The bonding features and environmental embrittlement of Ll2 intermetallic compounds

The discrete variational Xα method (DV Xα) was applied to calculate the bond order and charge transfer of Ll2 ordered Ni3X (X = Si, Al, Fe) compounds. We found that there is minor charge transfer between the constituent atoms, and X atoms lose some charge. The positively charged X atoms, which we call the reactive centers, bond with the oxygen atoms carrying some negative charge due to electrostatic force. A reaction driving force is proposed and analyzed. We apply the difference of the bonding capacity of X atoms in the bulk crystal and at the surface to predict the tendency of the generation of atomic hydrogen, which is considered the fundamental reason for environmental embrittlement. The possibility of hydrogen generation is in the order of Ni3Si > Ni3Al > Ni3Fe.