TiAl Intermetallic Alloy Research Articles

High Temperature Mechanical Properties of TiAl Intermetallic Alloy Parts Fabricated by Metal Injection Molding

High Temperature Mechanical Properties of TiAl Intermetallic Alloy Parts Fabricated by Metal Injection Molding

Long exposure test in air, conducted on super-lattice coatings at 850°C for 4,000 hours

Purpose– The purpose of this study is to investigate the high-temperature behavior of newly developed high-impulse power magnetron sputtering system (HIPIMS) coatings and compare them to the standard TiAlCr system deposited on to a Ti–Al intermetallic alloy. The corrosion test was performed in air for 4,000 hours at 850°C.Design/methodology/approach– In this study, air oxidation test was performed at high temperature. Design and methodology is described in detail in the methodology section in the submitted manuscript. The test was carried out by discontinuous exposure of the three different systems produced by different deposition technique. The exposed samples were investigated using scanning electron microscope coupled with energy dispersive X-ray spectroscopy. The exposed samples were investigated from the surface and cross-sections.Findings– The performed study shows that HIPIMS coatings had a much better oxidation resistance at a high temperature than that offered by the standard physical vapor deposition (PVD) system. HIPIMS costing developed Al–Cr oxide on the surface; however, cracks and detachments were found at the interface between the coating and the substrate. TiAlCr coating spalled off from the material due to the critical thickness reached; moreover, high brittleness and lack of adherence were found. Due to poor oxidation resistance, TiAlCr coating was discarded from the test after 3,000 hours of exposure.Originality/value– The work performed in this study was designed for 4,000 hours oxidation at 850°C. The long-term exposures are not commonly met in the research work due to the cost and time. The work clearly shows differences between new type of coatings and standard PVD system applied on TiAl lightweight alloy.

Effect of the Microstructure on the Fatigue Strength of a TiAl Intermetallic Alloy Produced by Additive Manufacturing

ABSTRACTIn this work we examine a Ti-48Al-2Cr-2Nb alloy obtained with an additive manufacturing technique by Electron Beam Melting (EBM) by conducting monotonic and cyclic loading experiments both on tension and compression samples for investigating the influence of the microstructure in strain accumulation process by fatigue loading. The residual strain maps corresponding to different applied stress levels, number of cycles and microstructures are obtained through the use of high-resolution Digital Image Correlation (DIC). The strain maps were overlaid with the images of the microstructure and detailed analyses were performed to investigate the features of the microstructure where high local strain heterogeneities arise. Such experiments, conducted ex-situ at room temperature, allow to characterize the effect of different microstructures on the strain accumulation process, and to clearly identify the role of the microstructural features of this TiAl intermetallic alloy on the fatigue initiation process.

Tribological Properties of TiAl Coating on Aluminum Alloy Surface by Supersonic Particles Deposition

In this study, Gamma TiAl intermetallic alloy protective coating was deposited on the surface of 5803 aluminum alloy by supersonic particles deposition. Researchers observed the surface morphology of the coating by SEM, and researched tribological properties of the substrate material and the coating by CETR friction and wear tester and Talysurf 5P-120 surface morphology tester. The results show that,the bonding mechanism of the Gamma TiAl intermetallic alloy coating and substrate is mechanical interlocking and metallurgical bonding. The abrasion mechanism of Gamma TiAl intermetallic alloy coating is abrasive Wear. The friction coefficients of the coating are all lower than 5083 alumium alloy. The friction volume of coatings reduced 36.7% than 5083 aluminum alloy with the friction loads 50N. TiAl coating has great function to anti-attrition and wearability.

Topography Prediction on Grinding of Emerging Aeronautical TiAl Intermetallic Alloys

Gamma-TiAl intermetallic materials are the focus of all leading aerospace / gas turbine manufacturers, as a replacement for some nickel-based superalloy components in parts of the engine subject to temperatures < 900°C. Although applicable for only a relatively narrow range of applications, titanium intermetallic materials are likely to play a significant role in the production of future aeroengines. This work presents the results from grinding tests on two types of Gamma TiAl alloys.

