Michael J. Zehetbauer

University of Vienna, Wien, Vienna, Austria

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Publications (67)82.39 Total impact

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    ABSTRACT: The release of excess volume upon recrystallization of ultrafine-grained Cu deformed by high-pressure torsion (HPT) was studied by means of the direct technique of high-precision difference dilatometry in combination with differential scanning calorimetry (DSC) and scanning electron microscopy. From the length change associated with the removal of grain boundaries in the wake of crystallite growth, a structural key quantity of grain boundaries, the grain boundary excess volume or expansion [Formula: see text] m was directly determined. The value is quite similar to that measured by dilatometry for grain boundaries in HPT-deformed Ni. Activation energies for crystallite growth of [Formula: see text] and [Formula: see text] are derived by Kissinger analysis from dilatometry and DSC data, respectively. In contrast to Ni, substantial length change proceeds in Cu at elevated temperatures beyond the regime of dominant crystallite growth. In the light of recent findings from tracer diffusion and permeation experiments, this is associated with the shrinkage of nanovoids at high temperatures.
    Acta Materialia 04/2014; 68(100):189-195. · 3.94 Impact Factor
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    ABSTRACT: The aim of the paper was to use recent experimental dislocation data for the development and testing of a dislocation mediated strength model based on that introduced by Scogna and Register for semicrystalline polymers. It is shown that the model can successfully describe measured and evaluated data of high density polyethylene (PE-HD) on the flow stress as function of strain rate and temperature. Similar coincidences of model fits with experimental yield stress data from literature for polypropylene and polyethylene-ethylenemethacrylic acid co-polymers (E/MAA) were found which suggests prevailing of dislocation mediated plasticity mechanisms also in these materials. It also turned out that two parameters of the model - namely the dislocation density and the lamella size – are not determined by the molecular chemistry but by the conditions of processing and/or sample preparation. Therefore the model allows for a reliable estimation of the dependence of the yield stress on these conditions.
    Polymer. 01/2014;
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    ABSTRACT: The mechanism of nanostructure formation during cryogenic and room-temperature milling of Cu and Cu–3wt%Zn was investigated using X-ray diffraction line profile analysis. For that, the whole powder pattern modeling approach (WPPM) was used to analyze the evolution of microstructural features including coherently scattering domain size, dislocation density, and density of planar faults. It was found that for all sets of experiments, structural decomposition is the dominant mechanism of nanostructure formation during cryomilling. During subsequent RT-milling, grain refinement still occurs by structural decomposition for pure copper. On the other hand, discontinuous dynamic recrystallization is responsible for nanostructure formation during RT-milling of Cu–3wt%Zn. This is attributed to lower stacking-fault energy of Cu–3wt%Zn compared to pure copper. Finally, room temperature milling reveals the occurrence of a detwinning phenomenon.
    Journal of Alloys and Compounds 01/2014; 588:138–143. · 2.73 Impact Factor
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    ABSTRACT: Fe0.05Co0.95Sb2.875Te0.125, a double-element-substituted skutterudite, was prepared by induction melting, annealing, and hot pressing (HP). The hot-pressed sample was subjected to high-pressure torsion (HPT) with 4 GPa pressure at 673 K. X-ray diffraction was performed before and after HPT processing of the sample; the skutterudite phase was observed as a main phase, but an additional impurity phase (CoSb2) was observed in the HPT-processed sample. Surface morphology was determined by high-resolution scanning electron microscopy. In the HP sample, coarse grains with sizes in the range of approximately 100 nm to 300 nm were obtained. They changed to fine grains with a reduction in grain size to 75 nm to 125 nm after HPT due to severe plastic deformation. Crystallographic texture, as measured by x-ray diffraction, indicated strengthening of (112), (102) poles and weakening of the (123) pole of the HPT-processed sample. Raman-active vibrational modes showed a peak position shift towards the lower energy side, indicating softening of the modes after HPT. The distortion of the rectangular Sb–Sb rings leads to broadening of Sb–Sb vibrational modes due to local strain fluctuation. In the HPT process, a significant effect on the shorter Sb–Sb bond was observed as compared with the longer Sb–Sb bond.
    Journal of Electronic Materials 01/2014; · 1.64 Impact Factor
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    ABSTRACT: Constant amplitude fatigue crack growth tests were carried out on commercial and high purity nanostructured copper processed by High Pressure Torsion (HPT). Due to strong grain refinement the HPT processed materials show higher tensile strength but also faster crack growth rates when compared to coarse grained material. Crack growth curves of nanostructured copper determined at different stress levels, however, showed that the occurrence of grain coarsening at low stress amplitudes leads to a retardation of crack growth in commercial and high purity HPT Cu. This effect was not observed for high purity HPT Cu with a bimodal microstructure. Crack propagation rates depend significantly on the coarsening phenomenon which on the other hand depends on the applied stress amplitude. A comparison of these results with cyclic deformation tests in the high cycle fatigue regime suggests that grain coarsening during crack growth depends more on the stored energy of the materials while a similar coarsening during cyclic deformation depends more on the activation enthalpy for annealing of defects.
