China low activation martensitic (CLAM) steel has been considered as the primary candidate structural material for application in fusion systems because of its good thermal conductivity and low thermal expansion ratio. In this work, the tensile behavior of the CLAM steel in liquid lead-bismuth eutectic was investigated to assess the compatibility of CLAM steel with liquid metal. The CLAM steel was tempered before test. The tensile tests were performed in liquid lead-bismuth eutectic and argon gas respectively at different temperatures ranging from 200 ℃ to 500 ℃ under different strain rates. All the specimens ruptured in ductile manner in argon gas environment, exhibiting obvious necking and dimples on the fracture surface. For those tested in liquid lead-bismuth eutectic, the specimens behaved ductile fracture when the test temperature was below 250 ℃, but fractured in brittle cleavage manner in the temperature range of 300~450 ℃. The embrittlement mainly occurred after necking, showing typical river pattern on the fracture surface with slight necking trace, and obvious cracking points were observed to initiate at the fracture edge and propagated towards the center of the specimen, namely, the appearance of the ductility trough that shows significant degradation in total elongation while no noticeable differences in strength compared with the tested specimens in argon gas environment. Furthermore, the brittle fracture disappeared and total elongation recovered when the tensile tests were performed out of the embrittlement temperature range. In slower strain rate tensile (SSRT) tests, the temperature range of the ductility trough greatly expanded and brittle fracture occurred at temperatures below 250 ℃. The results indicate that CLAM steel is susceptible to embrittlement in liquid lead-bismuth eutectic. This is because the contact of the liquid metal with the cracking tip leads to a decrease of the interfacial energy, which further reduces the critical cleavage stress and facilitates the brittle fracture. Both temperature and strain rate are evidenced in this work to have an effect on the embrittlement of CLAM steel.
Key words:
CLAM steel
;
liquid metal embrittlement
;
Pb-Bi eutectic
;
temperature
;
strain rate
在研究铁素体/马氏体(F/M)钢与液态金属的兼容性时发现,CLAM钢和EUROFER97均会受到Pb-Li合金的腐蚀[5,6]。除腐蚀问题外,原来塑性很好的金属材料在液态金属中会发生脆化现象,延伸率显著下降,断裂形式表现出脆性断裂行为。液态金属致脆(liquid metal embrittlement, LME)的机制复杂,虽然已有很多相关研究,但对其机理目前尚无统一的认识[7]。现有研究[8,9]表明,脆化现象是材料的组织状态、服役应力条件、接触状态、温度和应变速率等多种因素共同作用的结果。通常认为,液态金属脆化现象只是在一定的温度区间内才会出现,在低于或高于该温度区间时,脆化现象就会消失,且韧性会回复到与未接触液态金属时相当的水平[8,9]。此外,应变速率对液态金属脆化现象也有非常大的影响[10~12]。
Fig.2
Tensile curves of China low activation martensitic (CLAM) steel in Ar and LBE under tensile strain rate of 0.15 mm/min at 250 ℃ (a), 300 ℃ (b), 400 ℃ (c) and 500 ℃ (d)
Fig.4
Variations of strength of CLAM steel in Ar and LBE under different tensile rates of 0.15 mm/min (a) and 0.015 mm/min (b) (σs—yield strength, σb—ultimate tensile strength)
Fig.5
Variations of total elongation of CLAM steel in Ar and LBE under different tensile rates of 0.15 mm/min (a) and 0.015 mm/min (b) (δ— total elongation)
Fig.6
Macro (a, c, e, g) and micro (b, d, f, h) tensile fracture SEM images of CLAM steel in Ar under strain rate of 0.15 mm/min at 250 ℃ (a, b), 300 ℃ (c, d), 400 ℃ (e, f) and 500 ℃ (g, h)
Fig.7
Macro (a, c, e, g) and micro (b, d, f, h) tensile fracture SEM images of CLAM steel in LBE under strain rate of 0.15 mm/min at 250 ℃ (a, b), 300 ℃ (c, d), 400 ℃ (e, f) and 500 ℃ (g, h)
Fig.8
Macro (a, c, e, g) and micro (b, d, f, h) tensile fracture SEM images of CLAM steel in Ar under strain rate of 0.015 mm/min at 200 ℃ (a, b), 250 ℃ (c, d), 450 ℃ (e, f) and 500 ℃ (g, h)
Fig.9
Macro (a, c, e, g) and micro (b, d, f, h) tensile fracture SEM images of CLAM steel in LBE under strain rate of 0.015 mm/min at 200 ℃ (a, b), 250 ℃ (c, d), 450 ℃ (e, f) and 500 ℃ (g, h)
