Effect of enamel coating on long-term oxidation and hot corrosion behavior of Ti-24Al-17Nb-0.5Mo alloys

The effect of enamel coating on long-term isothermal oxidation at 700 ℃ and cyclic oxidation at 800 ℃ in air and hot corrosion resistance of Ti-24Al-17Nb-0. 5Mo in 75% (Na2SO4 K2SO4 ) 25% NaCl (mass fraction) molten mixed salts at 700 ℃ was investigated. The results indicate that Ti-24Al-17Nb-0.5Mo alloy exhibits relatively poor long-term oxidation resistance due to the formation of Al2O3 TiO2 Nb2O5 mixed scales and poor hot corrosion resistance due to the spallation of scales formed in molten (Ns, K)2 SO4 NaCl. Enamel coating can effectively protect Ti-24Al-17Nb-0.5Mo alloy from long-term oxidation at high temperature in air and remarkably improve the hot corrosion resistance of Ti-24Al-17Nb-0. 5Mo alloy, and can act as the barrier to suppress the migration of oxygen and corrosive ions into the substrate.     

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。     

Effects of yttrium on microstructures and properties of Ti- 17Al-27Nb alloy

1 Introduction With the rapid development in aerospace technology, the aero engine components have to endure larger force and higher temperature due to speed increase of aero-craft. Therefore, more requisitions have to be presented when selecting material…     

Oxidation characteristics of Ti3Al-Nb alloys and improvement in the oxidation resistance by pack aluminizing

The oxidation behavior of three T_i3-Al-Nb alloys: Ti-25Al-11Nb, Ti-24Al-20Nb, and Ti-22Al-20Nb was investigated in the temperature range of 700–900°C in air. The uncoated alloy Ti-25Al-11Nb showed the lowest weight gain with nearly parabolic oxidation rate; while the other two alloys had much higher weight gain, accompanied by excessive oxide scale spalling. The scale analysis, using XRD, SEMIEDAX, and AES revealed that the scale was a mixture of TiO₂, Al₂O₃, and Nb₂O₅ with the outer layer rich in TiO₂. The effect of variation in Al and Nb content on the oxidation behavior is discussed. A decrease in Al content of the alloy adversely affects the oxidation resistance; and it seems that a Nb content as high as 20 at.% is also not beneficial. Hence these alloys, especially Ti-24Al-20Nb and Ti-22Al-20Nb, should not be used in the as-received condition above 750°C. An attempt was made to improve the oxidation resistance of these alloys by pack aluminizing which led to the formation of an Al rich TiAl₃ surface layer doped with Nb. The coating process was gaseous-diffusion controlled with a parabolic Al deposition rate. The weight gains for the aluminized alloy specimens oxidized at 900°C in air were much lower than that of the uncoated specimens. The weight gains were further decreased in the case of Si-modified aluminized specimens. The scale analysis revealed an alumina-rich scale with some amount of titania doped with Nb. The improvement in the oxidation resistance of the pack-aluminized alloys at 900°C is attributable to the formation of the alumina-rich oxide scale. The addition of Si to the aluminizing pack seems to promote further the growth of an alumina-rich scale by lowering the oxygen partial pressure in the system.     

Dehydrogenation of Hydrogenated Ti-6Al-4V Alloy

对未置氢及置氢Ti-6Al-4V合金进行了TG/DSC试验,研究了置氢钛合金的除氢行为。结果表明,当温度超过600℃时,置氢钛合金的失重规律与未置氢钛合金具有较大的差别。当加热温度在600~900℃之间时,置氢钛合金的失重随着氢含量的增加而增加。这是由于置氢合金中的亚稳相发生了分解。不考虑合金氧化的影响,置氢钛合金的最大失重与合金中的氢含量一致。置氢钛合金的最佳除氢温度为750℃。对于不同氢含量的置氢钛合金,其除氢工艺是相同的。     

Microstructural control of Ti-Al-Mo-Nb alloy system with respect to variation of B

In the current study, phase stability of Ti-Al-Mo-Nb alloys was investigated, and the effect of B addition was examined for cast alloys. The fabricated cast alloys were mainly composed of α₂ / γ lamellar with a β phase, when they were heat treated at 1100 °C followed by air cooling, the alloy was composed of α2 / γ lamellar with γ+β necklace phase at the colony boundary for the Ti-45Al-3Mo-2Nb-1B alloy, and the colony size was refined to ~ 20 μm. In order to identify the effect of the microstructures on mechanical strength, compressive tests were performed on the fabricated alloys of Ti-45Al-3Mo-2Nb and Ti-45Al-3Mo-2Nb-1B at room temperature and at 800 °C. The microstructural variations and phase stability were discussed in terms of pseudo-binary phase diagram calculated by Pandat software?.     

