滲鋁處理對鎳基超合金Hastelloy X高溫循環氧化行為的影響

The Effect of Aluminizing to the Cyclic Oxidation Behaviors of the Hastelloy X Superalloys

本研究利用950℃-9小時的固體滲鋁處理，在鎳基超合金Hastelloy X試片表面擴散生成厚度約為75μm之NiAl合金層，其中有許多複雜而細小之析出相，同時在此合金層與基材之間有厚約10μm之中間擴散層生成。Hastelloy X基材與鋁化試片進行1100℃，每一循環時間2小時的高溫循環氧化實驗達98次之後發現，經鋁化處理之試片由於生成緻密之氧化鋁層而具有較佳的抗高溫氧化能力，不過高溫氧化與高溫擴散作用使得NiAl合金層的鋁元素漸漸耗竭，生成脫鋁之晶粒與連續層，而原本細小之析出相亦凝聚長大。由於中間擴散層與NiAl合金層以及基材三者之間的熱膨脹係數差異所致之熱應力，使得中間擴散層於多次高溫循環氧化之後開始產生橫向裂痕，對鋁化Hastelloy X的抗高溫氧化能力造成負面影響。

A pack aluminizing process at 950 ℃ for 9 hours has been employed on the Nickel-base superalloy Hastelloy X to deposit a 75 μm thick NiAl aluminide layer on the surface. An interdiffusion zone around 10 μm thick is found between the aluminide layer and the substrate. Lots of complex and fine precipitates are also observed in the NiAl layer. The cyclic oxidation tests of surface treated alloys and untreated substrate were conducted at 1100 ℃ for 196 hours. It is observed that the aluminizing process greatly enhances the cyclic oxidation resistance of Hastelloy X at 1100 ℃ due to a dense and protective alumina layer formed on the surface. Owing to the oxidation and diffusion effects at high temperature, the Al content of the NiAl layer was depleted to form some low Al containing grains and a continuous layer between the aluminide layer and substrate. Thermal stress induced transverse cracks in the interdiffusion zone were observed due to the difference of coefficients of thermal expansion among the substrate, aluminide layer and the interdiffusion zone. It is believed that these cracks are detrimental to the cyclic oxidation resistance of superalloy for long-term exposure.