High-Temperature Oxidation Properties of Ni2FeCoCrAlX High entropy Superalloys

• Main Authors: Chien, Feng-Chih, 簡逢志
• Other Authors: Kai, Wu
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/e5d4gz

碩士 === 國立臺灣海洋大學 === 材料工程研究所 === 106 === The main goal of this thesis is to investigate the high-temperature oxidation of four Ni2FeCoCrAlX high entropy superalloys (HESAs) in various oxygen-containing conditions. In the first part of the study, the oxidation behavior of Ni2FeCoCrAlX high-entropy superalloys (where X = 0, 0.5, 1.0 and 1.5) was studied over the temperature range of 700 ~ 900 oC in dry air. The oxidation kinetics of all the alloys followed the parabolic rate low, indicating that solid-state diffusion is the main controlling step of oxidation. The oxidation rates of all the alloys increased with increasing temperature, but decreased with increasing aluminum content. Based on microstructure and XRD analyses, the scales formed on the Al-free alloy consisted mostly of FeCr2O4 and NiCo2O4, while those of Al-containing alloys were composed of Al2O3. It was found that θ-Al2O3 formed on Al-containing alloys at 700-800 oC, while α-Al2O3 dominated at 900 oC. The formation of Al2O3 is responsible for the lower oxidation rates of Al-containing alloys. Secondly, the oxidation behavior of Ni2FeCoCrAl0.5(N5-0.5Al) was studied at various oxygen pressures from 10-4 to 1 atm at 900 °C. The oxidation kinetics of N5-0.5Al obeys the parabolic law at Po2 ≤ 0.21atm, having its oxidation rate increased with oxygen partial pressure. On the contrast, a mass-loss kinetics was observed at a pure oxygen pressure at 900 °C. The scale formed at Po2 ≤ 0.21atm consisted of an exclusine α-Al2O3 layer, having its thickness increased with oxygen partial pressure, while the oxide layer peeled off at a pure oxygen pressure. The scales formed at Po2 = 1atm were heterophasic, consisted of α-Al2O3, Cr2O3, (Fe,Cr)3O4, and (Ni,Co)3O4. Finally, the oxidation behavior of N5-0.5Al was studied at various ratios of CO/CO2 mixed gases at 700 - 900 °C. In general, the oxidation kinetics of the Ni2FeCoCrAl0.5 superalloy obeyed the parabolic law in all the gas mixture with its oxidation rate increased with increasing temperature and CO2 concentration. The scales formed on N5-0.5Al were also heterophasic, which were composed of intermixed α-Al2O3, Cr2O3, (Fe,Co)3O4, and (Fe,Ni)3O4 on top of the superalloy surface, and no continnous scale-layer was formed in any temperature and CO/CO2 gas mixture.