    Mechanics of Materials 12/2013; 67:38–45. · 2.23 Impact Factor
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    Science and Technology of Advanced Materials 10/2013; 14(5):5004-. · 3.75 Impact Factor
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    ABSTRACT: Several structural states of nanostructured high purity Ti with average grain size down to 100 nm were achieved by high pressure torsion (HPT) at temperatures 300 and 77 K. As a result of HPT processing, changes of crystallographic texture, of grain and crystallite size, and of the dislocation density have been measured and analyzed. Mechanical properties of the nanostructured Ti were studied by uniaxial compression at temperatures 300, 77, and 4.2 K. The texture components indicate simple shear deformation arising from HPT. With subsequent compression, the yield strength appears to be governed by the grain size rather than by crystallite size, dislocation density, and/or impurity content. Considerable changes of texture were observed after low temperature compressive deformation indicating that twinning markedly contributes to plasticity.
    Journal of Materials Science 07/2013; 48(13):4689-4697. · 2.31 Impact Factor
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    ABSTRACT: Defect structures in copper of different purity (nominally 99.99 and 99.999 wt. %) deformed via High Pressure Torsion (HPT) with varying processing parameters are investigated utilizing the radiotracer diffusion technique. While the degree of deformation is kept constant, the effects of applied quasi-hydrostatic pressure, processing temperature, post-deformation annealing treatments, and of the impurity concentration on the deformed samples are analyzed in terms of the formation of interconnected internal porosity. Furthermore, the anisotropy of the developing porosity network is examined. The porosity channels occurred to be interconnected along the direction parallel to the surface normal with a volume fraction of the order of a few ppm while no long-range penetration along the internal porosity could be detected when measured along the azimuthal or radial directions of a HPT processed sample.
    Journal of Applied Physics 01/2013; 114(18):183509-183509-10. · 2.21 Impact Factor
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    ABSTRACT: N-type skutterudite Sr0.07Ba0.07Yb0.07Co4Sb12 with ZT = 1.4 at 800 K was processed by high pressure torsion (HPT), a technique of severe plastic deformation (SPD) to produce a nanocrystalline material with many deformation induced lattice defects like dislocations and vacancies. As already shown previously, after HPT processing ZT similar to 1.8 was reached mainly due to a significantly reduced thermal conductivity (the lattice thermal conductivity reached almost the theoretical calculated minimum) although the electrical resistivity was higher. In this paper, the microstructural changes after HPT leading to such high ZT values were investigated. X-ray line profile analysis (XPA) before and after HPT was used to detect a smaller crystallite size and a high number of defects (dislocations and vacancies) resulting in an increase of the electrical resistivity but a significant decrease of the thermal conductivity after HPT processing. The decrease of the crystallite size could also be identified as the reason for enhanced microhardness, which means that Hall-Petch strengthening applies. In addition, for the first time, energy filtered transmission electron microscopy (TEM) was employed for the investigation of HPT processed skutterudites. Dislocations as well as grain boundaries of two types (polarised dipole walls and polarised tilt walls) could be directly observed, confirming what so far was assumed. Also for the first time thermal expansion was measured below and above room temperature and compared with the results before HPT revealing a slightly lower thermal expansion coefficient, the same Debye temperature but an Einstein temperature only half of that before HPT, the latter indicating lower frequencies of the filler atoms after HPT processing. Furthermore it could be shown that the decrease of the electrical resistivity after reaching a maximum runs parallel with a shrinking of the sample during thermal expansion measurements, proving that annealing out and closing of microcracks are responsible for this behaviour. (C) 2012 Elsevier B.V. All rights reserved.
    Journal of Alloys and Compounds 10/2012; 537:183-189. · 2.73 Impact Factor
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    ABSTRACT: High-pressure torsion (HPT) is a type of severe plastic deformation (SPD) that is highly suited to produce bulk ultrafine-grained and nanocrystalline materials, as it introduces many grain boundaries as well as dislocations and point defects. In this paper, HPT-mediated nanocrystallization was used to reduce the thermal conductivity and enhance the Seebeck coefficient of skutterudites. Both p- and n-type skutterudites have been processed by HPT with 4 and 5 GPa at temperatures up to 773 K, resulting in a strongly strengthened nanocrystalline structure, revealing oriented, lamellar-shaped crystallites with a size of ∼50 nm and an enhanced dislocation density. In comparison with ball-milled plus hot-pressed skutterudites, the HPT-processed samples show a reduction of the thermal conductivity up to 40%. This and the slightly higher Seebeck coefficient are the reasons why HPT proved to enhance the figure of merit (ZT) values up to a factor of 2, in spite of a markedly enhanced electrical resistivity.
