Michael J. Zehetbauer

University of Vienna, Wien, Vienna, Austria

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Publications (83)141.62 Total impact

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    ABSTRACT: Abstract A coarse grained biocompatible Ti–16.1Nb (wt.%) alloy was used to study the impact of severe plastic deformation on microstructural changes, phase transformations, and mechanical properties. The starting material, showing a rather low value of Young’s modulus (66 GPa), contained orthorhombic α” martensite. Hydrostatic pressure of 4 GPa solely yields a partial transformation to the ω-phase; increasing the pressure to 8 GPa increases the volume fraction of the ω-phase and causes a concomitant increase of Young’s modulus. By processing samples through high pressure torsion at room temperature, i.e. applying both hydrostatic pressure and shear deformation, a nanocrystalline structure was obtained. The samples almost exclusively contained the ω-phase and showed rather high values of Young’s modulus (up to 130 GPa) and hardness (up to 4.0 GPa). The ω-phase formed during high pressure torsion revealed stability upon unloading. However, upon heating to about 500°C the ω-phase decomposes into a phase mixture of hexagonal α and body centred cubic β phases which is still ultra-fine. Cold rolling and folding achieves a microstructure consisting of ω, α/α’ and α” phases. Concomitant decrease of grain size and increase of defect density yield a hardness (3.3 GPa) which is smaller than that of high pressure torsion but a Young’s modulus of about 100 GPa being closer to that of the initial material.
    Journal of Alloys and Compounds 04/2015; 628:434-441. · 2.73 Impact Factor
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    ABSTRACT: Bulk nanostructured Cu was prepared by in-situ consolidation through room temperature ball milling. The consolidated parts consist of hollow spheres having a diameter which increases with increasing the milling time. Microhardness maps reveal that the distribution of the hardness is relatively homogeneous after 2 h of milling. After 34 h the hardness is higher at the outer edge and decreases toward the inner edge and, finally, after 70 h the distribution is uniform again. Electron microscopic results show that the microstructure after 70 h of milling consists of two types of grains: elongated ultrafine grains with high density of defects and equiaxed nanosized grains produced by dynamic recrystallization. Continuous dynamic recrystallization is the dominant mechanism for the formation of the nanosized grains. Evidence for the occurrence of discontinuous dynamic recrystallization through twinning was also found in a few regions of the in-situ consolidated samples.
    Materials and Design 01/2015; 65:1083. · 3.17 Impact Factor
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    ABSTRACT: The best p-type skutterudites with ZT>1.1 so far are didymium (DD) filled, Fe/Co substituted, Sb-based skutterudites. DD0.68Fe3CoSb12 was prepared using an annealing -reacting – melting - quenching technique followed by ball milling and hot pressing. After severe plastic deformation via high-pressure torsion (HPT), no phase changes but particular structural variations were achieved, leading to modified transport properties with higher ZT values. Although after measurement-induced heating some of the HPT induced defects were annealed out, a still attractive ZT-value was preserved. In this paper we focus on explanations for these changes via TEM investigations, Raman spectroscopy and texture measurements. The grain sizes and dislocation densities, evaluated from TEM images, showed that (i) the majority of cracks generated during high-pressure torsion are healed during annealing, leaving only small pores, that (ii) the grains have grown, and that (iii) the dislocation density is decreased. While Raman spectra indicate that after HPT processing and annealing the vibration modes related to the shorter Sb-Sb bonds in the Sb4 rings are more affected than those related to the longer Sb-Sb bonds, almost no visible changes were observed in the pole intensity and/or orientation. Keywords: Skutterudites, severe plastic deformation, physical properties, TEM, Raman spectroscopy.
    Physical Chemistry Chemical Physics 12/2014; · 4.20 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 10/2014; 43(10):1-7. · 1.68 Impact Factor
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    Andreas Grill, Gerhard Krexner, Michael Zehetbauer
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    ABSTRACT: While microcrystalline Mg has very slow H-sorption kinetics even at temperatures as high as 400°C, nanostructuring of the metal leads to economically reasonable kinetics at lower temperatures as low as 150°C. The main method for nanostructuring used in this field is Ball Milling which is time and energy consuming and thus very expensive. [1] By Severe Plastic Deformation (SPD) and SPD like methods small grain sizes can be achieved in a more economical way. Cold Rolling (CR) of MgH 2 leads to good kinetics in hydrogen absorption and desorption [2]. Since MgH 2 has better resistance to oxidation, it can be deformed and handled completely at ambient air. It was shown that this material has a limited lifetime since after 100 cycles of ab-and desorption the capacity reduced to 50 % of the maximum capacity of the second cycle. (see Fig. 1) The stability of MgH 2 will be compared to the behaviour of other Mg based alloys after SPD including ZK60 where the stability of up to 1000 cycles were already shown by [3] . Fig.1 Hydrogen desorption from CR MgH 2 at 350°C – 100 cycles. Acknowledgment: This project is paid by the Austrian Climate and Energy Fonds and conducted in the Program "Energy of the Future". References: [1] Bellemare et al., Hydrogen storage properties of cold rolled magnesium hydrides with oxides catalysts, JAC, 512, 2012, 33-38. [2] Leiva et al., Nanostructured MgH2 prepared by cold rolling and cold forging, JAC, 509 Sup. 1, 444-448 [3] Krystian et al., Hydrogen storage properties of bulk nanostructured ZK60 Mg alloy processed by Equal Channel Angular Pressing, JAC, 509, 449-455
    The 6th International Conference on Nanomaterials by Severe Plastic Deformation, Metz, France; 06/2014
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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 structural and magnetic properties of polycrystalline ${rm Fe}_{100-{rm x}}{rm Mn}_{rm x}$ alloys with ${rm x}={38}$ , 42, 46, 50, and 55 atomic percent were investigated after a heat treatment and a cold rolling process. According to X-ray diffraction (XRD), all samples crystallize into a $gamma$ -phase. For cold rolled alloys ${rm x}={38}$ and 42, XRD analysis using pole figures showed no texture formation whereas cold rolled alloys with ${rm x}={46}$ and 50 exhibited texture components ${110}<1-12>$ and with ${rm x}={55}$ , ${011}<01-1>$ . Magnetization measurements clearly demonstrate an antiferromagnetic type of ordering. For both the as-cast as well as the cold rolled materials, magnetostriction measurements at room temperature gave nearly zero magnetostriction.
    IEEE Transactions on Magnetics 04/2014; 50(4):1-4. · 1.21 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 03/2014; · 3.77 Impact Factor
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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 03/2014; 588:138–143. · 2.73 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. · 2.19 Impact Factor
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    Journal of Materials Science 11/2012; 47(22):7717-7718. · 2.31 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: 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 09/2012; 125(6):4150-4154. · 1.40 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 05/2012; 60(8):3340 - 3349. · 3.94 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. · 3.94 Impact Factor
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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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    Materials Science Forum 12/2011; 702-703:370-373.

Publication Stats

1k Citations
141.62 Total Impact Points

Institutions

  • 1993–2015
    • University of Vienna
      • • Physics of Nanostructured Materials Group
      • • Fakultät für Physik
      Wien, Vienna, Austria
  • 2010
    • Graz University of Technology
      • Institut für Materialphysik
      Graz, Styria, Austria
    • AIT Austrian Institute of Technology
      Wien, Vienna, Austria
  • 1991
    • University of East Anglia
      Norwich, England, United Kingdom