Universität Stuttgart

Permanent URI for this communityhttps://elib.uni-stuttgart.de/handle/11682/1

Browse

Subject: search.filters.subject.530Institute: search.filters.UBSInstitut.Institut für MaterialwissenschaftInstitute: search.filters.UBSInstitut.Fakultätsübergreifend / Sonstige EinrichtungAuthor: search.filters.author.Hickel, Tilmann

Search Results

Now showing 1 - 7 of 7
  • Thumbnail Image
    ItemOpen Access
    Phonon lifetimes throughout the Brillouin zone at elevated temperatures from experiment and ab Initio
    (2019) Glensk, Albert; Grabowski, Blazej; Hickel, Tilmann; Neugebauer, Jörg; Neuhaus, Jürgen; Hradil, Klaudia; Petry, Winfried; Leitner, Michael
    We obtain phonon lifetimes in aluminium by inelastic neutron scattering experiments, by ab initio molecular dynamics, and by perturbation theory. At elevated temperatures significant discrepancies are found between experiment and perturbation theory, which disappear when using molecular dynamics due to the inclusion of full anharmonicity and the correct treatment of the multiphonon background. We show that multiple-site interactions are small and that local pairwise anharmonicity dominates phonon-phonon interactions, which permits an efficient computation of phonon lifetimes.
  • Thumbnail Image
    ItemOpen Access
    An insight into using DFT data for Calphad modeling of solid phases in the third generation of Calphad databases, a case study for Al
    (2019) Bigdeli, Sedigheh; Zhu, Li-Fang; Glensk, Albert; Grabowski, Blazej; Lindahl, Bonnie; Hickel, Tilmann; Selleby, Malin
    In developing the next generation of Calphad databases, new models are used in which each term contributing to the Gibbs energy has a physical meaning. To continue the development, finite temperature density-functional- theory (DFT) results are used in the present work to discuss and suggest the most applicable and physically based model for Calphad assessments of solid phases above the melting point (the breakpoint for modeling the solid phase in previous assessments). These results are applied to investigate the properties of a solid in the super- heated temperature region and to replace the melting temperature as the breakpoint with a more physically based temperature, i.e., where the superheated solid collapses into the liquid. The advantages and limitations of such an approach are presented in terms of a new assessment for unary aluminum.
  • Thumbnail Image
    ItemOpen Access
    Anomalous phonon lifetime shortening in paramagnetic CrN caused by spin-lattice coupling: a combined spin and ab Initio molecular dynamics study
    (2018) Stockem, Irina; Bergman, Anders; Glensk, Albert; Hickel, Tilmann; Körmann, Fritz; Grabowski, Blazej; Neugebauer, Jörg; Alling, Björn
    We study the mutual coupling of spin fluctuations and lattice vibrations in paramagnetic CrN by combining atomistic spin dynamics and ab initio molecular dynamics. The two degrees of freedom are dynamically coupled, leading to nonadiabatic effects. Those effects suppress the phonon lifetimes at low temperature compared to an adiabatic approach. The dynamic coupling identified here provides an explanation for the experimentally observed unexpected temperature dependence of the thermal conductivity of magnetic semiconductors above the magnetic ordering temperature.
  • Thumbnail Image
    ItemOpen Access
    Calculating free energies of point defects from ab initio
    (2018) Zhang, Xi; Grabowski, Blazej; Hickel, Tilmann; Neugebauer, Jörg
    The formation and lifetime of point defects is governed by an interplay of kinetics and thermodynamic stability. To evaluate the stability under process conditions, empirical potentials and ab initio calculations at T 1⁄4 0 K are often not sufficient. Therefore, various concepts to determine the full temperature dependence of the free energy of point defects with ab initio accuracy are reviewed. Examples for the importance of accurately describing defect properties include the stabilization of vacancies by impurities and the non-Arrhenius behaviour of vacancy formation energies due to anharmonic lattice vibrations.
  • Thumbnail Image
    ItemOpen Access
    A combined experimental and first-principles based assessment of finite-temperature thermodynamic properties of intermetallic Al3Sc
    (2021) Gupta, Ankit; Tas, Bengü; Korbmacher, Dominique; Dutta, Biswanath; Neitzel, Yulia; Grabowski, Blazej; Hickel, Tilmann; Esin, Vladimir; Divinski, Sergiy V.; Wilde, Gerhard; Neugebauer, Jörg
