High-temperature oxidation resistance of ceramic coatings on titanium alloy by micro-arc oxidation in aluminate solution

Ceramic coatings with aluminum titanuate (Al₂TiO₅) were prepared on Ti–6Al–4V alloy using pulsed bi-polar Micro-arc Oxidation (MAO). The micromorphology and phase composition of the micro-arc-oxidition ceramic coatings on the titanium alloy were characterized by X-ray powder diffraction (XRD), and scanning electron microscopy (SEM) respectively. The results revealed that the distinct discharge channels and pores on the surface of the micro-arc-oxidition coatings appeared, and these channels were connected in the molten state. The electrolyte concentration was inversely proportional to the coating hardness; additionally, the coating prepared with sodium aluminate and sodium hypophosphite concentrations of 4 ?g/L and 0.5 ?g/L, respectively, was the most refined after high-temperature sintering, and it was demonstrated to better prevent oxidation. Increasing the electrolyte concentration coincided with fluctuating coating thermal shock resistance. The thermal shock resistance of the coating respectively prepared with sodium aluminate, and the sodium hypophosphite concentrations of 4 ?g/L and 0.5 ?g/L was the highest. Additionally, the high-concentration coatings performed significantly better than the low-concentration coatings. The oxidation resistance of the coating samples was also significantly higher than that of the TC4 titanium alloy substrate. The adhesion strength between the coatings and the substrate with and without the sealing treatment was measured by tensile tests. Then, the high-temperature oxidation performance of the coating samples with and without the sealing treatment was investigated by conducting a high-temperature oxidation experiment at a calcinating temperature of 500 ?°C. The results indicate that the adhesion strength between the coatings and substrate was high for the as-prepared and sealed micro-arc oxidation samples regardless of whether they were calcined. The high-temperature oxidation mass increase curves for the sealed and unsealed coating samples calcined at 500 ?°C for 500 ?h revealed that the high-temperature-oxidation-induced mass increase of the coating samples sealed with a sodium silicate solution was much lower than that of the titanium alloy substrate. Thus, the sealing treatment significantly improved the high-temperature oxidation resistance of the TC4 titanium alloy. Lastly, the high-temperature oxidation behavior at 500 ?°C was analyzed and discussed.     

Corrosion mechanism of micro-arc oxidation treated biocompatible AZ31 magnesium alloy in simulated body fluid

The rapid degradation of magnesium(Mg) based alloys has prevented their further use in orthopedic trauma fixation and vascular intervention,and therefore it is essential to investigate the corrosion mechanism for improving the corrosion resistance of these alloys. In this work, the effect of applied voltage on the surface morphology and the corrosion behavior of micro-arc oxidation(MAO) with different voltages were carried out to obtain biocompatible ceramic coatings on AZ31 Mg alloy. The effects of applied voltage on the surface morphology and the corrosion behavior of MAO samples in the simulated body fluid(SBF) were studied systematically. Scanning electron microscope(SEM) and X-ray diffractometer(XRD)were employed to characterize the morphologies and phase compositions of coating before and after corrosion. The results showed that corrosion resistance of the MAO coating obtained at 250 V was better than the others in SBF. The dense layer of MAO coating and the corrosion precipitation were the key factors for corrosion behavior. The corrosion of precipitation Mg(OH)2and the calcium phosphate(Ca–P) minerals on the surface of MAO coatings could enhance their corrosion resistance effectively. In addition, the mechanism of MAO coated Mg alloys was proposed.     

Oxidation behavior of the Fe-36Al-0.09C-0.09B-0.04Zr alloy at 1250°C

To explore and study the Fe-Al system alloy presenting exceptional oxidation resistance at high temperature, the Fe-36Al-0.09C-0.09B-0.04Zr alloy was designed and developed. The microstructure and hardness of the backing at 1250°C were analyzed and measured. Thermodynamics and kinetics of the oxidation behavior were also analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy techniques. The results show that the microstructure of the Fe-36Al-0.09C-0.09B-0.04Zr alloy is FeAl phase at ambient temperature and is stable at 1250°C. It displays the excellent property of oxidation resistance because the oxide film has only the Al₂O₃ layer, and its oxidation kinetics curve obeys the parabolic law at 1250°C. The oxidation mechanism at 1250°C is presumed that in the early oxidation period, the alloy oxidizes to form a large number of Al₂O₃ and a little Fe₂O₃, then, the enrichment of Al caused by Fe oxidization combines with O to form Al₂O₃.     

