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Name Orava, Dr. Jiri
Department 3111
Address IFW Dresden
Helmholtzstraße 20
01069 Dresden
Phone Number +49-351-4659-644
Email j.orava(at)ifw-dresden.de

Key Research Activities

  • Physical-chemical properties of metallic glasses
  • Tailoing the relationship between structure and macroscopic properties of glasses and liquids
  • Synthesis and physical-chemical properties of chalcogenide glasses
  • Chalcogenide phase-change materials for non-volatile computer memory
  • Crystallization and stability of glass-forming liquids
  • Glasses and glass-ceramics for optics and photonics

 

List of Publications

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Research Highlights

Phase transformations in a Cu-Zr-Al metallic glass

A combination of conventional calorimetry, ultra-fast scanning calorimetry and resistive heating gives access to heating rates exceeding 6 orders of magnitude allowing to probe crystallization kinetics and mechanisms in a Cu-Zr-Al metallic glass. Continuous-heating-transformation and double-peak time-temperature-transformation diagrams are constructed and related to the formation of glass-crystal composites with enhanced ductility. The metastable B2-CuZr phase becomes dominantly formed at a heating rate of ~102 K s–1 and higher. A critical heating rate to bypass crystallization is ~10,000 K s–1. For isothermal annealing at >850 K, only one crystallization event is detected; for lower temperatures, a complex two-step transformation occurs.A combination of conventional calorimetry, ultra-fast scanning calorimetry and resistive heating gives access to heating rates exceeding 6 orders of magnitude allowing to probe crystallization kinetics and mechanisms in a Cu-Zr-Al metallic glass. Continuous-heating-transformation and double-peak time-temperature-transformation diagrams are constructed and related to the formation of glass-crystal composites with enhanced ductility. The metastable B2-CuZr phase becomes dominantly formed at a heating rate of ~102 K s–1 and higher. A critical heating rate to bypass crystallization is ~10,000 K s–1. For isothermal annealing at >850 K, only one crystallization event is detected; for lower temperatures, a complex two-step transformation occurs.

 

Q. Cheng, X. Hang, I. Kaban, I. Soldatov, W. H. Wang, Y. H. Sun & J. Orava,  Scr. Mater. 183 (2020) 61.

Fast-heating-induced formation of metallic-glass/crystal composites with enhanced plasticity

Bulk metallic glasses are known to have poor plasticity which limits their application as structural materials. Due to the lack of atomic periodicity in metallic glasses, their mechanical properties cannot be controlled the same way as in crystalline materials. Fast-heating-induced, a heating rate of 10 K s−1 and higher, crystallization of ductile nanocrystalline phase(s) leads to enhanced plasticity of metallic-glass/crystal composites. Here, an overview of controlling the microstructure on fast heating, the suggested crystallization mechanism of metastable phases and the principle of enhanced plasticity of the composites is presented and discussed, with a special focus on Cu–Zr-based metallic glasses.

 

J. Orava, I. Kaban, M. Benkocka, X. Han, I. Soldatov & A. L. Greer, Thermochimica Acta 677 (2019) 198.

Elemental re-distribution inside shear bands revealed by correlative atom-probe tomography and electron microscopy in a deformed metallic glass

A density variation in shear bands visible by electron microscopy is correlated with compositionally altered locations measured by atom-probe tomography in plastically-deformed Al85.6Y7.5Fe5.8 metallic-glass ribbons. Two compositionally distinct regions are identified along shear bands, one is Al-rich (~92 at.%), the other is Al- depleted (~82.5 at.%) and both regions show marginal concentration fluctuations of Y and Fe. The elemental re-distribution is observed within shear bands only, and no chemical exchange with the surrounding glassy ma- trix is observed.

 

S. Balachandran, J. Orava, M. Köhler, A. J. Breen, I. Kaban, D. Raabe, & M. Herbig, Scr. Mater. 168 (2019) 14.

Correlating ultrafast calorimetry, viscosity and structural measurements in liquid GeTe and Ge15Te85

Two distinct trends in the temperature dependence of viscosity, measured directly and inferred from calorimetry by analyzing crystallization kinetics, can be correlated with the temperature evolution of the height of the first peak of the x-ray total structure factor for liquid GeTe and Ge15Te85. The phase-change chalcogenide GeTe is a high-fragility liquid with the kinetic fragility value of 76, at the glass-transition temperature, being between those for liquid (Ag,In)-doped Sb2Te and Ge2Sb2Te5. The viscosity of the high-temperature liquid shows Arrhenius kinetics on cooling to the melting point, and the structure factor conforms to the fragile liquid. For liquid Ge15Te85, the temperature evolution of the structure factor suggests a transition in the temperature range of about 100 K above the melting. The crystallization shows a wide range of Arrhenius kinetics in the supercooled liquid region. This finding combined with the dynamic viscosity measurements is interpreted by invoking a weak fragile-to-strong crossover on cooling the liquid Ge15Te85. The differences in structures and dynamics of liquid GeTe and Ge15Te85 appear closely correlated to their distinctly different crystallization mechanisms.

 

H. Weber, J. Orava, I. Kaban, J. Pries & A. L. Greer, Phys. Rev. Mater. 2 (2018) 093405.

Photoluminescence in pulsed-laser deposited GeGaSbS:Er films

The complex dielectric function, the Er3+ Stokes emission at ≈1540 nm and the upconversion photoluminescence at ≈990 nm of pulsed-laser deposited thin-film GeGaSbS:Er3+ are studied. The linear-refractive-index dispersion is obtained by fitting spectroscopic ellipsometry data to the Sellmeier and the Cody-Lorentz parametrizations. The former model is used to calculate nonlinear refractive index. The Stokes emission intensity at ≈1540 nm, originating from the Er3+: 4I13/24I15/2 transition under a 980 nm pumping, diminishes with increasing refractive index and decreasing optical-bandgap energy of the films. An attempt to relate differences in the optical and photoluminescence properties to the atomic structure elucidated from Raman spectra analysis is presented. The emission probability is highest for the structure with the lowest fraction of (semi)metallic bonds, the highest content of homonuclear S-S bonds and the lowest Ge/S ratio. In such a film, the clustering of Er3+ ions, which is responsible for the intensity weakening, is suppressed. The film shows ≈990 nm upconversion photoluminescence originating from the Er3+: 4I11/24I15/2 transition under a 1550 nm excitation. The upconversion peak shape resembles that of the compositionally and structurally similar bulk glass.

 

L. Strizik, S. N. Yannopoulos, V. Benekou, J. Oswald, M. Pavlista, V. Prokop, T. Wagner & J. Orava, Opt. Mater. 85 (2018) 246.