Cyclic oxidation behavior of TiAl composites incorporated with TiB2 dispersoids

TiAl alloys incorporated in (0,3,5,10) wt.% TiB₂ dispersoids were manufactured via mechanical alloyingspark plasma sintering (MA-SPS), and their cyclic oxidation characteristics were studied at 800, 900 and 1000°C in air. The cyclic oxidation resistance of the prepared TiAl-TiB₂ composites effectively increased with increases in TiB₂ content. The oxide scale formed consisted of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (Al₂O₃+TiO₂) mixed layer. The scale adherence was relatively good, and much thinner oxide scales, when compared to TiB₂-free TiAl alloys, were formed on the prepared composites. The incorporated TiB₂ dispersoids oxidized to TiO₂ and B₂O₃ which evaporated during oxidation.     

The Oxidation of TiB2 Particle-Reinforced TiAl Intermetallic Composites

The oxidation kinetics of TiAl alloys with and without (3, 5, 10 wt.%) TiB₂ dispersoids were studied between 1073 and 1273 K in atmospheric air. The inert TiB₂ dispersoids effectively increased the oxidation resistance of TiAl alloys. The higher the TiB₂ dispersoids content, the more pronounced the effect. The oxide scale formed on TiAl–TiB₂ composites was triple-layered, consisting mainly of an outer TiO₂ layer, an intermediate Al₂O₃ layer, and an inner (TiO₂+Al₂O₃) mixed layer. No B₂O₃ was observed within the oxide scale because of its high vapor pressure. A thin Ti₃Al sublayer and discrete TiN particles were found at the oxide–substrate interface. During the oxidation of TiAl alloys with and without TiB₂ dispersoids, titanium ions diffused outwardly to form the outer TiO₂ layer, while oxygen ions transported inwardly to form the inner (TiO₂+Al₂O₃) mixed layer. The increased oxidation resistance by the addition of TiB₂ was attributed to the enhanced alumina-forming tendency and thin and dense scale formation.     

High temperature cyclic oxidation of aluminide layers on titanium

Aluminide layers containing TiAl₃ or TiAl on the surface were formed on titanium samples using the pack-cementation process. Cyclic oxidation (1 hr at the desired temperature and 20 min at room temperature) was carried out over the temperature range 850–1000°C. TiAl surfaces showed poor oxidation resistance compared to TiAl₃. The oxide morphology showed a peculiar protrusion formation on TiAl₃ surfaces at lower temperatures. A cross-section of the oxidized sample showed discrete Al₂O₃ and TiO₂ layers.     

Improving the high-temperature oxidation resistance of a β-γ TiAl alloy by a Cr2AlC coating

A Cr₂AlC coating was deposited on a β-γ TiAl alloy. Isothermal oxidation tests at 700 °C and 800 °C, and thermocyclic oxidation at 800 °C were performed in air. The results indicated that serious oxidation occurred on the bare alloy. Thick non-protective oxide scales consisting of mixed TiO₂ + α-Al₂O₃ layers formed on the alloy surface. The coated specimens exhibited much better oxidation behaviour by forming an Al-rich oxide scale on the coating surface during the initial stages of oxidation. This scale acts as diffusion barrier by effectively blocking the ingress of oxygen, and effectively protects the coated alloys from further oxidation.     

Oxidation characteristics of Ti-25Al-10Nb-3V-1Mo

Static oxidation kinetics of Ti-25Al-10Nb-3V-1Mo (atomic percent) were investigated in air over the temperature range of 650–1000°C using thermogravimetric analysis. The oxidation kinetics were complex at all exposure temperatures and displayed up to two distinct stages of parabolic oxidation. Breakaway oxidation occurred after long exposure times at high temperatures. Oxidation products were determined using x-ray diffraction techniques, electron microprobe analysis, and energy dispersive x-ray analysis. Oxide scale morphology was examined using scanning electron microscopy of the surfaces and cross-sections of oxidation specimens. The oxides during the parabolic stages were compact and multilayered, consisting primarily of TiO₂ doped with Nb, a top layer of Al₂O₃ and a thin bottom layer of TiN. The transition between the first and second parabolic stage is linked to the formation of a TiAl layer at the oxide-metal interface. Porosity also formed in the TiO₂ layer during the second stage, causing degradation of the oxide and breakaway oxidation.     

