First-principles Study On The Effect Of Alloying Elements On The Mechanical Properties Of Fe

During the working process of nuclear reactor,liquid metal embrittlement(LME)will occur after long-term contact between T91 steel and liquid lead-bismuth eutectic alloy(LBE),resulting in a sharp decline in the mechanical properties of T91 steel,threatening the safe operation of the reactor.However,with regard to the mechanism of Pb and Bi embrittlement of T91 steel,the reports in the literature are inconsistent and further in-depth studies are needed.In this paper,first-principles calculations based on density functional theory are used to study the effects of alloying elements Cr,Mo,Pb and Bi on the mechanical properties of BCC Fe bulk,surface and grain boundaries,respectively.By comparing and analyzing the obtained results with existing data in the literature,the embrittlement mechanism of Pb and Bi on the Fe matrix was proposed,and the in-depth discussion from the electronic structure level was carried out to deepen the understanding of the LME mechanism.The main results obtained are as follows:(1)The effects of Cr,Mo,Pb and Bi on the stacking fault energy and ductility of BCC Fe {110}<111> and {11(?)}<111> slip systems were studied.It is found that the alloying elements Cr and Mo have little effect on the surface energy,unstable stacking fault energy and ductility of BCC Fe,while Pb and Bi significantly reduce the surface energy,unstable stacking fault energy and ductility of BCC Fe,and Pb and Bi make the reduction of surface energy more significant,resulting in a decrease in the toughness of BCC Fe,and the influence of Bi on the ductility of BCC Fe is greater than that of Pb.(2)The effects of alloying elements Cr,Mo,Ni,Pb and Bi on the stability,cohesive strength and fracture toughness of BCC Fe∑3(111)[1(?)0]inclined symmetric grain boundaries were investigated.The results show that these alloying elements reduce the grain boundary energy and improve the stability of the grain boundary.Compared with Cr,Mo and Ni,Pb and Bi have larger segregation energy,indicating that Pb and Bi have oriented grain boundary segregation.Mo increases the bonding strength of grain boundaries,while Pb and Bi weaken the bonding strength of grain boundaries;the segregation of Pb and Bi at grain boundaries greatly reduces the fracture toughness of grain boundaries and makes grain boundaries brittle.(3)The effects of Pb and Bi on the separation work and fracture toughness of several common surfaces and grain boundaries in BCC Fe were investigated.It is found that Pb and Bi atoms reduce both the surface energy and grain boundary cohesion strength of BCC Fe,but reduce the surface energy more significantly.At the same time,both Pb and Bi grain boundary segregation reduce the fracture toughness of the grain boundary and make the grain boundary brittle;comparing different surface energies and separation work,it is found that BCC pure Fe is more prone to intergranular fracture,while the presence of Pb and Bi makes BCC Fe more prone to cleavage fracture.