Electronic structure and bonding of FCC iron embrittlement

S. SIMONETTI, D. REY SARAVIA, G. BRIZUELA, A. JUAN

Bratislava, Slovakia

Congreso; 8th Conference on Solid State Chemistry; 2008

IUPAC

Severe embrittlement can be produced in many metals by small amounts of hydrogen. The interactions of hydrogen with lattice imperfections are important and often dominant in determining the influence of this impurity on the properties of solids. Hydrogen-metal interactions have been shown to strongly control the mechanisms of hydrogen embrittlement and stress corrosion cracking of numerous metals and alloys. The interaction between two-hydrogen atoms and a g-Fe structure having a vacancy has been studied using a cluster model and a semi-empirical theoretical method. The energy of the system was calculated by the atom superposition and electron delocalization molecular orbital (ASED-MO) method. The electronic structure was studied using the concept of density of states (DOS) and crystal orbital overlap population (COOP) curves. For the study of a sequential absorption, the hydrogen atoms were positioned in their energy minima configurations, near to the octahedral sites neighboring the vacancy. The changes in the electronic structure of Fe atoms near the vacancy were also analyzed. The interactions mainly involve Fe 4s atomic orbitals. The contribution of Fe p and Fe 3d orbitals is less important. The Fe-Fe bond weakened as new Fe-H and H-H pairs were formed. The effect of H atoms is limited to its first Fe neighbors. The Fe nearest neighbours overlap population decreases to about 46 % with the introduction of the H atoms. Fe-H bonding is achieved at expense of weakening the metal-metal nearest bonds. There is no bonding between the H atoms but some H-H interaction is observed. The detrimental effect of H atoms on the Fe-Fe bonds can be related to the mechanism for embrittlement in g-Fe.