Papers by Keyword: Titanium Aluminide

Abstract: The effect of point defects on the shear rupture resistance in titanium aluminide is investigated by the density functional theory and pseudopotential methods. Vacancies, as well as substitution atoms – tungsten and chromium were considered as points defects. The shear was simulated in the(111) slip plane for two directions, namely [110] and [11-2]. It is shown that for a {111}<110> sliding system, vacancies significantly reduce the shear resistance. However, when alloying element occupies a titanium vacancy, it can partially compensate for this negative effect.

Abstract: The hot workability of gamma titanium aluminide alloy, Ti-48Al-2V-2Nb, was assessed in the cast condition through a series of compression tests conducted over a range of temperatures (1000 to 1175 °C) and at the strain rate of 10 S^-1. The mechanism of dynamics recrystallization has been investigated from SEM Z-contrast images and from the Electron backscattered diffraction EBSD as well. It has been observed that volume fraction of the recrystallized grains increases with increasing the deformation temperature. The major volume fraction of the recrystallized grains was observed in the shear band which was forming at an angle 45 ̊ with respect to the compression direction. The mechanism of breaking of the laths and the region of the dynamic recrystallization were also investigated from the SEM Z-contrast image and EBSD. The dynamic recrystallization occurred in the region of the broken laths and shear bands. The breaking of the laths was because of the kinking of the lamellae. The shear band, kinked lamellae and dynamic recrystallized region where all investigated simultaneously.

Abstract: In the present study, effects of heat treatment on microstructures and tensile properties of the cylindrical bars of Ti-48Al-2Cr-2Nb (at.%) alloy with unique layered microstructure consisting of equiaxed γ grains region (γ band) and duplex-like region fabricated by electron beam melting (EBM) were investigated. We found that it is possible to control width of the γ bands (W_γ) by heat treatments at 1100°C and 1190°C. The W_γ increases with decreasing heat treatment temperature. The bars heat-treated at 1190°C exhibit high elongation of 2.9% at room temperature (RT) with maintaining high strength. The RT elongation increases with increasing the W_γ because of increasing deformable regions. In contrast, the RT elongation of the bars decreases with increasing the W_γ when W_γ is very large. This is because the large γ band leads intergranular fracture. These results indicate that there is appropriate width for the γ band to obtain excellent tensile properties at RT.

Abstract: Intermetallic TiAl alloys based on the γ-TiAl phase are already used as engineering light-weight high-temperature materials in aircraft and automotive engines. Thereby, they partly substitute the twice as heavy Ni-base superalloys. Present applications are, for example, blades in the low-pressure turbine of advanced aero-engines, turbine wheels for turbocharger systems of car diesel engines as well as engine parts used in racing cars. All these applications require balanced mechanical properties, i.e. certain ductility at room temperature as well as defined creep strength at elevated temperatures. The first part of this paper reviews the alloy design strategy, which was used for the development of a β-solidifying γ-TiAl-based alloy, the so-called “TNM alloy”, which exhibits an excellent hot-deformability. In the meantime, the TNM alloy with the nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in atomic percent, at.%) is introduced in a particular eco-friendly and fuel-saving aero-engine, which is powering a medium-range aircraft since the beginning of 2016. In the second part of this work the microstructural parameters are highlighted, which influence the failure strain at room temperature and creep strength at elevated temperatures. It will be shown how the creep resistance can be improved by tailoring phase fractions as well as the spatial arrangement of the microstructural constituents.

Abstract: Quantum-mechanical calculations were used to investigate shear rupture in intermetallic titanium aluminide (TiAl) alloys in the presence of vacancy or chromium dopant. The substitution of both Ti and Al atoms by Cr atoms in the γ-TiAl crystal lattice was considered. The simulation of shear was carried out in the (111) slip plane along two directions, namely the [110] and [11-2]. The decrease in the shear resistance of the defects present in the γ-TiAl lattice was estimated. It was shown that when chromium occupies a titanium vacancy, it can compensate for this defect by increasing the shear modulus for the {111} <110> slip system.

Abstract: This work is devoted to investigation of the structure of Ti-TiAl₃ composites reinforced by TiB₂ or TiC hard particles and obtained by spark plasma sintering of elemental foils and ceramic powders. Sintering was carried out at the temperature of 830 oC under the pressure of 40 MPa during 10 minutes. Microstructure of the composites obtained was represented by alternated layers of titanium and intermetallic compound TiAl₃. Also, it was found that at the Ti-TiAl₃ interfaces thin intermediate layers were formed. Quantitative elemental analysis of these layers showed that Ti₃Al, TiAl, and TiAl₂ compounds, as well as Ti (Al) solid solution could be formed in these zones. Diffraction analysis did not reveal any transformations of initial reinforcing phases after sintering. Interlayers with titanium diboride had the average microhardness level of 3988 HV, and the average microhardness level of interlayers with TiC was 1610 HV.

