Diffusion creep of intermetallic TiAl alloys

The creep behaviour of γ-TiAl with L1₀ structure without second phases, γ-TiAl with precipitated particles of α₂-Ti₃Al with D0₁₉ structure, and γ-TiAl with the H-phase Ti₂AlC has been studied at low stresses in the temperature range 900–1200°C. The obtained data allow the construction of creep deformation mechanism maps for the studied alloys which may be used for an extrapolation of the observed creep behaviour. At higher stresses dislocation creep occurs in all alloys, which is well described by the Dorn equation with stress exponents in the range 3–5. Extended Coble creep with threshold stress was observed only for the studied two-phase alloys. A strong temperature dependence of the threshold stress for Coble creep was found for the TiAl alloy with carbide particles.     

采用自蔓延高温合成技术连接TiAl合金

采用自蔓延高温合成技术实现了TiAl合金的连接。在连接过程中采用了具有很高放热量的Ti-Al-C中间层以及外加电磁场辅助连接。连接接头包括3个典型的反应区域,靠近TiAl母材界面处发现了深灰色的TiAl,反应层,在中间层内观察到了TiC颗粒以及Ti-Al系化合物。直接连接时由于产物和反应物之间的比热差,杂质的气化和孔隙中束缚气体的释放而导致孔隙无法避免。为了提高致密度,在粉末压坯和TiAl母材之间添加了Ag-Cu钎料箔。在SHS反应过程中熔化的钎料改善了中间层对TiAl母材的润湿同时填充到了中间层反应产物的孔隙中,采用这种方法能够提高反应产物的致密度和连接质量。     

Environmental corrosion resistance of porous TiAl intermetallic compounds

Porous TiAl intermetallic compound, as a novel substitute for current inorganic porous material, offsets the shortages of both ceramics and metals. The environmental corrosion resistance of porous TiAl intermetallic compound was investigated. The kinetic equation for the cyclic oxidation of porous TiAl alloy at 600 ℃ is determined to be Δm2=1.08×10-5t. After total oxidation of 140 h, porous TiAl intermetallic compound shows more stability of pore structure and the mass gain of TiAl alloy is 0.042 g/m2, which is only 10.6% that of porous 316L stainless steel. The kinetic equation for the cyclic corrosion behavior of porous TiAl alloy in hydrochloric acid with pH=2 at 90 ℃ is determined to be Δm2=5.41×10-5t-2.08×10-4. After 50 h exposure, the mass loss of TiAl alloy is 0.049 g/m2, which is only 14.8% and 5.57% that of porous Ti and stainless steel, respectively. The kinetic equation in hydrochloric acid with pH=3 is determined to be Δm2=2.63×10-6t-3.72×10-6.     

Diffusion aluminide coatings for TiAl intermetallic turbine blades

Alloys based on intermetallic phases of a Ti–Al system are materials that, thanks to their resistance characteristics, can be widely used in automotive and aerospace applications. The main restriction for the use of Ti–Al materials is their insufficient oxidation resistance above 850 °C. Oxidation parameters might be improved by aluminide coatings based on TiAl₂ and TiAl₃ phases, which could induce the creation of an Al₂O₃ scale in the oxidation process. This type of aluminide could be deposited on the surface of TiAl alloys by various methods such as pack cementation, plasma spraying or magnetron sputtering. This article presents a new method of aluminide coating deposition on TiAl intermetallic alloys: out of pack technology. The investigated coating was deposited on turbine blades made of a Ti45Al5Nb intermetallic alloy. The surface morphology, structure, phase and chemical composition have been investigated using XRD phase analysis, SEM and EDS. The phase analysis showed that TiAl₃ and TiAl₂ were the main components of the deposited coating. An isothermal oxidation test of the TiAl turbine blades was conducted as well. After 1000 h of testing at 950 °C, the scale formed on the surface of the uncoated blades underwent spallation. The scale on the turbine blade with deposited aluminide coatings was very thin and no spallation was observed.     