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The microstructure of China Low Activation Martensitic steel (CLAM) and its influence on mechanical properties were investigated. The tensile test showed that the strength of CLAM (HEAT 0603A) was higher than that of HEAT 0408B at room temperature, and the reverse results were obtained at elevated temperatures. The results indicated that the microstructure was composed of dispersived carbide particles and lath martensite with high dislocation density. The main precipitation phases were Cr-rich M 23 C 6 carbides precipitated mainly along the lath boundaries and prior-austenite grain boundaries and Ta-rich MX particles precipitated mainly in the laths and lath boundaries. The finer lath was the main reason for the higher strength of HEAT 0603A compared with HEAT 0408B at room temperature; contrasted with the lower strength at high temperature. Heavier hot forging deformation degree was considered as the main possible reason for the decrease of martensite lath width in HEAT 0603A.
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The embrittlement effect of liquid lead–bismuth eutectic (LBE) on martensitic steel T91 has been studied by performing slow-strain-rate tensile (SSRT) tests in static LBE with about 102wppm oxygen at temperatures ranging from 25002°C to 42502°C. Two groups of samples were used. Group-I samples with microcracks on the lateral surfaces indicated clearly LBE embrittlement effect at temperatures 8230002°C, while Group-II samples without microcracks did not show the effect. The LBE embrittlement effect occurred after the necking of specimens started. The yield and ultimate tensile strengths and uniform elongation were not affected. SEM observations showed the specimens ruptured in a brittle fracture mode when the embrittlement occurred. It is concluded that the requirements for the susceptibility of LBE embrittlement effect on the T91 steel are: surface cracks or flaws, wetting and a certain level of stress concentration at crack tips.
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Abstract The addition of silicon to steels has been found to strongly increase the material’s resistance to corrosion in liquid lead–bismuth eutectic environment; however the mechanical properties of ferritic–martensitic elevated silicon steels are also strongly affected by the liquid metal environment.This paper discusses the assessment of the mechanical properties of silicon enriched high Cr steels T91, T91-Si, EP823, S2439 and S2440 when in contact with LBE. The stability and strength of the material’s oxide layer was examined by nano-indentation and the oxide layer of EP823 was found to be harder and stiffer than that of T91. Furthermore, the nano-indents caused cracking of the oxide layer at the matrix–oxide interface in EP823 while no cracks were observed due to nano-indentation in the T91 oxide layer. These findings are then related to the respective material’s susceptibility to liquid metal embrittlement.
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The effect of liquid lead–bismuth eutectic on 316L and T91 steels at 16002°C has been studied as a function of strain rate, using a centre cracked in tension specimen adapted for the study of crack propagation. Brittle fracture, characterized by elongated river cracks on all the fracture surfaces, indicates that T91 is sensitive to the embrittlement by LBE. This embrittlement effect is very pronounced at low deformation rate (6510 615 02mm02s 611 ). A ductile–brittle transition is observed in the high strain rate range investigated. In the transitory regime, there is a competition between the growth of dimples and brittle cracking induced by the liquid metal. Ductility recovery is complete at the highest investigated deformation rate. The mechanical properties of the 316L steel are not clearly affected by the presence of LBE, in spite of a modification in the plastic deformation mode which strongly affects fracture surfaces.
