SEM-EDS and XPS studies of the high temperature oxidation behaviour of Inconel 718

The oxidation in air under isothermal conditions of Inconel 718 has been in vestigated at 900 degrees C. We have studied the firsts stages of the high temperature oxidation mechanism. The oxidised samples were examined using a number of surface analytical techniques including XPS and SEM-EDS. The res ults have shown that minor alloying elements strongly influence the oxidati on behaviour of the alloy: Titanium diffuses rapidly across the inner Cr2O3 layer to form TiO2 in the extreme surface. Furthermore, intergranular oxides in the alloy may be rich in oxidised Ti beneath the oxide-alloy interface, Aluminium, even in small concentrations, leads to the formation of intergra nular Al2O3 in the alloy substrate. Manganese diffuses very rapidly across the initial oxide layer and induces the formation of manganese-substituted rhombohedral or spinel phases in the outer part of the oxide scale. Niobium does not diffuse in the chromia layer. Segregation of Nb at the oxi de-alloy interface leads to the formation of a quasi continuous scale of th e intermetallic phase Ni3Nb. Previous studies had shown that the rate of the external oxide scale growth and the intergranular oxide penetration during oxidation of Inconel 718 at 900 degrees C in air are lower than those of Inconel X or Incoloy 800, We suggest that the formation of a quasi continuous scale of the intermetallic phase Ni,Nb at the oxide-alloy interface could have an important contribut ion to the good oxidation resistance of Inconel 718.