Abstract: Pre-alloyed gamma titanium aluminide powders were fabricated by argon gas atomization. The powder was hot isostatic pressed (HIP) at 1200°Cand 150MPa for 3h to obtain full density compact. The properties of the powders and the HIP’ed compacts were investigated in this work. The microstructure of the powder exhibited dendrite and cellular mixed image, resulted from rapid solidification and independent of particle size. The microstructural transition from cell to dendrite could be readily represented in a droplet or the droplets of different size. The transition was the result of the decrease of undercooling. XRD analysis result showed that the powder consisted of α₂ phase and γ phase, coarse powder was mostly γ phase, while fine powder mostly α₂ phase. After HIP, the near gamma microstructure showed an average γ-TiAl grains of approximately 6μm.

Abstract: Pure powders of titanium, aluminium, nickel and ruthenium were mechanically alloyed and melted in a button arc furnace under an argon atmosphere to produce two alloys of composition Ti-52.5Al-10.0Ni (at.%) and Ti-52.5Al-10.0Ni-0.2Ru (at.%). The alloys were then cut and metallographically prepared. Scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) were used to characterize the samples. Thermogravimetric analysis (TGA) was used to analyze the oxidation behavior from room temperature up to 1050°C. The alloys were also oxidized in air at 1050°C for 120 hours. The Ti-52.5Al-10.0Ni (at.%) alloy formed dendrites of γ-TiAl (55.6 at.% Al) surrounded by a eutectic of γ-TiAl + Al₃NiTi₂ (τ₃) phases. The Ti-52.5Al-10.0Ni-0.2Ru (at.%) alloy formed dendrites of γ-TiAl (53.6 at.% Al) surrounded by a eutectic of γ-TiAl + Al₃NiTi₂ (τ₃). The ruthenium was mostly in solid solution (0.3 at.%) in the Al₃NiTi₂ (τ₃) phase, although traces of it were present in the dendrites (0.1 at.% Ru). When oxidized in air from room temperature to 1050°C, the as-cast Ti-52.5Al-10.0Ni-0.2Ru (at.%) had a mass gain of 0.60% and the as-cast Ti-52.5Al-10.0Ni (at.%) had a mass gain of 0.97%. Isothermal oxidation of both alloys at 1050°C for 120 hours formed mixed metal oxides of TiO₂+Al₂O₃ on the surface.

Abstract: Today conventional titanium-based alloys represent one third of the weight of modern aircraft engines and, are the second most used engine material following Ni-based superalloys. [1] Titanium aluminide alloys based on intermetallic phases γ (TiAl) and α₂ (Ti₃Al) and the most recent – orthorhombic titanium aluminide, are widely recognized as having the potential to meet the design requirements for high temperature applications. The outstanding thermo-physical and mechanical properties of these materials rely mainly on the strongly ordered nature and the directional bonding of the compounds. These involve: high melting point, above 1460°C, low density of 3,9-5 g/cm³, according the alloying degree, high elastic modulus (high stiffness), high yield strength and good creep resistance at high temperature, low diffusion coefficient, good structural stability at high temperature. The main objective of our paper are focussed on the short-term mechanical properties if Titanium niobium aluminide at 850°C. High temperatures mechanical properties evaluation was performed by tensile testing at temperature of 850°C on universal static and dynamic testing machine Instron 8802, equipped with high temperature system, for maximum 1000°C, and extensometer with a measuring basis of 40 mm. The mechanical tensile test was performed according the ASTM E8, with control of deformation and a testing rate of 10^-4 mmsec^.-1. Short-term behavior request of the support uncovered alloys, at 850°C has proved to be modest and it seems obvious that the alloys based on titanium aluminides cannot be used without protective coatings. Key words: titanium aluminides, high temperatures, mechanical properties

Abstract: Currently there is a continuous extension of application areas of titanium aluminides but their processing encounters a number of difficulties such as the precision of machining and modifying the properties of the surface layer. In the specialist literature [1, 2, 4, 7] different authors address the processing of these materials by conventional methods. In this context, this paper aims at finding the optimal regimes for the electrical discharge machining (EDM) of titanium aluminides Ti-40Al-5NB-3V and provides the analysis of the process parameters effect on cutting through wire EDM technological characteristics. In the case of wire EDM a relatively good output has been obtained as well as very good roughness of the processed surfaces. On the contrary, when considering the die sinking edm, due to very small output, several experimental measurements were performed on three machines – Sodick, ONA and ELER. Based on the experimental data, it has been established that the best output is to be obtained by reverse polarity machining.