Liquid lead-bismuth embrittlement effects on unirradiated and irradiated ferritic/martensitic steels for nuclear applications

In liquid metal spallation targets such as the MEGAPIE (the megawatt pilot experiment) target and the future ADS (accelerator driven system) spallation targets, which utilize liquid lead-bismuth eutectic (LBE) as the target material, liquid metal embrittlement (LME) effects on the target structural materials are considered as one of the critical issues that determine the lifetime of the targets. However, nowadays, the LBE embrittlement effects are not yet well understood, particularly under irradiation conditions. The aim of this work was: (a) to investigate the effects of LBE embrittlement on the mechanical properties of ferritic-martensitic (FM) steels (candidate materials for applications in spallation targets) in various states, and (b) to study the mechanisms of LBE embrittlement effects on the mechanical properties of FM steels. These two goals have been reached by studying LBE embrittlement effects on the mechanical properties of the T91 steel in various conditions: (i) the standard normalized and tempered condition, (ii) hardened by tempering at lower temperatures, and (iii) after irradiation under high energy proton and spallation neutron mixed spectrum. Mechanical tests such as slow-strain-rate tensile (SSRT) tests and 3-point bending tests have been performed to characterize the mechanical properties of the T91 steel and to determine the effects of LBE embrittlement on these mechanical properties. Microstructural analyses such as transition electron microscopy (TEM) and scanning electron mirocopy (SEM) observations have been conducted to obtain the microstructural information needed for understanding the embrittlement mechanisms. SSRT tests on the T91 steel in the standard metallurgical (SM) state, tempered at 760°C and denoted as HT760, revealed that it may encounter LBE embrittlement in the temperature range of 300 to 425°C. For the first time, a well defined "ductility trough" for a FM steel – LBE system was evidenced from the reduction of the fracture strain (or total elongation) of the T91 FM steel in this temperature range. SEM observations of the fracture surfaces showed that the fracture mode of the specimens suffering embrittlement effects is brittle transgranular cleavage. SSRT tests on the hardened T91 steel, tempered at 500 or 600°C and denoted as HT500 and HT600, respectively, showed more pronounced LBE embrittlement effects. In comparison to that of HT760, the "ductility troughs" of HT600 and HT500 cover a wider temperature range. The LME onset temperature TE is not exactly known but most probably lower than 150°C, which means close to the melting point temperature (125°C) of LBE. The ductility recovery temperature TR of HT500 is higher than that of HT760 and HT600. The results of SSRT tests on the HT600 and HT500 materials demonstrate clearly that the LBE embrittlement effects on the tensile properties of FM steels are strongly enhanced by hardening. Both the influenced temperature range and the degradation level increase with the hardening amount. The fracture mode of HT600 and HT500 materials tested in liquid LBE at T ≤ 450°C showed mainly brittle transgranular cleavage. SSRT tests on the T91 and F82H FM steels irradiated at temperatures in the range of 140-500°C to doses in the range of 0.048 to 20 dpa revealed significant irradiation-induced hardening and embrittlement effects (loss of ductility) as compared to the unirradiated ones. The SSRT tests performed on irradiated specimens in liquid LBE revealed that the specimens undergo a further reduction in ductility. The ductility of some irradiated specimens is reduced to a very low level of about 2-3%. 3-point bending tests on the T91 steel showed that the load-displacement curves of specimens tested in liquid LBE are different from those of specimens tested in Ar. The evaluated J-integral values of specimens tested in liquid LBE are always lower that of the specimens tested in Ar. It means that the crack propagation in a specimen tested in liquid LBE needs less energy as compared to a specimen tested in Ar. These results suggest that the LBE embrittlement is so-called "crack propagation controlled" rather than "crack nucleation controlled". SEM observations revealed that the crack propagation mode undergoes a ductile-to-brittle transition. In liquid LBE the fracture toughness of FM steels can be reduced by about 20% as compared to that in inert Ar environment. 3-point bending tests on the hardened T91 materials HT500 and HT600 demonstrated that the LBE embrittlement effects are much more drastic for hardened materials, similarly to what was observed as a result from SSRT tests. The JQ value is reduced by as much as 95% for both HT600 and HT500 materials tested in liquid LBE. The hardened materials exhibit unstable crack propagation behavior in liquid LBE in a wide temperature range of 150 to 500°C. 3-point bending tests on the irradiated T91 steel revealed not only irradiation-induced embrittlement effects but also LBE embrittlement effects. The fracture toughness of the irradiated specimens was found to decrease further as a result from testing in liquid LBE. SEM observations of the fracture surface showed a mixture of intergranular and transgranular fracture. The LME phenomena evidenced for the T91 steel – LBE couple could be interpreted using the model of adsorption-induced reduction in cohesion of atomic bonds combined with the Kelly-Tyson-Cottrell (KTC) criterion (σ/τ ≥ σmax/τmax). According to the KTC criterion, the tendency to fail by cleavage increases as (σmax/τmax) decreases. The σmax value can be greatly reduced by adsorption of liquid Pb or Bi atoms at crack tips, while the τmax value can be increased by hardening the steel by means of either deformation, or tempering, or irradiation. Practically, the present results indicate that a great attention should be paid to application of FM steels in liquid LBE, since the mechanical properties of FM steels, such as the ductility and the fracture toughness, can be substantially degraded by LBE embrittlement effects, particularly under irradiation conditions as the LBE embrittlement are enhanced by irradiation-induced hardening. Fundamentally, the observations made in this work suggest that the mechanism responsible for liquid LBE-induced embrittlement effects on FM steels can be essentially interpreted by using the adsorption-induced reduction in cohesion of atomic bonds model combined with the KTC criterion for brittle cleavage fracture.