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Eigenschaftsoptimiertes Warmumformen einer intermetallischen Titanaluminid-Legierung
Abstract
Die steigenden Anforderungen an Werkstoffe in Verbrennungsmotoren führen zum Einsatz innovativer Hochtemperaturleichtbauwerkstoffe mit geringer Dichte und hoher spezifischer Festigkeit bei hohen Temperaturen. Dieser Werkstoffklasse gehören auch intermetallische Titanaluminide an. Es handelt sich dabei um mehrphasige TiAl-Legierungen, deren komplexer Aufbau aus γ-TiAl, α2-Ti3Al und einem geringen Anteil an βo-TiAl Phase besteht. Durch gezielte Kombination von Wärmebehandlung und Warmumformung werden die mechanischen Eigenschaften optimiert, was vor allem auf den geringeren lamellaren Abstand innerhalb der α2/γ-Kolonien zurückzuführen ist. Die Untersuchungen der mechanischen Kennwerte aus Warmzug- und Kriechversuchen weisen auf das hohe Potential der intermetallischen Titanaluminide hin.
The 4th generation of intermetallic γ-TiAl based alloys, the so-called TNM alloy with its nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at.%), gained attention especially as high temperature lightweight structural materials. The scope of this study is to investigate the processability of a TNM-based powder with an Al content of 45 at.% by laser powder bed fusion, characterize the microstructural features and develop a tailored heat treatment for the as-built material to adjust a microstructure with balanced mechanical properties. The slightly increased Al content was chosen to counteract an Al loss during the process. Therefore, the chemistry of the powder was investigated after each building job, especially in terms of Al evaporation and O impurities. Based on an extensive parameter study with different preheating temperatures of the base plate, crack-free samples with relative densities of 99.9% were built. It could be shown that the process parameters have a direct influence on the microstructure in terms of phase composition, grain size, and lamellar spacing. The results of the post-processing heat treatment study enable the definition of parameters for adjusting an isotropic nearly lamellar γ or a fully lamellar microstructure, which corresponds to the respective microstructure obtained using conventional processing routes.
Tension specimens of a γ-TiAl based TNM-B1 alloy (nominal composition in at.%: Ti-43.5Al-4Nb-1Mo-0.1B) were coated with oxidation protective intermetallic Ti-Al-Cr layers containing yttrium and zirconium. The coated specimens were tensile tested in air at room temperature (RT) and 850 °C in the as-deposited condition and after a pre-oxidation treatment at 850 °C for 300 h in air. For comparison, uncoated specimens with and without pre-oxidation were also tested. The elongation after fracture of the TNM-B1 alloy was 0.23% at RT; it increased to 8.3% at 850 °C. The as-deposited coatings spalled off during tensile testing at RT. No coating spallation was observed at elevated temperature and after pre-oxidation due to improved adhesion by interdiffusion bonding. Pre-oxidation deteriorated the RT tensile properties of the γ-TiAl based alloy. The RT ductility determined for pre-oxidized coated specimens was even more reduced probably caused by the brittleness of the coatings initiating premature cracks. When tensile tested at 850 °C, the coated specimens exhibited considerably enhanced elongation after fracture in both the as-deposited and pre-oxidized conditions in comparison to the bare substrate alloy. The increase in ductility was attributed to the high environmental protection of the substrate by the coatings.
The Ti–46Al–6Nb (mole fraction, %) ingots that were directionally solidified by cold crucible were cyclic heat treated at 1330 °C in the α phase region. The microstructures and mechanical properties of the ingots before and after heat treatment were investigated. The results show that the large columnar grains are changed into equiaxed grains after heat treatment. The grain size decreases with increasing the cyclic times, which is caused by the recrystallization and the transition from the large grain of small lamellae to the small grain of large lamellae. Four times of cyclic heat treatment refines the grain size from 1.33 mm to 0.59 mm, nevertheless the lamellar spacing increases from 0.71 μm to 1.38 μm. Extending the holding time and increasing the cyclic times of heat treatment eliminate the β-segregation at the grain boundary and the interlamellar. The compression testing shows that the compressive strength of the directionally solidified ingot in the parallel and perpendicular directions are 1385.09 MPa and 1267.79 MPa, respectively, which are improved to 1449.75 MPa and 1527.76 MPa after two and four times of cyclic heat treatment, respectively, while that is 1180.64 MPa for the as-cast sample. The fracture mode of the sample after cyclic heat treatment is quasi-cleavage fracture.