    Acta Materialia. 03/2012; 60(5):2146–2157.
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    ABSTRACT: The grain boundary excess volume, i.e., the grain boundary expansion, e{GB}, was experimentally determined for high-angle grain boundaries in nickel using the direct technique of high-precision difference dilatometry. Values of e{GB}=(0.35±0.04)×10{-10}  m and e{GB}=(0.32±0.04)×10{-10}  m were obtained by measuring the removal of grain boundary volume upon grain growth for two different types of ultrafine-grained samples. The results are discussed in comparison to values obtained so far from indirect techniques and from computer simulations. It demonstrates the strength of the presented novel, direct approach for grain boundary expansion measurements.
    Physical Review Letters 02/2012; 108(5):055504. · 7.73 Impact Factor
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    ABSTRACT: Nanocrystalline Cu–30% Zn samples were produced by high energy ball milling at 77 K and room temperature. Cryomilled flakes were further processed by ultrahigh strain high pressure torsion (HPT) or room temperature milling to produce bulk artifact-free samples. Deformation-induced grain growth and a reduction in twin probability were observed in HPT consolidated samples. Investigations of the mechanical properties by hardness measurements and tensile tests revealed that at small grain sizes of less than ∼35 nm Cu–30% Zn deviates from the classical Hall–Petch relation and the strength of nanocrsytalline Cu–30% Zn is comparable with that of nanocrystalline pure copper. High resolution transmission electron microscopy studies show a high density of finely spaced deformation nanotwins, formed due to the low stacking fault energy of 14 mJ m–2 and low temperature severe plastic deformation. Possible softening mechanisms proposed in the literature for nanotwin copper are addressed and the twin-related softening behavior in nanotwinned Cu is extended to the Cu–30% Zn alloy based on detwinning mechanisms.
    Acta Materialia 01/2012; 60(8):3340 - 3349. · 3.94 Impact Factor
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    ABSTRACT: Wide angle X-ray experiments evaluated by recent developments of X-ray line profile analysis allow for the detection of the presence of dislocations as well as to determine their density in crystalline materials. The application to semicrystalline polymers not only provides information on the crystal size and the dislocations but—in combination with in situ deformation—also information on the evolution of these microstructural parameters. Investigations on cold rolled and on uniaxially compressed samples of α-phase isotactic polypropylene (α-iPP) as well as poly(3-hydroxybutyrate) (P3HB) are presented. The synchrotron experiments reveal a dislocation governed deformation process in α-iPP. P3HB, however, deforms by a process not including dislocation generation. Here, microcracking and strain localization in the amorphous phase seem to be the predominant deformation mechanisms. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012
    Journal of Applied Polymer Science 01/2012; 125(6):4150-4154. · 1.40 Impact Factor
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    ABSTRACT: For a long time the shift and broadening of Bragg profiles have been used to evaluate internal stresses and coherent domain sizes, i.e. the smallest crystalline region without lattice defects. Modern technology provides both enhanced detector resolution and high brilliance x-ray sources thus allowing measurements of x-ray peaks with a high resolution in space and time. In parallel to the hardware, also diffraction theories have been substantially improved so that the shape of Bragg profiles can be quantitatively evaluated not only in terms of the crystallite size and its distribution, but also in terms of the density, type and arrangement of dislocations, twins and stacking faults. Thus state-of-the-art x-ray line profile analysis enables the thorough characterization especially of nanostructured materials which also contain lattice defects. The method can be used also to prove the existence of dislocations in crystalline materials. Examples of nanostructured metals, polymers and even molecular crystals like fullerenes are given.
    JOM: the journal of the Minerals, Metals & Materials Society 01/2011; 63(7):61-70. · 0.99 Impact Factor
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    ABSTRACT: The release of excess volume upon recrystallization of ultrafine-grained Ni deformed by high-pressure torsion was measured with a high-precision difference-dilatometer employing constant heating rates in the range from 0.3 to 10Kmin−1. The kinetics of the recrystallization process was analyzed according to the Johnson–Mehl–Avrami–Kolmogorov theory adapted to the case of constant heating rates. An effective Avrami exponent of 2 and a value of 1.20eV for the activation energy of recrystallization was determined. Analysis by the Kissinger method yielded the same result for the activation energy.
    Journal of Alloys and Compounds 01/2011; 509. · 2.73 Impact Factor
