High temperature alloy chloridation at 850°C. Part I: Comparison of Ni‐based and Fe‐based alloy behaviour

Eight alloys were tested under Ar/Cl₂ atmosphere at 850°C for 15 min and 1 h. Their gross and net weights were evaluated together with the base metal consumption. Macroscopic and microscopic micrographs, associated with chemical analyses and X‐ray diffraction gave the composition and microstructure of the corrosion products. Huge differences were observed if one compared the nickel based alloy behaviour to that of the iron based alloy. Molybdenum and tungsten could also play a role, but it was not clearly defined until now. A tentative evaluation of the best candidates will be given, according the experimental conditions of this work and the chosen criteria. A corrosion index was established after each observation and helped to classify the different material behaviour under chloridation. The Ni‐based alloys possessed the best chloridation behaviour in the present conditions, followed by the Ni‐based alloys containing more Cr and finally by the Fe‐based materials. The chloridation behaviours were discussed according to the volatility of the chlorides species formed on the different alloys.     

High temperature oxidation behavior of FeCo‐based nanocrystalline alloys

This paper describes the dynamic and isothermal oxidation behavior of three different FeCo‐based Fe_38.5Co_38.5Nb₇Cu₁B₁₅, Fe₃₆Co₃₆Nb₇Si₁₀B ₁₁ and Fe_33.5Co_33.5Nb₇Si₁₅B₁₁ alloys and one traditional FINEMET Fe₇₃Nb₃Cu₁Si_15.5B_7.5 alloy. Dynamic and isothermal oxidation measurements in controlled oxidizing atmosphere were performed and the oxidation apparent energy as well as the oxidation behavior was obtained. SEM observations were carried out in order to characterize the oxide layer formed during the oxidation measurements. The apparent activation oxidation energy found for the Fe₃₆Co₃₆Nb₇Si₁₀B₁₁, Fe_33.5Co_33.5Nb₇Si₁₅B₁₁ and Fe₇₃Nb₃Cu₁Si_15.5B_7.5 alloys was about 35 kJ/mol and for the Fe_38.5Co_38.5Nb₇Cu₁B₁₅ alloy was about 70 kJ/mol.     

Oxidation‐led decomposition of hexagonal boron nitride coatings on alloy substrates at 900 °C: Ti‐6Al‐4V

Hexagonal boron nitride (h‐BN) coatings on Ti‐6Al‐4V substrates undergo complete decomposition in air at 900 °C. This fate is similar to that of this ceramic material on chromia‐former alloys, and unlike that of a mass of powder treated in isolation. As the ceramic and alloy oxidize concurrently, outwardly diffusing aluminum (III) ions but not the predominant titanium (IV) ions react with the boron trioxide that forms around the h‐BN basal plane peripheries. Resultant aluminum borate is incorporated into the growing scale and the boron trioxide diffusion barrier is depleted. By this mechanism, the oxidation of h‐BN is maintained at an enhanced rate, until both this material and its oxide completely decompose. Liberated nitrogen from the oxidation of h‐BN can enter the underlying scale as a randomly distributed solute in rutile solid solution. The post‐coating oxide‐atmosphere interface comprises elongated aluminum borate crystallites protruding through at the boundaries between 3–5 at% nitrogen‐doped rutile grains. It differs significantly from that of oxidized, uncoated Ti‐6Al‐4V, which is occupied by a thin α‐alumina layer atop rutile. This interface does not change with an additional 72 h of heat‐treatment.     