Oxidation behavior of in-situ Al2O3/TiAl composites at 900°C in static air

In-situ Al₂O₃/TiAl composites were successfully synthesized from the starting powders of Ti, Al, TiO₂ and Nb₂O₅. The oxidation behavior of the composites at 900°C in static air was investigated. The results indicate that the composite samples present a much lower oxidation mass gain. Under long-time intensive oxidation exposure, the formed oxide scale is multi-layer. The formation of the outer TiO₂ layer is fine and dense, the internal Al₂O₃ scale has good adhesiveness with the outer TiO₂ scale, and the TiO₂+Al₂O₃ mixed layer forming the protective oxide scale is favorable for the improvement of oxidation resistance. It is believed that the incorporation of Al₂O₃ particulates into the metal matrix decreases the coefficient of thermal expansion of the substrate, and forms a local three-dimensional network structure that can hold the oxide scale. The formation of the oxide scale with finer particle size, stronger adherence, less micro-defects and slower growth rate can contribute to the improvement of oxidation resistance. Nb element plays an important role in reducing the internal oxidation action of the materials, restraining the growth of TiO₂ crystals and promoting the stable formation of the Al₂O₃-riched layer, which is beneficial to improve the oxidation properties.     

电参数对钛合金微弧氧化法制备TiO2生物陶瓷涂层的影响

采用微弧氧化法在钛合金(Ti-6Al-4V)基体上制备TiO2生物陶瓷涂层.试验研究了不同微弧氧化电压和脉冲频率对涂层微观形貌,涂层厚度及相组成的影响.结果表明:涂层厚度随氧化电压的升高而增加,随脉冲频率的增加而减小.SEM观察显示:涂层微孔数目随电压的升高而增加,随脉冲频率的增加表面粗糙度增大.XRD相结构分析表明:涂层中的主要相为金红石型TiO2和锐钛矿型TiO2及CaTiO3.     

ZrO2 and ZrO2 -Y2 O3 coatings deposited by double pulsed plasma arc

A novel surface technique has been developed to produce ZrO₂ and ZrO₂-Y₂O₃ coatings on the surface of alloys by using double pulsed plasma arc to react with a solution film containing nano-oxide particles. These coatings exhibit smooth surface and excellent adhesion with substrate. The morphologies of the ceramic coatings and phases were analyzed. It was shown that the oxidation resistance of 18-8 stainless steel was markedly improved by applying ZrO₂ and ZrO₂-Y₂O₃ coatings.     

Effects of surface modification on isothermal oxidation behavior of HVOF-sprayed NiCrAlY coatings

NiCrAlY coatings were deposited on Ni-based superalloy by high-velocity oxygen-fuel spraying (HVOF). Surface modification by means of grit-blasted, shot-peened and ground methods was used in order to study the effect of surface conditions on the isothermal oxidation behavior of HVOF-sprayed NiCrAlY coatings at 1050 ℃. The results showed that surface modification had an obvious effect on the isothermal oxidation behavior of the coatings. There was a large decrease in growth rate compared with the as-sprayed coating. The scale formed on the grit-blasted and shot-peened coatings was a mixture of Al₂O₃ and NiCr₂O₄, while the oxide formed on the ground coating was composed mainly of Al2O3. After surface modification, the content of NiCr₂O₄ spinels decreased compared with the as-sprayed coating.     