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    ABSTRACT: It is shown that hydrogen storage materials nanostructured by Severe Plastic Deformation (SPD) retaining the bulk shape are capable to meet or even exceed the adsorption/desorption kinetics of their ball milled counterpart. The grain size of commercial magnesium alloy ZK60 was decreased by Equal Channel Angular Pressing (ECAP) down to 250nm – the smallest value ever achieved by this technique. The material exhibits high gravimetric hydrogen storage capacity of 6.6wt.% and rapid hydrogen desorption kinetics of less than 5min at 350°C. Furthermore, the influence of grain size, surface oxidation and of the addition of metallic chromium as catalyst on the storage capacity and kinetics was investigated. A new method for rapid activation in only one charging/discharging process was successfully tested. The long-term durability of the material was proven in a cyclic sorption/desorption test up to 1000 cycles; no deterioration in storage capacity or in kinetics was observed which is exceptional for nanomaterials for hydrogen storage. Finally, the pressure–composition isotherms of ZK60 were extended by new plateaus at low temperatures (200–260°C).
    Journal of Alloys and Compounds 01/2011; 509. · 2.73 Impact Factor
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    ABSTRACT: The mechanical properties of ultrafine grained (UFG) zirconium (grain size 200 nm) obtained by a combination of extrusion, drawing, and annealing, are studied experimentally under uniaxial compression at temperatures of 4.2-300 K and compared with the mechanical properties of coarse grained (CG) Zr. The evolution of the texture and microstructure of Zr during strain is studied by x-ray diffraction and transmission electron microscopy. The volume fractions of twinned material are determined for UFG and CG Zr. It is found that at room temperature and below, twinning activity is lower in coarse grained zirconium, but at the very lowest temperatures (4.2 K) the opposite effect is observed, i.e., increased twinning activity with decreasing grain size. The influence of internal thermal anisotropy stresses on twinning in CG and UFG zirconium is discussed. The effect of twinning on the mechanical properties of UFG Zr is analyzed.
    Low Temperature Physics 01/2011; 37:609-617. · 0.82 Impact Factor
  • Matthias Bönisch, Michael J Zehetbauer, M Krystian, D Setman, G Krexner
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    ABSTRACT: Recent investigations on palladium hydride (Pd-H) showed, for the first time, evidence of formation of vacancy-hydrogen (Vac-H) clusters during Severe Plastic Deformation (SPD) effected by High Pressure Torsion (HPT). Vacancy concentrations produced in Pd-H by this method are extraordinarily high. DSC-scans show that the thermal stability range of vacancies is extended by about 150K due to trapping of hydrogen leading to the formation of vacancy-hydrogen clusters. Recent experiments give evidence that the mobility of the H atoms and/or the vacancies is conditional for the formation of Vac-H clusters during HPT. Results furthermore indicate defect stabilization by hydrogen trapping not only for vacancy-type defects but also for dislocations and grain boundaries.
    Nanomaterials by Severe Plastic Deformation: NanoSPD5; 01/2011
  • J. Horky, G. Khatibi, B. Weiss, M. J. Zehetbauer
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    ABSTRACT: High cycle fatigue (HCF) life time curves and fatigue crack growth rates of bulk ultra-fine grained (UFG) copper deformed by high pressure torsion (HPT) were determined. Cu of two different purities as well as a bimodally structured HPT Cu were investigated. The results show increased HCF properties of the UFG materials compared to coarse grained (CG) Cu. Especially HPT Cu with lower purity shows enhanced fatigue resistance due to higher microstructural stability. Contrary, crack growth rates in HPT Cu are increased. In case of the high purity Cu, cyclic deformation induced coarsening of the UFG microstructure nearby the crack is found at threshold crack growth rates leading to a retardation of the fatigue crack propagation. Within these coarse grains typical fatigue surface slip marks as observed in CG Cu are found.
    Journal of Alloys and Compounds 01/2011; 509. · 2.73 Impact Factor
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    ABSTRACT: A high-intensity positron beam is used for specific in situ monitoring of thermally activated fast defect annealing in Cu and Ni on a time scale of minutes. The atomistic technique of positron-electron annihilation is combined with macroscopic high-precision length-change measurements under the same thermal conditions. The combination of these two methods as demonstrated in this case study allows for a detailed analysis of multistage defect annealing in solids distinguishing vacancies, dislocations, and grain growth.
    Physical Review Letters 10/2010; 105(14):146101. · 7.73 Impact Factor

Publication Stats

682 Citations
82.39 Total Impact Points

Institutions

  • 1993–2014
    • University of Vienna
      • • Fakultät für Physik
      • • Physics of Nanostructured Materials Group
      Wien, Vienna, Austria
  • 2010–2012
    • Graz University of Technology
      • Institut für Materialphysik
      Graz, Styria, Austria
  • 2009–2010
    • AIT Austrian Institute of Technology
      Wien, Vienna, Austria
    • Ufa State Aviation Technical University
      Oufa, Bashkortostan, Russia
  • 2007
    • Eötvös Loránd University
      • Department of Materials Physics
      Budapeŝto, Budapest, Hungary
  • 1991
    • University of East Anglia
      Norwich, England, United Kingdom