预氧化钛铁矿弱还原动力学研究

以钛铁矿为原料制备人造金红石是氯化法钛白生产的重要环节,探求合适的钛铁矿焙烧预处理方法以改善钛铁矿的物相组成和还原过程。这里对钛铁矿的预氧化、还原焙烧过程物相规律影响机理进行了系统研究。其结果如下:对两种典型的热力学稳定(氧化温度:T800℃和T≥800℃)预氧化矿在600~800℃温度范围内弱还原焙烧过程的研究结果表明预氧化矿的弱还原焙烧有两种相互竞争的反应路径。当还原温度高时(如750℃和800℃),预氧化生成的金红石参与还原反应而被消耗。在T800℃和T≥800℃下分别预氧化的钛铁矿都被还原成钛铁矿。当还原温度较低时(如600℃),金红石不参与反应。在T800℃预氧化钛铁矿的赤铁矿被还原为金属Fe和金红石;而在T≥800℃预氧化钛铁矿的假板铁矿被还原为钛铁矿。     

Y_2O_3改性细晶Ni_3Al合金涂层在900℃下的循环氧化性能

将Ni-Al和Ni-Al/Y2O3复合镀层分别在800°C下扩散3h制备纯Ni3Al和Y2O3改性的Ni3Al合金涂层,对其显微组织和氧化性能进行对比研究。SEM/EDAX和TEM分析表明,涂层中Y2O3的加入抑制了合金化过程中基体晶粒的长大。在900°C下循环氧化100h的结果表明,Y2O3的加入明显提高了Ni3Al合金涂层的抗循环氧化性能。对Y2O3影响Ni3Al涂层的结构和氧化性能的机理进行了分析。     

哈氏C-2000合金在800°C和1000°C的氧化行为(英文)

研究哈氏C-2000合金在(800°C,1000 h)和(1000°C,100 h)时的氧化行为。分别利用增重法、SEM、XRD和XPS表征氧化动力学和氧化膜的形貌特征。合金在800°C和1000°C时均基本满足抛物线速率规律。此外,由于退火孪晶提高了合金化元素及氧原子的扩散速率,因此,降低了合金的高温抗氧化性能。在800°C时,氧化膜微观结构主要是由NiO和Cr1.3Fe0.7O6组成。此外,初始退火孪晶结构在氧化后依然可见,且临近氧化膜附近出现了富Mo相。然而,在1000°C时,氧化膜的微观结构由细小的铬氧化物和粗大的镍氧化物颗粒通过相互镶嵌而构成,同时,在临近氧化膜处几乎无富Mo相出现。     

预氧化钒钛磁铁精矿的固态还原动力学及其机理

研究预氧化钒钛磁铁精矿固态还原反应的动力学,采用XRD、SEM和EDS等手段研究还原产物的显微结构和物相变化,在此基础上,对其固态还原机理进行研究。结果表明：以煤为还原剂,在还原温度为950~1100°C时,预氧化钒钛磁铁精矿的固态还原受界面化学反应控制,反应的表观活化能为67.719 k J/mol;预氧化钒钛磁铁精矿的还原历程可描述为：预氧化钒钛磁铁精矿→钛铁晶石→钛铁矿→亚铁板钛矿（Fe Ti2O5）→（FenTi1-n）Ti2O5。预氧化钒钛磁铁精矿在1050°C还原60 min后,还原产物中会形成M3O5型（M为Fe、Ti、Mg、Mn等）固溶体,存在于M3O5固溶体中铁的难还原性是限制预氧化钒钛磁铁矿还原的主要原因。     

