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Effects of the Additions of Si and Al on Enhancing High-Temperature Oxidation Resistance of Ti-, Nb-, and Mo-based Alloys
|Authors: ||Song-En Lin|
|Contributors: ||NTOU:Institute of Materials Engineering|
Ti-Si alloys;Nb-Si alloys;Mo-Si alloys;Al;high temperature oxidation;pesting;TiO2;SiO2;Nb2O5;α-Al2O3;MoO3;parabolic-rate law;volatilization
|Issue Date: ||2011-11-23T03:13:07Z
|Abstract: ||本研究主要探討鈦矽、鈮矽及鉬矽三種二元合金在700~1000℃下空氣之高溫氧化行為研究，並探討添加少量的Al元素對三種合金抗氧化之影響。 研究結果顯示三種鈦矽(鋁)合金在800~1000℃之氧化動力學皆遵守拋物線律，且氧化速率隨溫度上升而增加，顯示其氧化機制主要是由固態擴散主導。比較兩種鈦矽合金氧化速率後，可知Si含量的增加有助於提升鈦矽合金抗氧化能力。另一方面，(TiSi2)91Al9合金氧化後的主要氧化生成物為α-Al2O3，可知它的生成更進一步地提升鈦矽合金的抗氧化能力。 Nb-20Si合金在700~800℃之氧化動力學遵守多段式拋物線率；而900~1000℃之氧化動力學則因完全氧化而呈現無增重的持平反應。另一方面，藉由添Al改質之(Nb-20Si)90Al10合金在700~1000℃之氧化動力學皆遵守拋物線。但由實驗結果得知，改質後之試片經氧化後皆呈現粉末狀，顯示添加Al並無法防止鈮矽合金發生pesting現象。 另外，MoSi2合金在700℃以上易形成MoO3揮發相，進而產生嚴重的氧化失重反應。但添加少量的Al元素後，使鉬矽鋁三元合金之氧化動力學遵守抛物線律且有氧化增重反應，明顯地改善MoSi2合金之揮發特徵。由顯微結構分析得知，氧化後三元合金表面主要的氧化生成物為α-Al2O3，可知它的生成有助於提升MoSi2的抗氧化能力。|
The oxidation behabior of binary Ti-Si, Nb-Si, and Mo-Si alloys was studied over the temperature range of 700~ 1000℃ in dry air. In addition, the effect of Al addition on high temperature oxidation resistance of those alloys was also studied for comparative purpose. The results showed that the oxidation kinetics of three Ti-Si alloys at 800~1000℃ followed the parabolic-rate law, with their oxidation rates gradually increasing with temperature. This further indicates that diffusion is the rate-determining step during oxidation. It was also found that the higher the Si content the better the oxidation resistance. In addition, the scales formed on the (TiSi2)91Al9 consisted mostly of α-Al2O3, whose formation is responsible for the further improvement of oxidation resistance. The oxidation kinetics of the Nb-20Si alloy followed a multiple-stage parabolic-rate law at 700~800℃, while no-mass-gain kinetics were observed at 900-1000℃. In addition, the oxidation kinetics of the (Nb-20Si)90Al10 alloy also followed parabolic-rate law at 700~1000℃. The scales formed on the two alloys consisted of fragile, powder-like Nb2O5 structures, implying that the addition of Al can not effectively prevent the pesting phenomenon for the ternary alloy. The scales forms on MoSi2 were composed of volatile spieces of MoO3 during oxidation indicating that the pesting behavior was evident for the MoSi2 alloy. On the other hands, the oxidation kinetics of the (MoSi2)95Al5 ternary alloy followed the parabolic-rate law, indicating that the volatilization of MoO3 was significantly retarded. Based on microstructural analyses, the scales formed on the (MoSi2)95Al5 surface consisted mostly of α-Al2O3, whose formation is also responsible for the improvement of the oxidation resistance for the ternary alloy.
|Appears in Collections:||[材料工程研究所] 博碩士論文|
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