Changgong Meng

The bonding features and environmental embrittlement of Ll2 intermetallic compounds

The discrete variational X alpha method (DV X alpha) was applied to calculate the bond order and charge transfer of L1(2) ordered Ni(3)X (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.

Relationship between bonding characteristics and mechanical properties of L12 intermetallics

Abstract The bonding characteristics of A 3 B L1 2 intermetallics have been studied by the discrete variational X α cluster method. Two concepts, i.e. similarity of bonding strength and delocalization of bonding electrons, are introduced to explain the ductility or brittleness of the materials. The analysis of bonding characteristics shows that the strength of the Aue5f8A bond is similar to that of the Aue5f8B bond in A 3 B L1 2 intermetallics, and the delocalized degree of bonding electrons is different between the ductile and brittle intermetallics. This is a dominant factor in governing the ductile/brittle behavior of the materials. Environmental embrittlement in some intermetallics is related to the reactive centers in grain boundaries, which is due to the directional bonding and a small amount of charge transfer, and the reactive centers weaken the ability against the attack of the environmental species. The effect of stoichiometry on ductility contributes to the degree of delocalized bonding electrons. Mechanical strength of the materials is correlated with the covalent bond strength.