On two stages of brittle-to-ductile transition in TiAl intermetallic

The present paper has shown that the brittle-to-ductile transition in TiAl intermetallic ought to be considered as a two-stage phenomenon: (1) first noticeable increase in ductility (at T₁°C) is controlled by thermally activated relaxation processes in grain boundaries. In this case, the brittle fracture type is retained; (2) following an increase in ductility (at T₂=T₁+ΔT) is caused by thermally activated relaxation processes within the grains (in addition to ones in grain boundaries) which lead to the transition from brittle fracture to ductile one.     

Infrared brazing of TiAl intermetallic using BAg-8 braze alloy

TiAl intermetallic alloy joined by infrared brazing using BAg-8 braze alloy was investigated. The microstructural evolution of the brazed joint, shear strength and reaction kinetics across the joint was comprehensively evaluated. According to the experimental observations, silver would not react with the TiAl substrate, but copper reacted vigorously with the TiAl, forming continuous reaction layer. The consumption of copper from molten braze during infrared brazing resulted in depletion of the copper content from the braze. Therefore, chemical composition of the braze deviated from Ag-Cu eutectic into hypoeutectic with increased brazing time and/or temperature. Both AlCuTi and AlCu₂Ti phase were observed at the interface between BAg-8 and TiAl substrate for the specimen brazed at 950°C. By increasing the brazing temperature and time, the growth rate of AlCuTi phase was much faster than that of AlCu₂Ti phase. The maximum shear strength achieved 343 MPa for the specimen infrared brazed at 950°C for 60 s. Further increasing the brazing time resulted in excessive growth of brittle AlCuTi reaction layer, which greatly deteriorated the shear strength of the joint.     

TiAl合金微弧氧化膜的制备及抗氧化性能研究

利用微弧氧化的方法在γ-TiAl合金上制备陶瓷膜,探讨了氧化过程中电流密度与成膜速度的关系。利用扫描电镜研究了氧化膜的表面形态和截面结构,结果表明,氧化膜由三层组成,且氧化膜与基体间有良好的冶金结合;物相分析发现氧化膜中含有金红石、Al2TiO5和SiO2非晶成分,并且物相组成和分布与成膜时间及膜的深度无关;氧化膜的硬度随着氧化时间的增加而增加,极大值出现在距离膜基交界面约10μm附近的氧化膜中。1100℃下的高温氧化实验发现微弧氧化处理后样品的氧化速度只有原始基体材料的1／3,微弧氧化处理后提高了TiAl合金的抗氧化性。     

TiB2金属陶瓷与TiAl金属间化合物的扩散连接

对TiB2金属陶瓷与TiAl金属间化合物进行了扩散连接试验，研究了直接扩散连接和采用Ni为中间层进行扩散连接的接头界面结构及工艺参数对界面结构和连接性能的影响。直接扩散连接时，连接界面处生成了Ti(Cu，Al)2金属间化合物，采用Ni为中间层进行扩散连接时，界面处生成了单层TiAlNi2金属间化合物层和两层T1，Al，N2扩散层共三层结构。直接扩散连接时，连接温度T＝1223K，时间t=1．8ks，压力p＝80MPa时接头强度为103MPa；采用Ni为中间层时，连接温度T＝1273K，时间t=1．8ks，压力p=80MPa时接头强度为110MPa。     

γ—TiAl基合金的表面激光处理及超塑性扩散连接

较系统地研究了AiAl基合金激光快速熔凝组织细化特征及该类组织的超塑连接规律。结果表明,激光处理后试样表面熔凝区的组织主要为胞状枝晶组织,经后续热处理后转变为细小的等轴晶粒组织,并形成了良好的超塑连接条件。利用该表面组织对试样进行超塑扩散连接,探讨了连接温度,压力和时间对连接效果的影响。表面细化组织试样与整体细化组织试样的超塑连接具有基本相同的连接规律,在连接温度900℃,连接压力60MPa,连接时间1h条件下,可以实现试样的超塑扩散连接。     

Pore constitution and tortuosity factor of reactively synthesized porous TiAl intermetallic compound