Corrosion behavior of Fe(Ni)CrAIX alloys in an HCl−H2O−H2 gas mixture at 800°C

The high-temperature-corrosion behavior of a series of Fe(Ni)CrAlX-type alloys (where X=Zr and Hf, e.g.) has been studied in a gas mixture of 50% HCl-10% H₂O–H₂ at 800°C. The experimental results obtained indicated that Ni-base alloys had superior corrosion resistance to Fe-base alloys in this gas mixture. While the exposed Ni-base alloys showed weight gains due to the formation of oxides (e.g., Al₂O₃, Cr₂O₃) as well as CrCl₂, the Fe-base alloys exhibited substantial weight losses resulting from the formation and subsequent evaporation of FeCl₂. This study also demonstrated that Fe(Ni)Cr8AlX-type alloys, which contained high aluminum, had better chloridation resistance than Fe(Ni)25CrAlX-type alloys, which had high chromium. The improved performance of Fe(Ni)Cr8AlX-type alloys was due to the presence of a high level of aluminum which promoted formation of protective Al₂O₃. Although the presence of chromium in the alloys promoted the formation of Cr₂O₃, the high level of chromium adversely affected the chloridation resistance of Fe(Ni)25CrAlX-type alloys, due to the development of chloride (CrCl₂) at the interface of the oxide scale and alloy substrate.     

Effect of creep deformation on oxidation behavior of nickel‐based alloy with Re‐based diffusion barrier coating

Oxidation and creep behaviors of a Ni‐Mo‐based alloy (Hastelloy‐X) with a diffusion barrier coating consisting of duplex, inner Re‐based alloy, and outer β‐NiAl layers were investigated at 1243 K in air with an external tensile stress of 22.5 MPa. For comparison the alloys, as‐received, heat treated, and with the Ni‐aluminide coating, were oxidized under creep deformation. Creep rapture time for the diffusion barrier‐coated alloy was longer than those for the bare alloy and with the β‐NiAl‐coated alloy. After creep deformation to a strain of 3.5% for 190 h, it was found that the Re‐based alloy layer has few cracks and flaws and the β‐NiAl layer has the similar structure and composition before and after the creep test. The external scale mainly consisted of θ‐Al₂O₃ at the early stage of creep, and with further oxidation the external scale became a duplex layer, inner, equi‐axed α‐Al₂O₃, and outer, plate‐like θ‐Al₂O₃, which exfoliated significantly. It was concluded that the Re‐based alloy layer acts effectively as a barrier against inward Al diffusion and outward diffusion of alloy elements under creep deformation.     

Ti2AlNb合金不同温度下的高温氧化行为

研究了Ti₂AlNb合金在不同温度下的高温氧化行为。采用连续氧化增重的方法计算了合金氧化动力学规律;用XRD、SEM、EDS等手段研究氧化后的表面相结构和表面、截面形貌以及元素分布。结果表明,Ti₂AlNb合金在650 ℃和750 ℃下的连续氧化动力学曲线近似符合抛物线规律,而在850 ℃时符合直线规律。氧化层分层现象明显,且都未形成连续致密的Al₂O₃保护层。氧化产物主要为TiO₂,少量的Al₂O₃、Nb₂O₅,以及微量的AlNbO₄、Nb₂TiO₇、AlNb₂。氧化温度越高,分层越明显,富氧层越厚,危害性氧化物越多。     

Oxidation behaviour of particle reinforced MoSi2 composites at temperatures up to 1700°C.

Part III: Oxidation behaviour of optimised MoSi₂ composites The topic “Oxidation behaviour of particle reinforced MoSi₂ composites at temperatures up to 1700°C” is discussed in a three part publication. In the first part of this paper a literature survey on the oxidation behaviour of MoSi₂ and MoSi₂ composites has been given. In the second part an initial screening at 1600°C revealed those composites which may be suitable for high temperature applications. The low temperature oxidation behaviour of selected composites in the “pest” region was examined as well. Additionally, the effect of iron impurities on the high temperature behaviour of the composites was explained. The present part deals with a detailed investigation of an optimised MoSi₂/HfO₂ composite. These investigations include high temperature oxidation at 1500 to 1700°C and low temperature oxidation at 400 and 500°C.     