Electrochemical Behavior of Biomedical Titanium Alloys Coated with Diamond Carbon in Hanks’ Solution

Biomedical implants in the knee and hip are frequent failures because of corrosion and stress on the joints. To solve this important problem, metal implants can be coated with diamond carbon, and this coating plays a critical role in providing an increased resistance to implants toward corrosion. In this study, we have employed diamond carbon coating over Ti-6Al-4V and Ti-13Nb-13Zr alloys using hot filament chemical vapor deposition method which is well-established coating process that significantly improves the resistance toward corrosion, wears and hardness. The diamond carbon-coated Ti-13Nb-13Zr alloy showed an increased microhardness in the range of 850 HV. Electrochemical impedance spectroscopy and polarization studies in SBF solution (simulated body fluid solution) were carried out to understand the in vitro behavior of uncoated as well as coated titanium alloys. The experimental results showed that the corrosion resistance of Ti-13Nb-13Zr alloy is relatively higher when compared with diamond carbon-coated Ti-6Al-4V alloys due to the presence of β phase in the Ti-13Nb-13Zr alloy. Electrochemical impedance results showed that the diamond carbon-coated alloys behave as an ideal capacitor in the body fluid solution. Moreover, the stability in mechanical properties during the corrosion process was maintained for diamond carbon-coated titanium alloys.     

Effect of microstructure on tensile properties and fracture behavior of intermetallic Ti2AlNb alloys

The tensile properties and fracture behaviors of Ti-22Al-27Nb and Ti-22Al-20Nb-7Ta alloys were investigated in the temperature range of 25-800℃ Three typical microstructures were obtained by ifferent thermomechanical processing techniques.The results indicate that the duplex microstructure has an optimum combination of tensile yield strength and ductility both at room and elevated temperatures.Adding Ta to Ti2AlNb alloy can improve the yield strength,especially at high temperature while retain a good ductility.The study on crack initiation and propagation in dedformed microstructure of Ti2AlNb alloys indicates that microstructure has ikmportant effect on the tensile fracture mechanism of the alloys.The cracks initiate within primary O/α2 grains along O/B2 boundaries or O phase laths in B2 matrix,and propagate along primary B2 grain boundaries for the duplex microstructure.The fracture mode is transgranular with ductile dimples for the duplex and the equiaxed microstructures,but intergranular for the lath microstructure.     

粉末冶金Nb-35Ti-6Al-5Cr-8V合金组织演变及其力学行为

采用粉末冶金法对不同球磨时间的Nb-35Ti-6Al-5Cr-8V合金机械合金化粉末塑变行为,热压烧结材料的微观组织结构和力学行为进行了研究。研究结果表明：塑性良好的Nb-35Ti-6Al-5Cr-8V粉末随着球磨时间增加首先变形为大尺寸的片状、后经持续的加工硬化破碎成絮状;热压烧结能够制备微观组织可控晶粒细化的Nb-35Ti-6Al-5Cr-8V合金,合金由单一的Nbss相构成,Ti、Al、Cr、V元素固溶引起Nb晶格尺寸减小0.0685 ?;随着球磨时间增加合金晶粒明显细化进而显著提高了合金的维氏硬度和室温压缩强度,其变化符合材料硬度和强度的Hall-Petch规律。粉末冶金制备Nb-35Ti-6Al-5Cr-8V合金的各项力学性能明显优于熔铸法制备合金。     

粉末冶金碳化物强化铌合金组织演变及其力学行为

采用机械球磨与热压烧结相结合的粉末冶金法对不同球磨时间Nb-35Ti-6Al-5Cr-8V-5C合金的粉末变形行为,微观组织结构和力学行为进行研究。结果表明:随着球磨时间的增加,Nb-35Ti-6Al-5Cr-8V-5C复合粉末中的块状金属颗粒首先变形为片状后在碰撞挤压作用下破碎成絮状,TiC粉末均匀的分布于片状金属粉末表面;Nb-35Ti-6Al-5Cr-8V-5C合金由Nbss和(Nb,Ti)C两相构成,各合金碳化物体积分数均为11%左右,Ti元素主要分布于Nbss晶界和碳化物内,Al、Cr、V元素主要分布于Nbss晶粒内,Nbss和(Nb,Ti)C相尺寸均随球磨时间增加而尺寸减小;Nbss晶粒细化及强化相碳化物弥散化导致合金的室温压缩力学性能和塑性变形能力显著提高,压缩变形后合金Nbss与碳化物具有良好的界面结合能力,但是碳化物内部存在明显的近似平行分布的裂纹;数据对比表明,粉末冶金法制备Nb-35Ti-6Al-5Cr-8V-5C合金的力学性能优于电弧熔炼法。     