Laser processing of titanium aluminides

Using a Nd-YAG laser, laser processing of a series of Ti-Al alloys including pure Ti and Ti-Al intermetallic compounds has been studied. Scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), and optical microscopy were used to determine the surface morphological, chemical, and compositional characteristics of the laser-processed samples. Analysis of results showed that cracks along grain boundaries caused by rapid heating and cooling of laser processing were the dominant characteristics of the surface morphologies of the laser-processed samples. The Al content in the Ti-Al alloys played a very important role in crack initiation and/or development. The more Al content in the samples, the more severe the cracks that developed after laser processing under the same conditions. The experiments were conducted at ambient conditions, resulting in surface oxidation layers being observed on the processed samples. The XPS results indicated that the oxidation layer consisted of adsorbed O₂, Al₂O₃, TiO₂, and TiO. In addition, Al enrichment was found in the oxide film of TiAl as well as in the oxidation layers formed on the surfaces of TiAl and Ti₃Al intermetallics that were processed by the laser; this differs from the reported results for traditional oxidation of TiAl at elevated temperature.     

Effect of Nb on oxidation behavior of high Nb containing TiAl alloys

The isothermal oxidation behavior of Ti-45Al-8Nb and Ti-52Al-8Nb alloys at 900 °C in air was investigated. The early oxidation behaviors were studied by using XPS and AES. And the microstructure and the composition of the oxidation scale were studied by using XRD and SEM. The results show that the oxidation behavior of TiAl alloy is significantly improved by Nb addition. Nb substitutes for Ti in TiO₂ as a cation with valence 5, and thus to suppress TiO₂ growth. The (Ti,Nb)O₂-rich layer is a dense and chemically uniform which is more protective than the TiO₂ layer. Nb addition also lowers the critical Al content to form an external alumina. Nb₂Al phase is formed in the metallic matrix at the oxide–metal interface on the high Nb containing TiAl alloys.     

Alloying effects on properties of Al2O3 and TiO2 in connection with oxidation resistance of TiAl

In the present work, a first-principles method is used to calculate the oxidation energies of Al₂O₃ and TiO₂ as well as the formation energy of oxygen vacancy in TiO₂ containing various alloying elements, in order to shed some light on the alloying effects on the oxidation resistance of γ-TiAl. Our calculations demonstrate that almost all alloying elements increase the oxidation energies of Al₂O₃ and TiO₂. The alloying elements with number of d electrons from 2 to 5 in the forth and fifth rows of the periodic table (e.g., Zr, Nb, Mo, Hf, Ta, W) increase significantly the oxidation energy difference between Al₂O₃ and TiO₂, i.e., reduce the relative stability of Al₂O₃ to TiO₂. On the other hand, these alloying elements increase the formation energy of oxygen vacancy in TiO₂. The effects of other alloying elements are less significant or opposite. Observing the experimental mass gains of TiAl alloys and unalloyed TiAl due to oxidation, we find that the elements reducing the relative stability of Al₂O₃ to TiO₂ and increasing the formation energy of oxygen vacancy enhance the oxidation resistance of TiAl whereas others do not. Such correlations are rationalized by analyzing the alloying effects on the internal oxidation of Al in the γ-TiAl matrix and the diffusion of oxygen in TiO₂ surface scale.     

TEM investigations of the early stages of TiAl oxidation

The early stages of TiAl oxidation at 900°C and 1000°C in air have been investigated by transmission electron microscopy (TEM). The investigations revealed that at the beginning of oxidation, i.e., after 4 min, TiO₂ and Al₂O₃ grow in a preferential orientation on the -TiAl substrate. After 4 h of oxidation an oxide scale structure can already be found similar to that known from long-term oxidation. In addition, besides -Al₂O₃, the formation of a second aluminum oxide phase and of titanium nitrides is observed. The processes at the metal-oxide interface of oxidation in the early stages, consisting of a repeated cycle of Al₂O₃ formation, Al₂O₃ dissolution, outward migration of Al through the scale, and reprecipitation of Al₂O₃ in the outer scale, are described by a model. The four stages observed in the kinetics of TiAl oxidation are explained on the basis of the results obtained and the structure of the oxide scale.     

