-
Crystal structure identification with 3D convolutional neural networks with application to high-pressure phase transitions in SiO$_2$
Authors:
Linus C. Erhard,
Daniel Utt,
Arne J. Klomp,
Karsten Albe
Abstract:
Efficient, reliable and easy-to-use structure recognition of atomic environments is essential for the analysis of atomic scale computer simulations. In this work, we train two neuronal network (NN) architectures, namely PointNet and dynamic graph convolutional NN (DG-CNN) using different hyperparameters and training regimes to assess their performance in structure identification tasks of atomistic…
▽ More
Efficient, reliable and easy-to-use structure recognition of atomic environments is essential for the analysis of atomic scale computer simulations. In this work, we train two neuronal network (NN) architectures, namely PointNet and dynamic graph convolutional NN (DG-CNN) using different hyperparameters and training regimes to assess their performance in structure identification tasks of atomistic structure data. We show benchmarks on simple crystal structures, where we can compare against established methods. The approach is subsequently extended to structurally more complex SiO$_2$ phases. By making use of this structure recognition tool, we are able to achieve a deeper understanding of the crystallization process in amorphous SiO$_2$ under shock compression. Lastly, we show how the NN based structure identification workflows can be integrated into OVITO using its python interface.
△ Less
Submitted 8 May, 2024;
originally announced May 2024.
-
Thermodynamics of vacancies in concentrated solid solutions: From dilute Ni-alloys to the Cantor system
Authors:
Daniel Utt,
Alexander Stukowski,
Karsten Albe
Abstract:
The vacancy concentration at finite temperatures is studied for a series of (CoCrFeMn)$_{1-x_\mathrm{Ni}}$Ni$_{x_\mathrm{Ni}}$ alloys by grand-canonical Monte-Carlo (MC) simulations. The vacancy formation energies are calculated from a classical interatomic potential and exhibit a distribution due to the different chemical environments of the vacated sites. In dilute alloys, this distribution feat…
▽ More
The vacancy concentration at finite temperatures is studied for a series of (CoCrFeMn)$_{1-x_\mathrm{Ni}}$Ni$_{x_\mathrm{Ni}}$ alloys by grand-canonical Monte-Carlo (MC) simulations. The vacancy formation energies are calculated from a classical interatomic potential and exhibit a distribution due to the different chemical environments of the vacated sites. In dilute alloys, this distribution features multiple discrete peaks, while concentrated alloys exhibit an unimodal distribution as there are many different chemical environments of similar vacancy formation energy. MC simulations using a numerically efficient bond-counting model confirm that the vacancy concentration even in concentrated alloys may be calculated by the established Maxwell-Boltzmann equation weighted by the given distribution of formation energies. We calculate the variation of vacancy concentration as function of Ni content in the (CoCrFeMn)$_{x_\mathrm{Ni}}$Ni$_{1-x_\mathrm{Ni}}$ and prove the excellent agreement of the thermodynamic model and the results from the grand-canonical Monte-Carlo simulations.
△ Less
Submitted 25 June, 2021; v1 submitted 6 April, 2021;
originally announced April 2021.
-
The origin of jerky dislocation motion in high-entropy alloys
Authors:
Daniel Utt,
Subin Lee,
Yaolong Xing,
Hyejin Jeong,
Alexander Stukowski,
Sang Ho Oh,
Gerhard Dehm,
Karsten Albe
Abstract:
Dislocations in single-phase concentrated random alloys, including high- entropy alloys (HEAs), repeatedly encounter pinning during glide, resulting in jerky dislocation motion. While solute-dislocation interaction is well understood in conventional alloys, the origin of individual pinning points in concentrated random alloys is a matter of debate. In this work, we investigate the origin of disloc…
▽ More
Dislocations in single-phase concentrated random alloys, including high- entropy alloys (HEAs), repeatedly encounter pinning during glide, resulting in jerky dislocation motion. While solute-dislocation interaction is well understood in conventional alloys, the origin of individual pinning points in concentrated random alloys is a matter of debate. In this work, we investigate the origin of dislocation pinning in the CoCrFeMnNi HEA. In- situ transmission electron microscopy studies reveal wavy dislocation lines and a jagged glide motion under external loading, even though no segregation or clustering is found around Shockley partial dislocations. Atomistic simulations reproduce the jerky dislocation motion and link the repeated pinning to local fluctuations in the Peierls friction. We demonstrate that the density of high local Peierls friction is proportional to the critical stress required for dislocation glide and the dislocation mobility.
△ Less
Submitted 23 August, 2022; v1 submitted 22 July, 2020;
originally announced July 2020.
