Browsing by Author "Clemens, H"
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- ItemThe contribution of high-energy x-rays and neutrons to characterization and development of intermetallic titanium aluminides(Wiley-Blackwell, 2011-08-01) Schmoelzer, T; Liss, KD; Staron, P; Mayer, S; Clemens, HAbstract Intermetallic γ-TiAl based alloys are a novel class of lightweight structural materials that exhibit excellent high-temperature strength while having low density. These properties make them ideal candidates for replacing dense Ni base alloys currently used in the temperature range from 550 to 750 °C. Therefore, extensive research activities were conducted during the last 20 years to make this innovative class of materials fit for service. In this task, diffraction methods have been an important tool for promoting the development of TiAl alloys. The ability to perform experiments in situ and to determine phase fractions even in cases where two phases are present in ultrafine lamellar structures are only two examples for applications in which diffraction methods are indispensable. In this work, a review is given concerning the use of diffraction methods in the development of TiAl alloys. Different methods are introduced and highlighted by examples. This review lists the advantages of diffraction experiments and critically discusses the limits of the individual methods.© 2011, Wiley-Blackwell.
- ItemDirectional atomic rearrangements during transformations between the α- and γ- phases in titanium aluminides(Wiley-VCH Verlag Berlin, 2008-04) Liss, KD; Stark, A; Bartels, A; Clemens, H; Buslaps, T; Phelan, D; Yeoh, LAMaking movies in-situ at glowing temperatures up to 1300°C through a microscope (false color image) and from two-dimensional X-ray diffraction (movie frames) reveal the lattice correlations, gradients and intermediate structures during phase transformations in titanium aluminide. A quenched, α2-rich γ-based TiAl first approaches its equilibrium by α2 → γ on a heating ramp, disorders α2 → α and then evolves reversely γ → α, which are morphologically different processes. © 2008, Wiley-VCH Verlag Berlin
- ItemDynamic recovery and recrystallization during hot-working in an advanced TiAl alloy(Carl Hanser Verlag, 2011-01-01) Schmoelzer, T; Liss, KD; Rester, M; Yan, K; Stark, A; Reid, M; Peel, MJ; Clemens, HIntermetallic TiAl alloys are light-weight high-temperature materials and intended to partly replace Ni based alloys in jet engines. Due to difficult forming operations, component prices are high and limit the possible field of application. During hot-working, recovery and recrystallization effects determine the microstructural evolution and thereby the mechanical properties of the finished part as well as its behavior during deformation. To study the occurring experiments with high-energy X-rays were conducted. By means of this method, the dominating processes were identified. The results were validated through electron back scatter diffraction experiments. © 2011 CARL HANSER VERLAG
- ItemEvolution of composition and grain correlations upon phase transitions and micro-structural rearrangement processes followed in-situ by high energy x-ray diffraction(Materials Australia, 2007-07-03) Liss, KD; Yeoh, LA; Clemens, H; Chladil, HF; Bartels, A; Stark, A; Buslaps, TTwo-dimensional powder diffraction using high-energy synchrotron x-rays is a powerful tool for bulk studies of materials and a short introduction of its advantages is given here.
- ItemIn and ex situ investigations of the β-phase in a Nb and Mo containing gamma-TiAl based alloy.(Elsevier, 2008-06) Clemens, H; Chladil, HF; Wallgram, W; Zickler, GA; Gerling, R; Liss, KD; Kremmer, S; Guther, V; Smarsly, WIn β-stabilized Ti-43Al-4Nb-1Mo-0.1B alloy (composition in atomic percent) the correlation between the occurrence of β-phase and temperature was analyzed experimentally and compared to thermodynamic calculations. Results from in situ high-energy X-ray diffraction, texture measurements, heat treatments, scanning electron microscopy, and temperature-dependent flow stress measurements were used to study the evolution of the β-phase with temperature. Thermodynamic calculations based on the CALPHAD method were applied to correlate the phases developed in the β-solidifying TiAl based alloy under investigation. This alloy is characterized by an adjustable β-phase volume fraction at temperatures where hot-work processes such as forging and rolling are conducted. Due to a high volume fraction of β-phase at elevated temperatures the hot-extruded alloy can be forged under near conventional conditions. © 2008, Elsevier Ltd.
