M. Zehetbauer

University of Vienna, Wien, Vienna, Austria

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Publications (154)259.76 Total impact

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    ABSTRACT: The best p-type skutterudites so far are didymium filled, Fe/Co substituted, Sb-based skutterudites. Substitution at the Sb-sites influences the electronic structure, deforms the Sb4-rings, enhances the scattering of phonons on electrons and impurities and in this way reduces the lattice thermal conductivity.
    Acta Materialia 06/2015; 91. DOI:10.1016/j.actamat.2015.03.008 · 3.94 Impact Factor
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    ABSTRACT: NiAl is an intermetallic compound with a brittle-to-ductile transition temperature of about 300°C at ambient pressure. At standard conditions, it is very difficult to deform, but fracture stress and fracture strain are increased under hydrostatic pressure (HP). On account of this, deformation at low temperatures is only possible at high HP, as for instance used in high pressure torsion (HPT). In order to study the influence of HP on texture evolution, small discs of polycrystalline NiAl were deformed by HPT at different temperatures ranging from room temperature to 500°C and different HPs. The influence of HP is presented for deformation at room temperature and 500°C. It is found that HP affects the formability of the samples as well as texture and microstructure.
    IOP Conference Series Materials Science and Engineering 04/2015; 82(1). DOI:10.1088/1757-899X/82/1/012026
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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. DOI:10.1016/j.jallcom.2014.12.159 · 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. DOI:10.1016/j.matdes.2014.06.052 · 3.17 Impact Factor
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    ABSTRACT: Abstract This paper reports long-term hydrogen storage experiments on MgH2 and on the Mg alloy ZK60 following prior Severe Plastic Deformation (SPD). Although SPD processing leads to significant enhancements of hydrogen absorption and desorption rates in both materials, these are not necessarily stable with respect to repeated loading/unloading cycles. Cold rolled (CR) MgH2 shows a reduction of capacity by 30% after 100 cycles. In contrast, in ZK60 (Mg-5Zn-0.8Zr) processed by High Pressure Torsion (HPT), both kinetics and storage capacity are stable for at least 200 absorption/desorption cycles. Analysis by means of Johnson-Mehl-Avrami theory clearly suggests that in the case of CR MgH2 nucleation is followed by growth of extended MgH2 domains leading to a gradual deterioration of hydrogen diffusion and storage/release characteristics. In the case of HPT ZK60, however, no further growth occurs subsequent to nucleation thus allowing for permanently enhanced hydrogen diffusion and stable storage/release properties. These results can be understood in terms of the different density and stability of SPD-induced lattice defects acting as nucleation sites in both materials studied.
    International Journal of Hydrogen Energy 01/2015; DOI:10.1016/j.ijhydene.2015.05.145 · 2.93 Impact Factor
  • MRS Online Proceeding Library 01/2015; 1760. DOI:10.1557/opl.2014.963
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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; 17(5). DOI:10.1039/C4CP05230G · 4.20 Impact Factor
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    ABSTRACT: β-Type titanium alloys are promising materials for orthopaedic implants due to their relatively low Young’s modulus and excellent biocompatibility. However, their strength is lower than those of α- or α + β-type titanium alloys. Grain refinement by severe plastic deformation (SPD) techniques provides a unique opportunity to enhance mechanical properties to prolong the lifetime of orthopaedic implants without changing their chemical composition. In this study, β-type Ti–45Nb (wt%) biomedical alloy in the form of 30 mm rod was subjected to hydrostatic extrusion (HE) to refine the microstructure and improve its mechanical properties. HE processing was carried out at room temperature without intermediate annealing in a multi-step process, up to an accumulative true strain of 3.5. Significant microstructure refinement from a coarse-grained region to an ultrafine-grained one was observed by optical and transmission electron microscopy. Vickers hardness measurements (HV0.2) demonstrated that the strength of the alloy increased from about 150 to 210 HV0.2. Nevertheless, the measurements of Young’s modulus by nanoindentation showed no significant changes. This finding is substantiated by X-ray diffraction analyses which did not exhibit any phase transformation out of the bcc phase being present still before processing by HE. These results thus indicate that HE is a promising SPD method to obtain significant grain refinement and enhance strength of β-type Ti–45Nb alloy without changing its low Young’s modulus, being one prerequisite for biomedical application.
