基于氧化膜屈曲破坏的高温合金循环氧化动力学模型

高温合金循环氧化膜弯曲剥落行为等效为弹性基础梁失稳问 题,利用有限元特征值屈曲分析,提出高温合金循环氧化膜弯曲剥落理论,建立了高温合金 循环氧化动力学模型.Fe-Ni-Cr-Al合金高温循环氧化动力学计算结果与实验数据相符,证明 了氧化膜弯曲剥落模型的正确性.     

铁基高温金恒温氧化行为研究

采用OM,XRD及SEM/EDX研究了铁基高温合金GH 1140的高温静态空气氧化行为,得到了该合金的氧化动力学曲线、氧化膜形貌及其相组成,评价了合金的抗氧化性能并分析了其氧化机理.结果表明:铁基高温合金GH1140在750～900℃时氧化动力学遵循抛物线规律,无氧化膜剥落;在950℃氧化时氧化动力学近似遵从直线规律,且有少量氧化膜剥落;合金氧化过程中有内氧化现象发生,生成的氧化膜以Cr2O3为主;该合金可满足850℃以下高温使用要求.     

高温合金氧化膜破坏的界面断裂力学分析

依据双材料模型分析高温合金氧化膜的应力状态，确定了氧化膜弯曲剥落的最大挠度与氧化膜压应力间关系．利用界面断裂力学理论，建立了高温合金氧化膜剥落的弯曲图．镍基高温合金氧化表面剥落的Al2O3氧化膜，在界面粗糙度参数A为0.3—0.4，临界弯曲指数Пc为1和临界粘附性指数∑e为1．15时，弯曲剥落特性与实验结果相符．高温合金氧化膜的弯曲图合理解释了氧化膜的弯曲剥落过程。     

低合金耐热钢T23在高温超临界CO_2环境中的腐蚀特性研究

通过低合金耐热钢T23在650℃/25 MPa的高温超临界二氧化碳(s-CO_2)下的1000 h氧化实验获得了T23钢的氧化动力学特性,并借助SEM、XRD及EDS对氧化膜的形貌、物相和成分进行了分析。结果表明:T23钢在650℃/25 MPa的高温s-CO_2中的氧化动力学符合立方规律,时间指数0.30。氧化膜具有典型的双层结构,外层氧化膜为Fe_3O_4且氧化膜疏松多孔,内层氧化膜较为致密含有大量尖晶石结构的Fe_(3-x)Cr_xO_4。T23材料在高温超临界CO_2环境中形成的氧化膜较容易发生剥落,剥落腐蚀现象明显。     

γ-TiAl的循环氧化行为

采用增重法研究了γ－ＴｉＡｌ合金在１１２３Ｋ和１１７３Ｋ空气中循环氧化的动力学曲线，用配有能谱的扫描电子显微铎氧化层的形貌和显微结构进行了分析，用Ｘ射线衍射仪测定了氧化层中的残余应力，研究结果表明：温度是影响γ－ＴｉＡｌ合金循环氧化的重要因素，氧化膜的剥落发生在金属／氧化层界面，其剥落途径为：氧化层先脱附凸起，而后发生剥落。     

TC4钛合金高温氧化行为

研究了TC4钛合金在650、750、850℃的循环氧化和25~1000℃范围内的连续变温氧化的氧化行为。采用电子天平或综合热分析仪、XRD、SEM和EDS分析了合金的氧化动力学、氧化膜的物相、表面形貌、截面结构及元素分析。结果表明:650℃循环氧化和连续变温氧化动力学曲线符合抛物线规律、750℃循环氧化符合抛物线-直线混合规律、850℃循环氧化符合直线规律。氧化膜由薄而致密的Al_2O_3外层和厚而疏松的TiO_2内层组成。随氧化温度升高,氧化膜厚度增加,但出现裂纹或剥落。     

退火处理对喷丸处理1Cr18Ni9Ti合金选择氧化的影响

采用400℃退火处理对喷丸处理的1Cr18Ni9Ti合金进行了预处理，研究了不同处理的合金试样在900℃空气中的恒温连续氧化性能。扫描电镜(SEM)、X射线衍射(XRD)及氧化动力学结果发现，喷丸处理虽然使促进了铬的选择氧化，然而由于喷丸处理合金表面压应力增大，形成的氧化膜的抗剥落性能不好；经退火处理的喷丸试样表面的应力得以消除，因此合金表面形成了附着性良好的选择性氧化铬膜，其氧化增重与氧化膜剥落下降，起到了很好的保护作用，大大提高了合金的抗高温氧化性能。     

