Development of a turbulence-free casting technique for titanium aluminides

As part of the EC-funded IMPRESS project, a novel technique has been developed for casting titanium aluminides which combines clean melting under argon in an Induction Skull Melting furnace with tranquil tilt pouring into a pre-heated ceramic shell mould. This has enabled good quality castings to be produced which are free from the entrained bubble defects which are routinely found when turbulent mould filling processes such as gravity casting are used. Examples are given of the use of this technique to cast blades up to 40 cm long.     

Numerical study of crucial parameters in tilt casting for titanium aluminides

Numerical modeling of the tilt casting process for TiAl alloys was investigated to achieve a tranquil mould filling and TiAl castings free of defects. Titanium alloys are very reactive in molten state, so they are widely melted in cold crucible, e.g. the Induction Skull Melting (ISM) furnace. Then the crucible holding the molten metal together with the mould is rotated to transfer the metal into the mould——ISM+ tilt casting. This paper emphasizes the effect of crucial parameters on mould filling and solidif...     

Laser processing of titanium aluminides

Using a Nd-YAG laser, laser processing of a series of Ti-Al alloys including pure Ti and Ti-Al intermetallic compounds has been studied. Scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), and optical microscopy were used to determine the surface morphological, chemical, and compositional characteristics of the laser-processed samples. Analysis of results showed that cracks along grain boundaries caused by rapid heating and cooling of laser processing were the dominant characteristics of the surface morphologies of the laser-processed samples. The Al content in the Ti-Al alloys played a very important role in crack initiation and/or development. The more Al content in the samples, the more severe the cracks that developed after laser processing under the same conditions. The experiments were conducted at ambient conditions, resulting in surface oxidation layers being observed on the processed samples. The XPS results indicated that the oxidation layer consisted of adsorbed O₂, Al₂O₃, TiO₂, and TiO. In addition, Al enrichment was found in the oxide film of TiAl as well as in the oxidation layers formed on the surfaces of TiAl and Ti₃Al intermetallics that were processed by the laser; this differs from the reported results for traditional oxidation of TiAl at elevated temperature.     

The mechanical alloying of titanium aluminides

This article reviews the mechanical alloying of titanium aluminides carried out in the past decades. Research has proven that mechanical alloying is an efficient means to synthesize nano-structured and non-equilibrium titanium aluminides. Although fine-grained structures have been successfully produced, effort is still needed to reduce contamination and to consolidate nano-structural powders.     

Hot Salt Corrosion of titanium aluminides

There is considerable interest in the use of γ-TiAl within advanced gas turbines where they offer greater high temperature capability over conventional titanium at reduced weight. These factors would enable engines with greater thrust to weight ratio to be developed. Thus γ-TiAl offers the potential of replacing nickel based alloys within the high pressure compressor and potentially the fourth stage turbine. Service in both of these locations would require that the γ-TiAl be resistant, not only to oxidation, but hot salt corrosion. This paper presents a study of the hot salt corrosion resistance of γ-TiAl over the temperature range 500–700°C. At 700°C, laboratory tests have shown that corrosion rates in a salt ladened environment are some 20X that of equivalent oxidation for exposures out to 100 h. The morphology of attack is consistent with a corrosion mechanism involving the vapour phase transport of aluminium from within the alloy and formation of a non protective oxide scale. A model for the corrosion mechanisms is presented, involving intermediate chloride phases. The mechanism is believed to be self sustaining, requires little chloride present, and leads to the observed accelerated oxidation rates.     

Hydrogen permeation in gamma titanium aluminides

The permeation of hydrogen in gamma titanium aluminides was studied using the Devanathan–Stachurski (DS) cell. Thin disk-shaped samples of gamma titanium aluminide with three different microstructures were appropriately prepared and cathodically charged in an aqueous 0.1 N NaOH solution. The permeation current was monitored as a function of time. Permeation parameters were calculated using the time lag criterion (non-steady state lag). Values of the apparent diffusion coefficient of hydrogen in gamma titanium aluminide varying from 1.87 × 10⁻⁷ cm²/s to 3.75 × 10⁻⁶ cm²/s were obtained. This slight variation is attributed to differences in microstructure.     

