Effects of Pb and Bi on mechanical properties and fracture modes of bcc iron symmetrical tilt grain boundaries

The internal structural material T91 in the accelerator driven subcritical reactor system (ADS) has encountered the challenge of liquid metal embrittlement caused by Pb and Bi. Therefore, it is of utmost importance to gain a comprehensive understanding of their embrittlement mechanism. The effects of Pb and Bi on the surface energy of surfaces and separation work and fracture toughness of grain boundaries in body center cubic (BCC) Fe are examined using first-principles calculation. Pb and Bi atoms reduce the surface energy and grain boundary cohesion strength of BCC Fe, with a more significant decrease in surface energy. Meanwhile, the segregation of Pb and Bi in the grain boundary reduces the fracture toughness of the grain boundary. By comparing different surface energy and separation work, it is found that BCC pure Fe is more prone to intergranular fracture, while the presence of Pb and Bi causes cleavage fracture. This elucidates the low-temperature fracture mode of BCC-Fe and reveals the potential mechanism of brittle cleavage fracture caused by Pb and Bi in BCC-Fe. The derived results are discussed in terms of electronic structure and show good agreement with experimental results in the literature, ultimately contributing to a thorough understanding of the intrinsic mechanism of liquid metal embrittlement of T91 steel.