合金元素对Nb-Ti-Al-C合金氧化行为的影响

利用真空非自耗电弧炉制备不同Al含量的Nb-25Ti-8C合金,研究Nb-Ti-Al-C合金的组织结构及其高温氧化行为。研究表明,Nb-25Ti-8C-(0,5,10)Al合金由Nbss和(Nb,Ti)C两相构成;Nb-25Ti-8C-15Al合金由Nbss、(Nb,Ti)C和Nb3Al三相构成。800~1000℃氧化过程中,合金氧化膜为由Nb2O5,Ti O2,Al2O3,Nb O2及Ti Nb2O7多种氧化物构成的混合氧化膜。Ti、Al活性元素可优先与氧发生选择性氧化,抑制氧化物Nb2O5生成,提高氧化膜致密度和合金抗氧化性,并且氧化温度越高,Al元素改善铌合金抗氧化性能效果越明显。Nb-25Ti-8C合金800℃氧化时表现出良好的抗氧化性能,1000℃氧化时Nb-25Ti-8C-x Al合金的抗氧化性能明显优于C-103。随氧化温度升高,氧化膜中Nb2O5含量增加,导致氧化膜与合金基体的内应力增大,引起外层氧化膜脱落。碳化物中C元素以CO2形式挥发导致氧化层表面形成氧化空洞。     

NiAl-31Cr-2.9Mo-0.1Hf-0.05Ho定向共晶合金的高温氧化行为(英文)

采用SEM和XRD等分析手段对NiAl-31Cr-2.9Mo-0.1Hf-0.05Ho定向共晶合金的高温氧化行为进行研究。结果表明,在900~1100°C下合金表面生成连续的Al2O3氧化膜,从而使合金具有良好的抗氧化性能;在1150°C下合金表面的Al2O3氧化膜破裂,氧化增重升高。定向凝固工艺细化合金的组织以及微量稀土元素Ho的加入,均有利于在合金表面形成连续的Al2O3氧化膜。在氧化过程中,表面氧化膜存在着θ-Al2O3→α-Al2O3的相变过程,从而导致1000°C和1050°C氧化增重反常现象的出现。     

低品位钒渣在碳酸钠存在下的氧化过程

利用XRD,SEM/EDS和TG-DSC等手段对低品位钒渣在Na2CO3存在条件下的氧化过程进行检测。结果表明：钒渣的氧化温度范围为273至700°C,橄榄石相与尖晶石相彻底分解的温度分别是500和600°C,大部分水溶性的钒酸盐在500与600°C之间形成。当温度达到700°C以上时,钒酸盐富集相明显可见,但焙烧温度在800°C以上时,样品发生烧结,并且钒被形成的玻璃相包裹,导致其浸出率下降。同时,研究不同焙烧温度对钒浸出率的影响,并对浸出残渣进行分析。     

反应合成Ti_3SiC 2多孔材料的氧化行为（英文）

采用反应合成方法制备孔隙度为54.3%的高纯Ti_3SiC_2多孔材料,并研究其在400~1000°C下空气中的氧化行为。采用热重-差热分析法、扫描电镜、X射线衍射技术、能谱仪、拉曼光谱、BET比表面分析法和孔结构测试等研究Ti_3SiC_2多孔材料在氧化前后的氧化动力学、物相组成、微观形貌以及孔结构参数演变。结果表明:形成不同晶型TiO_2氧化产物是影响Ti_3SiC_2多孔材料抗氧化性及孔结构稳定性的主要因素。由于氧化产物体积应力以及热应力的存在,因此,在400~1000°C试验过程中试样表面均出现开裂现象。其中,在400~600°C下形成的锐钛矿型TiO 2会导致Ti_3SiC_2晶粒出现严重开裂,并引发快速氧化以及孔径和透气度的异常减小。600°C以上在氧化过程中主要形成金红石型TiO_2,开裂现象得以缓解,但是氧化膜的外延生长大幅降低了Ti_3SiC_2多孔材料孔隙的连通性。     

