Insights into fatigue crack propagation mechanism of T91 steel in liquid lead-bismuth eutectic at 150–450oC

摘要

Liquid Pb-Bi embrittlement of alloys can cause catastrophic brittle fracture, which is a critical issue for the development of lead-cooled fast reactor (LFRs) [1]. However, the micromechanism of liquid Pb-Bi embrittlement for alloys has remained absent in the past decades. The key equipment materials of LFRs are used in high temperature liquid lead-bismuth eutectic (LBE) environment, and corrosion fatigue is one of the main potential failure forms, and also the focus of design, safety rev iew, operation and life evaluation [2]. T91 steel is one of the preferred structural materials with excellent radiation dama ge resistance and good medium and high temperature properties [3]. Here we conducted fatigue crack propagation tests of T91 steel in liquid LBE at 150-450oC. The FCG rate increased gradually with the increase of temperature at 150–350oC, is firstly low at 450oC and then comparable with that at 350 oC. The macroscopic trans-granular cracks preferentially propagated along the deformation - induced low -angle grain boundaries (LAGBs) near the crack tip at the microscopic scale. We use aberration -corrected transmis sion electron microscopy to directly capture the segregation of Pb and Bi atoms ahead of the crack tip. Intergranular precipitation-enhanced wetting of matrix by liquid LBE results in the reduction of atomic bonding force and formation of micro -cracks along the LAGBs, which triggers brittle cracking.