    Journal of Materials Science 10/2014; 49(20). DOI:10.1007/s10853-014-8397-7 · 2.31 Impact Factor
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    ABSTRACT: A β-phase Ti–45Nb alloy was processed by several severe plastic deformation (SPD) methods as high-pressure torsion, cold rolling and folding, and hydrostatic extrusion to enhance its strength by achieving an ultrafine grained structure without affecting the Young’s modulus being close to that of bone material. Mechanical properties during processing were monitored by direct torque and Vickers hardness measurements, while the micro-/nano-structural evolution was investigated by transmission electron microscopy and X-ray line profile analysis. Simulations of both mechanical and micro-/nano-structural data were performed on the basis of the SPD work-hardening model by Zehetbauer. The simulations not only found a good agreement with the deformation-specific evolution of strength and density of individual dislocations but also well reflected mesoscopic structural quantities such as the sizes of cell/grain interiors and walls without introducing additional fitting parameters.
    Journal of Materials Science 10/2014; 49(19). DOI:10.1007/s10853-014-8320-2 · 2.31 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. DOI:10.1007/s11664-014-3179-1 · 1.68 Impact Factor
  • 08/2014; 63:012103. DOI:10.1088/1757-899X/63/1/012103
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    ABSTRACT: NiAl is an intermetallic compound with a brittle-to-ductile transition temperature at about 300°C and ambient pressure. At standard conditions, it is very difficult to deform, but fracture stress and fracture strain are increased under high hydrostatic pressure. On account of this, deformation at low temperatures is only possible at high hydrostatic pressure, as for instance used in high pressure torsion. In order to study the influence of temperature on texture evolution, small discs of polycrystalline NiAl were deformed by high pressure torsion at temperatures ranging from room temperature to 500°C. At room temperature, a typical shear texture of body centred cubic metals is found, while at 500°C a strong oblique cube component dominates. These textures can be well simulated with the viscoplastic self-consistent polycrystal deformation model using the primary and secondary slip systems activated at low and high temperatures. The oblique cube component is a dynamic recrystallization component.
    IOP Conference Series Materials Science and Engineering 08/2014; 63(1):012154. DOI:10.1088/1757-899X/63/1/012154
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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
  • R Grössinger, N Mehboob, M Zehetbauer
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    ABSTRACT: The effect of SPD treatment on the hysteresis properties of selected soft magnetic materials such as Fe3%Si and Fe17%Co steel is presented. The enhancement of the coercivity due to the mechanical deformation can be explained by the magnetoelastic energy Ehe frequency dependence of the coercivity and of the losses can be described using an eddy current based model.
    IOP Conference Series Materials Science and Engineering 06/2014; 60(1):012019. DOI:10.1088/1757-899X/60/1/012019
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    ABSTRACT: Investigation of the magnetostriction in Fe100-xMnx alloys with x = 45, 48, 50, 52 and 55, in as-cast, annealed and cold-rolled states was performed. The magnetostriction was measured by two methods: capacitance dilatometer and strain-gauge bridge. A careful structure characterization was made by XRD. For all samples the magnetostriction measured at room temperature was quasi-zero. Additionally, we found an orthorhombic structure for as-cast and annealed samples. These results are in strong disagreement with those reported in literature. Possible reasons for these disagreements are briefly discussed.
    IOP Conference Series Materials Science and Engineering 06/2014; 60(1):012006. DOI:10.1088/1757-899X/60/1/012006

Publication Stats

2k Citations
259.76 Total Impact Points

Institutions

  • 1993–2015
    • University of Vienna
      • • Physics of Nanostructured Materials Group
      • • Institut für Physikalische Chemie
      Wien, Vienna, Austria
  • 2014
    • Vienna University of Technology
      • Institute of Solid State Physics
      Wien, Vienna, Austria
  • 2004
    • Austrian Academy of Sciences
      Wien, Vienna, Austria
  • 1997
    • Charles University in Prague
      Praha, Praha, Czech Republic
  • 1991
    • University of East Anglia
      Norwich, England, United Kingdom