一种多孔镍基合金在850~1000°C下的氧化行为和机理（英文）

以一种多孔Ni-Cr-Al-Fe合金为研究对象,分别利用金相显微镜、扫描电子显微镜(SEM)及能谱仪(EDS)、X射线衍射(XRD)、X射线光电子能谱仪(XPS)等分析手段,研究其在850~1000°C温度范围内的氧化行为及机理。研究表明,该多孔合金在950°C和1000°C时呈伪抛物线型的氧化动力学曲线特征,其表面形成由外层Cr_2O_3/NiCr_2O_4和内层α-Al_2O_3构成的复杂氧化物结构。γ相的存在能促进氧化初期NiO/Cr_2O_3/NiCr_2O_4复合氧化物的形成,而多孔合金中众多快速扩散的通道有利于其氧化膜的发展。随着氧化时间的延长和氧化温度的升高,多孔合金的开孔率和渗透率均有所减小,但可控制在一定的范围内。     

铜基体上渗铝层的高温抗氧化性能和组织变化(英文)

以铜为基体,采用电镀镍和浆料包渗铝法,在800°C渗铝12 h制备出组织为Ni2Al3的单相渗层。采用SEM、XRD和光学显微镜,研究在1000°C下空气中氧化25~250 h,Ni2Al3渗层的高温抗氧化性能与组织转变行为。结果表明:Ni2Al3渗层在1000°C氧化250 h后,氧化增重分别为纯铜和镍镀层的1/15和1/2。氧化时间在25 h内时,渗层表面仍有Ni2Al3相;氧化50 h后,出现NiAl相;当氧化时间增加到100 h时,Ni2Al3相完全转变为NiAl相。当氧化到250 h时,渗层仍具有良好的抗高温氧化性能。     

Isothermal oxidation behavior of powder metallurgy beta gamma TiAl–2Nb–2Mo alloy

A new TiAl–2Nb–2Mo beta gamma alloy was synthesized by powder metallurgy process. HIP’ed and vacuum heat treated specimens were isothermally oxidized at 800 °C and 900 °C in air up to 500 h. The TiAl–2Nb–2Mo alloy oxidized parabolically up to 500 h at both 800 °C and 900 °C. The oxides consisted of outer TiO₂ layer, intermediate Al₂O₃ layer, and inner TiO₂ rich mixed layer and the oxidation mechanisms of the alloy were identical at both temperatures. During oxidation, the degradation of lamellar colonies formed a diffusion zone just below the oxide/substrate interface consisting of γ-TiAl matrix and dispersed beta phases which contained high concentration of Nb and Mo. The oxidation rate of the TiAl–2Nb–2Mo alloy is more sensitive to temperature than those of the Ti–48Al–2Nb–2Cr and Ti–48Al–2Nb–2Cr–W alloys.     

High-temperature oxidation behavior of multi-phase Mo-containing γ-TiAl-based alloys

Intermetallic titanium aluminides are potential materials for a number of high-temperature components used in aero and automotive engines. In particular, alloys solidifying via the β-phase are of great interest because they possess a significant volume fraction of the disordered body-centered cubic β_o-phase at elevated temperatures ensuring good processing characteristics during hot-working. Nevertheless, the practical use of such alloys at a temperature as high as 800 °C requires improvement of their oxidation resistance. Various attempts have been made including alloying with additional elements such as Nb, Cr, Mo etc. or applying the so-called fluorine effect. However alloying could not provide a sufficient oxidation resistance above 850 °C whereas the fluorine effect protects the base material against environmental degradation up to over 1000 °C. This paper aims to investigate the influence of the phase composition on the oxide scale morphology without and with fluorine effect. The results refer to the oxidation behavior of three β-solidifying γ-TiAl-based alloys in the cast and hot-isostatically pressed condition at 800 °C in air. The behavior of the TNM alloy (Ti–43.5Al–4Nb–1Mo–0.1B, in at.%) was compared with that of two Nb-free TiAl alloys which contain different amounts of Mo (3 and 7 at.%, respectively) and hence a different microstructure (α₂/β_o/γ vs. β_o/γ). During testing in dry synthetic air at 800 °C a mixed oxide scale develops on all three alloys. This behavior was changed via the fluorine effect, as demonstrated for previously investigated TiAl alloys with an Al-content higher than 40 at.% based on α₂/γ and α₂/β_o/γ phases. The oxidation resistance of the fluorine treated samples was significantly improved compared to the untreated samples. The reason for this is the change in the oxidation mechanism triggered by the small additions of fluorine in the subsurface zone of the investigated alloys. The results of isothermal oxidation tests at 800 °C in air are presented and discussed in view of chemical composition and microstructure, along with the impact of the phase composition on the efficiency of the fluorine effect. From a microstructural perspective the fluorine effect leads to the formation of an even thinner oxide scale on the β-phase compared to the γ-phase.     

