Ti50Al和Ti45Al8Nb合金高温初期氧化行为

利用SEM、XPS和AES研究了Ti50Al和Ti45Al8Nb(原子分数,%)合金在900 ℃空气中的初期氧化行为.结果表明:Nb的加入显著提高了合金的抗氧化性.氧化90 min后,Ti50Al合金表面为粗大的富TiO2颗粒,冷却过程中部分表层氧化膜脱落,脱落后内层为多孔、疏松的Ti和Al的混合氧化物(可能有氮化物);Ti45Al8Nb合金表面生成很薄的氧化膜,其外层为细小富Al2O3颗粒,Nb只存在于内层(富TiO2层)和混合层,在外层Al2O3内没有发现,Nb5 取代TiO2中的Ti4 ,降低氧空位浓度从而减慢氧的向内扩散,抑制TiO2的生成,同时促进了外层纯Al2O3层的形成,该层阻碍氧以及合金元素的进一步扩散.     

β相凝固铸造高Nb-TiAl合金的高温氧化行为

研究了β相凝固的铸造合金Ti45Al8Nb0.2W0.25Cr和Ti45Al8Nb0.2W0.25Cr0.2B0.02Y合金900℃×130 h的循环抗氧化性能,探讨了B、Y元素对合金抗氧化性能的影响。结果表明,0.2B0.02Y的添加能在一定程度上改善合金的抗氧化性能。B、Y的加入能够促进合金表层形成致密连续的Al2O3层,抑制氧元素的进一步入侵,Y的加入使得合金氧化层的抗剥落性能得到提高。分析计算得出了两种合金的氧化动力学曲线,氧化10 h后,Ti45Al8Nb0.2W0.25Cr合金的氧化动力学方程中的幂指数n=1.87,合金单位面积的氧化增重随氧化时间的变化规律介于直线规律和抛物线规律之间,即这种合金的氧化膜保护性不足。Ti45Al8Nb0.2W0.25Cr0.2B0.02Y合金氧化动力学方程中的幂指数n=2,合金单位面积的氧化增重随氧化时间的变化规律为抛物线规律,利于合金表面生成保护性的氧化膜,合金的抗氧化性较好。     

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。     

Nb对TiNiAl合金高温氧化行为的影响

对Ti50Ni44Al6和Ti50Ni41Al6Nb3合金在1073K循环氧化行为的测试表明,Nb的加入显著改善合金的高温抗氧化性能．Ti50Ni44Al6合金在1073K经过100h循环氧化后形成外层以TiO2为主并含有少量Al2NiO4、内层为TiNiO3的氧化层,合金的氧化动力学服从线性规律;Ti50Ni41Al6Nb3合金生成以TiO2为主的氧化膜,在外氧化层下面形成了一层富Nb和Al的复合氧化物,显著阻碍氧以及合金元素的扩散,降低了合金的氧化速率,合金高温氧化动力学遵从抛物线规律．     

W、B、Y微合金化高铌TiAl基合金高温抗氧化性研究

使用元素W、B、Y对Ti-45Al-8Nb合金进行了微合金化,研究了微合金化后高铌TiAl基合金在900℃静止空气中的断续氧化行为。结果表明,与Ti-45Al-8Nb合金相比,经过0.2B与0.1Y联合微合金化后合金的氧化增重小,氧化膜与基体的粘附性强,抗氧化性明显改善;经过0.2W与0.1Y微合金化后合金氧化增重明显,氧化膜容易脱落,合金抗氧化性下降;经过0.2B、0.2W、0.1Y联合微合金化后合金抗氧化性没有明显变化。对氧化膜进行的扫描电镜(SEM)、能谱分析(EDS)及X射线衍射(XRD)分析表明,联合添加B、Y促进了合金中的连续致密的Al2O3条带的形成,W、Y联合微合金化的合金中靠近基体处未形成连续的Al2O3条带,并且混合层中形成了较厚的低铌含量的TiO2层。W、B、Y联合微合金促进了混合层中富Al2O3层的形成。     

Ti-45Al-8(Nb,Hf,Y)-0.2B合金高温抗氧化性研究

设计了铌当量约为8的三种成分合金：Ti-45Al-8Nb-0.2B,Ti-45Al-4Nb-0.5Hf-0.1Y-0.2B,Ti-45Al-7Nb-0.1Hf-0.1Y-0.2B, (at.%),研究了这三种合金在900℃静止空气中的断续氧化行为。研究结果表明：Hf、Y联合微合金化的合金氧化膜与基体粘附性明显增强;低Nb/Hf比值的Ti-45Al-4Nb-0.4Hf-0.1Y-0.2B的合金氧化增重小、抗氧化性强,高Nb/Hf比值的Ti-45Al-7Nb-0.1Hf-0.1Y-0.2B合金氧化增重大,抗氧化性差。对氧化膜的扫描电镜(SEM)、能谱分析(EDS)及X射线衍射（XRD）分析表明,Hf、Y的联合加入促进了Al2O3膜的须状生长形态,从而提高了氧化膜与基体粘附性,低Nb/Hf比值的合金中形成了较厚的连续致密的Al2O3膜,提高了合金的抗氧化性;高Nb/Hf比值的合金内存在明显的外氧化现象,导致了该合金抗氧化性下降。     

