鎳基超合金CMSX4等溫及熱週期之高溫氧化行為

Isothermal and Cyclic Oxidation Behaviour of Nickel-Base Superalloy CMSX4

本研究探討航空渦輪引擎葉片材料CMSX4在等溫及熱週期環境下之高溫氧化行為。等溫測試於900至1100℃間於空氣中氧化100小時，熱週期實驗則以15分鐘為一週期，試片加熱至1100℃維持13分鐘後急速空冷2分鐘使溫度降至約180℃，測試時間為1200週期，同時並探討微量元素硫（S）含量高低對熱週期氧化行為之影響。實驗結果顯示：等溫環境下，在1000℃以上，CMSX4於表面形成氧化鋁（Al2O3）保護層控制氧化行為，氧化速率與氧化時間呈拋物線（parabolic）關係式。然而在900℃時，因無法形成連續狀的氧化鋁層，主要氧化物是成長速率較高的NiO與NiCr2O4為主，因而呈現低溫時較高的氧化速率現象。熱週期條件下，由於冷卻產生之熱應力，導致氧化層剝落現象，因而試片隨熱週期數增加而呈現重量損失結果，剝落氧化物為NiO，NiAl2O4及其它複雜氧化物。硫含量低於1 ppm的試片，在熱週期實驗中顯現較輕微的氧化層剝落現象，顯示降低硫含量至1 ppm以下，可改善氧化層與基材間的附著性。

An investigation of isothermal and cyclic oxidation of turbine blade material CMSX4 was conducted in air. Isothermal tests were carried out at temperatures between 900 and 1100℃for 100 hours. Cyclic oxidation was tested at 1100℃ under the conditions of a thermal cycle of 13 minutes at temperature and 2 minutes for air cooling by fan aided for 1200 cycles. The effect of sulphur content on the cyclic test was also investigated. The results of isothermal oxidation show that parabolic relation between oxidation rate and exposed time was revealed and the oxidation rate increased with the increasing temperatures except at 900℃. A protective alumina (Al2O3) layer formed beneath the external transition oxide scale at temperatures above 1000℃. However, at 900℃, NiO and NiCr2O4 were the major oxides instead of a continuous alumina sub-layer and then showed higher oxidation rate. For the cyclic test, spalling of oxide scale occurred due to the thermal stress produced during air cooling and the weight loss increased with the increasing thermal cycle numbers. The major oxides in the spalled scale were NiO, NiAl2O4 and some complex oxides. The low sulphur CMSX4 (S<1ppm) showed less scale spallation in the cyclic test. Therefore reducing sulphur below 1ppm can significantly improve the adherence of scale formed on CMSX4.