니켈기 초합금의 고온산화 저항성에 미치는 합금원소의 영향

Abstract

Ni-based superalloys are highly resistant to oxidation and wear and also they have remarkable mechanical properties at high temperatures. These alloys are suitably used in highly demanding industrial components such as gas turbine blades, jet engines, aerospace components and chemical plants. The mechanical properties and oxidation resistance of these alloys have been widely studied. However the effects of alloying elements on the oxidation behavior still need to be investigated particularly for the interactions between them, considering the complicated composition of superalloys. In this study, the effects of alloying elements on the oxidation resistance are examined by using statistical methods. High purity Ni-9.5Co-8Cr-2.5Mo-6W-(4∼8)Al-(2,3)Ti-(3∼6)Ta-0.01C-0.01B alloys were used to examine the effects of Al, Ti, and Ta on the oxidation of the alloys. Similarly, Ni-9.5Co-(8∼12)Cr-(1.5∼5.5)Mo-(4∼8)W-3Al-5Ti-3Ta-0.1C-0.01B alloys were used for the effects of Cr, Mo and W. Ni-(0∼15)Co-(8∼15)Cr-(0∼5)Mo-(0∼10)W-(3∼8)Al(0∼5)Ti-(0∼10)Ta-0.01C-0.01B were designed by Response Surface Method. The mass gain after cyclic oxidation at 850 ℃ or 1000 ℃ was analyzed statistically to evaluate the effects of alloying elements on the oxidation rate. The phase, composition, and structure of the oxide scales were examined by X-ray diffractometry, scanning electron microscopy, and X-ray energy dispersive spectrometry. TiO2, NiO, NiCr2O4, Cr2O3, Al2O3 were formed after oxidation and NiAl2O4 was found additionally at 1000 ℃. The alloys with high resistance to oxidation formed Cr2O3 at the outermost part of the scale or continuous Al2O3 layer on the alloy, as well as having little crack or spallation of scale. Al was highly effective in improving the oxidation resistance as it facilitates formation of continuous layer of Al2O3. The effectiveness of Al appears stronger with low concentration of Ti and Ta. Reducing Ti and Ta contents, especially for the alloys with low Al concentration, increased the oxidation resistance of superalloys also. Cr slightly increased the mass gain due probably to instability at the high temperatures. Co did not show clear effect on the oxidation rate. Mo and W are thought to be beneficial to the oxidation resistance since they increase stability of Cr2O3 or lowering thermal stress between the alloy and oxide.