Browsing by Author "Neugebauer, Jörg"

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Open Access
Ab initio based method to study structural phase transitions in dynamically unstable crystals, with new insights on the β to ω transformation in titanium
(2019) Korbmacher, Dominique; Glensk, Albert; Duff, Andrew Ian; Finnis, Michael W.; Grabowski, Blazej; Neugebauer, Jörg
We present an approach that enables an efficient and accurate study of dynamically unstable crystals over the full temperature range. The approach is based on an interatomic potential fitted to ab initio molecular dynamics energies for both the high- and low-temperature stable phases. We verify by comparison to explicit ab initio simulations that such a bespoke potential, for which we use here the functional form of the embedded atom method, provides accurate transformation temperatures and atomistic features of the transformation. The accuracy of the potential makes it an ideal tool to study the important impact of finite size and finite time effects. We apply our approach to the dynamically unstable β (bcc) titanium phase and study in detail the transformation to the low-temperature stable hexagonal ω phase. We find a large set of previously unreported linear-chain disordered (LCD) structures made up of three types of [111]β linear-chain defects that exhibit randomly disordered arrangements in the (111)β plane.
Open Access
Ab initio modelling of solute segregation energies to a general grain boundary
(2017) Huber, Liam; Grabowski, Blazej; Militzer, Matthias; Neugebauer, Jörg; Rottler, Jörg
We apply a quantum mechanical/molecular mechanical (QM/MM) multiscale approach to calculate the segregation energies of Mg and Pb to two kinds of grain boundaries in Al. The first boundary, a symmetric (310)[001] ∑5 tilt boundary, is also tractable using traditional QM calculations, and serves as a validation for the QM/MM method. The second boundary is a general, low-symmetry tilt boundary that is completely inaccessible to pure QM calculations. QM/MM results for both of these boundaries are used to evaluate the accuracy of empirical (EAM) potentials for the Al-Mg and Al-Pb alloy systems. Based on these results we develop a physical model for the segregation energy based on elastic interaction and bond breaking terms. Both MM calculations with the EAM potentials and the model work quantitatively well for describing Mg-GB interaction across a wide range of local environments. For Pb, MM performance is weaker and the model provides only qualitative insight, demonstrating the utility of a QM/MM approach.
Open Access
Accurate electronic free energies of the 3d, 4d, and 5d transition metals at high temperatures
(2017) Zhang, Xi; Grabowski, Blazej; Körmann, Fritz; Freysoldt, Christoph; Neugebauer, Jörg
Free energies of bulk materials are nowadays routinely computed by density functional theory. In particular for metals, electronic excitations can significantly contribute to the free energy. For an ideal static lattice, this contribution can be obtained at low computational cost, e.g., from the electronic density of states derived at T = 0 K or by utilizing the Sommerfeld approximation. The error introduced by these approximations at elevated temperatures is rarely known. The error arising from the ideal lattice approximation is likewise unexplored but computationally much more challenging to overcome. In order to shed light on these issues we have computed the electronic free energies for all 3d, 4d, and 5d transition elements on the ideal lattices of the bcc, fcc, and hcp structures using finite-temperature density-functional theory. For a subset of elements we have explored the impact of explicit thermal vibrations on the electronic free energies by using ab initio molecular dynamics simulations. We provide an analysis of the observed chemical trends in terms of the electronic density of states and the canonical d band model and quantify the errors in the approximate methods. The electronic contribution to the heat capacities and the corresponding errors due to the different approximations are studied as well.
Open Access
Anharmonicity in bcc refractory elements : a detailed ab initio analysis
(2023) Srinivasan, Prashanth; Shapeev, Alexander; Neugebauer, Jörg; Körmann, Fritz; Grabowski, Blazej
Explicit anharmonicity, defined as the vibrational contribution beyond the quasiharmonic approximation, is qualitatively different between the group V and group VI bcc refractory elements. Group V elements show a small and mostly negative anharmonic entropy, whereas group VI elements have a large positive anharmonic entropy, strongly increasing with temperature. Here, we explain this difference utilizing highly accurate anharmonic free energies and entropies from ab initio calculations for Nb and Ta (group V), and Mo and W (group VI). The numerically calculated entropies are in agreement with prior experimental data. The difference in behavior between the two sets of elements arises not from their high-temperature behavior but rather from the 0K　quasiharmonic reference state. We understand this by analyzing the 0K and the high-temperature phonon density of states and the electronic density of states. The qualitative difference disappears when the anharmonicity is instead referenced with a high-temperature effective harmonic potential. However, even for an optimized effective harmonic reference, the remaining effective anharmonicity is significant. The reason is that the anharmonicity in the bcc systems - carried by asymmetric distributions in the nearest neighbors - can never be accounted for by a harmonically restricted potential.
