strength increment calculated based on TEM observations and the nanohardness increment measured using nanoindentation are in excellent agreement. [ABSTRACT FROM AUTHOR]

Bulk samples of the GH3535 alloy weld metal have been characterized by transmission electron microscopy, X-ray diffraction and nanoindentation to determine their microstructural evolution and mechanical property changes after 8 MeV Ni 3+ ions irradiation. The irradiation experiments were carried out at room temperature and 600 °C, and the ion fluences correspond to a calculated peak damage dose of 0.5, 2 and 12 dpa, respectively. TEM results show the formation of solute clusters or dislocation loops with a number density of approximately 5–14 × 10 22 m −3 and sizes between 3 and 10 nm at room temperature due to irradiation induced defects and their evolution. Moreover, the peak shift with increasing ion dose observed in XRD diffraction patterns reveals the lattice distortion induced by ion irradiation. As far as the case of high temperature irradiation, several solute clusters with the same size were observed, whereas the number density was smaller than that of the former case. The calculated indentation values in irradiated samples were found to be much higher in comparison to the unirradiated one, indicating the dose dependent effect of irradiation on hardness. However, in the case of the ion irradiation at 600 °C, the hardness value of samples was significantly decreased. The relationship between ion irradiation induced microstructural evolution and the changes in the mechanical properties of this weld metal is discussed in the context of ion dose and irradiation temperature. [ABSTRACT FROM AUTHOR]

The effect of grain boundary engineering (GBE) on the Te diffusion along the surface grain boundaries was investigated in GH3535 alloy. It can be found that GBE treatment increases obviously the fraction of low-Σ coincidence site lattice (CSL) boundaries, especially the Σ3 ones, and introduces the large-size grain clusters. When the as-received (AR) and GBE-treated (GBET) specimens were exposed to Te vapor, only Σ3 boundaries were found to be resistant to Te diffusion. From the cross section and the surface, the fewer Te-attacked grain boundaries and the thinner corrosion layer can be observed in the GBET sample. The improvement of resistance to Te diffusion in the GBET sample can be attributed to the large size grain-clusters associated with high proportion of the Σ3 n boundaries. [ABSTRACT FROM AUTHOR]

The influence of yttrium (Y) addition on the tellurium (Te) induced intergranular embrittlement of Ni-16Mo-7Cr alloy was investigated. The results show that the addition of Y remarkedly reduces the diffusion depth of Te into the alloy, and thus improves the intergranular cracking resistance of the alloy. The YTe phases on the surface reaction layer and near-surface grain boundaries, plays an important role in fixing Te and slowing down the Te diffusion. Adding more than 0.05 wt% Y into the alloy does not continuously improve the intergranular embrittlement resistance, and even damages the mechanical properties. • Ni 17 Y 2 is formed near the primary M 6 C during casting process for Y-modified alloys. • Y can be segregated in reaction layer, promoting the formation of Y-Te compounds. • Ni 17 Y 2 phase on grain boundaries converted to YTe phase when it encountered Te. [ABSTRACT FROM AUTHOR]

• The constraint effects and mechanism on the creep crack growth of GH3535 alloy under the condition of W/B variation and constant were investigated. • Wedge cracking is the dominant form of crack initiation. • The deformation of the thick specimen is larger, which inhibits the wedge-shaped crack initiation. • Samples with small W/B or initial load are prone to crack branching. To clarify the influence of constraint effects on the creep crack growth behavior and mechanism of GH3535 alloy, creep crack growth tests on compact tension (CT) specimens of various sizes were performed at 650 °C. To reduce dispersion and more clearly compare the crack growth rate and crack initiation time under different constraints, an incremental polynomial was used to obtain the fracture parameter C * and the crack growth rate. Then the fracture parameter C * were used to correlate the crack growth rate and crack initiation time, respectively, and their relationships in double logarithmic coordinates were obtained. The results show that both the crack initiation time and the crack growth rate are affected by the constraint at the crack tip but remain linearly related to the fracture parameter C * in log–log scales. The essence of the constraint effect is that the structure hinders the plastic deformation of the crack tip. If the constraint is large, the plastic deformation corresponding to the crack tip will be small. However, the deformation accompanying the crack propagation of the alloy in this study will increase with increasing constraint. The crack growth rate at the middle thickness of the plain specimen without side grooves is greater than that at the near surface, and the crack profile is inverse parabolic shape, whereas the crack profile of the side-grooved specimen is relatively uniform. [ABSTRACT FROM AUTHOR]

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 M 12 C 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. [ABSTRACT FROM AUTHOR]