Ti-45Al-8(Nb,Hf,Y)-0.2B合金的高温抗氧化性

设计了铌当量约为8at%的3种成分合金:Ti-45Al-8Nb-0.2B,Ti-45Al-4Nb-0.5Hf-0.1Y-0.2B,Ti-45Al-7Nb-0.1Hf-0.1Y-0.2B(at%),研究了这3种合金在900℃静止空气中的断续氧化行为。研究结果表明:Hf、Y联合微合金化的合金氧化膜与基体粘附性明显增强;低Nb/Hf比值的Ti-45Al-4Nb-0.5Hf-0.1Y-0.2B合金的氧化增重小、抗氧化性强,高Nb/Hf比值的Ti-45Al-7Nb-0.1Hf-0.1Y-0.2B合金的氧化增重大,抗氧化性差。对氧化膜的扫描电镜(SEM)、能谱(EDS)及X射线衍射(XRD)分析表明,Hf、Y的联合加入促进了Al2O3膜的须状生长形态,从而提高了氧化膜与基体粘附性,低Nb/Hf比值的合金中形成了较厚的连续致密的Al2O3膜,提高了合金的抗氧化性;高Nb/Hf比值的合金内存在明显的外氧化现象,导致了该合金抗氧化性下降。     

微量Y和Hf对高铌TiAl基合金高温抗氧化性的影响

使用元素Hf和Y对Ti-45Al-8Nb合金进行了微合金化,研究了微合金化前后高铌TiAl基合金在900℃静止空气中的断续氧化行为。结果表明,与Ti-45Al-8Nb合金相比,0.5Hf(mol%)微合金化后合金的抗氧化性无明显变化,0.1Y(mol%)微合金化后合金抗氧化性增强,0.5Hf与0.1Y联合微合金化后合金氧化膜与基体粘附性明显增强,但合金氧化增重明显。对氧化膜进行的扫描电镜(SEM)、能谱分析(EDS)及X射线衍射(XRD)分析表明,单独添加Y及Hf、Y联合添加促进了合金中形成以Al2O3为主的连续致密的氧化层,Hf、Y联合微合金化的合金中存在局部内氧化现象降低了该合金的抗氧化性。     

基于卤素效应的阳极氧化技术提高Ti48Al5Nb合金抗高温氧化性能

采用电化学阳极氧化技术在含NH_4F的乙二醇电解液中对Ti48Al5Nb合金进行阳极氧化处理,以获得富铝含氟阳极氧化膜。研究了阳极氧化处理对Ti48Al5Nb合金在1000℃空气中的氧化行为及氧化膜组成和结构的影响。结果表明:阳极氧化处理的Ti48Al5Nb合金经高温氧化后表面可形成连续、致密的Al_2O_3氧化膜,且氧化膜与基体具有良好结合力,有效阻止了氧向内扩散,进而显著提高了合金的抗高温氧化性能。经1000℃氧化100 h后,阳极氧化试样增重由未经阳极氧化处理试样的26.73 mg/cm~2降至1.18 mg/cm~2。同时,阳极氧化处理改变了合金的氧化机制,抑制了氧化膜/基体界面处富Nb层的出现。阳极氧化提高Ti48Al5Nb合金抗高温氧化性能是由于氧化膜中F在高温氧化过程中表现出的"卤素效应"所致。     

稀土Y对Nb-Ti-Si基超高温合金组织及抗氧化性能的影响

采用真空非自耗电弧熔炼法制备了5种成分为Nb-22Ti-15Si-5Cr-3Al-3Hf-x Y(x=0,0.03,0.06,0.12,0.30 at%)的合金,并在1250℃下分别进行了1、10、20和50 h的高温氧化实验,研究Y含量对合金电弧熔炼态组织及其高温抗氧化性能的影响。结果表明:添加Y不改变电弧熔炼态合金相组成,组织仍由Nbss及γ-(Nb,X)5Si3相组成,但细化了共晶组织。高温氧化实验结果表明,不同Y含量合金在氧化不同时间后,氧化膜均由Nb2O5,Ti O2,Ti2Nb10O29和Ti Nb2O7组成;随着Y含量的增加,氧化膜中的孔洞和裂纹减少,合金单位面积氧化增重也逐渐减小,表明Y的添加能够显著提高合金的高温抗氧化性能。     