Van den BoschJ, SapundjievD, AlmazouziA. Effects of temperature and strain rate on the mechanical properties of T91 material tested in liquid lead bismuth eutectic[J]. J. Nucl. Mater., 2006, 356: 237
ABSTRACT The effects of temperature and strain rate on the susceptibility of one of the most promising candidate materials for ADS, Ferritic/ Martensitic steel (T91), to liquid metal embrittlement have been investigated in the temperature interval 150 to 450掳C, at strain rates between 1.10-3 s-1 and 1.10-6 s-1. The effect of the liquid lead bismuth eutectic has been evaluated by comparison between tests in the liquid metal and in Ar with 5% H2. Although the untreated T91 material did not undergo liquid metal embrittlement and there was no change in the tensile properties as function of temperature or strain rate due to the liquid metal, the pre-exposed T91, did show a decrease in total elongation. The total elongation of the untreated T91 specimens in liquid lead bismuth and in H2/Ar-gas mixture followed the same dependence on the temperature up to 245掳C. At temperatures above 375掳C no results are available from testing in controlled gas environment however reference data measured on the same batch of steel in air are used for comparison. The results are discussed in terms of self-healing mechanism and crack initiation processes.
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The present work aims to investigate the susceptibility of ferritic/martensitic steels of different strength to the embrittlement of liquid Pb–Bi eutectic (LBE). Slow strain rate tensile (SSRT) tests on specimens of the T91 steel in three tempering conditions at 500, 600 and 76002°C were conducted in Ar and in LBE at temperatures between 150 and 50002°C. For the specimens tempered at 76002°C (the normal tempering condition) the susceptibility of the steel to LBE embrittlement appeared at temperatures between 300 and 45002°C. With increasing the strength of specimens by lowering the tempering temperature, specimens tempered at 600 and 50002°C demonstrated more pronounced embrittlement effects, reflected by wider and deeper ‘ductility-troughs’. The results suggest that ferritic/martensitic steels with higher strength are more susceptible to LBE embrittlement. The LBE embrittlement effects can be attributed to the decrease of fracture stress resulted from the ‘weakening inter-atomic bond’ by LBE contacting at crack tips.
LiuJ, Huang QY, Jiang ZZ, et al.Effect of strain rate on the mechanical properties of CLAM steel in liquid PbLi eutectic[J]. Fusion. Eng. Des., 2013, 88: 2603
Abstract The liquid PbLi blanket is one of the most promising designs for fusion reactors and under extensive research in the world. China Low Activation Martensitic (CLAM) steel has been chosen as the primary candidate structural material for PbLi blanket in China. The effect of strain rate on mechanical properties of CLAM steel with and without liquid PbLi under Ar atmosphere has been investigated with slow strain rate tensile (SSRT) test at 480 °C. SEM observation showed the specimens tested in both environments exhibited ductility fracture with dimpled behavior. The primary tensile results tested with and without PbLi indicated that the uniform elongation of CLAM specimens was independent of strain rate, whereas the total elongation decreased with increasing strain rate. The strength properties tested with liquid PbLi under Ar increased with strain rate from 8.33 × 10616 s611 to 4.17 × 10614 s611, but which decreased at 8.33 × 10614 s611.
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ABSTRACT The susceptibility of T91 and EUROFER97 to liquid metal embrittlement (LME) in lead alloys has been examined under various conditions. These steels, similar in microstructure and mechanical properties in the unirradiated condition were tested for their susceptibility to LME as function of temperature (150–450 °C) and strain rate (1.10-3 – 1.10-6 s-1). Also, the influence of pre-exposure and surface stress concentrators was evaluated for both steels in, respectively, liquid PbBi and PbLi environment. To assess the LME effect, results of the tests in liquid metal environment are compared with tests in air or inert gas environment. Although both unirradiated and irradiated smooth ferritic–martensitic steels do not show any or little deterioration of mechanical properties in liquid lead alloy environment compared to their mechanical properties in gas as function of temperature and strain rate, pre-exposure or the presence of surface stress concentrators does lead to a significant decrease in total elongation for certain test conditions depending on the type of liquid metal environment. The results are discussed in terms of wetting enhanced by liquid metal corrosion or crack initiation processes.