Open 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.
Open Access
Approximating the impact of nuclear quantum effects on thermodynamic properties of crystalline solids by temperature remapping
(2022) Dsouza, Raynol; Huber, Liam; Grabowski, Blazej; Neugebauer, Jörg
When computing finite-temperature properties of materials with atomistic simulations, nuclear quantum effects are often neglected or approximated at the quasiharmonic level. The inclusion of these effects beyond this level using approaches like the path integral method is often not feasible due to their large computational effort. We discuss and evaluate the performance of a temperature-remapping approach that links the finite-temperature quantum system to its best classical surrogate via a temperature map. This map, which is constructed using the internal energies of classical and quantum harmonic oscillators, is shown to accurately capture the impact of quantum effects on thermodynamic properties at an additional cost that is negligible compared to classical molecular dynamics simulations. Results from this approach show excellent agreement with previously reported path integral Monte Carlo simulation results for diamond cubic carbon and silicon. The approach is also shown to work well for obtaining thermodynamic properties of light metals and for the prediction of the fcc to bcc phase transition in calcium.
Open Access
Atomic scale processes of phase transformations in nanocrystalline NiTi shape-memory alloys
(2016) Ko, Won-Seok; Maisel, Sascha B.; Grabowski, Blazej; Jeon, Jong Bae; Neugebauer, Jörg
Molecular dynamics simulations are performed to investigate temperature- and stress-induced phase transformations in nanocrystalline nickel-titanium shape-memory alloys. Our results provide detailed insights into the origins of the experimentally reported characteristics of phase transformations at the nanoscale, such as the decrease of the transformation temperature with grain size and the disappearance of the plateau in the stress-strain response. The relevant atomic scale processes, such as nucleation, growth, and twinning are analyzed and explained. We suggest that a single, unified mechanism - dominated by the contribution of a local transformation strain - explains the characteristics of both temperature- and stress-induced phase transformations in nanocrystalline nickel-titanium.
Open Access
Atomistic migration mechanisms of atomically flat, stepped, and kinked grain boundaries
(2016) Hadian, Raheleh; Grabowski, Blazej; Race, Christopher Peter; Neugebauer, Jörg
We studied the migration behavior of mixed tilt and twist grain boundaries in the vicinity of a symmetric tilt ⟨111⟩ Σ7 grain boundary in aluminum. We show that these grain boundaries fall into two main categories of stepped and kinked grain boundaries around the atomically flat symmetric tilt boundary. Using these structures together with size converged molecular dynamics simulations and investigating snapshots of the boundaries during migration, we obtain an intuitive and quantitative description of the kinetic and atomistic mechanisms of the migration of general mixed grain boundaries. This description is closely related to well-known concepts in surface growth such as step and kink-flow mechanisms and allows us to derive analytical kinetic models that explain the dependence of the migration barrier on the driving force. Using this insight we are able to extract energy barrier data for the experimentally relevant case of vanishing driving forces that are not accessible from direct molecular dynamics simulations and to classify arbitrary boundaries based on their mesoscopic structures.
Open 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.
Open Access
Chemically induced local lattice distortions versus structural phase transformations in compositionally complex alloys
(2021) Ikeda, Yuji; Gubaev, Konstantin; Neugebauer, Jörg; Grabowski, Blazej; Körmann, Fritz
Recent experiments show that the chemical composition of body-centered cubic (bcc) refractory high entropy alloys (HEAs) can be tuned to enable transformation-induced plasticity (TRIP), which significantly improves the ductility of these alloys. This calls for an accurate and efficient method to map the structural stability as a function of composition. A key challenge for atomistic simulations is to separate the structural transformation between the bcc and the ω phases from the intrinsic local lattice distortions in such chemically disordered alloys. To solve this issue, we develop a method that utilizes a symmetry analysis to detect differences in the crystal structures. Utilizing this method in combination with ab initio calculations, we demonstrate that local lattice distortions largely affect the phase stability of Ti-Zr-Hf-Ta and Ti-Zr-Nb-Hf-Ta HEAs. If relaxation effects are properly taken into account, the predicted compositions near the bcc–hcp energetic equilibrium are close to the experimental compositions, for which good strength and ductility due to the TRIP effect are observed.
