Papers by Keyword: Reactive Element Effect

Abstract: In this study, the effects of alloying with Cu and external doping with CeO2 on the oxidation of nickel were evaluated. The materials studied were pure Ni, Ni-5wt% Cu, and Ni with the surface doped with CeO2 by pulsed laser deposition (PLD). The oxidation kinetics were measured using thermogravimetric analysis (TGA). The oxidation microstructures were observed by scanning electron microscopy (SEM) compositional analysis was performed with energy dispersive x-ray analysis (EDS) and sputtered neutrals mass spectrometry (SNMS). Phase identification was performed using X-ray diffraction (XRD). The Cu additions had a negligible effect on the oxidation kinetics but Cu was found to be present in the outer portions of the scale in significant concentrations. Doping with CeO2 resulted in a significant decrease in the NiO growth rate. The scales on doped Ni grew primarily inward whereas those on the undoped Ni grew primarily outward. Deposition of the CeO2 dopant onto Ni with a thin, preformed NiO layer produced a similar reduction in the subsequent NiO growth rate. This suggests that the poisoned interface model, proposed by Pieraggi and Rapp, does not describe the effect of the CeO2 dopant. Mechanisms are presented to attempt to describe the above observations.

Abstract: The oxidation behavior of Pt+Hf-modified γ-Ni+γ′-Ni3Al alloys containing up to 20 at.% Pt and either 15 or 20 at.% Al was studied by oxidizing the alloys in air at 1150°C under both isothermal and thermal cycling conditions. It was found that the co-addition of Pt and Hf was extremely beneficial to oxidation resistance, to the extent that Ni-20Al-20Pt-Hf and Ni-20Al-10Pt-Hf alloys (all compositions are in at.%) oxidized at significantly slower rates than that of a Ni-50Al-15Pt β-NiAl alloy. A Ni-20Al-5Pt-Hf alloy also showed good oxidation resistance, with the steady-state oxidation rate being almost the same as that obtained for the β alloy. Over a period of up to 500 one-hour oxidation cycles, no oxide spallation from the modified γ+γ′ alloys was observed. From cross-sectional SEM examination coupled with X-ray diffraction analyses, it was found that a compact and planar exclusive scale layer of α-Al2O3 formed on the Ni-20Al-20Pt-Hf alloy. By contrast, the Ni-20Al-10Pt-Hf and Ni-20Al-5Pt-Hf alloys formed a very thin outer layer of NiAl2O4 and a planar inner layer of α-Al2O3. The thickness of the inner Al2O3 layer increased with increasing oxidation time relative to that of the NiAl2O4 layer, meaning that the latter primarily formed during the initial stages of scale formation. Both NiO and NiAl2O4 were found in the scales formed on the Ni-20Al-Hf and Ni-15Al-0~10Pt-Hf alloys, with the thickness of these oxide layers decreasing with increasing Pt content in the alloys. Further, it was found that the extent of internal HfO2 formation decreased significantly with increasing Pt content, to the extent that no HfO2 was found in the oxidized Ni-20Al-20Pt-Hf alloy. Inferences for the observed beneficial effects of Pt promoting protective Al2O3 formation and decreasing the tendency for Hf to oxidize in γ+γ′ alloys are discussed.