The Effect of Si3N4 Additions on the Oxidation Resistance of TiAl Alloys

The oxidation kinetics of TiAl alloys with and without 3 and 5 wt.%additions of Si₃N₄ particles were studied at 1173 and1273 K in 1 atm of air. The Si₃N₄ dispersions wereunstable in the matrix phase, so that some of them reacted with titaniumduring sintering to form Ti₅Si₃ and dissolvednitrogen. The oxide scale formed on TiAl–Si₃N₄alloys consisted of an outer TiO₂, an intermediate(Al₂O₃+TiO₂), and an inner(TiO₂+Al₂O₃) mixed layers. The enhancedalumina-forming tendency, the presence of discrete SiO₂ particlesbelow the outer TiO₂ layer, and the improved scale adhesion bySi₃N₄ dispersions were attributable mainly to theincreased oxidation resistance compared to the Si₃N₄-freeTiAl alloys. Marker experiments showed that, for TiAl–Si₃N₄ alloys, the primary mode of scale growth was the outward diffusion oftitanium ions for the outer scale and the inward transport of oxygen ionsfor the inner scale.     

Effect of SiC, Si3N4 and TiB2 additions on the oxidation resistance of TiAl alloys

The oxidation behavior of TiAl alloys containing dispersed particles of (5, 10, 15 wt.%) SiC, (3,5 wt.%) Si₃N₄ or (3, 5, 10 wt.%) TiB₂ was studied between 800 and 1200°C in atmospheric air. The TiAl−(SiC, Si₃N₄) alloys oxidized to TiO₂, Al₂O₃, and SiO₂. The TiAl−TiB₂ alloys oxidized to TiO₂, Al₂O₃, and B₂O₃ which evaporated during oxidation. Improvement in oxidation resistance accompanied by thin, dense scale formation due to the addition of dispersoids originated primarily from the enhanced alumina-forming tendency, improved scale adhesion by oxide grain refinement owing to the beneficial effect of dispersoids, and the incorporation of SiO₂ within the oxide scale in the case of TiAl−(SiC, Si₃N₄) alloys.     

Oxidation resistance of TiAl3-Al composite coating on orthorhombic Ti2AlNb based alloy

The TiAl₃-Al composite coating on orthorhombic Ti₂AlNb based alloy was prepared by cold spray. Oxidation in air at 950 °C indicated that the bare alloy exhibited poor oxidation resistance due to the formation of TiO₂/AlNbO₄ mixture and intended to scale off at the TiO₂ rich zone. A nitride layer about 2 µm was formed under the oxide layer. The oxygen invaded deeply into the alloy and caused severe microhardness enhancement in the near surface region. The TiAl₃-Al composite coating exhibited parabolic oxidation kinetics and showed no sign of degradation after oxidized up to 1098 h at 950 °C in air under quasi-isothermal condition. No scaling of the coating was observed after oxidized at 950 °C up to the tested 150 cycles. The major oxide in the oxidized coating was Al₂O₃. The AlTi₂N, TiAl and small amount of TiO₂ were also observed in the oxidized coating. The EPMA and microhardness tests showed that inward oxygen diffusion was prevented by the interlayer, which was formed between the composite coating and the substrate during heat-treatment. Microstructure analyses demonstrated that the interlayer play a major role in protecting the substrate alloy from high temperature oxidation and interstitial embrittlement.     

The Effect of Niobium-Ion Implantation on the Oxidation Behavior of γ -TiAl Alloys in Static and Flowing Air

-TiAl (Ti–50Al at.%) alloys were implanted with Nb ions at an acceleration energy of 50 keV, at a dose of 1.2×10¹⁷ ions/cm². The cyclic-oxidation behavior of the unimplanted and Nb⁺-implanted TiAl specimens was investigated at 850°C in static air and in air with a flow velocity of 12.0 m/s (1000 ml/min). In static air, the unimplanted TiAl specimen showed rapid oxidation during a transition period of about 80 hr, after which partial scale spallation occurred and a net mass loss was observed. In flowing air, the whole scale spalled off after each cycle. On the other hand, Nb-ion implantation led to the formation of an adherent protective Al₂O₃ scale during oxidation in both static and flowing air, thereby significantly improving the cyclic-oxidation resistance of -TiAl alloys. A remarkable deference in the initial-oxidation behavior between unimplanted and Nb⁺-implanted specimens was also observed. A mixed TiO₂/A₂O₃ scale on the unimplanted specimen developed at a high growth rate during the very initial stage of oxidation. In contrast, the initial scale growth rate was significantly decreased by Nb-ion implantation and an Al₂O₃-rich layer was found present as the inner part of the initial scale on Nb⁺-implanted TiAl. Flowing air appeared to cause severe scale spallation during oxidation of unimplanted TiAl, but not to have any influence on the adhesion of the scale on Nb⁺-implanted TiAl.     