Effects of the manufacturing process on fracture behaviour of cast TiAl intermetallic alloys

The ? -TiAl based intermetallic alloys are interesting candidate materials for high-temperature applications with the efforts being directed toward the replacement of Ni-based superalloys. TiAl-based alloys are characterised by a density (3.5-4 g/cm3) which is less than half of that of Ni-based superalloys, and therefore these alloys have attracted broad attention as potential candidate for high-temperature structural applications. Specific composition/microstructure combinations should be attained with the aim of obtaining good mechanical properties while maintaining satisfactory oxidation resistance, creep resistance and high temperature strength for targeted applications. Different casting methods have been used for producing TiAl based alloys. In our experimental work, specimens were produced by means of centrifugal casting. Tests carried out on several samples characterised by different alloy compositions highlighted that solidification shrinkage and solid metal contraction during cooling produce the development of relevant residual stresses that are sufficient to fracture the castings during cooling or to produce a delayed fracture. In this work, crack initiation and growth have been analysed in order to identify the factors causing the very high residual stresses that often produce explosive crack propagation throughout the casting.

High temperature oxidation behaviour of TiAl–Cr–Nb–Mo alloys

The oxidation behaviour of six different TiAl–Cr–Nb–Mo alloys was investigated by thermogravimetric method. Isothermal experiments were carried out at 900 °C and 1000 °C in air.The aim of this work was to evaluate the effect of aluminium and of other alloying elements on the oxidation behaviour of TiAl intermetallic alloys. The alloy's microstructure and composition as well as the composition distribution of the oxide scales were analysed by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-Ray Diffractometry (XRD). The study highlighted that the oxidation resistance of the considered TiAl-based alloys is strongly affected by its aluminium content, while it seems that an increase of Cr, Nb and Mo concentrations does not affect the oxidation behaviour of the alloy. Yttrium addition, at least in the tested quantity, has a detrimental effect since it promotes the formation of a TiO2–Al2O3 mixed oxide instead of a compact and thin alumina layer.

Irradiation creep of candidate materials for advanced nuclear plants

In the present paper, irradiation creep results of an intermetallic TiAl alloy and two ferritic oxide dispersion strengthened (ODS) steels are summarized. In situ irradiation creep measurements were performed using homogeneous implantation with α- and p-particles to maximum doses of 0.8dpa at displacement damage rates of 2–8×10−6dpa/s. The strains of miniaturized flat dog-bone specimens were monitored under uniaxial tensile stresses ranging from 20 to 400MPa at temperatures of 573, 673 and 773K, respectively. The effects of material composition, ODS particle size, and bombarding particle on the irradiation creep compliance was studied and results are compared to literature data. Evolution of microstructure during helium implantation was investigated in detail by TEM and is discussed with respect to irradiation creep models.

An in-situ high-energy X-ray diffraction study on the hot-deformation behavior of a β-phase containing TiAl alloy

In engineering materials, microstructural evolution during hot-working critically determines the properties of the finished part. Intermetallic TiAl alloys are no exception and numerous attempts have been made to improve their performance by subjecting them to harmonized hot-working steps. In the current work a novel in-situ diffraction technique along with conventional microscopic methods were employed to characterize the behavior of the individual phases at two different deformation temperatures. A so-called TNM™ alloy with a nominal composition of Ti-43.5 Al-4 Nb-1 Mo-0.1 B (in at%), which exhibits an adjustable fraction of disordered β-phase at elevated temperatures, was deformed isothermally at 1220 °C and 1300 °C. At 1220 °C three phases (α,β,γ) are present in thermodynamic equilibrium which reduces to two (α,β) at 1300 °C. It was possible to observe in-situ the individual behavior of the involved phases during deformation and the phenomena which accommodate the defects generated by hot-working. Results of post-mortem microscopic investigations were used to confirm the findings. The results of the in-situ experiments give unique insights into the hot-deformation behavior of multi-phase TiAl alloys, which can be used for specific process optimization and for further alloy development.