Oxidation behavior of micro-sized Al2O3–TiC–Co composites prepared from cobalt-coated powders

The oxidation behavior of hot-pressed Al₂O₃–TiC–Co composites prepared from cobalt-coated powders has been studied in air in the temperature range from 200 °C to 1000 °C for 25 h. The oxidation resistance of Al₂O₃–TiC–Co composites increases with the increase of sintering temperature at 800 °C and 1000 °C. The oxidation surfaces were studied by XRD and SEM. The oxidation kinetics of Al₂O₃–TiC–Co composites follows a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. The mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations.     

High-temperature oxidation and hot corrosion of Cr2AlC

High-temperature oxidation and hot corrosion behaviors of Cr₂AlC were investigated at 800–1300 °C in air. Thermogravimetric–differential scanning calorimetric test revealed that the starting oxidation temperature for Cr₂AlC is about 800 °C, which is 400 °C higher than other ternary transition metal aluminum carbides. Thermogravimetric analyses demonstrated that Cr₂AlC displayed excellent high-temperature oxidation resistance with parabolic rate constants of 1.08 × 10⁻¹² and 2.96 × 10⁻⁹ kg² m⁻⁴ s⁻¹ at 800 and 1300 °C, respectively. Moreover, Cr₂AlC exhibited exceptionally good hot corrosion resistance against molten Na₂SO₄ salt. The mechanism of the excellent high-temperature corrosion resistance for Cr₂AlC can be attributed to the formation of a protective Al₂O₃-rich scale during both the high-temperature oxidation and hot corrosion processes.     

Comparison between oxidation kinetics of HVOF sprayed WC–12Co and WC–10Co–4Cr coatings

In this study, the high temperature oxidation behavior of HVOF-sprayed WC–12Co and WC–10Co–4Cr coatings were investigated. To explore the oxidation mechanism, thermo-gravimetric analysis (TGA) was applied for isothermal treatments in the range of 500–800 °C for 3 h. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the structural changes and microstructural evolutions during oxidation tests. The TGA experiments showed negligible oxidation mass gains at 500 °C for both coatings. At higher temperatures, i.e. 700 and 800 °C, the oxidation mass gains of WC–12Co were found to be much higher than those for WC–10Co–4Cr coating, respectively. The higher oxidation resistance of WC–10Co–4Cr coating probably results from the formation of compact chromium oxide layers and higher MWO₄ type tungstate (M: Co and/or Cr) to tungsten trioxide (WO₃) ratios which provide lower porosity and consequently more efficient passivation effect against oxidation. The time dependent mass gain of WC–12Co coating obeys the linear law within temperature range of 600–800 °C with apparent oxidation activation energy of ~ 104 kJ/mol. As for the oxidation of WC–10Co–4Cr coating, a negligible deviation from linear law was observed possibly due to the presence of chromium oxide and higher tungstate to tungsten trioxide ratio which hinders the diffusion process through the scales compared with WC–12Co coating. The apparent activation energy for oxidation of the WC–10Co–4Cr coating was found to be ~ 121 kJ/mol.     

