Browsing by Author "Yeoh, LA"

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    Directional 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, LA
    Making 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
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    Evolution 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, T
    Two-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.
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    In 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, T
    Quantitative 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.
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    In situ synchrotron high-energy x-ray diffraction analysis on phase transformations in Ti-Al alloys processed by equal-channel angular pressing
    (Wiley-Blackwell, 2009-11) Liss, KD; Whitfield, RE; Xu, W; Buslaps, T; Yeoh, LA; Wu, XL; Zhang, DL; Xia, KN
    Mixtures of 47-Al and 53-Ti powders (atomic %) have been consolidated using back pressure equal-channel angular pressing starting with both raw and ball-milled powders. In situ synchrotron high-energy X-ray diffraction studies are presented with continuous Rietveld analysis obtained upon a heating ramp from 300 K to 1075 K performed after the consolidation process. Initial phase distributions contain all intermetallic compounds of this system except Al, with distribution maxima in the outer regions of the concentrations (α-Ti, TiAl3). Upon annealing, the phase evolution and lattice parameter changes owing to chemical segregation, which is in favour for the more equilibrated phases such as γ-TiAl, α(2)-Ti3Al and TiAl2, were followed unprecedentedly in detail. An initial δ-TiH2 content with a phase transition at about 625 K upon heating created an intermediate β-Ti phase which played an important role in the reaction chain and gradually transformed into the final products. © 2009, Wiley-Blackwell.
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    In-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, LA
    Phase 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.
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    In-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, T
    The 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.
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    Towards the thermodynamic equilibrium of titanium aluminides after consolidation by back pressure equal channel angular pressing
    (ARC Centre of Excellence for Design in Light, 2007-12) Xu, W; Wu, X; Whitfield, R; Liss, KD; Buslaps, T; Yeoh, LA; Zhang, DL; Xia, K
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    Two dimensional high energy x-ray powder diffraction
    (Australian Institute of Physics, 2006-12-04) Yeoh, LA; Liss, KD; Buslaps, T
    High-energy synchrotron radiation (around 100 keV) combined with a two-dimensional detector boasts the advantage of high penetration power for bulk studies of materials and simple setup geometries. Typically, the diffraction pattern is obtained for a several millimeter thick sample using a transmission geometry setup and the resulting concentric Debye-Scherrer rings are recorded on a flat detector. The radii of the rings and their intensities reflect the structure and phase composition of the sample and the ring morphology can be evaluated, revealing grain statistics. Grain correlations across phase boundaries or domain relations can also be identified and sometimes a local reciprocal lattice of a crystallite can be mapped. Furthermore, anisotropies in intensity and ring radius reveal texture and lattice strain, respectively. Data acquisition times for these patterns range from below a second to few minutes, allowing for in-situ registration during temperature cycles, resulting in a huge amount of individual diffraction patterns which have to be evaluated. As a result, algorithms need to be scripted to automate and batch-process the data evaluation. Currently code is being written in SCILAB, a public available software package, to extract the 1D pattern and other parameters for further analysis with greater efficiency. This method can also be extended to the study of many other processes; such as thermo-mechanical deformation in light metals, like titanium aluminides, magnesium, oxidation layers from steel processing and amorphous materials to mention a few.