Corrosion behavior of Fe‐Si metallic coatings added with NiCrAlY in an environment of fuel oil ashes at 700°C

Electrochemical potentiodynamic polarization curves and immersion tests for 300 h at 700°C in a furnace have been used to evaluate the corrosion resistance of Fe‐Si metallic coatings added with up to 50 wt.% of NiCrAIY. The corrosive environment was fuel oil ashes from a steam generator. The composition of fuel oil ashes includes high content of vanadium, sodium and sulfur. The results obtained show that only the addition of 20 wt.% NiCrAlY to the Fe‐Si coating improves its corrosion resistance. The behavior of all tested coatings is explained by the results obtained from the analysis of every coating using electron microscopy and energy dispersive X‐ray analysis.     

新型Cr18Ni31Al合金的高温抗氧化性

利用氧化增量法测得新型Cr18Ni31Al合金的不同温度下的高温氧化动力学曲线,使用X射线衍射仪(XRD)、扫描电镜(SEM)和能谱仪(EDS)对合金表面高温氧化膜的形貌及组成进行了分析和检测。结果表明,新型Cr18Ni31Al合金的高温氧化动力学曲线为Δm。700 ℃和800 ℃氧化后,氧化膜均由Fe₂O₃和NiCr₂O₄组成;900 ℃氧化后,氧化膜表面有尖晶石结构的Fe(Cr, Al)₂O₄氧化物生成。＝atⁿ     

Phase equilibria in the Ni–Cr–Ti system at 850°C

The isothermal cross-section through the ternary phase diagram Ni–Cr–Ti at 850°C was constructed by means of diffusion couples and equilibrated alloys. No ternary phases exist in the system at this temperature. The topology of the isotherm is largely determined by the presence of the TiCr₂-Laves phases which are in equilibrium with the binary Ti–Ni intermetallics. About 10 at.% of Ni can be dissolved in the hexagonal β-TiCr₂ at 850°C, and the solubility of nickel in cubic α-TiCr₂ is approximately 4 at.%. A small amount of nickel or chromium increases the stability of the b.c.c. β-Ti structure. At this temperature the β-Ti(Ni)-based solid solution can dissolve up to 18 at.% of Cr.     

Field exposure of FeCrAl model alloys in a waste‐fired boiler at 600°C: The influence of Cr and Si on the corrosion behaviour

The aim of this study was to examine the performance of FeCrAl model alloys in a waste‐fired boiler and investigate the influence of chromium and silicon content on the corrosion behaviour. The investigation was executed by utilising an air‐cooled probe, giving a material temperature of 600°C throughout a 672 hr exposure. The material loss measurements were performed by utilizing an ultrasonic thickness gauge in combination with scanning electron microscopy analysis. It was found that increasing the chromium content significantly reduced the overall material loss of the FeCrAl model alloys but further accelerated the corrosion attack on the windward side. Simultaneously, the increased chromium content caused embrittlement of the material. Minor additions of silicon drastically reduced the material loss of the FeCrAl model alloys, whereas the sample ring with no silicon present was completely deteriorated. The trends observed in this field study correlated well with what has been observed in previous laboratory studies. A state‐of‐the‐art alloy in the present environment, Inconel 625, was simultaneously exposed and showed similar performance to the silicon‐containing FeCrAl model alloys with ≥10 wt% Cr.     