La对Ni-20Cr-18W-1Mo基高温合金1373 K/100 h氧化行为的影响（英文）

对有无La元素的Ni-20Cr-18W-1Mo基高温合金进行了1373 K/100 h的等温氧化试验,分别采用扫描电镜(SEM),X射线能谱(EDX)和X射线衍射(XRD)分析了氧化试样的氧化膜表面和截面形貌特征。结果表明,La元素的添加能够极大地提高基体和氧化膜之间的结合力,使得Ni-20Cr-18W-1Mo基高温合金在1373 K时的抗氧化性提高,相比于未添加La元素的Ni-20Cr-18W-1Mo基高温合金,添加La元素的高温合金具有更小的抛物线氧化速率常数κ"和较弱的氧化膜剥落,同时,形成的表面氧化膜更加致密纤细。     

Effect of Lanthanum on Oxidation Behavior of Ni-20Cr- 18W-1Mo Alloys at 1373 K for 100 h in Air

对有无La元素的Ni-20Cr-18W-1Mo基高温合金进行了1373 K/100 h的等温氧化试验,分别采用扫描电镜(SEM),X射线能谱(EDX)和X射线衍射(XRD)分析了氧化试样的氧化膜表面和截面形貌特征。结果表明,La元素的添加能够极大地提高基体和氧化膜之间的结合力,使得Ni-20Cr-18W-1Mo基高温合金在1373 K时的抗氧化性提高,相比于未添加La元素的Ni-20Cr-18W-1Mo基高温合金,添加La元素的高温合金具有更小的抛物线氧化速率常数κ"和较弱的氧化膜剥落,同时,形成的表面氧化膜更加致密纤细。     

3种典型轧辊材料氧化行为的原位观察

轧辊表面氧化膜的形成及剥落对轧辊的消耗及产品质量有显著影响。采用VL2000DX-SVF18SP型超高温激光共聚焦显微镜观察了高铬铸铁、高铬铸钢和高速钢在连续加热及等温过程中其表面氧化膜形成的特征，同时对比研究了这3种材料在循环加热、冷却后的氧化行为。结果表明，高铬铸铁和高铬铸钢开始形成氧化膜的温度较低，约为300 ℃，但氧化膜致密均匀，抑制了高温及等温过程氧化膜的生长；而高速钢氧化膜开始形成温度较高，氧化膜快速形成的温度约为480 ℃，但氧化膜均匀性差，具有较快的生长速率，高速钢的抗氧化性显著低于高铬铸铁和高铬铸钢。     

A New High-Temperature,Oxidation-Resistant Ti-Based Material

Ti₃Al(O)–Al₂O₃ composites were fabricated in situ using a mechanical-alloying and reaction-sintering technique. Their oxidation behaviors were studied in air at 700–900°C. The oxidation rates were much lower than those of Ti₃Al. The behaviors of isothermal and cyclic oxidation were very similar. The oxide scales that formed exhibited excellent spallation resistance under all testing conditions. No scale cracking or spallation could be observed, even along the edges or corners of the samples, implying that growth and thermal stresses generated during heating and cooling periods had been effectively released. The mechanisms of the decrease in oxidation rate and the improvement on spallation resistance are discussed based on microstructure studies. This composite has advantages of light-weight, simple fabrication from inexpensive materials, and superior high-temperature oxidation resistance; it may provide opportunities to be used in some high-temperature structural applications.     

The use of thin layer activation in high-temperature cyclic oxidation studies

Thin layer activation (TLA) has been applied to a study of the high-temperature cyclic oxidation behaviour of pure and yttrium-implanted chromium, taking advantage of the high area selectivity of the technique.On pure chromium, two different sample areas, i.e. corners and the central part of a large surface area, were selected for activation. Due to this area selectivity, it was possible to distinguish the spallation behaviour of the oxide scale formed on flat from that formed on highly curved surfaces. In particular, the TLA data showed that the oxide scale formed during high-temperature cyclic oxidation near corners and edges was more prone to cracking and spallation.In the case of yttrium-implanted specimens, activation was performed only at the centre of the implanted zone, avoiding the distorting effect of the non-implanted parts of the sample. The area selectivity of TLA made it possible to study more accurately the beneficial effect of ion implantation on the cyclic oxidation behaviour than by conventional thermogravimetry. Due to the complementary character of TLA with respect to the conventional thermogravimetry, a more complete and better understanding of the cyclic oxidation performance of materials can be obtained.     

盐雾腐蚀对Al-Si涂层循环氧化行为的影响

研究了盐雾腐蚀对Ni基高温合金Al-Si防护涂层循环 氧化行为的影响.结果表明，涂层在盐雾腐蚀后，表面Al2O3膜受到破坏，出现大量的腐蚀坑，且蚀坑区域Al含量明显下降.在循环氧化过程中，腐蚀坑内重新生成的Al2O3膜粗糙且易剥落，蚀坑区域出现γ'-Ni3Al相的时间明显提前，从而降低了涂层的耐循环氧化能力，缩短了涂层的寿命.     