The pore constitution and tortuosity factor of porous TiAl intermetallic were studied on the basis of the variation behavior of pore structure parameters and the discrete particle model. The pore formation mechanism of porous TiAl is mainly ascribed to three aspects: the clearance space in green compact, the diffusive pores in the reaction process and the phase transition pores, resulting in the open porosities of 5.6%, 42.9% and 1.3%, respectively. According to the Hagen?Poiseuille equation, the tortuosity factor of porous TiAl is determined in the range of 1.3?2.2. Based on the discrete particle model and the variation rule of the tortuosity factor, the tortuosity factor depends mainly on the parameters of fabrication constant, particle shape factor, clearance distance and powder particle size. The quantitative relationships among them have been established, which can be used as the basis for adjusting the pore structure of porous intermetallics.     

Bonding interface between TiAl intermetallic compound and Ti under the electric field

TiAl intermetallic compound was synthesized and bonded together with Ti substrate synchronously by using fieldactivated and pressure-assisted synthesis (FAPAS) process.The effect of electric field on t...     

TiAl合金激光熔覆复合材料涂层耐磨性研究

利用预涂NiCr-Cr3C2复合粉末对γ-TiAl合金（简称TiAl合金）进行激光熔覆处理,制得了以Cr7C3、TiC硬质相为耐磨增强相,以γ-NiCrAl镍基固溶体为基体的复合材料涂层,研究了原始TiAl合金和激光熔覆涂层的室温和高温（600℃）滑动磨损性能,并讨论了原始TiAl合金和所制备涂层的室温和高温滑动磨损机理。结果表明：激光熔覆复合材料涂层均具有较好的室温和高温滑动磨损耐磨性。室温下涂层的耐磨性先随着其中硬质耐磨增强相体积分数的增加而提高,但当耐磨相体积分数过高时,由于涂层脆性增大,其耐磨性反而下降。     

Fabrication of TiAl intermetallic phases by heat treatment of warm sprayed metal precursors

Warm Spraying is an atmospheric coating process based on high-velocity impact bonding of powder particles. By decreasing the temperature of combustion gas via mixing with nitrogen the oxidation of feedstock powder can be effectively controlled. This is particularly important for Ti-based coating materials, which rapidly oxidize at elevated temperatures.In this study, Ti–Al composite coatings were fabricated by the Warm Spray process using a mixture of titanium and aluminum powders as a feedstock and applying a two-stage heat treatment at 600 and 1000 °C to obtain intermetallic phases. The microstructure, chemical and phase composition of the deposited and heat-treated coatings were investigated using SEM, EDS and XRD. The experimental results show that TiAl₃ was the first intermetallic phase formed during the first-stage heat treatment. The growth of TiAl₃ layer occurred mainly by diffusion of Al into Ti particles. Significant porosity that developed during the heat treatment was caused mainly by Kirkendall effect. After the second-stage heat treatment, a coating layer with TiAl as the dominant phase was obtained with about 20 vol % porosity.     

Effect of a sputtered TiAlCr coating on the oxidation resistance of TiAl intermetallic compound

The effect of a sputtered TiAlCr coating on the oxidation resistance of TiAl intermetallic compound was investigated in static air. The bare TiAl alloy exhibited poor isothermal and cyclic-oxidation resistance at 800–1000°C due to the formation of TiO₂-base scales which tend to spall during cooling. A sputtered Ti-50Al-10Cr coating remarkably improved the oxidation resistance of TiAl, due to the formation of an adherent Al₂O₃ scale at 800–1000°C. After long-term oxidation (at 900°C for 1000 hr), TiAlCr coating still provided excellent protection for the TiAl alloy. Minor interdiffusion occurred due to the inward diffusion of Cr, while no Kirkendall voids were found at the coating/ substrate interface. In contrast, NiCrAlY and CoCrAlY coatings reacted extensively with the TiAl alloys. Moreover, the TiAlCr coating alloy is based on -TiAl and TiAlCr Laves phases, which may offer improved mechanical properties. The TiAlCr coating exhibited a better combination of oxidation resistance and substrate compatibility than conventional aluminide and MCrAlY coatings.     