Open 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
Open Access
Correlation analysis of strongly fluctuating atomic volumes, charges, and stresses in body-centered cubic refractory high-entropy alloys
(2020) Ishibashi, Shoji; Ikeda, Yuji; Körmann, Fritz; Grabowski, Blazej; Neugebauer, Jörg
Local lattice distortions in a series of body-centered cubic alloys, including refractory high-entropy alloys, are investigated by means of atomic volumes, atomic charges, and atomic stresses defined by the Bader charge analysis based on first-principles calculations. Analyzing the extensive data sets, we find large distributions of these atomic properties for each element in each alloy, indicating a large impact of the varying local chemical environments. We show that these local-environment effects can be well understood and captured already by the first and the second nearest neighbor shells. Based on this insight, we employ linear regression models up to the second nearest neighbor shell to accurately predict these atomic properties. Finally, we find that the elementwise-averaged values of the atomic properties correlate linearly with the averaged valence-electron concentration of the considered alloys.
Open Access
Development and application of a Ni-Ti interatomic potential with high predictive accuracy of the martensitic phase transition
(2015) Ko, Won-Seok; Grabowski, Blazej; Neugebauer, Jörg
Phase transitions in nickel-titanium shape-memory alloys are investigated by means of atomistic simulations. A second nearest-neighbor modified embedded-atom method interatomic potential for the binary nickel-titanium system is determined by improving the unary descriptions of pure nickel and pure titanium, especially regarding the physical properties at finite temperatures. The resulting potential reproduces accurately the hexagonal-close-packed to body-centered-cubic phase transition in Ti and the martensitic B2−B19′ transformation in equiatomic NiTi. Subsequent large-scale molecular-dynamics simulations validate that the developed potential can be successfully applied for studies on temperature- and stress-induced martensitic phase transitions related to core applications of shape-memory alloys. A simulation of the temperature-induced phase transition provides insights into the effect of sizes and constraints on the formation of nanotwinned martensite structures with multiple domains. A simulation of the stress-induced phase transition of a nanosized pillar indicates a full recovery of the initial structure after the loading and unloading processes, illustrating a superelastic behavior of the target system.
Open Access
Efficient approach to compute melting properties fully from ab initio with application to Cu
(2017) Zhu, Li-Fang; Grabowski, Blazej; Neugebauer, Jörg
Applying thermodynamic integration within an ab initio-based free-energy approach is a state-of-the-art method to calculate melting points of materials. However, the high computational cost and the reliance on a good reference system for calculating the liquid free energy have so far hindered a general application. To overcome these challenges, we propose the two-optimized references thermodynamic integration using Langevin dynamics (TOR-TILD) method in this work by extending the two-stage upsampled thermodynamic integration using Langevin dynamics (TU-TILD) method, which has been originally developed to obtain anharmonic free energies of solids, to the calculation of liquid free energies. The core idea of TOR-TILD is to fit two empirical potentials to the energies from density functional theory based molecular dynamics runs for the solid and the liquid phase and to use these potentials as reference systems for thermodynamic integration. Because the empirical potentials closely reproduce the ab initio system in the relevant part of the phase space the convergence of the thermodynamic integration is very rapid. Therefore, the proposed approach improves significantly the computational efficiency while preserving the required accuracy. As a test case, we apply TOR-TILD to fcc Cu computing not only the melting point but various other melting properties, such as the entropy and enthalpy of fusion and the volume change upon melting. The generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional and the local-density approximation (LDA) are used. Using both functionals gives a reliable ab initio confidence interval for the melting point, the enthalpy of fusion, and entropy of fusion.
Open Access
Finite-temperature interplay of structural stability, chemical complexity, and elastic properties of bcc multicomponent alloys from ab initio trained machine-learning potentials
(2021) Gubaev, Konstantin; Ikeda, Yuji; Tasnádi, Ferenc; Neugebauer, Jörg; Shapeev, Alexander V.; Grabowski, Blazej; Körmann, Fritz
An active learning approach to train machine-learning interatomic potentials (moment tensor potentials) for multicomponent alloys to ab initio data is presented. Employing this approach, the disordered body-centered cubic (bcc) TiZrHfTax system with varying Ta concentration is investigated via molecular dynamics simulations. Our results show a strong interplay between elastic properties and the structural ω phase stability, strongly affecting the mechanical properties. Based on these insights we systematically screen composition space for regimes where elastic constants show little or no temperature dependence (elinvar effect).