Protection of titanium aluminides by FeCrAlY Coatings

The effectiveness of Fe+15%Cr+5%Al+0.3%Y coatings. produced by sputter ion plating, in improving the oxidation behaviour of γ-2 Ti₃Al at 800°C and γ-TiAl at 900° and 1000°C. during cyclic exposures in air of up to 1000 hours duration, has been studied. The 45-135 μm coatings were tested in both the as-coated condition and following densification by peening. The kinetics of attack and of spallation were followed gravimetrically, while the reaction product compositions were examined by a range of surface analytical techniques. Oxidation protection was afforded by the formation on the coating surface of a γ-Al₂O₃ oxide scale but chemical reactions at the coating-substrate interface with accompanying voidage development, eventually caused the coating to fail mechanically. To surmount this problem the potential of several ceramic interlayers produced by the same coating route, and interposed as diffusion barriers between the FeCrAlY overlayers and the Ti₃Al substrate was examined in 1000 hour oxidation tests at 800°C.     

The melt spinning of gamma titanium aluminides

Rapid solidification shows great promise for the production of gamma titanium aluminides, and one of the most interesting rapid solidification techniques is melt spinning. In this work, melt-spun Ti-5SAl and Ti-48Al-2Cr-2Ta (atomic percent) were produced; the resulting alloys had uniform microstructure and composition. In addition, the investigation showed the utility of a model developed to describe heat transfer and fluid flow in melt spinning.     

The effects of hydrogen on titanium aluminides

Advanced aerospace systems such as the national aerospace plane are slated to use hydrogen as a fuel. Thus, caution must be exercised in the use of materials which can exhibit degradation in mechanical properties on exposure to hydrogen. Although recent work shows that hydrogen has adverse effects on titanium aluminides, it can also be used to advan tage to improve processing and refine microstructure using the thermochemical processing technique.     

Quasi-static chip formation of intermetallic titanium aluminides

As a result of the development of new materials for high temperature applications the potential for mass reduction and increased process temperatures is constantly being expanded. Intermetallic γ-TiAl alloys can meet these demands to a large extent. The properties necessary for these applications have an adverse effect on the machinability however and render intermetallic titanium aluminides as difficult to machine materials. Cutting operations tend to produce damaged surfaces which are unsuitable for the intended applications. As the basis for a reliable and economic cutting technology, the chip formation of the intermetallic TiAl alloy TNBV5 has been examined in quasi-static cutting experiments. Observations showed that increased workpiece temperatures lead to a transition of the chip formation from segmented to continuous chips. By decreasing the undeformed chip thickness crack-free surfaces could be produced at low workpiece temperatures. In this case other mechanisms than the thermal activation of slip systems must be the reason for the observed large plastic deformations. The theory that hydrostatic pressure leads to this behavior is substantiated by the results of finite element simulations. This offers the possibility for damage free machining at lower cutting speeds, thus enabling the use of conventional tool materials at an acceptable tool life.     

Gamma titanium aluminides as prospective structural materials

This status report focuses on specific products of two γ-TiAl alloys that are advancing toward structural applications for 550–750°C service in advanced turbine engines. These are low-pressure turbine blade, transition-duct beam and radial diffuser castings for engine components, and corner-beam and closeout beam castings for the outlet-nozzle of a very large engine. Also included are current development of sheet corrugations for formed subcomponents and the perfection of cast turbine wheels for automotive turbochargers. In the current implementation stage, alloy composition, desired process and component definition are the important introductory issues. Then, the engineering technology that must be developed is discussed for the desired final product and a match of cost and benefit. Balancing better performance with the acceptance constraints is the key. Cost is a major constraint along with real and perceived risk. Within five to ten years, systematic reduction of certain hardware costs will occur as familiarity builds and enters into the production stage.     

The oxidation and protection of gamma titanium aluminides

The excellent density-specific properties of the gamma class of titanium aluminides make them attractive for intermediate-temperature (600–850 °C) aerospace applications. The oxidation and embrittlement resistance of these alloys is superior to that of the α₂ and orthorhombic classes of titanium aluminides. However, since gamma alloys form an intermixed Al₂O₃TiO₂ scale in air rather than the desired continuous Al₂O₃ scale, oxidation resistance is inadequate at the high end of this temperature range (i.e., greater than 750–800°C). For applications at such temperatures, an oxidation-resistant coating will be needed; however, a major drawback of the oxidation-resistant coatings currently available is severe degradation in fatigue life by the coating. A new class of oxidation-resistant coatings based in the Ti-Al-Cr system offers the potential for improved fatigue life.     