The mechanism of oxidation of dilute NiAl alloys at 800–1200°C

The oxidation behaviour of dilute NiAl alloys at 800–1200°C in flowing oxygen at 1 atm pressure has been studied using kinetic measurements, optical and scanning electron microscopy and electron probe micro-analysis. The oxidation rates of Ni0.5 to 4%Al alloys are greater than the corresponding values for nickel at 1000 and 1200°C, but less at 800°C. The increased rates at the higher temperatures are largely due to increases in the total cation vacancy concentration in the scale, although internal oxide formation can make a significant contribution to the oxidation rate. The decreased rates at 800°C are almost certainly due to a build-up of Al₂O₃ particles at the oxide/alloy interface. The roles played in the oxidation processes by doping, internal oxidation, blocking effects in the oxide, dissociation of NiO and gaseous transport of oxygen within the scale are considered in detail and related to the oxidation rates of the various alloys.     

Glass coatings on stainless steels for high-temperature oxidation protection: Mechanisms

The oxidation behavior of a martensitic stainless steel with or without glass coating was investigated at 600–800 °C. The glass coating provided effective protection for the stainless steel against high-temperature oxidation. However, it follows different protection mechanisms depending on oxidation temperature. At 800 °C, glass coating acts as a barrier for oxygen diffusion, and oxidation of the glass coated steel follows linear law. At 700 or 600 °C, glass coating induces the formation of a (Cr, Fe)₂O₃/glass composite interlayer, through which the diffusion of Cr³⁺ or Fe³⁺ is dramatically limited. Oxidation follows parabolic law.     

Steam oxidation and its potential effects on creep strength of power station materials

The oxidation behaviour of several commercial alloys in simulated steam was investigated and compared with that of model alloys with systematic variations of selected alloying elements in the temperature range of 550°C to 650°C. Also, creep tests of specimens pre‐oxidized in steam or in flue gas were carried out in order to study the interaction between creep and corrosion. The corrosion products were characterised by optical microscopy, XRD, SEM/EDX and Raman spectroscopy. It was found that the oxidation resistance of the materials increased with increasing chromium content. High chromium materials exhibited the best oxidation behaviour, whereas the low chromium materials formed thick, multilayered oxide scales, prone to spallation. Anomalous temperature dependences were found in materials with intermediate chromium contents. Creep tests showed a significant reduction in the creep strength of both alloys was caused by a 1000 h thermal exposure treatment at 650°C for P92 and at 800°C for Alloy 800. An additional, though much smaller reduction in strength was observed for the specimens that had been oxidized for 1000 h at 650 or 800°C prior to testing. Further testing is required for confirmation of this effect.     

Oxidation Behavior of Tribaloy T-800 Alloy at 800 and 1,000 °C

The oxidation behavior of Co-based Tribaloy T-800 alloy has been studied isothermally in air at 800 and 1,000 °C, respectively. The results showed that the oxidation mechanism was dependent on the exposure temperature. The oxidation of the alloy followed subparabolic oxidation kinetics at 800 °C. The oxide scale at this temperature exhibited a multi-layered structure including an outer layer of Co oxide, a layer composed of complex oxide and spinel, a nonuniform Mo-rich oxide layer, an intermediate mixed oxides layer and an internal attacked layer with different protrusions into Laves phase. During 1,000 °C exposure, it followed linear kinetics. The oxidation rendered a relatively uniform external Cr-rich oxide layer coupled with a thin layer of spinel on the top surface and voids at local scale/alloy interface and intergranular region together with internal Si oxide at 1,000 °C.     

Cyclic oxidation behavior of TiCrN coating deposited on a steel substrate by the Arc-ion plating method

The cyclic oxidation behavior of a TiCrN coating deposited on a steel substrate was studied at 700, 800, 900 and 1000°C in atmospheric air, by repetitively exposing the specimen to thermal cycles of 2 hr-heating and subsequent quenching to room temperature. The coating displayed good cyclic oxidation resistance up to 800°C, with relatively small weight gains, but above 900°C, the coating displayed a drastic diminishment in oxidation resistance, accompanied by large weight gains. Cracks were observed particularly above 900°C throughout the oxide scale that consists of TiO₂, Cr₂O₃ and/or Fe₂O₃.