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Reduction of aqueous silver nitrate by hydrazine dihydrochloride in weakly alkaline solution results in a polydisperse colloid that is stable for many months without addition of any stabilizing compounds. The average size of the predominantly spherical particles depends on the initial concentration of silver ions, ranging between 40 and 70 nm in diameter. The colloidal solutions exhibit a characteristic absorption in the blue region of the visible spectrum and are not turbid below a formal silver concentration of 4.5 x 10(-4) M. With colloids prepared from 1.5 x 10(-4) M silver(I), the SERS spectra of dyes such as nile blue A could be recorded from a solution with concentrations as low as 10(-10) M, whereas no SERS signal was observed for dye concentrations higher than 10(-4) M. The maximum signal intensity was obtained at a concentration of about 10(-7) M. With colloids prepared from greater than or equal to3 x 10(-4) M silver(I), no SERS signal was obtained from highly diluted solutions, but the concentration limit for the maximum signal intensity of around 10(-7) M became even sharper. The thus prepared silver colloids can therefore be recommended for qualitative detection of certain organic compounds-in the parts per billion range as well as for a semiquantitative determination in the parts per million range.
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Two potential problems are encountered in the case of intimate contact between liquid metals and metallic substrates: grain boundary wetting and liquid metal embrittlement (LME) which both induce a degradation of the mechanical properties. Tensile tests were carried out on a 9% Cr 1% Mo martensitic (Grade 91) steel in a liquid lead environment at temperatures ranging between 623 and 773 K. The Grade 91 steel was submitted to heat treatments in order to modify its hardness and also to produce either ferritic or martensitic grains. Smooth and notched specimens were used. We found out that by combining adapted heat treatments and the notch effect, it is possible to create conditions severe enough that lead to LME. Our experimental observations (transgranular failure) are compatible with the expectations of traditional mechanisms based on a reduction of the surface energy and/or adsorption induced chemical bond softening at the steel surface in contact with liquid lead.
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Martensitic steel T91 will be used for the liquid lead bismuth eutectic (LBE) container of the MEGAPIE target. The irradiation assisted LBE corrosion and embrittlement effects on the behaviours of T91 steel have been studied by performing the LiSoR experiments, where T91 steel was irradiated with 72 MeV protons to doses up to 0.2 dpa at temperatures above 300 掳C in flowing LBE with or without mechanical stress. Tensile tests on the T91 steel after irradiation demonstrated that the irradiation assisted LBE embrittlement was not evident at such a low irradiation dose. The irradiation produced small defect clusters and dislocation loops were observed in the inner tensile-stressed specimens (ITS-specimens) of LiSoR-3 and LiSoR-4. The main features of the dislocation structure in the ITS-specimen of LiSoR-2 and the test-section tubes (TS-tubes) were dislocation tangles and dislocation networks, and small defect clusters and dislocation loops were hardly observed due to the high irradiation temperatures. The TEM observations support the results of the tensile tests.
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For the R&D of high power spallation targets, one of the key issues is understanding the behavior of structural materials in the severe irradiation environments in spallation targets. At PSI, several experiments have been conducted using the targets of the Swiss spallation neutron source (SINQ) for studying radiation damage effects induced by high energy protons and spallation neutrons. As well, experiments have been performed to investigate liquid lead-bismuth eutectic (LBE) corrosion and embrittlement effects on T91 steel under irradiation with 72聽MeV protons. In this paper, an overview will be given showing a selection of results from these experiments, which include the mechanical properties and microstructure of ferritic/maretensitic (FM) steels (T91, F82H, Optifer etc.) and austenitic steels (EC316LN, SS 316L, JPCA etc.) irradiated to doses higher than ever attained by irradiation in a spallation environment, and the behaviors of T91 irradiated with 72聽MeV protons in contact with flowing LBE.
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ABSTRACT The TWIN ASTIR irradiation program [1] was aimed at determining the separate and possibly synergistic effects of a liquid lead–bismuth eutectic (LBE) environment and neutron irradiation. The materials in one capsule were irradiated in contact with LBE to a dose of about 1.5 dpa at a temperature between 460 and 490 °C and subsequently tested in liquid lead–bismuth eutectic environment at temperatures between 200 °C and 450 °C. This paper discusses the tensile results of T91 and 316L with and without irradiation while in contact with liquid lead–bismuth eutectic. The ferritic–martensitic T91 was softened by 50–100 MPa while the austenitic 316L steel showed a slight decrease in total elongation. Although no irradiation hardening was observed at the elevated irradiation temperature, the ferritic–martensitic T91 steel was prone to a certain extent of liquid metal embrittlement at 200 °C, 350 °C and 450 °C which was not affected by irradiation. The decrease in total elongation of the 316L is thought to be due to the neutron irradiation effect.