Advanced intermetallic multi-phase γ-TiAl based alloys, such as TNM alloys with a nominal composition of Ti–43.5Al–4Nb–1Mo–0.1B (in at.%), are potential candidates to replace heavy Ni-base superalloys in the next generation of aircraft and automotive combustion engines. Aimed components are turbine blades and turbocharger turbine wheels. Concerning the cost factor arising during processing, which – additionally to material costs – significantly influences the final price of the desired components, new processing solutions regarding low-cost and highly reliable production processes are needed. This fundamental study targets the replacement of hot-working, i.e. forging, for the production of turbine blades. But without forging no grain refinement takes place by means of a recrystallization process because of the lack of stored lattice defects. Therefore, new heat treatment concepts have to be considered for obtaining final microstructures with balanced mechanical properties in respect to sufficient tensile ductility at room temperature as well as high creep strength at elevated temperatures. This work deals with the adjustment of microstructures in a cast and heat-treated TNM alloy solely by exploiting effects of phase transformations and chemical driving forces due to phase imbalances between different heat treatment steps and compares the mechanical properties to those obtained for forged and heat-treated material.
A robust processing route at low cost is an essential requirement for high-temperature materials used in automotive engines. Because of their excellent high-temperature properties, their low density, high elastic modulus as well as high specific strength, intermetallic γ-TiAl based alloys are potential candidates for application in advanced automotive turbochargers. So-called 3rd generation alloys, such as TNM™ alloys with a nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at%), are multi-phase alloys consisting of γ-TiAl, α2-Ti3Al and a low volume fraction of βo-TiAl phase. In this paper a novel hot-processing route, which is a combination of a one-shot hot-forging step and a controlled cooling treatment, leads to mechanical properties required for turbocharger turbine wheels. The observed strength can be attributed to the small lamellar spacing within the α2/γ colonies of the nearly lamellar microstructure. In order to analyze the microstructure and the prevailing phase fractions microscopic examinations and X-ray diffraction measurements were conducted. The mechanical properties were determined by hardness measurements as well as tensile and creep tests. The evolution of the microstructure during the hot-forming process is described and its relation to the obtained mechanical properties.
After 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.
Neue innovative Hochtemperaturleichtbauwerkstoffe, die eine geringe Dichte und hohe spezifische Festigkeit aufweisen, kommen aufgrund von Effizienzsteigerung in der Turboladereinheit moderner Verbrennungsmotoren zum Einsatz. Die hier untersuchte intermetallische Titanaluminid-Legierung, die sogenannte TNMTM-Legierung, ist mehrphasig aufgebaut, bestehend aus Kolonien mit abwechselnden g-TiAl und a2-Ti3Al Lamellen, umrandet von einem Saum aus bo-TiAl-Phase. Im Rahmen dieser Arbeit wurden die wichtigsten thermophysikalischen Eigenschaften der TNMTM-Legierung experimentell ermittelt und mit den Kennwerten anderer Werkstoffe verglichen. Des Weiteren wird das Gefüge nach einem Warmumformprozess sowie nach einem Kriechversuch beschrieben. Das Potential für eine Turboladeranwendung wird durch abschließende statische Zugversuche demonstriert.
Development and processing of high-temperature materials is the key to technological progress in engineering areas where materials have to meet extreme requirements. Examples for such areas are the aerospace and automotive industries. New structural materials have to be stronger, stiffer and lighter to withstand the extremely demanding conditions in the next generation of aero- and automotive engines. Intermetallic -TiAl based alloys exhibit numerous attractive properties which meet these demands. These properties include high melting point, low density, high specific elastic modulus, good oxidation and burn resistance, and high specific strength up to application temperatures of 700 to 800°C. Thus, current -TiAl based alloys outperform advanced Ti-based alloys and have the potential to replace heavy Ni-based superalloys.
A set of general guidelines for structure refinement using the Rietveld (whole-profile) method has been formulated by the International Union of Crystallography Commission on Powder Diffraction. The practical rather than the theoretical aspects of each step in a typical Rietveld refinement are discussed with a view to guiding newcomers in the field. The focus is on X-ray powder diffraction data collected on a laboratory instrument, but features specific to data from neutron (both constant-wavelength and time-of-flight) and synchrotron radiation sources are also addressed. The topics covered include (i) data collection, (ii) background contribution, (iii) peak-shape function, (iv) refinement of profile parameters, (v) Fourier analysis with powder diffraction data, (vi) refinement of structural parameters, (vii) use of geometric restraints, (viii) calculation of e.s.d.'s, (ix) interpretation of R values and (x) some common problems and possible solutions.
Titanium aluminides have proven their suitability for automotive applications. This paper evaluates the technical benefits of titanium aluminides for rotating and oscillating components based on first engine tests. These promising results and the limited availability of high quality titanium aluminide components in acceptable quantities and at reasonable costs have led to extensive research efforts. This paper summarizes the current state of titanium aluminides with regard to material properties, production process and quality of cast prototypes, as well as cost. Critical topics and the demands for future research and development will be outlined. These topics will be elaborated on the basis of the potential use of titanium aluminides for valves and turbocharger wheels. Besides these components, possible other applications in automotive combustion engines will be presented. Finally, the strategy of promoting titanium aluminides into series production will be discussed.