组织结构对片层TiAl合金氧化行为影响

以Ti-48Al-2Cr-2Nb(at%)合金为研究对象,通过热处理获得近片层和全片层两种显微组织,并在800℃空气条件下,进行了100 h的抗氧化性实验。利用OM、SEM、TEM、XRD和EDS对试样的微观结构、相组成及微区成分进行了分析。恒温氧化动力学结果表明:在800℃空气条件下,对于近片层和全片层两种Ti Al合金组织,其恒温氧化动力学曲线符合近抛物线规律,且近片层组织合金的抗氧化性能优于全片层组织。基于氧化动力学曲线、组织结构、氧化膜结构、氧化表面形貌分析,表明Ti Al合金的片层组织结构对其氧化行为具有不可忽视的影响。     

Martensitic transformation of Ti-18Nb(at.%) alloy with zirconium

The addition of 3%～9% Zr on the martensitic transformation of Ti-18Nb(at.%) alloy was investigated. The results of microstructure and X-ray diffraction (XRD) analysis show that the phase constitution of as-quenched Ti-18Nb-9Zr(at.%) alloy consists of the retained matrix and martensite, while that of the other three alloys is single martensite. No trace of athermal phase was found in any of the as-quenched alloys. Unlike the effect of Nb addition on the martensitic transformation start temperature Ms of Ti-1...     

B元素对Ti-46Al和Ti-46Al-5Nb合金柱状晶组织的影响

B元素对Ti-46Al和Ti-46A1-5Nb(原子分数,%)合金的柱状晶组织均有明显的细化作用,且对后者的细化效果更显著.这一现象可归结为:B元素在Ti-46Al-5Nb合金中的溶解度较低,硼化物析出量增加,柱状晶组织进一步细化.     

INFLUENCE OF ALLOYING ELEMENTS（Nb，Mo，V） ON MICROSTRUCTURE OF Ti_3Al BASE ALLOYS

ＩＮＦＬＵＥＮＣＥＯＦＡＬＬＯＹＩＮＧＥＬＥＭＥＮＴＳ（Ｎｂ,Ｍｏ,Ｖ）ＯＮＭＩＣＲＯＳＴＲＵＣＴＵＲＥＯＦＴｉ_３ＡｌＢＡＳＥＡＬＬＯＹＳ￥ＳＯＮＧＤａｎ;ＤＩＮＧＪｉｎｊｕｎ;ＷＡＮＧＹａｎｄｏｎｇ（ＡｎａｌｙｓｉｓａｎｄＴｅｓｔｉｎｇＣｅｎｔｅ?..     

Ion Implantation as a tool to study the oxidation behaviour of TiAl-based intermetallics

TiAl-based Intermetallic alloys are being considered as structural materials for high-temperature applications due to their low density and substantial mechanical strength at high temperatures. The effect of various elements added by ion implantation on the oxidation behaviour of near-γ Ti-48Al-2Cr and Ti-48Al-2Cr-2Nb (compositions are given in at-%) in air at 800°C has been studied. Ions implanted (10¹⁷ Ions/Cm²) were Al, Ti, Cr, Mo, Y, Mn, Pt, Nb, and Si in case of Ti-48Al-2Cr and Nb in case of Ti-48Al-2Cr-2Nb. Comparison was made with the oxidation behaviour of Ti-48Al2Cr-2Nb and Ti-47Al-2Cr-0.2Si alloys in which quaternary elements were added by alloying. It is Concluded that ion implantation can serve as a research tool to study, in the frame of screening tests, the influence of various elements on the corrosion behaviour of materials.     

The role of martensitic transformation in corrosion fatigue failure of Ti-22Nb-6Ta and Ti-22Nb-6Zr (at %) medical alloys

The effect of cyclic dynamic loading on the kinetics of changes in the electrode potentials of Ti-22Nb-6Ta and Ti-22Nb-6Zr (at %) superelastic medical alloys in a model solution simulating the bone tissue environment in a living organism is studied with the use of electrochemical chronopotentiometry. An increase in the applied load from 450 to 780 MPa is found to affect dramatically the character of changes in the electrode potentials of the alloys during exposure. Ti-22Nb-6Ta alloy exhibits higher resistance to corrosion fatigue cracking than does Ti-22Nb-6Zr alloy. It is shown that the superelasticity of the alloys is responsible for the deceleration of the corrosion fatigue cracking due to the blocking effect of martensite crystals formed in the structure during the loading half-cycle, which is confirmed by the low rate of the potential decrease after the nucleation of corrosion fatigue cracks.     