Improvement of oxidation resistance of Nb–Ti–Si based alloys with additions of Al,Cr and B at different temperatures

The effects of Cr, Al and B addition on the microstructure and high-temperature oxidation behaviors (at 1200, 1250 and 1300 ?°C) of Nb–Ti–Si based alloys were investigated. The results showed that the addition of Cr stabilized α-Nb₅Si₃, while Al promoted the formation of β-Nb₅Si₃ and adding B promoted the formation of γ-Nb₅Si₃. Among the three elements, Al and Cr were beneficial to oxidation resistance at 1200 ?°C, and B was favorable to the oxidation resistance at 1300 ?°C. At 1250 ?°C, Al and B had the same effects on the improvement of oxidation resistance. The ratio of these alloying elements might play an important role in enhancing oxidation resistance. The oxidation resistance of the three kinds of silicides was compared, and the sequence was: γ-Nb₅Si₃> β-Nb₅Si₃> α-Nb₅Si₃. To predict the effects of the investigated alloying elements on the oxidation resistance of Nb–Ti–Si based alloys in a wider range of concentration, an artificial neural network (ANN) model was established, showing excellent accuracy and generalization ability. With the instructions of the ANN model, the oxidation resistance can be optimized with less additions of alloying elements.     

Growth characteristics of directionally solidified Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/YAG/ZrO<Subscript>2</Subscript> ternary hypereutectic <Emphasis Type="Italic">in situ</Emphasis> composites under ultra-high temperature gradient

Oxide eutectic ceramic in situ composites have attracted significant interest in the application of high-temperature structural materials because of their excellent high-temperature strength, oxidation and creep resistance, as well as outstanding microstructural stability. The directionally solidified ternary Al₂O₃/YAG/ZrO₂ hypereutectic in situ composite was successfully prepared by a laser zone remelting method, aiming to investigate the growth characteristic under ultra-high temperature gradient. The microstructures and phase composition of the as-solidified hypereutectic were characterized by using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The results show that the composite presents a typical hypereutectic lamellar microstructure consisting of fine Al₂O₃ and YAG phases, and the enriched ZrO₂ phases with smaller sizes are randomly distributed at the Al₂O₃/YAG interface and in Al₂O₃ phases. Laser power and scanning rate strongly affect the sample quality and microstructure characteristic. Additionally, coarse colony microstructures were also observed, and their formation and the effect of temperature gradient on the microstructure were discussed.     

ZrO_2-supported α-MnO_2:Improving low-temperature activity and stability for catalytic oxidation of methane

The single phaseα-Mn O_2and in-situ supportedα-Mn O_2/Zr O_2with different ratios of Mn/Zr were synthesized by one-pot hydrothermal method.They showed superior activity for catalytic oxidation of methane and even better than that of 1%Pt/Al_2O_3.The T_(50)of Mn O_2/Zr O_2catalysts with different ratios of Mn/Zr were located in the range of 315–335°C at a WHSV of 90 L g~(-1)h~(-1),whereas that of Pt/Al_2O_3was 380°C.After sulfur ageing,the Mn O_2/Zr O_2catalysts with Mn/Zr ratio of 2:1(2Mn O_2/1Zr O_2)and 1:1(1Mn O_2/1Zr O_2)exhibited satisfying sulfur resistance in comparison to the pure Mn O_2.The 2Mn O_2/1Zr O_2catalyst also showed acceptable catalytic stability,and the addition of 10 vol%CO_2had no obvious negative effect on its stability,whereas the addition of2.6 vol%H_2O caused slight but reversible decreasing methane oxidative activity.     

Microstructural evolution and mechanical properties of Nb-Si-Cr ternary alloys with a tri-phase Nb/Nb5Si3/Cr2Nb microstructure fabricated by spark plasma sintering

In this work, pure Nb, Nb₅Si₃ and Laves Cr₂Nb compound powders were used as raw materials to prepare Nb-Si-Cr ternary alloys by spark plasma sintering (SPS). A comprehensive estimation of the microstructure and properties, including room temperature fracture toughness, high temperature strength and oxidation resistance, of the Nb-Si-Cr ternary alloys as a function of the Nb/Nb₅Si₃/Cr₂Nb phase volume fraction combinations was conducted. The results showed that Nb-Si-Cr ternary samples with the relative density larger than 98.42% were obtained by SPS processing, and the samples all consisted of Nb, Nb₅Si₃ and Cr₂Nb phases that were distributed homogeneously. The fracture toughness K_Q of the Nb/Nb₅Si₃/Cr₂Nb microstructure, which was dominated by the Nb phase, naturally increased with the Nb fraction. As expected, the room-temperature Vickers hardness and the high-temperature strength of the bulk alloys increased monotonically with the increasing of the stiffening Nb₅Si₃ fraction. Interestingly, the binary Cr₂Nb phase played a positive role in the high temperature strength and oxidation resistance. Finally, the fracture modes of the typical Nb/Nb₅Si₃/Cr₂Nb microstructures under bending and compression conditions at room and high temperatures as well as the oxidation mechanism are described and discussed.     

