Evolution of grain structure in nickel oxide scales

In systems such as the oxidation of nickel, in which grain-boundary diffusion in the oxide can control the rate of oxidation, understanding of the factors governing the grain structure is of importance. High-purity mechanically polished polycrystalline nickel was oxidized at 700°C, 800°C, and 1000°C for times up to 20 hr in 1 atm O₂. The scale microstructures were examined by parallel and transverse cross section transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Texture coefficients were found by X-ray diffraction (XRD). Each grain in the transverse section grain boundary networks was systematically analyzed for width parallel to the Ni-NiO interface and perpendicular length, for boundary radius of curvature and for number of sides. The variation of these parameters with depth in the scale was examined. In particular, grains were increasingly columnar (i.e., with ratio of grain length to width >1) at higher temperatures and longer times. Columnar grain boundaries tended to be fairly static; the columnar grain width was less than the rate controlling grain size predicted from the oxidation rate. The mean boundary curvature per grain provided a guide to the tendency for grain growth, except in the region of the Ni-NiO interface, where the boundaries were thought to be pinned.     

氧化膜应力研究进展

综述了氧化膜应力产生与释放机理,重点介绍了氧化膜应力原位测试技术的新进展及稀土绎氧化膜和的影响,探讨了氧化膜应力研究的发展方向。     

磁控溅射法制备FeCrAlY薄膜表征及其在高温水环境中的抗腐蚀性能研究

目的 在锆合金基底上沉积FeCrAlY薄膜,研究其高温水蒸气环境中的抗腐蚀性能。方法 采用磁控溅射的方法在锆合金基底与玻璃基底上沉积FeCrAlY薄膜。使用XRD、SEM对沉积的FeCrAlY薄膜进行结构、形貌表征,使用XPS对FeCrAlY薄膜高温水蒸气腐蚀前后的元素价态变化进行表征和分析。结果 在900 ℃高温水蒸气环境中测试了锆合金与锆合金沉积FeCrAlY薄膜的腐蚀性能。发现与锆合金相比,锆合金沉积FeCrAlY薄膜的样品表现出了良好的抗腐蚀性能,氧化增重少于锆合金样品的30%。结论 磁控溅射法制备的FeCrAlY薄膜在高温水蒸气环境下对锆合金基底起到了很好的保护作用。高温水蒸气腐蚀过程中,由于第三元素效应的发生,即FeCrAlY薄膜中的铬元素促使铝元素析出到FeCrAlY薄膜表面,在高温水蒸气环境下氧化生成氧化铝钝化膜,氧化铝钝化膜起到了阻挡层的作用,减缓了腐蚀的进行。     

Improvement of thermally grown oxide layer in thermal barrier coating systems with nano alumina as third layer

A thermally grown oxide (TGO) layer is formed at the interface of bond coat/top coat. The TGO growth during thermal exposure in air plays an important role in the spallation of the ceramic layer from the bond coat. High temperature oxidation resistance of four types of atmospheric plasma sprayed TBCs was investigated. These coatings were oxidized at 1000 °C for 24, 48 and 120 h in a normal electric furnace under air atmosphere. Microstructural characterization showed that the growth of the TGO layer in nano NiCrAlY/YSZ/nano Al₂O₃ coating is much lower than in other coatings. Moreover, EDS and XRD analyses revealed the formation of Ni(Cr,Al)₂O₄ mixed oxides (as spinel) and NiO onto the Al₂O₃ (TGO) layer. The formation of detrimental mixed oxides (spinels) on the Al₂O₃(TGO) layer of nano NiCrAlY/YSZ/nano Al₂O₃ coating is much lower compared to that of other coatings after 120 h of high temperature oxidation at 1000 °C.     

DEFLECTION METHOD FROM TWO SIDES OXIDATION TO STUDY EFFECT OF Y ON INTERNAL STRESS OF OXIDE SCALES

Au investigation was carried ont of the effect of Y addition upon the internal stressof Al_2O_3 scale formed during oxidation of sputtering coating on Co-30Cr-6Al alloyand the growth stress of oxidized film on Fe-23Cr-5Al alloy heating up to 900℃ inair,using a specially designed deflection method from thin strip specimen with coat-ings,ion-implanted Y on one side and oxidized onto both sides.Results indicate thatY may decrease the internal stress of oxide scale on 2×10~(17) Y~+/cm~2 implanted Co-30Cr-6Al coating,and increase one on 2×10~(16) Y~+/cm~2 implanted Fe-23Cr-5Al alloy.This seems due to whether or not Y promotes the plastic deformation of oxide scale.Inaddition,at initial oxidation stage,the change of growth stress of oxide scale formed onY-implanted Co-30Cr-6Al coating may be related to the influence of Y on oxidationprocess of the coating.     