Kurzfassung
Der vorliegende Artikel versteht sich als Fortsetzung des Beitrages “Intermetallische γ-Titanaluminide aus metallographischer Sicht” von H. Clemens und F. Jeglitsch in Prakt. Metallogr. 37 (2000) 194-217. Dieser zweite Teil enthält neben der Motivation zur Entwicklung dieser Werkstoffklasse Angaben zu Eigenschaften und Einsatzmöglichkeiten. Überdies wird die Zusammensetzung moderner mehrphasiger Legierungen beschrieben und auf ihre Verarbeitung eingegangen. In diesem Zusammenhang wird die Entwicklungsstrategie der derzeit aktuellen γ-TiAl-Basislegierungen vorgestellt. In der Folge wird die Mikro- und Nanostruktur nach Herstellung, Verarbeitung und mehrstufiger Wärmebehandlung erörtert. Zu jedem dieser Punkte werden Fallbeispiele aus metallographischen Untersuchungen gezeigt, die unter Verwendung metallkundlicher und metallphysikalischer Ansätze interpretiert werden.
aluminide in industrial applications, a quick and affordable method of quantitatively analysing their microstructures is required In the presented work it was able to demonstrate on a powder metallurgical manufactured TNM TM alloy of nominal composition Ti-43.5Al-4Nb- 1Mo-0.1B (at.%), that by electrolytic-polishing and colour etching a quick and cost effective quantitative microstructural analysis may be carried out via light-optical microscopic images In doing so, the phase fractions and microstructural constituents of the various types of microstructures present are determined using complementary analysing techniques. Both light-optical and scanning electron microscopic images were captured from each of three different types of microstructures These were then quantitatively evaluated using an image analysis program. The results were compared with those obtained from X-ray diffraction experiments The possibilities and limits of the quantitative phase evaluation of light-optical microscopic images of colour etched microstructures are also explained and their relationship to the choice of parameters used for the colour etching and electro-polishing operations discussed.
Because of the small "deformation window" hot-working of γ-TiAl alloys is a complex and difficult task and, therefore, isothermal forming processes are favoured. In order to increase the deformation window a novel Nb and Mo containing γ-TiAl based alloy (TNM™ alloy) was developed. Due to a high volume fraction of β-phase at elevated temperatures the alloy can be hot-die forged under near conventional conditions, which means that conventional forging equipment with minor and inexpensive modifications can be used. With subsequent heat-treatments balanced mechanical properties can be achieved. This paper summarizes our progress in establishing a "near conventional" forging route for the fabrication of γ-TiAl components. The results of lab scale compression tests and forging trials on an industrial scale are included. In addition, the mechanical properties of forged and heat-treated TNM™ material are presented.
Intermetallic γ-TiAl based alloys of the TNM™ alloy family attain their excellent processing characteristics by a high β-phase content present at hot-working temperatures. Subsequent to hot-working the β-phase content is decreased by a heat treatment step performed at temperatures where the β-phase fraction exhibits a minimum. In this study, in- and ex-situ experiments were conducted on three alloys with different contents of β/β0 stabilizing elements. The course of phase fractions as a function of temperature as well as phase transition temperatures were determined by means of in-situ high-energy X-ray diffraction experiments. Additionally, dynamic scanning calorimetry investigations were performed to obtain complementary data on the transition temperatures. Quantitative metallography was conducted on heat treated and quenched specimens to acquire additional information on the dependence of the phase fractions on temperature. By neutron diffraction experiments the ordering temperatures of the constituent phases were determined. It was shown that the experiments yielded consistent results which differ significantly from ThermoCalc simulations for which a commercial TiAl database was used. The differences between the experimental results and the thermodynamic predictions are discussed.
Attempts to utilize gamma Ti aluminide in non-aerospace applications have centered on components for which weight reduction would improve overall equipment performance, namely exhaust valves and turbocharger turbines for automotive engines. As a result of extensive research that has recently been conducted on materials, manufacturing processes, and application tests for those two types of components, both have reached the point of practical utilization. However, the development of low-cost manufacturing processes is still needed for expanded practical application in the future.
Conventional high Nb bearing γ-TiAl based alloys exhibit a strong tendency to segregations because of their peritectic solidification path which cause severe problems regarding the hot-working behaviour. Thermodynamic calculations based on the CALPHAD method were applied to define a β-solidifying TiAl based alloy, which exhibits an adjustable β/B2-phase volume fraction at temperatures where hot-work processes are performed. Therefore, this alloy can be forged under near conventional conditions.
Verringerung der Schadstoff-emissionen von leichten Kraftfahrzeugen
- Europäische Union
Europäische Union Gesetzgebung, Verringerung der Schadstoff-emissionen von leichten Kraftfahrzeugen, http://europa.eu/legisla-tion_summaries/environment/air_pollution/index_de.htm (1. Juni 2012)
Ergebnisse für Wirtschaftsbereiche und private Haushalte, www.statistik.at (29
- Luftemissionsrechnung Austria
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Panel on Climate Change - Aviation and the Global Atmosphere
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