The effect of Al addition on Te diffusion behavior of a Ni-16Mo-7Cr base superalloy was investigated at 800 °C for 100 h. The results show that Ni 3 Te 2 and Cr 3 Te 4 are the mainly reaction products formed on the surface of Al-modified alloy. Although no aluminum telluride is detected both on the surface and in the alloy, the addition of Al promotes the formation of compact and continuous Cr 3 Te 4 layer on the surface, which effectively inhibits the diffusion of Te into the alloy. Moreover, Al can also promote the γ' phase formation, and thus improve the tensile property of the alloy significantly. • Effect of Al on intergranular embrittlement behavior of GH3535 alloy in Te vapor was investigated. • Al addition can significantly decrease the diffusion depth of Te into the alloy. • Al addition can promote the formation of Cr 3 Te 4 in the reaction layer. • The optimum content of Al should be 1 % in the alloy. [ABSTRACT FROM AUTHOR]

The catastrophic intergranular cracking had been observed in the low-Cr alloy components in the primary loop of molten salt reactor. Such cracking was related to the coupling effect of fission product tellurium (Te) and fuel salts, whose atomic-level mechanism has puzzled researchers for many decades. In this work, combining multi-scale characterization techniques and theoretical calculations, it was found that Te segregation at grain boundaries can reduce the diffusion barrier of active element along grain boundaries and cause the nano-voids, which further evolve into the intergranular cracking. • Dissolution of active elements causes the formation of nanovoids at grain boundaries. • Nanovoids evolve into the intergranular cracking in the manner of local collapse. • Te segregation at grain boundaries reduces the diffusion barrier of active element. • Te segregation induces intergranular corrosion of alloys in molten salt. [ABSTRACT FROM AUTHOR]

• Corrosion behavior of GH3535 in NaOH, HF and H 2 O 2 solution was studied firstly. • NaOH solution is chemically inert towards the GH3535. • GH3535 suffered pitting corrosion and intergranular corrosion in HF solution. • Oxidation and further reaction in HF solution accelerate the fall-off of carbides. • Fall-off of carbides cause the pitting corrosion of GH3535 in H 2 O 2 solution. Corrosion behaviors of GH3535 immersed in NaOH, HF and H 2 O 2 solutions were investigated at RT for 600 h. The micro-morphology of surface and cross section was systematically characterized by SEM, EDS, EPMA, XRD and Nano Indenter. GH3535 showed excellent corrosion resistance in NaOH solution, but suffered serious pitting corrosion and slight grain boundary corrosion in HF solution, whereas only distinct hole corrosion in H 2 O 2 solution due to the depletion of carbides. Element depletion, fall-off of carbides, decrease of hardness and Young's modulus of GH3535 were detected in HF and H 2 O 2 solution. However, NaOH solution is chemically inert towards GH3535. Corrosion mechanism was also explored based on the experimental measurement and thermodynamic calculation. Results not only provide the theoretical guidance for understanding the anticorrosion mechanism of GH3535 and purifying the molten salt, also expand the application of GH3535. [ABSTRACT FROM AUTHOR]

The behavior and micromechanism of creep crack growth (CCG) of GH3535 alloy at 650 to 750 °C have been investigated experimentally. The relationships among the effective stress intensity factor, fracture parameter C ⁎, and CCG rate were obtained and the effect of temperature on the crack initiation time were analyzed. The results show that it is suitable to use the fracture parameter C ⁎ to correlate the CCG rate at different temperatures and that the CCG rate and fracture parameter C ⁎ have a good linear relationship in a narrow band in the log-log scale. This linear relationship does not depend on temperature when using the fracture parameter C ⁎ to correlate the crack initiation time. During crack propagation, intergranular propagation and dimple transgranular fracture occurred successively. The crack growth is found to be accompanied by oxidation. • Creep crack growth behavior of GH3535 allov used as structural material of TMSR at different temperatures was investigated. • The relationships between crack gowth rate crack initiation time and C* parameter are independent of temperature. • The propagation mechanism of GH3535 at different temperatures was found by observing the fracture and crack tip morphology. • Oxidation promotes the crack growth and the interaction between crack growth and oxidation is described. [ABSTRACT FROM AUTHOR]

• Hot-rolled specimens possess the more Cr 3 Te 4 and MnTe in the scales. • Te tends to be trapped by the primary M 6 C carbides to form flake-like Cr 3 Te 4. • Hot-rolled specimens possess the shallower and fewer Te penetration and cracks. The tellurium (Te) corrosion behaviors of the hot-rolled GH3535 alloy have been investigated systematically in this study as compared with the standard solid-solution treated one. In the hot-rolled alloy, the higher proportion of Cr 3 Te 4 and MnTe can be observed in the corrosion scales. Beneath the scales, Te was found to enrich in the primary M 6 C carbides rather than at the grain boundaries. The blocking effect by the protective Cr 3 Te 4 and MnTe and the trapping effect by the primary M 6 C carbides jointly contribute to the better Te corrosion and cracking resistance of the hot-rolled alloy. [ABSTRACT FROM AUTHOR]