Arno Bartels

HIGH-ENERGY X-RAYS: A TOOL FOR ADVANCED BULK INVESTIGATIONS IN MATERIALS SCIENCE AND PHYSICS

The combination of these techniques is a strong issue for the construction and development of future instruments.

General aspects of the thermomechanical

This article represents a survey on the thermomechanical treatment (TMT) of two-phase intermetallic TiAl compounds. Nearly all of the important parameters, such as temperature, cooling rate, forging ratio, and direction, of the TMT steps were examined. The aim of this investigation was to understand the effects of the different parameters and thus to provide the knowledge of how to achieve all desired types of microstructures depending on the application purpose. Along the way, it was necessary to overcome the brittleness of TiAl compounds in order to permit further working and deformation processes, such as near net shape forming and sheet rolling.We obtained the best ductility with homogeneous equiaxed microstructures with true plastic X tensile strains at room temperature of more than 6 pct for Ti-48Al-2Cr (atomic percent). In the case of equiaxed microstructures, the Hall-Petch relationship could be confirmed for both the yield and the fracture stress. Therefore, the microstructures causing highest strength and high ductility and the way to achieve these properties were elaborated and are discussed in detail here.

Experimental studies and thermodynamic simulation of phase transformations in high Nb containing γ-TiAl based alloys

Abstract Solid-state phase transformations and phase transition temperatures in Ti-45 at.% Al and Ti-45 at.% Al-(5, 7.5, 10) at.% Nb alloys were analyzed experimentally and compared to thermodynamic calculations. Results from scanning electron microscopy, high-energy and conventional X-ray diffraction as well as differential scanning calorimetry were used for the characterization of the prevailing phases and phase transformations. For the prediction of phase stabilities and phase transition temperatures, thermodynamic calculations using the CALPHAD method were conducted. In order to achieve better agreement between calculated and experimental results, a commercially available database was modified using our own results from thermo-physical measurements and annealing treatments.

Optimizing the properties of TiAl sheet material for application in heat protection shields or propulsion systems

Abstract The present work represents a survey on the production of TiAl sheet material on an industrial scale and on the properties of these sheets. Using different final heat treatments, near-gamma and duplex microstructures have been produced. The mechanical properties as a function of the microstructure have been examined at room temperature (RT) and 700 °C. Additionally, the parameter field of temperature and strain rate for which superplastic forming of the sheet material is possible has been established. Mechanical testing at RT revealed that the strength of Ti-48Al-2Cr sheet material is mainly governed by grain size if the microstructure is homogeneous. The volume fraction and distribution of the second phase (α2-Ti3Al) appears to be of minor influence. Room temperature ductilities between 0.25 and 0.051 have been measured. At 700 °C the duplex microstructures show slightly improved strength compared to near-gamma microstructures. Depending on the deformation rate, fracture strains between 0.11 and 0.80 have been detected. Superplastic behaviour of Ti-48Al-2Cr (at.%) is governed by dynamic recrystallization or dynamic grain growth, depending on the initial grain size and the deformation parameters temperature and strain rate.

Micromechanical mechanism of texture formation in γ-TiAl

Abstract Texture evolution in TiAl depends on the type of deformation such as uniaxial compression, tension or rolling as well as on the initial microstructure. Cast TiAl shows a lamellar microstructure with an alignment of the lamellae and exhibits a strong texture. Deformation parallel to the aligned lamellae results in a plane-strain deformation which leads to a brass type texture. From room temperature up to 700 °C, compression tests of TiAl specimens with equiaxed microstructures result in -fiber textures where the component is shifted to . At higher temperatures the orientation is caused by dynamic recrystallization. In rolled sheets of TiAl with equiaxed microstructure the main texture component is a modified cube texture {100} where the tetragonal c-axis is aligned in transverse direction of the sheet. This texture component is caused by dynamic recrystallization and appears to be stable against further rolling.

