Oxidation Resistance of Pt Containing .GAMMA.-Ni+.GAMMA.'-Ni3Al Alloys

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  • Izumi Takeshi
    Department of Materials Science and Engineering, Iowa State University
  • Gleeson Brian
    Department of Materials Science and Engineering, Iowa State University

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Other Title
  • Ｐｔ含有γ‐Ｎｉ＋γ′‐Ｎｉ３Ａｌ合金の耐酸化性
  • Pt含有γ-Ni+γ´-Ni3Al合金の耐酸化性
  • Pt ガンユウ ガンマ Ni ガンマ Ni3Al ゴウキン ノ タイサンカセイ
  • Oxidation Resistance of Pt Containing γ-Ni+γ′-Ni3Al Alloys

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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-0.4Hf and Ni-20Al-10Pt-0.4Hf 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-0.3Hf alloy also showed relatively good oxidation resistance, with the oxidation rate being almost the same as that obtained for Ni-50Al-15Pt 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-0.4Hf alloy. By contrast, Ni-20Al-10Pt-0.4Hf and Ni-20Al-5Pt-0.3Hf alloys formed a very thin outer layer of NiAl2O4 and a planar inner layer of α-Al2O3. The thickness of this inner Al2O3 layer increased with increasing oxidation time relative to that of the NiAl2O4 layer, suggesting that the latter primarily formed during the initial stages of scale formation. Both NiO and NiAl2O4 were found in the outer scales formed on the Ni-20Al-0.3Hf 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 in Pt content, to the extent that no HfO2 was found in the oxidized Ni-20Al-20Pt-0.4Hf 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 will be presented.<br>

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