单极性脉冲电流密度对铝合金MAO膜相结构和微结构的影响

利用微弧氧化(MAO)技术在铝合金表面沉积陶瓷膜, 采用X射线衍射仪(XRD), 扫描电子显微镜(SEM), 电子能谱(EDS)和显微硬度方法, 研究了MAO过程中单极性脉冲电流密度与氧化膜力学性能和化学成分的关系. 发现在较高电流密度下制备的陶瓷膜中含有较多的α-Al₂O₃相, 而在较低电流密度下制备的陶瓷膜主要由γ-Al₂O₃相组成. 对陶瓷膜截面上不同厚度处的膜层化学成分分析表明, 组成氧化膜的主要物质α-Al₂O₃和γ-Al₂O₃的比例随深度的变化呈现出复杂的变化规律, 这一结果与相关的文献报道有所不同.     

Experimental studies and modeling for the transition from internal to external oxidation of three-phase Nb-Si-Cr alloys

A modified oxidation model, which assumes the cylindrical-like morphology of Cr_2Nb phase, has been proposed to examine the conditions to form external scale of three-phase Nb-base alloys in this paper. The oxidation behavior of three-phase Nb-base alloys was tested at 1250 ℃ for 1 h and 7 h, and the experimental results were compared with the simulated results. The results show that the external CrNbO_4 scale can be achieved by lengthradius ratio and volume fraction increasing for Cr_2Nb phase. The simulated results are consistent with the experimental results.     

Fabrication and characterization of nanocrystalline Al/Al12（Fe,V）3Si alloys by consolidation of mechanically alloyed powders

The aim of this study was to produce bulk nanocrystalline Al/Al12（Fe,V）3Si alloys by mechanical alloying （MA） and subsequent hot pressing （HP） of elemental powders. A nanostructured Al-based solid solution was formed by MA of elemental powders for 60 h. After HP of the as-milled powders at 550℃ for 20 min, the Al12（Fe,V）3Si phase was precipitated in a nanocrystalline Al matrix. Scanning electron microscopy （SEM） images of the bulk samples represented a homogeneous and uniform microstructure that was superior to those previously obtained by rapid solidification-powder metallurgy （RS-PM）. Nanostructured Al-8.5Fe-1.3V-1.7Si and Al-11.6Fe-1.3V-2.3Si alloys ex-hibited high HV hardness values of～205 and～254, respectively, which are significantly higher than those reported for the RS-PM counter-parts.     

Rapid reactive synthesis of TiAl3 intermetallics by thermal explosion and its oxidation resistance at high temperature

Porous TiAl₃ intermetallics were synthesized from Ti-75 at.% Al elemental powder mixtures using an energy-saving and rapid reactive method of thermal explosion (TE). The results demonstrated that the actual temperature of the compact climbed rapidly from 673 °C to 1036 °C within 24 s, indicating that an obvious TE reaction occurred during sintering process. The video graphs suggested that the TE in Ti–Al system behaved instant occurrence and overall heating whether from axial or radial direction. The silver wires and NaCl particles that pressed on the surface of the sample disappeared due to the heavy heat released during TE reaction. Only pure TiAl₃ phases were synthesized in TE products and the open porosity of 55.4% was easy to obtain. After high-temperature treatment at 1000 °C, large amounts of sintering-neck formed and then improved the compressive strength of porous TiAl₃ materials. Moreover, the mass gain curve of porous TiAl₃ intermetallics oxidized at 650 °C for 120 h exhibited the parabolic oxidation rate law. XPS analysis confirmed that the strong O 1s peak was 531.4 eV which was the typical binding energy of Al₂O₃. Therefore, the excellent oxidation resistance of porous TiAl₃ foams would be considered as good candidate materials for prolonging the service life at high temperatures.     