TiAl基金属间化合物电子束焊接接头组织特征及断裂形貌

焊接材料: Ti-48Al-2Cr-2Nb     

The internal stress state in lamellar PST-crystals of the intermetallic alloy TiAl after compressive deformation

The microstructure of γ-based TiAl alloys often exhibits grains with an internal lamellar structure consisting of alternating γ-TiAl and α₂-Ti₃Al lamellae. Even in undeformed material, internal coherency stresses, due to the lattice misfit between the two phases and the tetragonality of the γ-phase are present. The goal of this work was to investigate the evolution of internal strains induced by compressive deformation of PST-specimens of the alloy TiAl at room temperature and at 800°C. The results of X-ray measurements of different peak profiles, performed with a special high-resolution double-crystal diffractometer are reported, which revealed the dependence of the deformation-induced internal strains on the orientation of the lamellar boundaries with respect to the axis of deformation.     

Fabrication of TiAl components by means of hot forging and machining

The work reported here is aimed at the supply of large TiAl components having complex shapes and high dimensional precision, so as to expand the applications for TiAl alloys. The fabrication of such components requires rough forming by hot forging, followed by machining, a method which cannot be used with conventional TiAl alloys. Accordingly, a new Ti–42Al–5Mn (at.%) alloy, which featuring capabilities for hot forging and machining was developed, and in this study hot forging and machining tests were performed from an industrial perspective. As a result, it was ascertained that the objective components could be fabricated using this new alloy.     

Additive manufacturing of lightweight,fully Al-based components using quasicrystals

This article addresses a still-open question in additive manufacturing of lightweight metallic components. We find that using Al-based quasicrystalline particles allows extending the selective laser sintering technology to produce Al-based parts of any complex shape. The process consists in selectively melting a powder blend containing a binder and quasicrystalline particles to construct a three-dimensional pre-form. In a second step, the pre-form is infiltrated by a conventional aluminium alloy in a furnace through a thermal cycle under an inert atmosphere. The aluminium alloy reacts with the quasicrystalline particles, inducing phase transformations and ending with a manufactured product of good mechanical properties that is harmless to the environment and cost-effective.     

Dual surface and bulk control by Nb of the penetration of environmental elements in TiAl intermetallic alloys

X-ray photoelectron spectroscopy (XPS) and micro-hardness measurements were combined to study the effect of Nb doping on the high temperature environmental interactions of α₂-Ti₃Al intermetallic alloys with pure oxygen and oxygen/nitrogen gas mixtures. The XPS data show a pre-oxidation stage in the first minutes of low pressure exposure to the gaseous environments at 650 °C, characterized by the absence of oxidation of the metal constituents of the alloy, the adsorption on the surface, and the sub-surface penetration of atomic oxygen and nitrogen. The XPS measurements of the surface and sub-surface concentrations reveal the effect of niobium on the surface adsorption of oxygen and nitrogen and on sub-surface penetration, showing that nitrogen is disfavored. The micro-hardness data show a bulk effect of niobium that counteracts the near-surface embrittlement caused by the penetrating environmental elements. The surface and bulk effects of niobium combine to mitigate the embrittlement of the material by nitrogen.     

Dynamic dislocation behaviour in the intermetallic compounds NiAl, TiAl and MoSi2

The dynamic behaviour of dislocations in NiAl, TiAl and MoSi₂ on ‘easy’ slip systems is studied by in situ straining experiments in a high-voltage electron microscope. At elevated temperatures, the dislocations are smoothly bent as in NiAl and TiAl or sometimes show superkinks as in MoSi₂, and they move in a viscous way. It is suggested that this dynamic behaviour as well as the flow stress anomaly are connected with the formation of atmospheres around the dislocations. A model is proposed assuming that the lowest energy configuration of a dislocation may require a certain number of antisite defects or other point defects in the dislocation core. This cloud of disordered structure may follow partly the moving dislocations to induce an additional friction, analogous to other diffusion controlled mechanisms. The view of atmospheres controlling the dislocation mobility in intermetallics at elevated temperatures is supported by measurements of the dependence of the strain rate sensitivity on the strain rate itself.