Optimized coating procedure for the protection of TiAl intermetallic alloy against high temperature oxidation

The interest for TiAl intermetallic alloys is growing in the last years because of their excellent ratio between mechanical properties and density. However, the application is restricted at temperatures up to 873–973 K by the resistance to oxidation. For this reason it is of great importance to study protective coatings able to raise its temperature of performance above 1073 K.A TiAl3 layer was obtained on the surface of Ti–Al intermetallic samples, depositing an Al coating by arc Physical Vapour Deposition, followed by a thermal treatment. The microstructure and the composition of the coating were characterised before and after the thermal treatment by Scanning Electron Microscopy, X-Ray Diffraction, Focused Ion Beam and Glow Discharge Optical Emission Spectroscopy. The mechanical behaviour of the thermal treated layer was investigated by means of scratch tests and nano-indentation.Oxidations at 1123 K were carried out on coated and uncoated samples, in order to study the effect of the coating on the oxidation resistance of the samples. The aluminide coating provides a protection against oxidation in air at the considered temperatures due to its ability to form a continuous Al2O3 scale on the surface. The TiAl3 layer phase transformations occurring during high temperature exposition were also investigated.

Preparation of TiAl Intermetallic Alloy by Gelcasting

TiAl intermetallic alloy was prepared by gelcasting. The alloy powders were synthesized through reaction diffusion. The microstructure was investigated by XRD and SEM. The results showed that titanium aluminide powders were synthesized from elemental powders under the condition of 600°C for 3h. The solid loading of metal slurry was 45%. After gelcasting, drying and sintering at 1480°C for 2h, TiAl with the relative density of 93% was successfully prepared.

Novel protection solutions against environmental attack for light weight high temperature materials

ABSTRACTThe use of light weight structural materials such as titanium in transport systems like aero planes leads to a significant reduction in fuel consumption. However, titanium and its alloys cannot be used at elevated temperatures above 500°C for several reasons. Today aero engine compressors are made of a mixture of light Ti- and heavy Ni-alloys. The improvement of Ti-alloys to withstand the conditions in the high pressure compressor i.e. temperatures above 500°C would enable the manufacturing of a compressor from titanium as a whole with all its associated benefits. Intermetallic TiAl-alloys are another class of light weight materials for several high temperature applications. The use of TiAl as low pressure turbine (LPT) blades in the last sections of a large jet engine could save up to 150 kg of weight. In the last sections of the LPT the temperature is quite moderate (max. 650°C). The improvement of the high temperature capability of TiAl would allow its use in hotter sections of the engine with additional weight reduction. Similarly, the response performance of TiAl-turbocharger rotors in automotive engines would be much faster compared to the heavy Ni-based alloys used today. Furthermore higher rotation speeds are possible. Due to the novel so called fluorine effect the oxidation mechanism of TiAl can be altered. Fluorine-treated TiAl-components are protected by an alumina layer formed during high temperature exposure in oxidizing environments. This effect can be transferred to Ti-base materials if they are enriched with aluminum in a thin surface zone. The concepts and the results of high temperature exposure experiments of treated Ti- and TiAl-specimens are presented in this paper. They are discussed in the view of a use for real components.

