Browsing by Author "Stark, A"
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- 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-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.
- 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.