    We present a first-principles assessment of the finite-temperature thermodynamic properties of the intermetallic Al3Sc phase including the complete spectrum of excitations and compare the theoretical findings with our dilatometric and calorimetric measurements. While significant electronic contributions to the heat capacity and thermal expansion are observed near the melting temperature, anharmonic contributions, and electron–phonon coupling effects are found to be relatively small. On the one hand, these accurate methods are used to demonstrate shortcomings of empirical predictions of phase stabilities such as the Neumann–Kopp rule. On the other hand, their combination with elasticity theory was found to provide an upper limit for the size of Al3Sc nanoprecipitates needed to maintain coherency with the host matrix. The chemo-mechanical coupling being responsible for the coherency loss of strengthening precipitates is revealed by a combination of state-of-the-art simulations and dedicated experiments. These findings can be exploited to fine-tune the microstructure of Al-Sc-based alloys to approach optimum mechanical properties
  • Thumbnail Image
    ItemOpen Access
    Temperature dependence of the stacking-fault Gibbs energy for Al, Cu, and Ni
    (2018) Zhang, Xi; Grabowski, Blazej; Körmann, Fritz; Ruban, Andrei; Gong, Yilun; Reed, Roger C.; Hickel, Tilmann; Neugebauer, Jörg
    The temperature-dependent intrinsic stacking fault Gibbs energy is computed based on highly converged density-functional-theory (DFT) calculations for the three prototype face-centered cubic metals Al, Cu, and Ni. All relevant temperature-dependent contributions are considered including electronic, vibrational, magnetic, and explicit anharmonic Gibbs energy contributions as well as coupling terms employing state-of-the-art statistical sampling techniques. Particular emphasis is put on a careful comparison of different theoretical concepts to derive the stacking fault energy such as the axial-next-nearest-neighbor-Ising (ANNNI) model or the vacuum-slab approach. Our theoretical results are compared with an extensive set of previous theoretical and experimental data. Large uncertainties in the experimental data highlight the necessity of complementary parameter-free calculations. Specifically, the temperature dependence is experimentally unknown and poorly described by thermodynamic databases. Whereas calphad derived data shows an increase of the stacking fault energy with temperature for two of the systems (Cu and Ni), our results predict a decrease for all studied systems. For Ni, the temperature induced change is in fact so strong that in the temperature interval relevant for super-alloy applications the stacking fault energy falls below one third of the low temperature value. Such large changes clearly call for a revision of the stacking fault energy when modeling or designing alloys based on such elements.
  • Thumbnail Image
    ItemOpen Access
    Melting properties of the refractory metals V and W and the binary VW alloy fully from first principles
    (2024) Zhu, Li-Fang; Srinivasan, Prashanth; Gong, Yilun; Hickel, Tilmann; Grabowski, Blazej; Körmann, Fritz; Neugebauer, Jörg
    We investigate the melting properties of the bcc refractory metals V and W, and the disordered equiatomic VW alloy from first principles. We show that thermal vibrations have a large impact on the electronic density of states (DOS) and thus considerably affect the electronic contribution to the free energy. For W, the impact of vibrations on the electronic free energy of solid and liquid is different. This difference substantially impacts the computed melting point and also triggers a large electronic heat capacity difference between solid and liquid. For V, although vibrations likewise affect the electronic free energy, the effect on the melting properties cancels out to a large degree. For the binary VW alloy we observe a similar impact as for W, but slightly weaker. The underlying physics is explained in terms of the electronic DOS of the solid and liquid phases. Based on our accurate first-principles results, we reveal critical limitations of the Sommerfeld approximation in predicting the electronic heat capacity difference between solid and liquid. Our results thus prompt us to scrutinize this approximation which is often used in phase diagram parametrizations in the CALPHAD approach, as well as for materials, such as W, that have a large electronic DOS difference between solid and liquid at the melting temperature.