Vapor phase codeposition of Cr and Si on Nb-base in situ composites by pack cementation process

In order to identify suitable halide activators and pack compositions for codepositing Cr and Si to form diffusion coatings on Nb-base in situ composites by the pack cementation process, thermochemical calculation was taken to analyze the vapor pressure of halide species generated at high temperatures. NH4Cl, NaF and CrCl3·6H2O were selected as the halide salts. The results of thermochemical calculations suggested that the pack powder mixtures, which contained Cr, Si, halide salts and Al2O3, may be activated by NH4Cl and NaF. According to the thermochemical calculations, the pack powder mixture of 12Cr-6Si-5NH4Cl-77Al2O3 (wt%) activated by NH4Cl was formulated and coating deposition experiments were carried out at 1200 and 1300℃. With adequate control of pack compositions and deposition conditions, it was found that codeposition of Cr and Si could indeed be achieved at these temperatures. The coating has a three-layer structure, of which was mainly composed of Cr2(Nb,X) (X represents Ti and Hf elements), Nb5Si3 and (Nb,Cr)3Si. Then the kinetics of coating growth process affected by temperature was studied. The experimental results of the oxidation showed that the coating can efficiently prevent substrate from oxidizing.     

Grain refinement of Mg-Li-Al cast alloys by adding typical master alloys

Commercial Al-3Ti-1C and Al-5Ti-1B master alloys were added in order to refine the grains of Mg-Li-Al alloys.The effects of their addition levels on grain refinement of Mg-Li-Al cast alloy were investigated and the mechanism of the grain refinement was discussed.The results showed that the addition of Al-3Ti-1C master alloy reduced the grain size of LA141 cast alloy from 900μm to 400μm,while the addition of Al-5Ti-1B master alloy reduced the grain size of LA51 cast alloy from 500μm to 240μm.The grain ref...     

Oxidation behavior of high-strength FeCrAl alloys in a high-temperature supercritical carbon dioxide environment

The oxidation behavior of three high-strength FeCrAl alloys was investigated in supercritical carbon dioxide environment at 650 ℃. After exposure for 500 h, the weight gains of the FeCrAl alloys gradually decreased with increasing Al content. The oxide scales are primarily composed of α-Al_2O_3 and spinel oxides. With increasing Al content, the amount of α-Al_2O_3 increases and the C content decreased in the oxide scale and sub-scale matrix.Moreover, larger(Nb,Mo)C carbides formed in the sub-scale matrix and their number decreased with the increase of Al content.     

Inter-phase selective corrosion of γ-TiAl alloy in molten salt environment at high temperature

Hot corrosion behavior of Ti-48Al-2Cr-2Nb alloy in molten salt environment of 75%Na_2SO_4 and 25%NaCl(mass fraction) at 800℃was studied using oxidation kinetics analysis,scanning electron microscopy,X-ray energy dispersive and X-ray diffraction analyses.The inter-phase selective corrosion,which caused the corrosion pits initiated from lamellar interfaces and the preferential corrosion ofα_2 phases,was observed during the hot corrosion of the alloy.The common controls of thermodynamic and kinetics in corr...     

Cyclic oxidation behavior of b-NiAlDy alloys containing varying aluminum content at 1200 1C

b-NiAlDy cast alloys containing varying aluminum content were prepared by arcmelting. The microstructures and cyclic oxidation behavior of the alloys at 1200 1C were investigated. Grain refinement was achieved by increasing aluminum content in the alloy, which is beneficial to selective oxidation. The Ni–55Al–0.1Dy alloy showed excellent cyclic oxidation resistance due to the formation of a continuous, dense and slow-growing oxide scale. In contrast to this, severe internal oxidation as well as large void formation at the scale/alloy interface occurred in the Ni–45Al–0.1Dy alloy. The aluminum content dependence of the reactive element effects in b- NiAlDy was established that Dy doping strengthened the scale/alloy interface by pegging mechanism in high-aluminum alloys but accelerated internal oxidation in low-aluminum alloys during high-temperature exposure.