Study of interface structure of the oxide grown on nickel alloys by APFIM

Abstract

This paper reports a study of the interface structure of the oxide in NiCrSi alloy as determined by the use of a field ion microscope atom probe (AP-FIM). This technique is well suited for the study of atomic distribution on an atomic scale at the surface and in the interface between the oxide and the alloy. AP depth profiles show that the fluctuation in concentration of Si and Cr atoms as a function of depth is not unusual but that of Ni is. The region of low concentration of nickel is an interface in the oxide. The interface contains CrO, NiO, CrO₁ and NiO₂. The following mechanism of oxidation of the alloy was deduced. First, an oxygen molecule, adsorbed on the surface of the alloy, dissociates into oxygen atoms (O₂→ 2O. Then the oxygen atoms diffuse into the interior via grain boundaries forming the oxide (M + O→MO). As the concentration of oxygen is continuously increased, dioxide (MO + O→MO₂) is formed.