ANSTO :: Browsing by Author "Dippenaar, RJ"

Browsing by Author "Dippenaar, RJ"

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Defect dynamics in polycrystalline zirconium alloy probed in situ by primary extinction of neutron diffraction
(American Institute Physics, 2013-02-13) Kabra, S; Yan, K; Carr, DG; Harrison, RP; Dippenaar, RJ; Reid, M; Liss, KD
After alpha+beta-zirconium has fully transformed into beta-phase upon heating, the intensities of all beta-Zr Bragg reflections decrease simultaneously as a function of time. It is shown that this effect represents a transition from the kinematic to the dynamic theory of diffraction due to the ever increasing crystal perfection driven by thermal recovery of the system. The best fitting coherent crystallite size of 30 mu m and other microstructural features are verified by in situ laser scanning confocal microscopy. This effect of primary extinction in neutron diffraction has been employed to further investigate the crystal perfection kinetics. Upon further heating, crystal recovery is identified as a process of dislocation annihilation, suffering from lattice friction. Upon cooling, precipitating alpha-Zr induces strain into the perfect beta-crystallites, re-establishing the kinematic diffraction intensities. An Avrami analysis leads to the estimations of nucleation time, consumption of nucleation sites and lower-dimensional growth. Such technique bears great value for further investigation on all metal systems annealed close to the melting temperature. © 2013, American Institute of Physics.
Direct, time-resolved in-situ observation of dynamic recyrstallization and related phenomena in the bulk of zirconium alloy
(Australian Institute of Physics, 2009-02-04) Liss, KD; Garbe, U; Schambron, T; Almer, JD; Li, HJ; Yan, K; Dippenaar, RJ
Not available
From single grains to texture
(Wiley-VCH Verlag GmbH & Co. KGaA, 2009-10) Yan, K; Liss, KD; Garbe, U; Daniels, JE; Kirstein, O; Li, HJ; Dippenaar, RJ
Structural materials, such as metals, ceramics, and their composites are most often polycrystalline. The nature, morphology, and composition of their microstructure determine in large measure the mechanical properties of the final product, and the art to design novel materials is to find particular arrangements which make them harder, more shock absorbing, heat resistant, or self-recovering upon damage and aging. The understanding of the basic processes and their interplay in a polycrystalline structure are most important for improved simulation of plastic deformation and to predict their thermo-mechanical behavior. © 2009, Wiley-VCH Verlag GmbH & Co. KGaA
Hydrostatic compression behavior and high-pressure stabilized β-phase in γ-based titanium aluminide intermetallics
(Multidisciplinary Digital Publishing Institute, 2016-07-15) Liss, KD; Funakoshi, K; Dippenaar, RJ; Higo, Y; Shiro, A; Reid, M; Suzuki, H; Shobu, T; Akita, K
Titanium aluminides find application in modern light-weight, high-temperature turbines, such as aircraft engines, but suffer from poor plasticity during manufacturing and processing. Huge forging presses enable materials processing in the 10-GPa range, and hence, it is necessary to investigate the phase diagrams of candidate materials under these extreme conditions. Here, we report on an in situ synchrotron X-ray diffraction study in a large-volume press of a modern (α2 + γ) two-phase material, Ti-45Al-7.5Nb-0.25C, under pressures up to 9.6 GPa and temperatures up to 1686 K. At room temperature, the volume response to pressure is accommodated by the transformation γ → α2, rather than volumetric strain, expressed by the apparently high bulk moduli of both constituent phases. Crystallographic aspects, specifically lattice strain and atomic order, are discussed in detail. It is interesting to note that this transformation takes place despite an increase in atomic volume, which is due to the high ordering energy of γ. Upon heating under high pressure, both the eutectoid and γ-solvus transition temperatures are elevated, and a third, cubic β-phase is stabilized above 1350 K. Earlier research has shown that this β-phase is very ductile during plastic deformation, essential in near-conventional forging processes. Here, we were able to identify an ideal processing window for near-conventional forging, while the presence of the detrimental β-phase is not present under operating conditions. Novel processing routes can be defined from these findings. © 2016, The Authors
In 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, H
Hot-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
In-situ characterization of lattice structure evolution during phase transformation of Zr-2.5Nb
(Wiley-Blackwell, 2011-09-01) Yan, K; Carr, DG; Kabra, S; Reid, M; Studer, AJ; Harrison, RP; Dippenaar, RJ; Liss, KD
The alpha-beta phase transformation behavior of Zr-2.5Nb (in mass%) has been characterized in real time during an in situ neutron diffraction experiment. The Zr-2.5Nb material in the current study consists, at room temperature, of alpha-Zr phase (hcp) and two beta phases (bcc), a Nb rich beta-Nb phase and retained, Zr rich, beta-Zr(Nb) phase. It is suggested that this is related to a quench off the equilibrium solubility of Nb atoms in the Zr bcc unit cells. Vegard's law combined with thermal expansion is applied to calculate the composition of the beta-phase, which is compared with the phase diagram, revealing the system's kinetic behavior for approaching equilibrium. © 2011, Wiley-Blackwell.