Investigation on the oxidation behaviour of gamma titanium aluminides coated with thermal barrier coatings

In the present study, the applicability of thermal barrier coatings (TBCs) on γ‐TiAl alloys was investigated. Two alloys with the chemical compositions of Ti‐45Al‐8Nb‐0.2B‐0.15C and Ti‐45Al‐1Cr‐6Nb‐0.4W‐0.2B‐0.5C‐0.2Si were used. Before TBC deposition, the specimens were pre‐oxidised in laboratory air or low partial pressure oxygen atmosphere. Yttria partially stabilised zirconia top coats were then deposited using electron‐beam physical vapour deposition (EB‐PVD). The oxidation behaviour of the γ‐TiAl specimens with TBC was studied by cyclic oxidation testing in air at 850 and 900 °C. Post‐oxidation analysis of the coating systems was performed using scanning electron microscopy with energy‐dispersive X‐ray spectroscopy (EDS). No spallation of the TBC was observed for pre‐oxidised specimens of both alloys when exposed to air at 850 °C for 1100 cycles of 1 h dwell time at high temperature. SEM micrographs of the thermally grown oxide scale revealed outer mixed TiO₂/Al₂O₃ protrusions with a columnar structure. The protrusions contained small particles of zirconia and a low amount of about 0.5 at% zirconium was measured by EDS analysis throughout this outer oxide mixture. The TBCs exhibited excellent adherence on the oxide scale. Intercolumnar gaps and pores in the root area of the TBC were filled with titania and alumina. Below the outer columnar oxide scale, a broad porous zone of predominant titania was observed. The transition region between the oxide scale and substrate consisted of a discontinuous nitride layer intermixed with alumina particles and intermetallic phases rich in niobium formed at the nitride layer/substrate interface. When thermally cycled at 900 °C, the oxide scales on the alloy Ti‐45Al‐8Nb‐0.2B‐0.15C pre‐oxidised in low partial pressure oxygen spalled off after 540 cycles. For the sample with TBC, spallation was observed after 810 cycles. Failure occurred in the thermally grown oxide near the oxide/nitride layer interface. Microstructural examinations revealed again oxide scales with columnar structure beneath the zirconia top coat and good adherence of the TBC on the thermally grown oxides formed at 900 °C.     

The isothermal and cyclic high temperature oxidation behaviour of Ti–48Al–2Mn–2Nb compared with Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr

The isothermal and cyclic oxidation behaviour of Ti–48Al–2Mn–2Nb (at%) were studied at high temperatures in air in comparison with the intermetallic alloys Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr. Tests were performed in air between 800 and 900°C. At 800°C Ti–48Al–2Mn–2Nb showed an excellent oxidation resistance under isothermal and cyclic conditions, comparable with Ti–48Al–2Cr–2Nb, and superior to Ti–48Al–2Cr. At 900°C the isothermal oxidation rate of Ti–48Al–2Mn–2Nb was similar as found for Ti–48Al–2Cr–2Nb, but much lower as that of Ti–48Al–2Cr. Upon cooling the oxide scale formed on Ti–48Al–2Mn–2Nb was prone to spallation. During the cyclic oxidation at 900°C, a steady state condition is reached for both niobium bearing materials, with a net linear mass loss rate, due to spallation and (re-)growth of the oxide scale. The linear mass loss rate for the Ti–48Al–2Mn–2Nb was higher than that of Ti–48Al–2Cr–2Nb, indicative of a higher susceptibility for spallation. During the initial stage of oxidation of all tested materials a complex multi-phased and multi-layered scale was formed consisting of α-Al₂O₃, TiO₂ (rutile), TiN and Ti₂AlN. After longer exposure times the outer scale was dominated by TiO₂. In case of the niobium containing materials no loss of protectivity of the oxide scale was found during the growth of the outer TiO₂ layer (under isothermal conditions). Two-stage oxidation experiments with isotope tracers were performed to study the oxidation mechanism in more detail.     

V/Nb对Ti-45Al-9(V, Nb, Y)合金抗氧化性能的影响

βγ-TiAl合金具有良好的高温变形能力,为TiAl合金的发展开辟了新的途径。成功制备了不同x=V/Nb(x=1,1.5,2,3.5)的βγ-TiAlTi-45Al-9(V,Nb,Y)合金,研究了上述合金在800℃静止空气中的氧化行为。结果表明:当x=1时,Ti-45Al-9(V,Nb,Y)合金中形成条带状、连续致密的Al2O3氧化层,显著提高了合金的抗氧化能力。随着x=V/Nb的增加,Al2O3氧化层厚度变薄,合金的抗氧化能力下降。     

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.     