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Abstract In the present work, LBE (lead–bismuth eutectic) embrittlement effects on ferritic/martensitic (FM) steels have been studied by conducting slow-strain-rate tensile (SSRT) testing on T91 and F82H steels either in Ar or in liquid LBE after irradiation to doses up to about 20 dpa in a mixed spectrum of spallation neutrons and high energy protons. SSRT tests were performed at temperatures in the range of 150–500 °C. Tests in Ar revealed significant irradiation-induced hardening and embrittlement effects (loss of ductility) as compared to the unirradiated ones. Tests in LBE showed additional embrittlement effects induced by LBE, which increased with irradiation-induced hardening. As a consequence, the fracture strain of irradiated specimens was reduced to a very low level of about 2–3%. A combination of the model of adsorption-induced reduction in cohesion of atomic bonds and the Kelly–Tyson–Cottrell criterion (σ/τ 82 σmax/τmax) for brittle cleavage fracture was used to interpret qualitatively the observation reported in the present paper.
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Abstract Although the risk of embrittlement of materials exposed to liquid metals has been recognized for many years, its prediction reMayns problematical insofar as the knowledge of the mechanisms involved in the phenomenon is limited. However, Liquid Metal Embrittlement (LME) is of prime interest because the risk of damage exists wherever the handling of liquid metals is required in various industrial or scientifical fields (chemical plants, power-producing systems, soldering process, ...). The interest for this phenomenon needs thus to be pursued. The present paper reviews experimental results about the occurrence of LME and the influence of different parameters, and presents a number of mechanisms which have been proposed to explain LME.
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Abstract A mechanism is advanced for the reduced fracture stress and ductility of metals tested in a liquid metal environment compared to the corresponding properties in air. It is proposed that if the stress necessary to extend a crack is calculated on the basis of the rupture of atomic bonds by tensile stresses at the crack tip one can account for the observed effects. Experimental data on polycrystalline cadmium wetted by liquid gallium are used to illustrate application of the proposed mechanism. The mere act of coating the walls of a cleavage crack with liquid metal does not stabilize the crack even though previous thermodynamic interpretations of the effect of the environment on surface energy suggest otherwise.
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The T91 steel exhibited in general a high degree of ductility. However, even with oxygen saturated LBE, it has been possible to observe LME at low strain rate. Furthermore, low oxygen content in LBE and an increase in temperature promoted the LME. It turns out that the strain rate appeared as the critical parameter for the occurrence of LME of the T91 steel in LBE.
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The liquid sodium embrittlement susceptibility of the T91 steel in a standard metallurgical state has been studied with varying dissolved oxygen levels. The experimental procedure consists of a pre-exposure wetting step in static sodium before mechanical testing in liquid sodium. The oxygen impurity plays the role of a wetting promoter that facilitates LME of the T91 steel rather than taking part in the fracture process. Brittle grain boundary decohesion is observed using transmission electron microscopy on focused ion beam prepared foils. This settles the issue about the susceptibility of steels to liquid metal embrittlement in liquid sodium.
Martı́n FJ, SolerL, HernándezF, et al. Oxide layer stability in lead-bismuth at high temperature[J]. J. Nucl. Mater., 2004, 335: 194
Materials protection by ‘in situ’ oxidation has been studied in stagnant lead–bismuth, with different oxygen levels (H 2 /H 2 O ratios of 0.3 and 0.03), at temperatures from 535 °C to 600 °C and times from 100 to 3000 h. The materials tested were the martensitic steels F82Hmod, EM10 and T91 and the austenitic stainless steels, AISI 316L and AISI 304L. The results obtained point to the existence of an apparent threshold temperature above which corrosion occurs and the formation of a protective and stable oxide layer is not possible. This threshold temperature depends on material composition, oxygen concentration in the liquid lead–bismuth and time. The threshold temperature is higher for the austenitic steels, especially for the AISI 304L, and it increases with the oxygen concentration in the lead–bismuth. The oxide layer formed disappear with time and, after 3000 h all the materials, except AISI 304L, suffer corrosion, more severe for the martensitic steels and at the highest temperature tested.
