Microstructural Investigation of LME Crack Initiated in Ferritic/Martensitic Steel T91 Loaded in Liquid Lead-Bismuth Eutectic at 300 °C

Abstract

Liquid lead-bismuth eutectic (LBE) is one of the candidate materials for advanced nuclear systems. The structural materials used in contact with LBE are selected according to the resistance to liquid metal corrosion, irradiation embrittlement, and compatibility with the coolant. However, simultaneous presence of mechanical strain and LBE environment can induce liquid metal embrittlement (LME) in these materials. In this study, a specimen from candidate ferritic-martensitic steel T91 was tested by Constant Extension Rate Tensile (CERT) test exposed to PbBi environment with oxygen concentration 6 × 10−6 wt % at 300 °C up to rupture. Post-test examination using scanning electron microscopy (SEM) showed a deep crack indicating features of LME in the plastic strained region of the tested specimen. Further investigations focused on characterization of the fracture path and microstructure determination using focused ion beam (FIB) and energy dispersive X-ray spectrometry/electron backscatter diffraction (EDX/EBSD). This observation revealed that the dominant LME failure mode of the observed crack is translath or transgranular and the crack stopped at the high-angle grain boundary. The role of oxides in the crack initiation is discussed.

Environmentally assisted cracking of T91 ferritic-martensitic steel in heavy liquid metals

“…The paper summarizes the key results of the new type of experiments for the T91 steel exposed in two lowflow HLM environments (Pb, PbBi-LBE) as well as the data, which were partially published in previous papers (Duchoň et al, 2018;Hojná et al, 2018a,b,c;Halodová et al, 2019). The conditions for occurrence (initiation) of the LME/EAC in the T91 in contact with the HLM are explained and discussed in the paper.…”

“…Two crack types appearing only in LBE were observed as LME/EAC: (i) the edge cracks recognized owing to their cleavage-like surface morphology; and (ii) few open cracks that occurred in the region of maximum plastic strain and stress. The depth of the one of the open cracks ( Figure 6D) was measured using FIB, as referred elsewhere (Halodová et al, 2019). The crack size was 46 μm in depth, but only about 20 μm on the surface.…”

“…To verify the sensitivity of the T91 to the LME/EAC in simulated HLM operational conditions with application of the allowable stresses and oxygen-controlled chemistry, a new type of experiments was performed recently. The testing was focused on studying the LME/EAC crack initiation conditions in static HLM (Duchoň et al, 2018;Hojná et al, 2018a,b,c;Halodová et al, 2019). The sensitivity tests in the dynamic HLM are still to be performed.…”

Wetting Behavior of LBE on Corroded Candidate LFR Structural Materials of 316L, T91 and CLAM

“…In addition, the formation of the surface corrosion layer separates the LBE from the material matrix, and its non-wetting characteristic is not conducive to the interdiffusion of elements between LBE and the matrix of material, which is beneficial to reduce the corrosion rate of the material surface. However, this hindering effect is not permanent [ 8 ]. In practical applications, the flowing LM helps LBE to overcome the energy barrier of the oxide structure on the uneven surface, leading to the penetration of the LBE.…”

“…At first, the wettability between material surface and LBE is a key parameter affecting the liquid metal embrittlement (LME) behavior of the structural materials. Generally, materials with poor LBE wettability possess a lower probability of LME [ 3 , 4 , 5 , 6 , 7 , 8 ]. Secondly, the surface wettability is directly related to the corrosion properties of the materials.…”