Oxidation Behaviour of TBC Systems on γ-TiAl Based Alloy Ti–45Al–8Nb

The lifetime of thermal barrier coating (TBC) systems on gamma titanium aluminides was determined in the temperature range between 850 °C and 950 °C under cyclic oxidation conditions in air. Coupons of the alloy Ti–45Al–8Nb (at.%) were coated by pack aluminizing. A subset of samples was subsequently annealed at 910 °C for 312 h in argon. During this heat treatment, the two-phase (Nb,Ti)Al₃ plus TiAl₂ microstructure of the coating transformed into single phase γ-TiAl. On pre-oxidised aluminized, annealed and bare samples, TBCs of yttria partially stabilized zirconia were deposited using electron-beam physical vapour deposition (EB-PVD). No spallation of the TBCs was observed in cyclic oxidation tests at 850 °C for up to 3,000 cycles of 1 h dwell time at high temperature. The two-phase aluminide coating provided effective oxidation protection due to the formation of a continuous alumina scale. The lifetime of this TBC system exceeded 1,400 cycles at 950 °C, whereas an aluminized and annealed sample failed after approximately 500 cycles. The TBC on bare substrate failed when thermally cycled at 900 °C. In contrast, no spallation occurred with an aluminized and annealed specimen at this temperature during the maximum exposure length of 1,000 cycles, probably related to an increased aluminium concentration in the subsurface region. EB-PVD zirconia top coats were well adherent to the alumina scale and the thermally grown mixed oxides. Failure of the TBC systems observed with bare and annealed samples was associated with spalled oxide scales formed on γ-TiAl.     

Lifetime of Thermal Barrier Coatings Deposited on γ-TiAl Based Alloys Using Intermetallic Ti–Al–Cr Bond Coats with Additions of Yttrium and Zirconium

Thermal barrier coatings (TBC) of yttria stabilized zirconia were deposited on the γ-TiAl based alloy Ti–45Al–8Nb (at.%) using electron-beam physical vapor deposition. The bond coats used were 10 μm thick intermetallic Ti–Al–Cr layers with additions of the reactive elements Y and Zr produced by magnetron sputtering. Cyclic oxidation tests at 900 and 1,000 °C in air revealed excellent oxidation resistance of the Ti–Al–Cr–Y bond coat associated with the precipitation of Y-rich particles in the thermally grown alumina scale as well as in the intermetallic layer. A less protective behavior was found with the zirconium containing bond coat. Lifetimes exceeding 1,000 1-h cycles were determined for both TBC systems at 900 °C. Edge chipping of the zirconia topcoat occurred at 1,000 °C. As observed by cross-sectional examination, a continuous alumina scale was still present on the samples with Ti–Al–Cr–Y bond coat, whereas the Ti–Al–Cr–Zr layer was severely degraded and a thick mixed oxide scale formed after 1,000 cycles at 1,000 °C.     

Oxidation behavior of gamma alloys designed for high temperature applications

The oxidation behavior of gamma alloys was investigated in air and scale spallation was observed. The gamma alloys were superior in oxidation behavior to binary TiAl alloys in both isothermal and cyclic exposure. The mass gain of the alloy with high Nb content was smaller than low Nb alloys at 760 and 815 °C, the difference between them decreased at 870 °C. The improved oxidation resistance of the alloys is attributable to Nb addition resulting from reduced TiO₂ growth through the so-called doping effect. Not only the alloys with low Nb but also the high Nb alloys suffered from scale spallation, especially after longer and/or higher temperature exposure. The density of the scale formed on the gamma alloys was found to be about one half of bulk oxide (Al₂O₃, TiO₂) and was comparable to that formed on binary TiAl alloys. The coefficient of the thermal expansion (CTE) of the gamma alloys was larger than that of the oxides and the alloying addition reduced the CTE value of the alloys. Although the Nb addition suppressed the oxide growth and reduced the CTE difference between alloy and oxides, the prevention of the scale spallation could not be attained by single addition of Nb.     

钛铝基合金与4种陶瓷界面反应的研究

借助SEM和差热分析(DTA),研究了Ti48Al2Cr2Nb合金与Y2O3、ZrO2(Y2O3稳定)、ZrO2(MgO稳定)和锆英砂4种陶瓷耐火材料界面反应后金属侧的显微组织,并测定了Ti48Al2Cr2Nb合金与Y2O3、ZrO2(Y2O3稳定)、ZrO2(MgO稳定)和锆英砂4种陶瓷耐火材料的初始反应温度.结果表明,Ti48Al2Cr2Nb合金与锆英砂反应后的显微组织最粗大,为菊花状,而与Y2O3反应产物最细小均匀,为颗粒状;4种陶瓷材料对Ti48Al2Cr2Nb合金的初始反应温度依次为:1500,1400,1380和820℃.