Helmut Clemens

Processing and applications of intermetallic γ-Tial-based alloys

Development and processing of high-temperature materials is the key to technological advancements in engineering areas where materials have to meet extreme requirements. Examples for such areas are the aerospace and spacecraft industry or the automotive industry. New structural materials have to be “stronger, stiffer, hotter, and lighter” to withstand the extremely demanding conditions in the next generation of aircraft engines, space vehicles, and automotive engines. Intermetallic γ-TiAl-based alloys show a great potential to fulfill these demands.

Designed fully lamellar microstructures in a γ-TiAl based alloy: adjustment and microstructural changes upon long-term isothermal exposure at 700 and 800 °C

Abstract The evolution of microstructure after annealing within the α phase field and subsequent cooling through the (α+γ) phase field has been investigated for a commercial Ti–46.5at.%Al–4at.% (Cr, Nb, Ta)-B alloy. Due to the presence of (Ti, Ta) borides, controlled fully lamellar microstructures can be adjusted exhibiting colony sizes smaller than 150 μm. In order to study the dependence of lamellar spacing on cooling rate, the latter was varied in the range of 1–200 K min −1 . The obtained microstructures reached from very fine lamellar structures with a Widmannstatten-type morphology at cooling rates higher than 200 K min −1 through undisturbed fully lamellar structures to a lamellar structure with primary γ-grains at colony boundaries at cooling rates below 4 K min −1 . It is demonstrated that the mean interface spacing of the Ti–46.5at.%Al–4at.% (Cr, Nb, Ta)-B alloy decreases with increasing cooling rate following an exponential expression with an exponent of −0.39. Since the adjustment of undisturbed fully lamellar microstructures with narrow lamellar spacing requires relatively high cooling rates, a condition far from thermodynamic equilibrium is obtained which provides one of the driving forces responsible for structural changes in lamellar microstructures upon exposure at elevated temperatures. It is shown that the α 2 volume fraction increases significantly with increasing cooling rate. The thermal stability during isothermal exposure to air at temperatures of 700 and 800xa0°C up to 3500 h was examined by means of optical microscopy, scanning and transmission electron microscopy.

Microstructural stability and creep behavior of a lamellar γ-TiAl based alloy with extremely fine lamellar spacing

Abstract Lamellar Ti–46Al–1.5Cr–2Mo–0.25Si–0.3B (in at.%) specimens with two different extremely fine lamellar spacings were produced and investigated. The average lamellar spacing was determined to be 200 and 35 nm, respectively. Creep tests at 700xa0°C have shown a distinct primary creep regime for both material conditions. After annealing for 24 h at 1000xa0°C the primary creep strain for both materials is significantly decreased. The steady-state creep for the specimens with the wider lamellar spacing appears to be similar to the creep behavior prior to annealing while the creep rate of the material with the previously smaller lamellar spacing is significantly higher. Optical microscopy and TEM-studies show that the microstructure of the specimens with the wider lamellar spacing is nearly unchanged, whereas the previously finer material was completely recrystallized to an equiaxed γ microstructure with a low creep resistance. The dissolution of the fine lamellar microstructure was also observed during creep tests at 800xa0°C as manifested in an acceleration of the creep rate.

On the origin of acoustic emission during room temperature compressive deformation of a γ-TiAl based alloy

Abstract In this study a two-phase γ-TiAl based alloy with near gamma microstructure was deformed in uniaxial compression at room temperature. The deformation experiments were carried out at a strain rate of 2×10−4 s−1. The acoustic emission during deformation was monitored continuously using a root mean square voltmeter. Different annealing experiments between deformation cycles combined with transmission electron microscopy investigations revealed the formation of mechanical twins as the decisive mechanism for the acoustic emission.

Creep behavior of γ -TiAl sheet material with differently spaced fully lamellar microstructures

Abstract Recent investigations have shown that the interface spacing in fully lamellar microstructures has a major influence on the creep behavior of γ-TiAl based alloys [Scr. Mater. 37 (1998) 1025; Scr. Mater. 35 (1996) 1391; Mater. Sci. Eng. A239–240 (1997) 419; Intermetallics 7 (1999) 171]. In order to study the dependence of interface spacing on creep, fully lamellar microstructures exhibiting different interface spacings but comparable colony size were adjusted in Ti–46.5 at.% Al–4 at.% (Cr, Nb, Ta, B) sheet material by appropriate heat treatments. Creep tests were conducted in a temperature range of 700–800xa0°C and stresses between 100 and 260 MPa. The results indicate that the primary creep strain as well as the minimum creep rate decreases with decreasing interface spacing. In addition, apparent activation energies and stress exponents were determined as a function of the interface spacing in order to describe the creep controlling mechanisms. A model assumption, which considers the limitation of the free dislocation path by stored dislocations as well as by geometrical obstacles was applied to explain the role of the interface spacing on primary creep strain and secondary creep rate.

Internal stress measurements by high-energy synchrotron X-ray diffraction at increased specimen-detector distance

High-energy X-ray diffraction has recently been shown to be a viable technique to measure volume-averaged lattice strains in the bulk of metallic polycrystals at increased speed compared to neutron diffraction. The established procedure is to irradiate the sample under investigation with monochromatic X-rays (∼100 keV) and to record complete diffraction rings with an area detector. The lattice strains are obtained by analyzing the minute distortions of these rings. In the present paper we present first results obtained using a setup in which two area detectors are positioned at a large distance (7 m) from the specimen. Although only segments of the rings can be recorded this way, this approach offers a number of advantages. In situ tensile tests were performed on a γ-TiAl-based alloy as an example to demonstrate the potential of the method. Both materials science aspects as well as consequences for further method development will be discussed.