Characterization of oxide scales formed on low carbon steel between 1100 and 1250 °C in air

A low carbon steel was oxidized isothermally at temperatures of 1100–1250 °C for up to 2 h in air, and the oxide scales were examined. Several hundred micrometer-thick scales formed owing to poor oxidation resistance at high temperatures. Scales consisting mainly of Fe₂O₃, Fe₃O₄, and FeO spalled easily, owing to the formation of voids and cracks in the scales. All the alloying elements were also oxidized and incorporated in the iron oxide scales, depending on their local concentration in the matrix.     

Residual stress relaxation in typical weld joints and its effect on fatigue and crack growth

Many factors influence the fatigue and crack growth behavior of welded joints. Some structures often undergo fairly large static loading before they enter service or variable amplitude cyclic loading when they are in service. The combined effect of both applied stress and high initial residual stress is expected to cause the residual stresses relaxation. Only a few papers seem to deal with appropriate procedures for fatigue analysis and crack growth by considering the combined effect of variable amplitude cyclic loading with residual stresses relaxation. In this article, some typical welded connections in ship-shaped structures are investigated with 3-D elastic-plastic finite element analysis. The effect of residual stress relaxation, initial residual stress, and the applied load after variable amplitude cyclic loading is revealed, and a formula for predicting the residual stress at hot spot quantitatively is proposed. Based on the formula, an improved fatigue procedure is introduced. Moreover, crack growth of typical weld joints considering residual stresses relaxation is studied.     

铝合金表面氧化层在燃烧气氛中的退化行为

目的通过对300、400、500℃热空气和燃烧气氛中5052铝合金表面氧化层退化行为的分析,研究基于火灾后金属材料表面产物分析来确定火场中是否存在助燃剂的方法。方法使用高温氧化实验装置对火场中的燃烧气氛进行模拟,用光学显微镜、扫描电子显微镜(SEM)、X射线衍射仪(XRD)和精密分析天平对5052铝合金在300、400、500℃空气和燃烧气氛中表面氧化物的退化行为进行研究,并对氧化物层的组织结构进行分析。结果 5052铝合金表面氧化物层与外界环境气氛和温度密切相关,在温度较低的热空气中表现出较好的抗氧化性能,表面生成的氧化物层完整致密;高温燃烧气氛使5052铝合金的氧化速度显著提高,表面生成的氧化物层结构疏松,并随时间的延长出现失稳氧化。结论火灾中铝合金表面氧化层的微观结构记录了火场的相关信息,这些信息不但是对传统残留物宏观形貌证据的有效补充,还可作为火场中是否有助燃剂存在的辅助证据使用。     

Formation mechanisms of keying or pegging yttrium oxide and increased plasticity of alumina scale on FeCrAlY

Formation of Keying clusters is presented in conjuction with its effects on the adherence of the alumina scale and nonuniform stress distribution in the scale. In addition, the specific nature of point defects and various aspects of scale plasticity are considered with respect to their role on the oxidation behavior and mechanism.     

耐高温氧化Fe-Cr-Ni中熵合金氧化层的微结构与力学性能分布

目的 开发有优异抗高温氧化性的低成本Fe-Cr-Ni中熵合金,研究其高温氧化层微结构与力学性能分布。方法 通过连续氧化增重试验研究了Fe-Cr-Ni中熵合金在1 150～1 240℃空气中的氧化动力学,结合SEM、EDS、XRD、XPS等分析技术与微米划痕试验,对经1 150℃氧化4 h后的氧化层形貌、成分、相分布与微米力学性能分布进行了分析。结果 Fe-Cr-Ni中熵合金在空气中高温氧化增重遵循抛物线规律,氧化激活能为417.64 kJ/mol,抗氧化性优异;经1 150℃氧化4 h后,Fe-Cr-Ni中熵合金氧化层形成双层结构,外层主要由致密的Mn(Fe/Cr)₂O₄尖晶石氧化物、Fe₂O₃和少量MnO₂组成,内层由致密且连续的Cr₂O₃组成,在基体与氧化层界面弥散分布着SiO₂内氧化物颗粒。氧化层的力学分布特征体现了其分层结构,外层与内层的结合力约为10 N,氧化内层与基体的结合性好,结合力约为24 N。随...     