Solid state phase equilibria in the Fe–Ga–Sb ternary system at 600 °C

Solid state phase equilibria in the ternary Fe–Ga–Sb diagram were determined at 600 °C using experimental techniques such as X-ray diffraction, electron probe microanalysis and scanning electron microscopy. Very limited solid solutions were measured in the binary constituent Fe–Ga and Fe–Sb compounds except for the -phase (Fe_≈2.55Sb₂) which extends from 42 to 48 at.% Sb. In the Fe-rich part of the diagram, a ternary phase Fe_tGa_2−xSb_x (2.15≤t≤2.80) was evidenced which corresponds in fact to a solid solution into which Ga and Sb substitute one another on the same hexagonal sublattice. This phase, which can be truly considered as a pseudo-binary one since its origin results from the -phase, shows an extended homogeneity range with a Ga-rich limit corresponding to the formula Fe_tGa_0.8Sb_1.2. Moreover, it crystallizes in hexagonal symmetry with a disordered structure derivative from the NiAs-type (B8₁). This pseudo-binary phase is in thermodynamic equilibrium with all the binaries of the system except FeGa₃. The main result of the ternary Fe–Ga–Sb diagram remains the existence of a diphasic region between the Fe_tGa_2−xSb_x phase (1.2≤x≤1.6; 2.15≤t≤2.80) and the semiconductor GaSb. Nevertheless, at 600 °C, this pseudo-binary phase does not extend up to the Fe₃GaSb composition which is stoichiometric in Ga and Sb. Finally, a comparative study has been made with the three other ternary systems Fe–Ga–As, Ni–Ga–As and Ni–Ga–Sb previously reported, and the consequences for the solid state interdiffusions in Metal/III–V semiconductor heterostructures are discussed.     

高熵合金抗高温氧化机理研究展望

近年来高熵合金因具有许多优于传统合金的性能备受瞩目,而高温氧化问题大大限制了其发展应用。多元组成使高熵合金的高温氧化过程不同于单一金属,不同氧化阶段的动力学规律有很大不同。氧化前期多种元素发生氧化反应,氧化物种类和氧化膜结构随时间发生变化,直到稳定氧化阶段氧化产物才固定存在。本文从初期选择性氧化、过渡态氧化和稳定氧化期3个阶段深入剖析高熵合金高温氧化各个过程的详细机理,并总结相应的改善高温抗氧化性能的方法,为高熵合金材料设计和性能调控提供重要的理论依据。     

The effect of surface treatment on the corrosion behavior of a pure Ta sheet in an equimolar NaCl–KCl melt at 850°C in air. Part 1: Laser surface melting

The corrosion behavior of pure Ta and laser surface melting–treated Ta sheets was investigated in an equimolar NaCl–KCl melt at 850°C in air. The results show that the predominant factors involved in the consumption of materials were reactions that form sodium‐containing oxides and Ta₂O₅. High corrosion stress led to slip of the untreated Ta surface grains, further resulting in preferential corrosion along slip bands. The slip of surface grains substantially deteriorated the bonding between the corrosion scale and matrix, thus leading to rapid degradation of the corrosion scale. The melt layer consisted of a thin Ta₂O₅ film, and the inner dendrite layer demonstrated self‐healing characteristics during corrosion and presented a better hot corrosion resistance than the untreated Ta owing to the excellent bonding at the scale and melt layer interface.     

Phase relationships at 500 °C in the Y---Pr---Mg system

Phase equilibria in the ternary system Y---Pr---Mg have been studied by X-ray powder diffraction analysis, optical and scanning electron microscopy, and electron probe microanalysis. The isothermal section at 500 °C has been determined in the range 50–100 at.% Mg. The phase equilibria are characterized by the formation of the following rather extended solid solutions: (Y_xPr_1−x)Mg (continuous solid solution of cP2-CsCl type) and Y_xPr_1−xMg₃ (0 x 0.5) of cF16-BiF₃ type. A cubic phase of (Y_xPr_1−x)Mg₂, (cF24-CuMg₂ type) derived from the binary PrMg₂ phase by a substitution of Pr with Y has been observed in a narrow range close to x ≈ 0.6. A solid solution based on the binary YMg₂ phase, i.e. (Y_xPr_1−x)Mg_{2 + y} of hP12-MgZn₂ type (0.79 x 1, for y = 0), extends into the ternary field. The ternary compound Y_xPr_{1 − x}Mg₅ (0.3 x 0.9) of the cF440-GdMg₅ type has been established. Finally, the Y---Pr---Mg section has been compared with the similar Y---La---Mg section studied previously and with an isothermal section predicted on the basis of the systematic trends of the rare earth alloy properties.