In-situ diffraction studies related to thermo-mechanical processes in metals and alloys
(Australian Institute of Nuclear Science and Engineering, 2016-11-29) Liss, KD; Li, X; Dippenaar, RJ
Both neutron and synchrotron high-energy X-rays have penetrating power into metals, and intensities are competitive for ex- and in-situ studies of thermo-mechanical processes. They bear great potential in order to speed up materials design by orders of magnitude. The present contribution will enhance novel pioneering experiments on selected metal systems and showcase the complementarity between neutrons and X-rays and to other in-situ techniques, such as the Laser Confocal Scanning Microscope. Neutrons bear the advantage of averaging over larger volumes and therefore, are less dependent on grain statistics, leading to good, quantitative phase analysis and texture measurements. Phase evolutions are studied upon application of high temperature and high pressure. The neutron contrast, different to X-rays has been employed to investigate order-disorder transitions in titanium-aluminides. Moreover, dynamical theory of diffraction leads to the study of the smallest distortions and their kinetics at high temperature in zirconium and titanium alloys. Synchrotron X-rays allow t focusing on a small number of crystallites, showing up traces of grain evolution in reciprocal space, such as grain rotation, grain growth, phase correlations, dynamic recovery and decrystallization such as in a Materials Oscilloscope.
In-situ studies of γ-based Ti-Al alloys using synchrotron x-ray and neutron diffraction
(Australian Institute of Physics, 2017-02-02) Li, X; Liss, KD; Dippenaar, RJ
Because of their low density, good strength, corrosion resistance and high service temperature γ-based titanium aluminides are being considered as materials of choice in the aerospace and automobile industry. In our study, we focus on a series of interesting γ-based Ti-Al alloys with respect to the lattice evolution of Ti-45Al-7.5Nb-0.25C (at. %) during heating under a pressure of 10 GPa followed by synchrotron radiation; order-disorder transitions and shortrange-order in polysynthetic twinned Ti-Al single crystals using neutron scattering and phase transformation and texture evolution of high pressure torsion samples of Ti-45Al-7.5Nb by using complementary X-ray and neutron diffraction. Lattice parameter evolution can be separated into four contributions: thermal expansion, composition, order parameter and pressure. Diffuse scattering at reciprocal lattice positions provides evidence of short-range order, even in the disordered phase at very high temperature. Structural and atomic disorder is largely introduced by severe plastic deformation. These findings are fundamental to develop an improved understanding of the complex phase transformations. We report our first results of experiments performed under extremely high-pressure, relevant to advanced manufacturing processes.