Thickness measurements of spinel and chromia layers in stainless steel oxide scales by X-ray diffractometry

An X-ray diffractometric technique has been used to determine the relative thicknesses of the two phases present in the duplex oxide formed on 20% Cr-25%Ni-Nb stabilized steel at 1123 K in CO ₂ .At times < 100 hr an outer spinel layer grows faster than an inner Cr ₂ O ₃ -rich layer, while after 500 hr the Cr ₂ O ₃ layer is the faster growing. The results indicate that parabolic kinetics do not pertain to the growth of the Cr ₂ O ₃ layer during early oxidation (<500 hr), the rate being controlled by an interface reaction between chromia and the outer spinel.     

高温屈服极限对高强钢焊接变形及残余应力影响的仿真研究

由于缺乏材料的高温数据,进行焊接仿真时所取的高温屈服极限通常有差异。采用双椭球热源模型,对某高强度结构钢的高温屈服极限设计了五种方案进行焊接仿真计算,研究了高温屈服极限对焊接变形和残余应力的影响规律。结果表明:高温屈服极限差异对焊接变形与焊接残余应力有不同的影响;高温屈服极限差异对变形影响较大,越靠近焊缝影响越明显;对残余应力影响较小,仅在近缝区的局部区域产生影响。焊接变形与残余应力均随高温屈服取值的增大而增大。     

外加压应力对纯Ni在800 ℃空气中氧化行为的影响

采用热重(TGA)方法测定纯Ni在800 ℃无应力及外加压应力为10和20 MPa时的氧化动力学曲线,通过扫描电镜观察有无压应力作用下的氧化膜表面和截面形貌,分析压应力作用下氧化动力学的变化和氧化膜的失效形式,从金属空位扩散的角度讨论压应力对纯Ni高温氧化行为的影响.结果表明:纯Ni在800 ℃时氧化,外加压应力增大其氧化速率,这是由于外加压应力对氧化膜结构的影响造成的;无压应力作用时,氧化膜为脊状氧化物;而在压应力作用下,氧化膜由粒状NiO组成,这增大晶界扩散的短路扩散;且压应力促进空位在氧化膜/金属界面的聚集、粗化,弱化界面,改变氧化膜的失效形式.     

真空荧光显示屏阵列材料氧化过程的研究

研究了真空荧光显示屏阵列材料FeNi42Cr6合金在高温湿氢气氛中的氧化行为。其氧化过程为：首先形成Cr2O3，然后(Fe,Mn)Cr2O4氧化物形核、生长，形成完整氧化膜，成熟氧化膜由颗粒状刚玉型氧化物Cr2O3和块状尖晶石型(Fe,Mn)Cr2O4氧化物组成。实验同时表明，阵列板电阻率随氧化膜厚度增加而增大，电阻率过高会导致与之焊接的Ni丝熔断，氧化膜厚度应控制在1μm～2μm。借助扫描电镜、X射线衍射研究了氧化时间、氢气流量、氢气露点等工艺参数对阵列板氧化增重、氧化膜相结构及、氧化膜表面形貌的影响。得出氧化温度为950℃，时间40min～60min，氢气露点(dp)35℃，流量8L／mm为最佳阵列材料氧化工艺。     

Structural and morphological studies of the growth of MoO3 scales during high-temperature oxidation of molybdenum

The oxidation of polycrystalline Mo plates and of Mo(100), Mo(110), and Mo(111) single-crystal plates in pure oxygen at 8 × 10⁴ and 2.7 × 10³ Pa, at 743–1023 K leads to the growth of orthorhombic MoO₃ only, as shown by X-ray diffraction and SEM observations. The stable oxides MoO₂ and Mo₄O₁₁ were not identified. At each side of the molybdenum plate, the oxide scale is a stacking of MoO₃ crystals with their [100] axes oriented normal to the surface of the initial Mo plate. The MoO₃ crystals are very thick in the [010] direction, compared with the well-known shape of the MoO₃ crystals grown from the vapor phase. Two main factors determine the oriented growth of MoO₃ crystals from Mo oxidation. A growth mechanism involving a structural rearrangement of the Mo atoms at the reactional interface and oxygen diffusion through the oxide is proposed.     