Design, Processing, Microstructure, Properties, and Applications of Advanced Intermetallic TiAl Alloys

AbstractAfter almost three decades of intensive fundamental research and development activities, intermetallic titanium aluminides based on the ordered γ‐TiAl phase have found applications in automotive and aircraft engine industry. The advantages of this class of innovative high‐temperature materials are their low density and their good strength and creep properties up to 750 °C as well as their good oxidation and burn resistance. Advanced TiAl alloys are complex multi‐phase alloys which can be processed by ingot or powder metallurgy as well as precision casting methods. Each process leads to specific microstructures which can be altered and optimized by thermo‐mechanical processing and/or subsequent heat treatments. The background of these heat treatments is at least twofold, i.e., concurrent increase of ductility at room temperature and creep strength at elevated temperature. This review gives a general survey of engineering γ‐TiAl based alloys, but concentrates on β‐solidifying γ‐TiAl based alloys which show excellent hot‐workability and balanced mechanical properties when subjected to adapted heat treatments. The content of this paper comprises alloy design strategies, progress in processing, evolution of microstructure, mechanical properties as well as application‐oriented aspects, but also shows how sophisticated ex situ and in situ methods can be employed to establish phase diagrams and to investigate the evolution of the micro‐ and nanostructure during hot‐working and subsequent heat treatments.

Effect of conditions of pulsed electron-beam melting for Al (film)/Ti (substrate) systems on phase formation and properties of Ti-Al surface alloys

Findings of comparative investigations are presented for phase formation, characteristics of hardening, tribological properties, and oxidation resistance of Ti-Al intermetallic surface alloys formed by multiple alternation of deposition of an Al film (0.1–1 μm) on a Ti substrate and pulsed liquid-phase mixing of Al and Ti by intense low-energy (∼15 keV) electron beams with microsecond (∼3 μs) and submillisecond (100 μs) duration. It is found that microsecond synthesis is effective for formation of γ(TiAl) oxidation-resistant surface alloys. In turn, submillisecond synthesis allows one to form Ti3Al surface alloys with enhanced wear resistance.

The Friction and Wear Properties of Conventional and Nanostructured ZrO<sub>2</sub>-7wt.%Y<sub>2</sub>O<sub>3</sub> Thermal Barrier Coatings Deposited on TiAl Intermetallic Alloy by Plasma Spraying

It has attracted very strong interests in enhancing the wear resistance of the TiAl intermetallic alloy. In this paper, both plasma-sprayed conventional and nanostructured ZrO2-7wt.%Y2O3 thermal barrier coatings (TBCs) were prepared on TiAl alloy, and microhardness of coating were investigated. Meanwhile, the dry sliding friction tests on the substrate, the traditional coating, and the nanostructured coating were evaluated by ball-on-disk tribometer at room temperature. The results show that the microhardness of nanostructured coating is higher than that of the conventional one. Lower friction coefficient and higher wear resistance than the original TiAl alloy is achieved in the coatings under wear test conditions. Due to the existence of nanostructured microstructures, the nanostructured coating exhibits the lowest friction coefficient, and the best wear resistance. The wear mechanism of the original TiAl substrate is severe adhesive wear. However, the wear mechanisms of the TBCs are mild ploughing and severe adhesion.

Hot Deformation of Cast and Extruded TiAl: An <i>In Situ</i> Diffraction Study

Intermetallic TiAl alloys are a class of innovative high-temperature materials which are developed to replace the substantially denser Ni-base alloys in low-pressure turbine blades of jet engines. By streamlining the production process of these parts, a substantial decrease in production costs can be achieved. To this end, a profound knowledge of the microstructural processes occurring during hot deformation is a prerequisite. To investigate the microstructural development during forming operations, cast and extruded as well as only cast specimens were hot-deformed and the microstructural development investigated in-situ by means of a novel diffraction method. This powder diffraction method utilizes the behavior of individual reflection spots on the Debye-Scherrer rings for deriving the materials response to the deformation imposed. It was found that the behavior of the two specimens is rather similar, although the starting microstructures show pronounced differences.