Effects of minor yttrium addition on hot deformability of lamellar Ti-45Al-SNb alloy

The effects of 0.3%（molar fraction, the same below） yttrium addition on hot deformability of lamellar Ti-45Al-5Nb alloy were investigated by simulated isothermal forging tests. The ingots with the nominal compositions of Ti-45Al-5Nb and Ti-45Al-5Nb-0.3Y were prepared by induction skull melting. Simulated isothermal forging tests were conducted on Gleeble 1500D thermo-simulation machine using a 6 mm in diameter and 10 mm in length compressive specimen at the deformation temperatures of 1 100, 1 150, 1 200 ℃ and strain rates of 1.0, 0.1, 0.01 s^-1. The results show that yttrium addition remarkably improves hot deformability of Ti-45Al-5Nb alloy. An appropriate hot deformation processing parameter of Ti-45Al-5Nb-0.3Y alloy is determined as 1 200 ℃, 0.01 s^-1. The flow stresses are decreased by yttrium addition under the same compressive conditions. The activation energies of deformation Q are calculated as 448.6 and 399.5 kJ/mol for Y-free and Y-containing alloys, respectively. The deformed microstructure observation under 1 200 ℃, 0.01 s^-1 condition indicates that Ti-45Al-5Nb-0.3Y alloy shows more dynamic recrystallization. The improvement of hot deformability of Ti-45Al-5Nb-0.3Y alloy induced by yttrium addition should be attributed to that the smaller the original lamellar colonies, the lower the deformation resistance and activation energy of deformation are, and the more the dynamic recrystallization is.     

Microstructure and mechanical properties of low- and heavy-alloyed intermetallic alloys based on γ-TiAl + α2-Ti3Al

The microstructure and mechanical properties of the Ti-43.7Al-3.2(Nb,Cr,Mo)-0.2B alloy in the as-cast state (after gasostatistic processing) and of the Ti-45Al-8Nb-0.2C alloy after hot extrusion at temperatures corresponding to the ?? + ?? phase field followed by heat treatment have been studied. The extruded heavy-alloyed alloy has demonstrated significantly higher plastic/mechanical properties at room temperature with close values of the plasticity/tensile strength and long-term strength at elevated temperatures. A comparison of the results with literature data has shown the properties of the as-cast Ti-43.7Al-3.2(Nb,Cr,Mo)-0.2B to be similar to or superior to those of the best-known casting ?? (TiAl) + ??₂ (Ti₃Al) alloys.     

Oxidation of orthorhombic titanium aluminide Tl-22AL-25NB in air between 650 and 1000 °C

The oxidation behavior of orthorhombic titanium aluminide alloy Ti-22Al-25Nb was studied in air between 650 and 1000 °C by isothermal thermogravimetry and postoxidation scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction. Microhardness measurements were performed after exposure to gage hardening due to nitrogen and oxygen ingress. The parabolic rate constant of Ti-22Al-25Nb was of the same order as conventional titanium alloys and Ti₃Al-based titanium aluminides at and below 750 °C. Between 800 and 1000 °C, the oxidation resistance of Ti-22Al-25Nb was as good as that of γ-TiAl based aluminides; however, the growth rate changed from parabolic to linear after several tens of hours at 900 and 1000 °C. The mixed oxide scale consisted of TiO₂, AlNbO₄, and Al₂O₃, with TiO₂ being the dominant oxide phase. Underneath the oxide scale, a nitride-containing layer formed in the temperature range investigated, and at 1000 °C, internal oxidation was observed below this layer. In all cases, oxygen diffused deeply into the subsurface zone and caused severe embrittlement. Microhardness measurements revealed that Ti-22Al-25Nb was hardened in a zone as far as 300 μm below the oxide scale when exposed to air at 900 °C for 500 h. The peak hardness depended on exposure time and reached five times the average hardness of the bulk material under the above conditions.     

Ti-22Al-26Nb合金热变形本构方程建立及软化行为研究

作为最具潜力的航空航天高温结构材料,Ti2AlNb基合金具有高的比强度和良好的高温蠕变性能。本文对热轧态Ti-22Al-26Nb合金高温变形中的力学行为和再结晶行为进行研究,建立其高温本构关系模型,对其中呈现出的动态再结晶多应力峰值曲线特征（以1000℃,0.1s-1为例）进行拟合分析。结果表明：基于双曲正弦函数建立Ti-22Al-26Nb合金的高温本构关系模型的精度较高,最大误差为2.6%,可以很好地描述合金在高温变形时各热力学参数之间高度非线性的复杂关系,由修正的Avrami方程预测得知再结晶体积分数与应变呈现典型的再结晶动力学增长趋势,揭示了该合金高温变形过程中复杂的软化行为。