Charakterisierung einer β-erstarrenden γ-TiAl-Basislegierung

ZusammenfassungHerkömmliche hoch Nb-haltige Titanaluminid-Legierungen zeigen auf Grund ihrer peritektischen Erstarrung relativ starke Seigerungen im Gusszustand als auch nach dem Strangpressen. Damit verbunden sind auch lokale Schwankungen in der Mikrostruktur, die zu nicht reproduzierbaren mechanischen Eigenschaften führen. Die Legierung Ti-43Al-4Nb-1 Mo-0,1B (At. %) zeigt im Gusszustand ein homogenes und feines Gefüge, was auf die Erstarrung über die kubische β-Phase zurückzuführen ist. Durch zusätzliche Wärmebehandlungen konnte gezeigt werden, dass unterschiedliche Gefügemodifikationen und ein noch feineres und homogeneres Gefüge einstellbar sind. Durch Zugversuche bei Raumtemperatur, 700°C und 800°C konnte gezeigt werden, dass die Ti-43Al-4Nb-1 Mo-0,1B-Legierung höhere Festigkeiten und plastische Dehnungen aufweist als herkömmliche hoch Nb-haltige Legierungen.AbstractConventional high Nb bearing γ-TiAl based alloys exhibit relatively strong tendency to segregations because of their peritectic solidification path. This leads to local inhomogeneities of the microstructure which is combined by non-reproducible mechanical properties. The β-solidified Ti-43Al-4Nb-1Mo-0.1B alloy (composition in atomic percent) shows a more homogeneous and fine microstructure in the cast and extruded state. With several heat treatments different modifications of the microstructure were adjusted and a reduction of the mean grain size was achieved. In tensile tests at room temperature, 700°C and 800°C the alloy Ti-43Al-4Nb-1Mo-0.1B shows higher strength and elongations than conventional high Nb containing titanium aluminides.

Microstructure and properties of Ti-48Al-2Cr after thermomechanical treatment

Abstract Ingots of Ti-48Al-2Cr exhibit a strong casting texture with chemical as well as structural inhomogeneities resulting in low ductility and anisotropic formability. To improve these properties, a thermomechanical treatment is required. A two-pass upset forging method to achieve the required fine-grained equiaxed microstructure is presented. The possibilities of influencing the grain size and the corresponding mechanical properties by variation of the time and temperature in the final heat treatment are shown. The yield stress at room temperature, as given by the Hall-Petch relationship, can be controlled in the range between 360 and 495 MPa by producing grain sizes between 30 μm and 2.7 μm respectively. Tensile and compression tests were performed as a function of temperature. Typical characteristics of this material are the negative strain rate sensitivity between 200 °C and 500 °C with corresponding strain aging effects.

Herstellung lamellarer Gefügetypen in intermetallischen TiAl- Legierungen und deren Charakterisierung

Kurzfassung Im Rahmen dieser Arbeit wurden zwei lamellare Gefügetypen in γ-TiAl-Basislegierungen untersucht. Zum einen erfolgte die Herstellung der voll lamellaren Gefüge mittels einer Glühung im α-Einphasengebiet und anschließend langsamer Abkühlung. Das resultierende Gefüge besteht aus lamellaren γ-TiAl/α2-Ti3Al-Kolonien, wobei diese die ehemaligen α-Körner darstellen. Innerhalb einer Kolonie sind die γ/α2-Lamellen parallel angeordnet und weisen eine bestimmte kristallographische Orientierungsbeziehung auf. Zum anderen konnte mit Hilfe der Massivtransformation und anschließender Auslagerungsglühung eine neuartige lamellare Mikrostruktur eingestellt werden, die auf konventionellem Weg nicht realisierbar ist. Dabei scheiden sich aus dem massiv transformierten γm feine α2-Ti3Al-Lamellen aus, die in einem Winkel von 70,5° zueinander orientiert sind. Neben einer ausführlichen Beschreibung der Mikrostruktur werden die Orientierungsbeziehungen beider lamellarer Gefügetypen während der Entstehung beschrieben und die Keimbildung der γ-Lamellen in α-Körnern mittels Transmissionselektronenmikroskopie (TEM) untersucht.

Properties of two-phase intermetallic (Ti, Nb)3 (Al, Si) + (Ti, Nb)5 (Si, Al)3 P/M bulk and sheet material

Abstract Intermetallic Ti-based alloy powder with nominal composition Ti 23.5Al 5.5Si 10Nb (at.%) was produced by argon gas atomization. This dual phase alloy is composed of the α 2 -(Ti, Nb) 3 (Al, Si)-matrix phase and a eutectic mixture of α 2 -(Ti, Nb) 3 (Al, Si) + ξ-(Ti, Nb) 5 (Si, Al) 3 . The powder was compacted by hot isostatic pressing, subsequently hot rolled into sheets, and heat treated. The microstructures of the HIPed compacts and of the sheet materials were metallographically characterized and correlated with tensile properties at temperatures between 20 and 1000°C. Due to networks of the brittle ξ-silicide phase, the HIPed state showed no ductility at 20°C. Break-down of the silicide network during rolling led to an improved ductility at 20°C. After post rolling annealing, the yield strengths at 20 and 700°C were between 750-490 MPa and 700-420 MPa for specimens taken parallel and perpendicular to the rolling direction, respectively. These values are considerably higher than those reported for γ-TiAl sheets. At 1000°C the sheet material could be superplastically deformed.

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.