Oxide film growth on manganese in neutral aqueous solutions

Abstract

The open circuit potential of the Mn electrode is followed in aerated solutions of Cl^?, Br^?, I^?, SO?&SetFont Typeface="44";24, CrO?&SetFont Typeface="44";24, CO?&SetFont Typeface="44";23 and NO?2 with different concentrations, till steady state values are attained. In all solutions studied, the steady state potentials are approached from negative values indicating oxide film growth. The rate of oxide film thickening is determined from the linear relationship between the open circuit potential and the logarithm of immersion time, t, as evident from the relation: E?=?a+b logt, where a and b are constants. Oxide film growth is assumed to occur by ion conduction under a high field. The rate of oxide film thickening depends on anion type and concentration. The concentration of the inhibitive anions, CrO&SetFont Typeface="44";2?4, CO&SetFont Typeface="44";2?3 and NO?2 that can withstand a certain concentration of the aggressive ions, Cl^?, Br^?, and I^?, varies according to the relation: logC_inh.?=?A+n logC_agg., where A and n are constants.     

Deformation of oxide scales on Fe‐20Cr alloy under compressive stress

The behavior of oxide scales on Fe‐20Cr alloy was investigated in constant load at 1173 K in air. The objective was to understand the effect of mechanical loading on the deformation of oxide scales. It was carried out mainly by comparison of surface morphologies of oxidized specimens observed by scanning electronic microscope (SEM). In all cases, the oxide scales formed on specimens without stress showed a flat surface. However, oxide scales formed on specimens after 20 h of oxidation under a stress of 8 MPa (strain 3.07%) showed slip band characteristics. Furthermore, for longer oxidation time up to 65 h, slip bands formed under a low stress of 2 MPa (strain 1.19%), and grain boundary sliding became the main surface feature for the specimens subjected to a stress of 5 MPa (strain 2.53%). Two typical deformation forms are illustrated in the work.     

TC4合金特高温下的氧化行为研究

通过Gleeble-1500热模拟机研究了特高温下TC4合金的氧化行为。结果表明，1300℃以上的氧化随着氧化温度的升高和保温时间的延长，氧化增重持续增加，并远远大于1150℃时的氧化增重：氧化产物中的V2O5具有挥发性，在合金的外表层无法形成保护性的Al2O3氧化膜，次外层中TiO2氧化膜和Al2O3氧化膜在低于1150℃温度时有一定的抗氧化作用，在1300℃以上形成的氧化膜以疏松的TiO2为主。     

Differences in growth mechanisms of oxide scales formed on ODS and conventional wrought alloys

The oxidation behavior in air of two ODS alloys, MA 754, a chromia former, and MA 956 an alumina-scale former, has been compared with that of conventional wrought model alloys with similar compositions. The main effects on scale properties of both oxide types due to oxide dispersions were found to be improved adherence, decreased growth rates, and enhanced selective oxidation. In addition to metallography, X-ray diffraction, energy dispersive X-ray analysis of the scales, and studies of scale morphology, the detailed growth mechanisms of the oxide layers were studied using an¹⁸O tracer technique. The results show that the oxides on the conventional alloys grow by both metal and oxygen transport, and that the addition of oxide dispersions suppresses the outward scale growth. This change in growth mechanism is a possible explanation for the observed improved scale adherence, decreased growth, and enhanced selective oxidation in the yttria-containing alloys.     

氧化时间及铬含量对Fe-Cr钢高温氧化行为的影响

为了测定不同氧化时间以及铬含量对高温条件下钢材表面氧化铁皮组织和厚度的影响,将Fe-5Cr钢与Fe-10Cr钢在1000℃空气条件下氧化60～180 min,采用增重法绘制其氧化动力学曲线,并利用光学显微镜、能谱仪和X射线衍射仪对氧化铁皮的断面形貌和物相进行研究.结果表明,两试验钢氧化初期为气-固反应,中后期为气...