-
Experimental and theoretical study of tracer diffusion in a series of (CoCrFeMn)$_{100-x}$Ni$_x$ alloys
Authors:
Josua Kottke,
Daniel Utt,
Mathilde Laurent-Brocq,
Adnan Fareed,
Daniel Gaertner,
Loic Perriere,
Lukasz Rogal,
Alexander Stukowski,
Karsten Albe,
Sergiy V. Divinski,
Gerhard Wilde
Abstract:
Tracer diffusion of all constituting elements is studied at various temperatures in a series of (CoCrFeMn)$_{100-x}$Ni$_x$ alloys with compositions ranging from pure Ni to the equiatomic CoCrFeMnNi high-entropy alloy. At a given homologous temperature, the measured tracer diffusion coefficients change non-monotonically along the transition from pure Ni to the concentrated alloys and finally to the…
▽ More
Tracer diffusion of all constituting elements is studied at various temperatures in a series of (CoCrFeMn)$_{100-x}$Ni$_x$ alloys with compositions ranging from pure Ni to the equiatomic CoCrFeMnNi high-entropy alloy. At a given homologous temperature, the measured tracer diffusion coefficients change non-monotonically along the transition from pure Ni to the concentrated alloys and finally to the equiatomic CoCrFeMnNi alloy. This is explained by atomistic Monte-Carlo simulations based on a modified embedded-atom potentials, which reveal that local heterogeneities of the atomic configurations around a vacancy cause correlation effects and induce significant deviations from predictions of the random alloy model.
△ Less
Submitted 20 March, 2020;
originally announced March 2020.
-
Flat Electronic Bands in Long Sequences of Rhombohedral-stacked Multilayer Graphene
Authors:
Hugo Henck,
Jose Avila,
Zeineb Ben Aziza,
Debora Pierucci,
Jacopo Baima,
Betül Pamuk,
Julien Chaste,
Daniel Utt,
Miroslav Bartos,
Karol Nogajewski,
Benjamin A. Piot,
Milan Orlita,
Marek Potemski,
Matteo Calandra,
Maria C. Asensio,
Francesco Mauri,
Clément Faugeras,
Abdelkarim Ouerghi
Abstract:
The crystallographic stacking order in multilayer graphene plays an important role in determining its electronic properties. It has been predicted that a rhombohedral (ABC) stacking displays a conducting surface state with flat electronic dispersion. In such a flat band, the role of electron-electron correlation is enhanced possibly resulting in high Tc superconductivity, charge density wave or ma…
▽ More
The crystallographic stacking order in multilayer graphene plays an important role in determining its electronic properties. It has been predicted that a rhombohedral (ABC) stacking displays a conducting surface state with flat electronic dispersion. In such a flat band, the role of electron-electron correlation is enhanced possibly resulting in high Tc superconductivity, charge density wave or magnetic orders. Clean experimental band structure measurements of ABC stacked specimens are missing because the samples are usually too small in size. Here, we directly image the band structure of large multilayer graphene flake containing approximately 14 consecutive ABC layers. Angle-resolved photoemission spectroscopy experiments reveal the flat electronic bands near the K point extends by 0.13 Å-1 at the Fermi level at liquid nitrogen temperature. First-principle calculations identify the electronic ground state as an antiferromagnetic state with a band gap of about 40 meV.
△ Less
Submitted 25 June, 2018; v1 submitted 10 August, 2017;
originally announced August 2017.
-
Local segregation versus irradiation effects in high-entropy alloys: Steady-state conditions in a driven system
Authors:
Leonie Koch,
Fredric Granberg,
Tobias Brink,
Daniel Utt,
Karsten Albe,
Flyura Djurabekova,
Kai Nordlund
Abstract:
We study order transitions and defect formation in a model high-entropy alloy (CuNiCoFe) under ion irradiation by means of molecular dynamics simulations. Using a hybrid Monte-Carlo/molecular dynamics scheme a model alloy is generated which is thermodynamically stabilized by configurational entropy at elevated temperatures, but partly decomposes at lower temperatures by copper precipation. Both th…
▽ More
We study order transitions and defect formation in a model high-entropy alloy (CuNiCoFe) under ion irradiation by means of molecular dynamics simulations. Using a hybrid Monte-Carlo/molecular dynamics scheme a model alloy is generated which is thermodynamically stabilized by configurational entropy at elevated temperatures, but partly decomposes at lower temperatures by copper precipation. Both the high-entropy and the multiphase sample are then subjected to simulated particle irradiation. The damage accumulation is analyzed and compared to an elemental Ni reference system. The results reveal that the high-entropy alloy---independent of the initial configuration---installs a certain fraction of short-range order even under particle irradiation. Moreover, the results provide evidence that defect accumulation is reduced in the high-entropy alloy. This is because the reduced mobility of point defects leads to a steady state of defect creation and annihilation. The lattice defects generated by irradiation are shown to act as sinks for Cu segregation.
△ Less
Submitted 4 October, 2017; v1 submitted 10 April, 2017;
originally announced April 2017.