Effects of chromia coatings on the high-temperature behavior of F17Ti stainless steel in air. Effect of rare-earth-element oxides

The MOCVD technique was used to form thin films of chromia and/or of a rare-earth-element oxide on the surface of F17Ti stainless-steel samples. These oxides were deposited alone or together, successively or by codeposition. Coated samples were then subjected to high-temperature oxidation tests, either under isothermal conditions at 1273 K for 50 h, or under cyclic conditions at 1223 K for 40 cycles. An improved oxidation behavior was observed in every case, especially with codeposited Ln₂O₃-Cr₂O₃. Weight gain decreased during oxidation, compared with an uncoated sample, and scale spallation, generally observed under cyclic conditions, disappeared. Longer time experiments were also carried out. Large differences were observed, depending on the sample surface treatment.     

The Isothermal and Cyclic Oxidation Behavior of a Titanium Aluminide Alloy at Elevated Temperature

The isothermal and cyclic oxidation behavior of Ti-47Al-2Mn-2Nb with 0.8 vol.% TiB₂ particle-reinforced alloy was investigated in air between 700 and 1000 °C. In the study, the kinetics of isothermal and cyclic oxidation were performed by using a continuous thermogravimetric method which permits mass change measurement under oxidation conditions. The oxide scales and substrates were characterized by scanning electron microscopy with energy-dispersive x-ray analysis and x-ray diffraction. At 700 and 800 °C, the alloy showed an excellent oxidation resistance under isothermal and cyclic conditions. After exposure to air above 800 °C, the outer scale of the alloy was dominated by a fast-growing TiO₂ layer. Under the coarse-grained TiO₂ layer was the Al₂O₃-rich scale, which was fine-grained. At 900 and 1000 °C, the extent of oxidation increased clearly. The oxidation rate follows a parabolic law at 700 and 800 °C. However, the alloy, upon isothermal oxidation at 900 °C, can be divided into several stages. During the cyclic oxidation at 900 and 1000 °C, partial scale spallation takes place, leading to a stepwise mass change.     

High-temperature oxidation of a 20/25 stainless steel in high-pressure carbon dioxide

Kinetics of high-temperature oxidation, and of oxide spallation, are reported for a 20Cr-25Ni Nb-stabilized stainless steel, exposed in carbon dioxide at 40 bar pressure in the temperature range 1115–1240 K. Gross weight-gain kinetics were parabolic with respect to time, with an activation energy of 370±16 kJ/mol. Oxidation rates are shown to be consistent with diffusion control in the inner chromia layer of a two-layer oxide. Intergranular oxidation of silicon was observed beneath the oxide scale. Oxide spallation occurred at all temperatures and increased linearly with gross weight gain. At 1240 K 50% of the oxide which formed spalled at the first weighing. Thereafter, the fraction spalling decreased to about 10%. The difference is attributed to a change in the oxide composition from an M₂O₃ structure to M₃O₄, which requires greater compressive stresses to induce spallation during cooling.     

Effect of minor alloying elements La,C and B on the cyclic oxidation behavior of Ni-Cr-W-Mo superalloys

The cyclic oxidation behavior of Ni-Cr-W-Mo base alloys with various La, C and B contents is investigated at 1150 °C in ambient air with 15 min of high-temperature exposure and 5 min of air cooling. Oxidation resistance is evaluated by the weight change during cyclic oxidation. The cross-section of the oxide scale is observed by scanning electron microscopy after the cyclic oxidation test. The oxide scale mainly consists with spinels and a chromia layer. NiWO₄ oxide particles and NiO are also observed in some areas. The addition of La improves the cyclic oxidation resistance significantly. However, the addition of 0.03 wt% B reduces the beneficial effect of La. The additions of B and C increase the spallation at the initial stage such that severe weight loss is observed. However, the spallation is reduced at the later stage. The addition of a proper amounts of B and C can be beneficial to improve the cyclic oxidation resistance of Ni-Cr-W-Mo alloys.     

The Effect of Deformation Substructure on the High-Temperature Oxidation of Inconel 625

The high-temperature, isothermal-oxidation behavior of a superalloy was studied in the as-rolled and deformed conditions. The microstructural changes occurring during the oxidation of samples were examined using optical, scanning electron microscopy (SEM), fine-probe EDS microanalysis, and X-ray diffraction techniques. The topography of the oxide layers formed in the as-rolled and cold-deformed specimens exposed at various temperatures and time intervals is also examined. The kinetics and microstructural results are presented for the comparative study of the structural changes occurring during high-temperature oxidation. It was found that a Cr₂O₃ external layer was adherent and uniform on the rolled specimens in comparison to the scattered and preferential oxide developed on the deformed specimens. The latter can be attributed to the concurrent dynamic changes occurring in the deformed substructure that subsequently lead to breaking and spallation of the oxide.