TiO2纳米粒子对铝锂合金微弧氧化膜结构及性能的影响

为改善微弧氧化膜层的耐蚀性及力学性能,向电解液中添加TiO₂纳米粒子后对2297铝锂合金进行了微弧氧化。利用SEM、XRD、EDS、辉光放电表征技术及电化学测试技术,分析了TiO₂纳米粒子对微弧氧化膜结构、力学性能及耐蚀性的影响。结果表明：添加TiO₂纳米粒子后,微弧氧化膜层变得平坦致密。随着TiO₂纳米粒子添加量的提高,膜层表面放电通道的孔径逐渐减小,数量逐渐增多。TiO₂纳米粒子会抑制熔融Al₂O₃与电解液中$ {\rm{SiO}}_{\rm{3}}^{{\rm{2 ^- }}}$的接触,所以膜层中Si元素的含量随TiO₂纳米粒子添加量的增加而逐渐下降(原子数分数从初始的10.27%下降到了3.10%)。显微硬度测试结果表明,TiO₂纳米粒子的引入增加了膜层的致密度及平整度,所以膜层的硬度得到了提升(添加1 g/L TiO₂纳米粒子后硬度提高了15%)。电化学测试结果显示,当微弧氧化的其它条件相同时,TiO₂纳米粒子的适量添加会提升膜层的耐蚀性,但过量添加时,由于膜层放电通道数量的增多等原因,其耐蚀性下降。     

Highly ordered self-organized TiO2 nanotube arrays prepared by a multi-step anodic oxidation process

Highly ordered TiO₂ nanotube arrays were prepared using a self-templating multi-step anodic oxidation process in a fluoride-containing electrolyte. The microstructures, chemical compositions, and phases of the self-organized TiO₂ nanotube arrays were analyzed by FESEM, XPS, and XRD, respectively. Hexagonal packing density in TiO₂ nanotube arrays significantly improved after the the multi-step anodic oxidation. The area densities of the hexagonal TiO₂ nanotube arrays increased approximately 3 times from the first to second step in the anodic oxidation steps process (4.9 μm⁻² to 16.4 μm⁻²), but there was no difference between the second and third step (16.4 μm⁻² to 16.0 μm⁻²). The as-anodized TiO₂ nanotube array had an amorphous structure and it transformed to an anatase phase during the annealing process at 450 °C for 1 h. The as-anodized TiO₂ nanotube arrays adsorbed the fluoride, hydrocarbon groups (CH), hydroxyl groups (OH, C-OH), and carboxyl groups (O = C-OH) on their surfaces.     

Sulfidation processing and Cr addition to improve oxidation resistance of TiAl intermetallics in air at 1173 K

High temperature oxidation properties of TiAl- (1,2,4 and 10) Cr and 40Ti-56Al–4Cr alloys, which were sulfidized at 1173 K for 86.4 ks at 1.3 Pa sulfur partial pressure in a H₂–H₂S gas mixture, were investigated at 1173 K in air for up to 2.7 Ms. The sulfidation processing formed a (Cr,Ti)Al₂ layer between a TiAl₃ (TiAl₂ included) layer and a Ti-rich sulfide scale by selective sulfidation of Ti. Oxidation of the sulfidation-processed alloys was examined for up to 2.7 Ms in air under isothermal and room temperature to 1173 K heat cycle conditions. In both oxidation experiments the sulfidation processed TiAl–10Cr alloy showed very good oxidation resistance up to 2.7 Ms, due to the formation of a continuous Ti(CrAl)₂ Laves layer, which was changed from (Cr,Ti)Al₂ and has a composition of 28.7Cr–36.2Al–35.1Ti, between the layers of protective Al₂O₃ (TiO₂ included) and TiAl₂, which was changed from TiAl₃. The sulfidation processed TiAl, TiAl–4Cr, and 40Ti–56Al–4Cr alloys showed better oxidation resistance than conventional TiAl based alloys, but displayed localized oxidation. The Ti(Cr,Al)₂ Laves on the sulfidation processed TiAl–4Cr alloy was discontinuous, leading to a localized oxidation after long oxidation. The sulfidation processed 40Ti–56Al–4Cr alloy oxidized faster than the sulfidation processed TiAl–10Cr alloy due to the formation of an Al₂O₃ and TiO₂ mixture, although the TiAl₂ layer remains. It was concluded that the Ti(Cr,Al)₂ Laves layer between the oxide scale and alloy substrate caused the good oxidation resistance.     

