Oxidation Behavior of Pt+Hf-Modified γ-Ni +γ'-Ni3Al Alloys
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.
Shigeji Taniguchi, Toshio Maruyama, Masayuki Yoshiba, Nobuo Otsuka and Yuuzou Kawahara
T. Izumi and B. Gleeson, "Oxidation Behavior of Pt+Hf-Modified γ-Ni +γ'-Ni3Al Alloys", Materials Science Forum, Vols. 522-523, pp. 221-228, 2006