Progress in the understanding of gamma titanium aluminides

During the last two years remarkable improvements have been made in the properties (such as toughness and creep) and processing technology of gamma titanium aluminides, making them potentially viable engineering alloys for high-temperature structural applications. These achievements were made possible by a greater understanding of both the fundamental and the practical aspects of these aluminides, such as phase relationships, the effects of alloying elements, deformation mechanisms, microstructure evolution and processing. This article reviews the current understanding of the above specific aspects and the processing-microstructure-property relationships, and identifies pacing problems and applications.     

Direct production of alloys based on titanium aluminides

Direct production of alloys based on titanium aluminides has been demonstrated via two-step aluminothermic reduction of titanium tetrachloride (TiCl₄). In the first step, TiCl₄ is reduced by aluminium to titanium trichloride (TiCl₃) at a temperature below the boiling point of TiCl₄ and at 1 atm under argon. The resulting TiCl₃ intermediate product is then reacted with aluminium at temperatures up to around 1000 °C, leading to synthesis of a Ti–Al alloy powder. In addition, alloying additives such as Nb, Cr, Mo and C have also been included in various concentrations.     

Tracer diffusion of niobium and titanium in binary and ternary titanium aluminides

This study presents recent results on titanium (Ti) and niobium (Nb) diffusion in binary (α₂-Ti₃Al and γTiAl) and ternary high-Nb-containing (γTiAl-10Nb) Ti aluminides. Three different techniques were used to measure self-diffusion and solute (Nb) diffusion. The radiotracer technique combined with mechanical sectioning by grinding was applied at higher temperatures, whereas ion-beam sputtering or secondary ion mass spectrometry was used to analyze the short penetration profiles at lower temperatures. The results measured by these different techniques for different penetration depths are very consistent. Nb is a slower diffuser compared to Ti in the ternary as well as in the binary Ti aluminides. The heavy alloying of γTiAl with 10at.% Nb, however, enhances significantly both the Ti and the Nb diffusivities. The diffusion mechanisms for Ti and Nb in ordered Ti aluminides are discussed.     

Oxidation behavior of titanium aluminides of high niobium content

The oxidation behaviour of two titanium aluminides of composition Ti–24Al–20Nb and Ti–24Al–27Nb were studied in oxygen in the range 1125–1325 K. The parabolic rate constants of the alloys in the present investigation were higher than that of titanium aluminides of lower Nb content [Ti–24Al–11Nb and Ti–24Al–15Nb (Roy TK, Balasubramaniam R, Ghosh A. Metall Mater Trans A 1996;27A:3993)]. The major constituent of the oxides was TiO₂ while Nb₂O₅ and Al₂O₃ were the minor constituents. The formation of Nb₂O₅ as a separate phase is possibly related to the lower oxidation resistance of these alloys compared to titanium aluminides of lower Nb content.     

Gamma titanium aluminides: Their status and future

Gamma alloys, based on the gamma titanium aluminide (y-TiAl) intermetallic compound, are emerging as a revolutionary engineering material for high-temperature structural applications. This article discusses the historical background as well as the status and future prospects of gamma alloy technology in the areas of alloy development/ design, process development, and applications.     

Severe plastic deformation and hydrogenation of the titanium aluminides

Severe plastic deformation (mechanical activation in a hydrogen atmosphere and shear under pressure) effects on the hydrogenation of two titanium aluminides Ti₃Al and TiAl-type alloy (B2) have been studied. It is shown that a conventional hydrogenation of the bulk samples allows forming the hydrides with high hydrogen content and high desorption temperature: TiAl (773 K, 1.76 mass%) and Ti₃Al (1043 K, 2.94 mass%). In comparison with the conventional hydrogenation, the mechanical activation in a hydrogen atmosphere at room temperature allows one to obtain the hydrides of the TiAl (B2) and Ti₃Al alloys with the reduced desorption temperature: TiAl (453 K, 1.96 mass%) and Ti₃Al (531 K, 2.6 mass%). The shear under pressure has been applied to the sample before hydrogenation also leads to a reduction of the desorption temperature; however, it produces the phase transformation in the Ti₃Al intermetallic compounds, which lowers the observed a maximum hydrogen content.