- ItemIn situ characterization of a Nb and Mo containing γ-TiAl based alloy using neutron diffraction and high-temperature microscopy(Wiley-VCH Verlag Berlin, 2009-11) Watson, IJ; Liss, KD; Clemens, H; Wallgram, W; Schmoelzer, T; Hansen, TC; Reid, MIn recent times, novel titanium aluminides containing the bcc β-phase at high temperatures are being developed for improved hot-working capabilities, however, predictions of the phase diagrams are merely uncertain. Here we present in-situ neutron studies, which are particularly sensitive to the atomic disorder in the ordered phases. Complementary laser scanning confocal microscopy is employed for in-situ microstructural investigations. © 2009, Wiley-VCH Verlag Berlin
- ItemIn situ diffraction experiments for the investigation of phase fractions and ordering temperatures in Ti-44 at% Al-(3-7) at% mo alloys(Wiley, 2011-04-01) Schmoelzer, T; Mayer, S; Sailer, C; Haupt, F; Guther, V; Staron, P; Liss, KD; Clemens, HBeing a strong beta stabilizer, Mo has gained importance as an alloying element for so-called beta/gamma-TiAl alloys. Intermetallic TiAl-based alloys which contain a significant volume fraction of the body-centered cubic beta-phase at elevated temperatures have proven to exhibit good processing characteristics during hot-working. Unfortunately, the effect of Mo on the appearing phases and their temperature dependence is not well known. In this work, sections of the Ti-Al-Mo ternary phase diagram derived from thermodynamic calculations as well as experimental data are presented. The phase transition temperatures stated in these phase diagrams are compared with the results of high-temperature diffraction studies using high-energy synchrotron radiation. Additionally, the disordering temperature of the beta(o)-phase is determined. © 1999-2020 John Wiley & Sons, Inc.
- ItemIn situ high-energy X-ray diffraction study and quantitative phase analysis in the α+γ phase field of titanium aluminides(Elsevier, 2007-12) Yeoh, LA; Liss, KD; Bartels, A; Chladil, HF; Avdeev, M; Clemens, H; Gerling, R; Buslaps, TQuantitative atomic structure and phase analysis in the titanium aluminide intermetallic system of composition Ti–45Al–7.5Nb–0.5C (at.%) was conducted in situ by use of high-energy X-ray diffraction from a synchrotron and evaluated using the Rietveld method, implementing a model for atomic order in the α-phase which describes the order to disorder transition α2→α at the eutectoid temperature. The order parameter exhibits unexpected behavior and is entangled with the competition of different kinetic processes. © 2007, Elsevier Ltd.
- ItemIn situ study of dynamic recrystallization and hot deformation behavior of a multiphase titanium aluminide alloy(American Institute of Physics, 2009-12-01) Liss, KD; Schmoelzer, T; Yan, K; Reid, M; Peel, MJ; Dippenaar, RJ; Clemens, HHot-compression tests were conducted in a high-energy synchrotron x-ray beam to study in situ and in real time microstructural changes in the bulk of a beta-solidifying titanium aluminide alloy. The occupancy and spottiness of the diffraction rings have been evaluated in order to access grain growth and refinement, orientation relationships, subgrain formation, dynamic recovery, and dynamic recrystallization, as well as phase transformations. This method has been applied to an alloy consisting of two coexisting phases at high temperature and it was found that the bcc beta-phase recrystallizes dynamically, much faster than the hcp alpha-phase, which deforms predominantly through crystallographic slip underpinned by a dynamic recovery process with only a small component of dynamic recrystallization. The two phases deform to a very large extent independently from each other. The rapid recrystallization dynamics of the beta-phase combined with the easy and isotropic slip characteristics of the bcc structure explain the excellent deformation behavior of the material, while the presence of two phases effectively suppresses grain growth. © 2009, American Institute of Physics
- ItemIn-situ characterization of phase transformations and microstructure evolution in a γ-TiAl based alloy(The Minerals, Metals & Materials Society (TMS), 2008-06-01) Liss, KD; Bartels, A; Clemens, H; Stark, A; Buslaps, T; Phelan, D; Yeoh, LAPhase diagrams and microstructures of titanium aluminides are rather complex and, so far, little data were observed in-situ at elevated temperatures. We report on two-dimensional high energy X-ray diffraction and complementary laser scanning confocal microscopy to characterize the appearing phases and to follow the phase evolution in-situ and in real time. As an example, the microstructure evolution of a quenched γ-TiAl alloy, consisting of α2-Ti3Al grains at room temperature, has been followed in both reciprocal and direct space as a function of temperature up to 1400°C. At 700 – 800°C extremely fine γ-laths are formed in α2-grains occurring through an oriented rearrangement of atoms. Streaks linking reflections of both phases testify from coherent lattice and orientation gradients in the transforming crystallite. At temperatures around the eutectoid temperature recrystallization effects and the γ->α phase transition take place leading to grain refinement.
- ItemAn in-situ high-energy X-ray diffraction study on the hot-deformation behavior of a β-phase containing TiAl alloy(Elsevier Science Ltd., 2013-08-01) Schmoelzer, T; Liss, KD; Kirchlechner, C; Mayer, S; Stark, A; Peel, MJ; Clemens, HIn 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. © 2013, Elsevier Ltd.
