Al-Hatab, KhA; Al-Bukhaiti, MA; Krupp, U; Kantehm, M
Ni-base superalloy IN 718 was cyclically oxidized in laboratory air at temperatures ranging from 750 to 950 degrees C for up to 12 cycles (14 h/cycle). The kinetic behaviour as well as the surface morphology, and the oxide phases of the scales were characterized by means of weight gain measurements, cyclic oxidation kinetics, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analysis techniques. The results showed that as the oxidation temperature increased, the oxidation rate, the external scale thickness, and internal oxidation zone increased. It was suggested that the oxidation rate was controlled by the diffusion of substrate elements in the alloy and the inward diffusion of oxygen through the oxide scale. The oxidation kinetics followed a sub-parabolic rate law and, the activation energy of oxidation was 249 +/- 20 kJ mol(-1). The scaling process was controlled mainly by the diffusion of chromium, titanium, manganese, and oxygen ions through the chromia scale. IN 718 showed low weight gain and very slow reaction rates of substrate elements at 750 degrees C. At 850 degrees C, a continuous and very thin oxide scale was formed. At 950 degrees C, XRD and EDS-elemental mapping analysis revealed that a complex oxide scale had formed. It consisted of an outermost layer of TiO(2)-MnCr(2)O(4) spinels, inner layer of Cr(2)O(3), and the inner most layer composed of Ni(3)Nb enriched with Nb, Ti and Al oxides underneath the chromia layer. The oxide scale at this temperature seemed to be thicker layer, significant spallation and volatilization had apparently occurred, and greater internal corrosion was identified. The doping effect of titanium was observed, where it was found to be diffused through the chromia scale to form TiO(2) at the oxide-gas interface as well as internally and at the oxide alloy interface. The amount of rutile (TiO(2)) at the oxide surface increased with temperature. In view of Mn contents in the alloy, the manganese-chromium spinel oxide was inferred to have played an important role in cyclic oxidation behaviour of IN 718, where the change in oxidation kinetic was noted. The Al contents would cause internal Al-rich oxide formation at grain boundaries.