Thermodynamic Calculations of Phase Equilibria and Phase Fractions of a β-Solidifying TiAl Alloy using the CALPHAD Approach

ABSTRACTThe CALPHAD (CALculation of PHAse Diagrams) method is widely recognized as a powerful tool in both scientific and industrial development of new materials and processes. For the implementation of consistent databases, where each phase is described separately, models are used which are based on physical principles and parameters assessed from experimental data. Such a database makes it possible to perform realistic calculations of thermodynamic properties of multi-component systems. However, a commercial available TiAl database can be applied for thermodynamic calculations to both conventional Ti-base alloys and complex intermetallic TiAl alloys to describe experimentally evaluated phase fractions as a function of temperature. In the present study calculations were done for a β-solidifying TiAl alloy with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at. %), termed TNMTM alloy. At room temperature this alloy consists of ordered γ-TiAl, α2-Ti3Al and β0-TiAl phases. At a certain temperature α2 and β0 disorder to α and β, respectively. Using the commercial database the thermodynamic calculations reflect only qualitative trends of phase fractions as a function of temperature. For more exact quantitative calculations the commercial available thermodynamic database had to be improved for TiAl alloys with high Nb (and Mo) contents, as recently reported for Nb-rich γ-TiAl alloys. Therefore, the database was modified by experimentally evaluated phase fractions obtained from quantitative microstructure analysis of light-optical and scanning electron micrographs as well as conventional X-ray diffraction after long-term heat treatments and by means of in-situ highenergy X-ray diffraction experiments. Based on the CALPHAD-conform thermodynamic assessment, the optimized database can now be used to correctly predict the phase equilibria of this multi-component alloying system, which is of interest for applications in automotive and aircraft engine industry.

Ab-initio calculations of electronic structure and optical properties of TiAl alloy

The electronic structures and optical properties of TiAl intermetallic alloy system are studied by the first-principle orthogonalized linear combination of atomic orbitals method. Results on the band structure, total and partial density of states, localization index, effective atomic charges, and optical conductivity are presented and discussed in detail. Total density of states spectra reveal that (near the Fermi level) the majority of the contribution is from Ti-3d states. The effective charge calculations show an average charge transfer of 0.52 electrons from Ti to Al in primitive cell calculations of TiAl alloy. On the other hand, calculations using supercell approach reveal an average charge transfer of 0.48 electrons from Ti to Al. The localization index calculations, of primitive cell as well as of supercell, show the presence of relatively localized states even above the Fermi level for this alloy. The calculated optical conductivity spectra of TiAl alloy are rich in structures, showing the highest peak at 5.73 eV for supercell calculations. Calculations of the imaginary part of the linear dielectric function show a prominent peak at 5.71 eV and a plateau in the range 1.1–3.5 eV.

Measurements of fluorine depth-profiles on TiAl turbine blades using ion beam analytical techniques

Intermetallic TiAl alloys are foreseen to substitute Ni-based alloys in several high-temperature applications such as turbine blades for aeronautics. Because of their low density the mass of these components could be reduced by half. However, a mixed oxide scale of TiO2 and Al2O3 which provides no oxidation protection is growing at temperatures above 700 °C. By means of the halogen-effect the high-temperature oxidation resistance of TiAl alloys can be improved by orders of magnitude. Therefore fluorine was introduced into turbine blades using two different chemical fluorination methods. The application of a fluorine treatment promotes the growth of a pure and dense alumina scale which prevents the alloy from increased oxidation. In previous work it has been shown that an appropriate fluorine content after oxidation and its location beneath the surface are indicators of a successful fluorine effect. In the present work, the fluorine content was measured before and after oxidation of TNB alloy as a function of depth by using proton induced gamma-ray emission (PIGE) in a specially designed vacuum chamber at the 2.5 MV van-de-Graaff accelerator at the IKF. Additionally, composition and thickness of the oxide scale was determined by Rutherford backscattering spectrometry (RBS). The ion beam techniques are non-destructive and thus offer a method for quality assurance of the halogen treatment.

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 TiAl 2 and TiAl 3 phases, which could induce the creation of an Al 2O 3 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 TiAl 3 and TiAl 2 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.