Characterisation and oxidation of LVOF sprayed Al2O3–40%TiO2 coating on superni 601 and superni 718 superalloys at 800 and 900°C

Failure of components due to high temperature oxidation is the major degradation mechanism in boiler and gas turbine industries. Superalloys having superior mechanical properties and creep resistance are used in these applications but lack resistance to oxidation under aggressive environments. Protective coatings are used to improve their oxidation resistance in such applications. In the present investigation, Al₂O₃–40%TiO₂ coating was deposited on superni 718 and superni 601 superalloys by low velocity oxy fuel process. The as sprayed coating was characterised for microhardness, surface roughness, scanning electron microscopy and X-ray diffraction analysis. High temperature oxidation behaviour of Al₂O₃–40%TiO₂ coated and uncoated superni 718 and superni 601 superalloys has been evaluated at the elevated temperatures of 800 and 900°C for total duration of 50 cycles under cyclic conditions. Each cycle consisted of keeping the samples for 1 h at the elevated temperature followed by 20 min cooling in ambient air. Al₂O₃–40TiO₂ coating in the as sprayed condition showed the presence of Al₂O₃–TiO₂, α-Al₂O₃, TiO₂ as the main phases. Al₂O₃–40%TiO₂ coating on superni 718 and superni 601 superalloys has shown a lower oxidation rate as compared to those of uncoated superalloys. However, the oxidation rate of the coating was not steady due to the occurrence of spallation/sputtering at various stages. The coating was found adherent on the substrate superalloys throughout the study.     

Oxidation Behavior of a High-Nb-Containing TiAl Alloy with Multilayered Thermal Barrier Coatings

Oxidation of TiAl alloys has been recognized as an obstacle for high-temperature applications such as aero engine and gas turbine. Substantial efforts have been made to improve oxidation resistance of TiAl alloys at elevated temperatures. In this study, multilayered thermal barrier coatings are prepared to protect a high-Nb-containing TiAl alloy from oxidation by air plasma spraying. The combination of Al₂O₃-13wt.%TiO₂ ceramic coatings and NiCoCrAlY metallic coatings can improve thermal stability and increase the service lifetime of coatings. The fully melted TiO₂ particles distribute in ceramic coatings uniformly and act as sealing pores and microcracks, which decrease porosity of the ceramic coatings and reduce diffusion channels of oxygen atoms. The porosity of surface and cross-section morphology are 5.5?±?0.8 and 5.1?±?0.8%, respectively. The results of oxidation experiment carried out at 800 and 900 °C for 100 h indicate that the coatings can effectively protect a high-Nb-containing TiAl alloy from oxidation. The mass gain of the high-Nb-containing TiAl alloys with coatings is lower than that of the one without coatings. The ceramic coatings retard diffusion of large amount of oxygen atoms, and bond coatings avoid to be excessively oxidized. Thus, the multilayered thermal barrier coatings exhibit an excellent long-term stability.     

Effects of processing parameters on the synthesis of TiO2 nanofibers by electrospinning

TiO² nanofibers were synthesized by an electrospinning technique using polyvinyl pyrrolidone and titanium tetraisopropoxide as precursors. The effects of processing parameters including the precursor ratio, calcination time, temperature and atmosphere were investigated. The calcination temperature determines the TiO₂ phases as either anatase or rutile. The diameter of the synthesized TiO₂ nanofibers is not sensitive to the calcination atmosphere or the time. However, the surface microstructure of the synthesized nanofibers depends highly on calcination atmosphere. Calcination in an N₂ atmosphere produces smooth surfaces. In contrast, surfaces that are more granular evolve when they are calcined in an O₂ atmosphere. In addition, less Ti precursor in the electrospinning solution results in slim nanofibers.     

阳极氧化法制备TiO2膜在模拟深海热液区的耐腐蚀性能

以恒压阳极氧化方法在钛基体上制备TiO₂氧化膜,使用水热釜模拟深海热液区的条件研究其耐腐蚀性能。采用XRD、SEM、接触角测定仪对氧化膜以及腐蚀试样产物进行晶型、表面结构、化学成分和亲疏水性能测定,使用动电位扫描方法对其进行极化曲线测试。结果表明,钛试样和阳极氧化钛试样在模拟深海环境条件下,经过腐蚀反应在表面都生成了一层非致密的TiO₂ 膜,对基体并不能起到保护作用,而阳极氧化生成的致密TiO₂ 膜对基体能够起到很好的保护作用。经腐蚀后钛试样表面有TiH₂相的形成,腐蚀电位负移0.45 V。而阳极氧化钛试样表面没有TiH₂相的形成,且腐蚀电位负移较小,表现出良好的耐腐蚀性能。