On the role of twinning during room temperature deformation of γ-TiAl based alloys

The present work dealing with the micromechanical modeling of the deformation behavior of γ-TiAl based alloys is based on a strong interaction between numerical simulation facilities and detailed experimental investigations. Main attention is devoted to a near-γ microstructure which exhibits equiaxed γ-grains and α2-phase located at grain boundaries and triple junctions. The purpose is an investigation of the deformation mechanisms of polycrystalline near-γ material using both computational finite element simulations and acoustic emission (AE) measurements. A three-dimensional micromechanical model, initially developed to describe lamellar γ-TiAl, has been adjusted to simulate the deformation behavior of polycrystalline near-γ material. The contribution of deformation twinning to the total plastic deformation is computed investigating compression tests at room temperature. Concerning the onset of twinning the results of our simulation are compared with experimental data obtained from AE measurements. Investigations of the influence of various heat treatments on the twinning evolution during loading at room temperature were carried out. The deformation mechanisms were analyzed using optical microscopy, SEM and TEM.

Strain Rate Dependence of the Deformation Mechanisms in a Fully Lamellar γ-TiAl-Based Alloy

Abstract The intention of this study was to investigate the mechanical properties of an engineering γ-TiAl-based alloy with fully lamellar microstructure under quasistatic and dynamic compression at room temperature and strain rates ranging from 5.0 × 10-3 s−1 to 4.0 × 103 s−1. The microstructure of undeformed and deformed specimens was investigated by means of optical and electron microscopy. The results of the compression tests exhibit a slight strain rate sensitivity of the postyield stress. The fracture behavior of the studied γ-TiAl alloy strongly depends on the applied strain rates. Transmission electron microscopy observations conducted on deformed specimens revealed the existence of a high number of tangled ordinary dislocations, curved superdislocations and mechanical twins. The analysis of the deformation texture confirms the high activity of superdislocations and mechanical twinning during dynamic compression testing.

Eigenschaftsoptimiertes Warmumformen einer intermetallischen Titanaluminid-Legierung

ZusammenfassungDie 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.AbstractThe demand of advanced light-weight high-temperature materials with a low density and good specific high-temperature strength for the application in advanced combustion engines leads to the implementation of intermetallic titanium aluminides. These TiAl-based alloys are multi-phase alloys consisting of γ-TiAl, α2-Ti3Al and low volume fractions of βo-TiAl phase. In the present work a new processing route was established which comprises a combination of heat-treatment and hot-forging to improve the mechanical properties. The observed increase in strength can be attributed to a small lamellar spacing within the α2/γ-colonies. In order to analyze phase fractions and to determine mechanical properties X-ray diffraction measurements, hardness tests as well as tensile and creep tests were conducted.

Texture Evolution during Deformation of a Mo-Hf-C Alloy Studied with Electron Backscatter Diffraction

The powder metallurgically produced alloy MHC (Mo-Hf-C) has excellent properties at elevated temperatures, e.g. high strength, and has a high recrystallization temperature. In order to study the recrystallization behavior of MHC, a detailed knowledge of the texture evolution during deformation is important. Therefore, samples were deformed with a deformation dilatometer at different temperatures, with different true strains and at a constant strain rate. Afterwards the microstructure was analyzed with scanning electron microscopy and electron backscatter diffraction. The investigations showed that the volume fraction of <111> directions, related to the loading direction, increases strongly and the fraction of <001> directions increases less strongly with increasing true strain below 1350u2009℃. This happens at the cost of other textural components. Additionally, the results revealed that below 1350u2009℃ the fraction of the <111> directions is independent of the deformation temperature for a deformation with a true strain of φu2009u2009=u20090.92. This behavior changes for a deformation at temperatures above 1350u2009℃. The volume fraction of other textural components in relation to the deformation direction increases at costs of the <111> directions. Furthermore, it was revealed that this behavior is mainly controlled by the Hf-content in solid solution.ZusammenfassungDie pulvermetallurgisch hergestellte Molybdänbasislegierung MHC (Mo-Hf-C) ist bekannt für ihre exzellenten Eigenschaften bei hohen Temperaturen, wie z.xa0B. hohe Festigkeit, und weist eine hohe Rekristallisationstemperatur auf. Um das Rekristallisationsverhalten dieser Legierung gezielt studieren zu können, braucht man detaillierte Kenntnisse über die Texturentwicklung während der Umformung. Hierfür wurden Proben mit Hilfe eines Umformdilatometers bei verschiedenen Temperaturen mit verschiedenen Umformgraden und einer konstanten Umformgeschwindigkeit verformt. Die anschließende Charakterisierung der Mikrostruktur erfolgte mit einem Rasterelektronenmikroskop mittels Elektronenrückstreubeugung. Die Untersuchungen ergaben, dass unter 1350u2009℃ mit steigendem Umformgrad die Volumsfraktion der <111> Richtungen, bezogen auf die Stauchrichtung, stark zunimmt, während der Anteil der <001> Richtungen nur schwach zunimmt. Dies geschieht auf Kosten aller anderen Texturkomponenten. Zusätzlich wurde für diesen Temperaturbereich aufgezeigt, dass der Anteil der <111> Richtungen bei einer Umformung mit φu2009=u20090,92 temperaturunabhängig ist. Dieser Mechanismus ändert sich bei einer Umformung über 1350u2009℃. Hier steigt die Volumsfraktion aller anderen Texturkomponenten auf Kosten der <111> Richtungen. Dieses Verhalten ist hauptsächlich vom gelösten Hf-Gehalt abhängig.