- ItemIn-situ study of phases and microstructures of titanium aluminides(Australian Institute of Physics, 2006-12-04) Liss, KD; Yeoh, LA; Bartels, A; Clemens, H; Phelan, D; Buslaps, TThe phase diagrams and microstructures of titanium aluminides are rather complex and little or no data were observed during in-situ experiments at elevated temperatures up to 1400 °C, where different processes compete, such as recovery, phase transitions, recrystallization, twinning, crystallite growth, segregation and more. Two-dimensional high energy X-ray diffraction is a powerful method to characterize the phase composition and modern synchrotron sources are strong enough to follow the evolution of the material in real time. Besides texture relations as well as grain and phase correlations, we followed the coherent transition from the alpha to the gamma phase and vice versa. A streak of diffuse scattering appears in reciprocal space bridging reciprocal lattice points of both phases and disappears after the transition completed. This proves, that the phase transition is a well ordered process. Furthermore, the system has been observed in a Laser Scanning Confocal Microscope in situ and in real time, relating the micro structure to the diffraction pattern. Fine laths of the lamellar alpha/gamma grains are created during the phase transition and coarsen as a function of time. The unique combination of the two complementary in-situ techniques was used for the first time and reveals novel, consistent information on a phase transformation in a real solid.
- ItemPhase fractions, transition and ordering temperatures in TiAl-Nb-Mo alloys: an in- and ex-situ study.(Elsevier, 2010-08) Schmoelzer, T; Liss, KD; Zickler, GA; Watson, IJ; Droessler, LM; Wallgram, W; Buslaps, T; Studer, AJ; Clemens, HIntermetallic 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. © 2010, Elsevier Ltd.
- ItemPhase transition and ordering behavior of ternary Ti-Al-Mo alloys using in-situ neutron diffraction(HANSER eLibrary, 2011-06-01) Kabra, S; Yan, K; Mayer, S; Schmoelzer, T; Reid, M; Dippenaar, RJ; Clemens, H; Liss, KDNeutron diffraction has been used for in-situ. investigations to elucidate the phase transformation behavior of two Mo-containing TiAl alloys with compositions of Ti-44Al-3Mo and Ti-44Al-7Mo (in at.%). Five different phases are present in these alloys. These include three ordered phases at room temperature, namely alpha(2), beta(0) and gamma and two disordered phases, alpha and beta, which occur at higher temperatures. The sequence of the three phase transformations in each alloy has been determined. The phase transformation and disordering/ordering temperatures were determined on heating and cooling from the diffracted peak intensities. The neutron experiments are particularly sensitive to the order disorder transitions in TiAl alloys, which are compared with the overall phase fractions obtained from previous high energy X-ray diffraction. Hysteresis and undercooling effects are observed for the various phase transformations and depend on the nature of atomic rearrangements. © 2011 Carl Hanser Verlag GmbH & Co. KG
- ItemPhase transition and ordering temperatures of TiAl-Mo alloys investigated by in-situ diffraction experiments(Trans Tech Publications, 2010-08-02) Schmoelzer, T; Mayer, S; Haupt, F; Zickler, GA; Sailer, C; Lottermoser, L; Guther, V; Liss, KD; Clemens, HIntermetallic TiAl alloys with a significant volume fraction of the body-centered cubic β-phase at elevated temperatures have proven to exhibit good processing characteristics during hot-working. Being a strong β stabilizer, Mo has gained importance as an alloying element for so-called β/γ-TiAl alloys. Unfortunately, the effect of Mo on the appearing phases and their temperature dependence is not well known. In this work, two sections of the Ti-Al-Mo ternary phase diagram derived from experimental data are shown. These diagrams are compared with the results of in-situ high-temperature diffraction experiments using high-energy synchrotron radiation. © 2020 by Trans Tech Publications Ltd.
- ItemPrecipitation, recovery, phase transition and recrystallization processes of massively transformed TiAI scrutinized by ex- and in-situ high-energy X-ray diffraction(Australian Institute of Physics, 2005-01-31) Liss, KD; Bystrzanowski, S; Bartels, A; Buslaps, T; Clemens, H; Gerling, R; Schimansky, FP; Stark, AHigh-energy synchrotron radiation above 100 keV is a novel and sophisticated probe to access the volume properties of materials. A Debye-Scherrer method is shortly presented for the measurements of textures, strain and composition. Low density, high specific yield strength, good oxidation resistance and good creep properties at elevated temperatures make intermetallic γ-TiAl-based alloys top candidates as structural materials for advanced jet and automotive engines as well as for future hypersonic vehicles. The mechanical properties depend strongly on composition, thermo mechanical processing and subsequent heat treatments. The present study examines the recrystallization processes of a massively transformed specimen of Ti45AI46Nb9 upon a heat ramp from room temperature to 1400 deg C. The registered Debye-Scherrer rings are rich of features relating to thermal expansion, phase changes, domain and phase coherences, chemical separation and much more which will be regarded in detail.