Open Access
GB code: a grain boundary generation code
(2018) Hadian, Sherry; Grabowski, Blazej; Neugebauer, Jörg
Grain boundaries (GBs) are crystalline borders between single crystals in materials microstructure. They play an important role in mechanical, chemical or electronic response of materials and are therefore essential to materials science and physics.
Open Access
Impact of asymmetric martensite and austenite nucleation and growth behavior on the phase stability and hysteresis of freestanding shape-memory nanoparticles
(2018) Ko, Won-Seok; Grabowski, Blazej; Neugebauer, Jörg
Martensitic transformations in nanoscaled shape-memory alloys exhibit characteristic features absent for the bulk counterparts. Detailed understanding is required for applications in micro- and nanoelectromechanical systems, and experimental limitations render atomistic simulation an important complementary approach. Using a recently developed, accurate potential we investigate the phase transformation in freestanding Ni-Ti shape-memory nanoparticles with molecular-dynamics simulations. The results confirm that the decrease in the transformation temperature with decreasing particle size is correlated with an overstabilization of the austenitic surface energy over the martensitic surface energy. However, a detailed atomistic analysis of the nucleation and growth behavior reveals an unexpected difference in the mechanisms determining the austenite finish and martensite start temperature. While the austenite finish temperature is directly affected by a contribution of the surface energy difference, the martensite start temperature is mostly affected by the transformation strain, contrary to general expectations. This insight not only explains the reduced transformation temperature but also the reduced thermal hysteresis in freestanding nanoparticles.
Open Access
A machine learning approach to model solute grain boundary segregation
(2018) Huber, Liam; Hadian, Raheleh; Grabowski, Blazej; Neugebauer, Jörg
Even minute amounts of one solute atom per one million bulk atoms may give rise to qualitative changes in the mechanical response and fracture resistance of modern structural materials. These changes are commonly related to enrichment by several orders of magnitude of the solutes at structural defects in the host lattice. The underlying concept - segregation - is thus fundamental in materials science. To include it in modern strategies of materials design, accurate and realistic computational modelling tools are necessary. However, the enormous number of defect configurations as well as sites solutes can occupy requires models which rely on severe approximations. In the present study we combine a high-throughput study containing more than 1 million data points with machine learning to derive a computationally highly efficient framework which opens the opportunity to model this important mechanism on a routine basis.
Open Access
Migration mechanisms of a faceted grain boundary
(2018) Hadian, Raheleh; Grabowski, Blazej; Finnis, Michael W.; Neugebauer, Jörg
We report molecular dynamics simulations and their analysis for a mixed tilt and twist grain boundary vicinal to the Σ7 symmetric tilt boundary of the type {123} in aluminum. When minimized in energy at 0K, a grain boundary of this type exhibits nanofacets that contain kinks. We observe that at higher temperatures of migration simulations, given extended annealing times, it is energetically favorable for these nanofacets to coalesce into a large terrace-facet structure. Therefore, we initiate the simulations from such a structure and study as a function of applied driving force and temperature how the boundary migrates. We find the migration of a faceted boundary can be described in terms of the flow of steps. The migration is dominated at lower driving force by the collective motion of the steps incorporated in the facet, and at higher driving forces by the step detachment from the terrace-facet junction and propagation of steps across the terraces. The velocity of steps on terraces is faster than their velocity when incorporated in the facet, and very much faster than the velocity of the facet profile itself, which is almost stationary. A simple kinetic Monte Carlo model matches the broad kinematic features revealed by the molecular dynamics. Since the mechanisms seem likely to be very general on kinked grain-boundary planes, the step-flow description is a promising approach to more quantitative modeling of general grain boundaries.
Open Access
Performance of the standard exchange-correlation functionals in predicting melting properties fully from first principles: application to Al and magnetic Ni
(2020) Zhu, Li-Fang; Körmann, Fritz; Ruban, Andrei V.; Neugebauer, Jörg; Grabowski, Blazej
We apply the efficient two-optimized references thermodynamic integration using Langevin dynamics method [Phys. Rev. B 96, 224202 (2017)] to calculate highly accurate melting properties of Al and magnetic Ni from first principles. For Ni we carefully investigate the impact of magnetism on the liquid and solid free energies including longitudinal spin fluctuations and the reverse influence of atomic vibrations on magnetic properties. We show that magnetic fluctuations are effectively canceling out for both phases and are thus not altering the predicted melting temperature. For both elements, the generalized gradient approximation (GGA) and the local-density approximation (LDA) are used for the exchange-correlation functional revealing a reliable ab initio confidence interval capturing the respective experimental melting point, enthalpy of fusion, and entropy of fusion.