Hydrogen and carbon interaction in a FeNi alloy with a vacancy

The bonding of hydrogen and carbon to Fe and Ni in a 50:50 alloy is analysed using density functional calculations. The changes in the electronic structure of a L10 alloy upon C and H introduction at a vacancy region are addressed and a comparison with H or C in pure metals is drawn. H in bulk FeNi alloy with a vacancy locates at a tetrahedral site shifted towards the vacancy. Instead, C prefers an octahedral site (Fe based). The vacancy acts as strong traps of both C and H. Fe–Ni atoms are initially more strongly bonded to each other due to the vacancy formation. Consequently, the Fe–Fe, Fe–Ni and Ni–Ni bond strengths are diminished as new metal–C or metal–H bonds are formed. The most affected bond is the Fe–Ni, whose overlap population decreases by 72%. An analysis of the orbital interaction reveals that the Fe–H bonding involves mainly the Fe 4s, H 1s and Ni 4s orbitals. In the case of the sequential absorption, the C–H interaction is almost zero at a distance of 2.72 Å. The main interactions of these interstitials are developed with either Fe or Ni. We also consider the absorption sequence (H first or C first) and its influence on the electronic structure. Our results could be relevant to understand some steps of the carburization process during thermal cracking operations where hydrogen atoms are present. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)