ISSN:
1662-9752
Source:
Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
Notes:
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 1150ºC under bothisothermal and thermal cycling conditions. It was found that the co-addition of Pt and Hf wasextremely beneficial to oxidation resistance, to the extent that Ni-20Al-20Pt-Hf and Ni-20Al-10Pt-Hfalloys (all compositions are in at.%) oxidized at significantly slower rates than that of a Ni-50Al-15Ptβ-NiAl alloy. A Ni-20Al-5Pt-Hf alloy also showed good oxidation resistance, with the steady-stateoxidation rate being almost the same as that obtained for the β alloy. Over a period of up to 500one-hour oxidation cycles, no oxide spallation from the modified γ+γ′ alloys was observed. Fromcross-sectional SEM examination coupled with X-ray diffraction analyses, it was found that acompact and planar exclusive scale layer of α-Al2O3 formed on the Ni-20Al-20Pt-Hf alloy. Bycontrast, the Ni-20Al-10Pt-Hf and Ni-20Al-5Pt-Hf alloys formed a very thin outer layer of NiAl2O4and a planar inner layer of α-Al2O3. The thickness of the inner Al2O3 layer increased with increasingoxidation time relative to that of the NiAl2O4 layer, meaning that the latter primarily formed duringthe initial stages of scale formation. Both NiO and NiAl2O4 were found in the scales formed on theNi-20Al-Hf and Ni-15Al-0~10Pt-Hf alloys, with the thickness of these oxide layers decreasing withincreasing Pt content in the alloys. Further, it was found that the extent of internal HfO2 formationdecreased significantly with increasing Pt content, to the extent that no HfO2 was found in theoxidized Ni-20Al-20Pt-Hf alloy. Inferences for the observed beneficial effects of Pt promotingprotective Al2O3 formation and decreasing the tendency for Hf to oxidize in γ+γ′ alloys arediscussed
Type of Medium:
Electronic Resource
URL:
http://www.tib-hannover.de/fulltexts/2011/0528/02/13/transtech_doi~10.4028%252Fwww.scientific.net%252FMSF.522-523.221.pdf
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