Martensitic phase transformation and deformation behavior of Fe–Mn–C–Al twinning-induced plasticity steel during high-pressure torsion
(Wiley Online Library, 2014-02-05) Yan, K; Bhattacharyya, D; Lian, Q; Kabra, S; Kawasaki, M; Carr, DG; Callaghan, MD; Avdeev, M; Li, HJ; Wang, Y; Liao, XZ; Langdon, TG; Liss, KD; Dippenaar, RJ
The transformation between the face centered cubic austenitic and hexagonal close-packed martensitic phases during high-pressure torsion processing was observed in a Fe–Mn–C–Al twinning-induced plasticity steel. This phase transformation was not found in the same material processed by unidirectional compressive and tensile deformation. Initiated by the high-pressure loading, the martensite phase initially increased with torsional strain but diminished subsequently. Texture evolution of the austenitic phase was compared with the ideal texture distribution of face-centered cubic materials after shear deformation.© 2014, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Neutron and x-ray studies of TiAl-Nb intermetallics undergone high-pressure torsion
(Australian Institute of Nuclear Science and Engineering, 2016-11-29) Li, X; Dippenaar, RJ; Kawasaki, M; Liss, KD
High-pressure torsion is a severe-plastic-deformation technique rendering the bulk metallic material into an ultrafine microstructure. Not only the application of high pressure in the 6 GPa range bears potential for phase transformation, moreover huge plastic shear strains of 10 to 100 is achieved by torsion processing. In a first study, pellets of y-based Ti-45Al-7.5Nb have been processed under 6 GPa at room temperature, (i) with pressure loading and unloading only, (ii) with 5 turns of torsion and (ii) 10 turns. The material, which usually is brittle and hard to deform, was successfully processed and showed ductility under these conditions. First investigations by neutron and X-ray diffraction are presented, emphasizing the complementarity of both kinds of quantum beams. While X-rays determine the overall structure, such as close-packing, neutrons are particularly sensitive to the order parameter in the constituting y-TiAl and a2-Ti3Al intermetallic phases. It is found that the atomic order decreases on larger amount of processing. Also structural transformations regarding the atomic packing take place. These preliminary examinations open routes for physical understanding and recovery of the occurring crystallographic transformation and microstructural arrangements.
Phase 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, KD
Neutron 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
Steels and intermetallics under extreme conditions
(Australian Institute of Physics, 2016-02-02) Liss, KD; Dippenaar, RJ; Akita, K; Funakoshi, K; Reid, M; Suzuki, H; Shobu, T; Higo, Y; Saitoh, H; Zhang, S; Tomato, Y
Materials are being designed and engineered for ever superior mechanical and operational properties, such as steels for lighter cars and energy-absorbing behaviour in an accident, and titanium aluminides for lighter airplane turbine blades. The manufacturing of such materials may involve processes at extreme conditions, under high pressure or high temperature. Examples are high-pressure torsion and near net-shape forging. Therefore, it becomes eminently important to know and understand the phase diagrams of such materials at extreme conditions. Structural changes may open processing windows, while elevated mechanical properties are conserved under less extreme conditions. Here, we present first phase diagram studies on high-manganese steels and on titanium aluminides by in-situ synchrotron X-ray diffraction in a large-volume cell.
Thermomechanical processing of titanium alloys
(Australian Institute of Physics, 2012-02-01) Thoennessen, L; Liss, KD; Dippenaar, RJ; Dehghan-Manshadi, A
Fine tuning the properties of titanium alloys will be a major challenge for future light weight structural applications in the aerospace industry [1]. The near-β titanium alloy Ti-5553 of composition Ti-5Al-5V-5Mo-3Cr (mass-%) exhibits excellent harden ability and strength characteristics combined with high fracture toughness and excellent high cycle fatigue behavior [2]. Another alloy which shows a good combination of properties, is the α+β titanium alloy Ti-6242 of composition Ti-6Al-2Mo-4Sn-2Zr (mass-%). The mechanical properties of both kinds of titanium alloys are very sensitive to their microstructure which is strongly influenced by the applied parameters during thermomechanical treatment. To control the microstructure during the processing, it is very important to have knowledge about the thermodynamics and kinetics of the phase transformations taking place under different conditions [3]. The intended research is aimed at unveiling the hot-deformation mechanisms and the development of microstructure in the aforementioned titanium alloys. This will be achieved by the comparison of results from both ex situ experiments (such as Gleeble testing, dilatometry and electron microscopy) and in situ studies. The in situ studies shall be conducted by the use of confocal microscopy and modern diffraction methods such as high intensity neutron diffraction and high energy synchrotron radiation. At the end of the research project we expect to fully understand the thermomechanical processes and precipitation behaviors of those two alloys. This understanding will help us to design new industrial processing routes for aerospace parts with better combination of strength and toughness.

Browsing by Author "Dippenaar, RJ"

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