D. Holec, L. Zhou, H. Riedl, C. M. Koller, P. H. Mayrhofer, M. Friak, M. Sob,
F. Koermann, J. Neugebauer, D. Music, M. A. Hartmann and F. D. Fischer,
Atomistic modeling-based design of novel materials,
Advanced Engineering Materials 19 (2017) 1600687.

Modern materials science increasingly advances via a knowledge-based development rather than a trial-and-error procedure. Gathering large amounts of data and getting deep understanding of non-trivial relationships between synthesis of materials, their structure and properties is experimentally a tedious work. Here, theoretical modeling plays a vital role. In this review paper we briefly introduce modeling approaches employed in materials science, their principles and fields of application. We then focus on atomistic modeling methods, mostly quantum mechanical ones but also Monte Carlo and classical molecular dynamics, to demonstrate their practical use on selected examples.

Z. Pei, X. Zhang, T. Hickel, M. Friak, S. Sandloebes, B. Dutta, and J. Neugebauer,
Atomic structures of twin boundaries in hexagonal close-packed metallic crystals
with particular focus on Mg,
npj Computational Materials 3 (2017) 6.

We have investigated twin boundaries in double-lattice hexagonal close-packed metallic materials, focusing on their atomic geometry. Combining accurate ab-initio methods and large-scale atomistic simulations we address the following two fundamental questions: (i) What are the possible intrinsic twin boundary structures in hcp crystals? (ii) Are these structures stable against small distortions? In order to help end a decade-long controversy over the experimental observations of the atomic structures of twin boundaries, we have determined the energetics, spectra, and transition mechanisms of the twin boundaries. Our results confirm that the mechanical stability controls structures which are observed.

P. Dey, R. Nazarov, B. Dutta, M. Yao, M. Herbig, M. Friák, T. Hickel, D. Raabe, and J. Neugebauer:
Ab initio explanation of disorder and off-stoichiometry in Fe-Mn-Al-C kappa-carbides,
Phys. Rev. B 95 (2017) 104108.

Carbides play a central role for the strength and ductility in many materials. Simulating the impact of these precipitates on the mechanical performance requires knowledge about their atomic configuration. In particular, the C content is often observed to substantially deviate from the ideal stoichiometric composition. In this work,we focus on Fe-Mn-Al-C steels, for which we determined the composition of the nanosized kappa carbides (Fe,Mn)3AlC by atom probe tomography in comparison to larger precipitates located in grain boundaries. Combining density functional theory with thermodynamic concepts, we first determine the critical temperatures for the presence of chemical and magnetic disorder in these carbides. Second, the experimentally observed reduction of the C content is explained as a compromise between the gain in chemical energy during partitioning and the elastic strains emerging in coherent microstructures.

Martin Friák, Monika Všianská, David Holec, Martin Zelený, Mojmír Šob,
Tensorial elastic properties and stability of interface states associated with Σ5(210) grain boundaries in Ni3(Al,Si),
Science and Technology of Advanced Materials 18 (2017) 273.

Grain boundaries (GBs) represent one of the most important types of defects in solids and their instability leads to catastrophic failures in materials. Grain boundaries are challenging for theoretical studies because of their distorted atomic structure. Fortunately, quantum-mechanical methods can reliably compute their properties. We calculate and analyze (tensorial) anisotropic elastic properties of periodic approximants of interface states associated with GBs in one of the most important intermetallic compounds for industrial applications, Ni3Al, appearing in Ni-based superalloys. Focusing on the Σ5(210) GBs as a case study, we assess the mechanical stability of the corresponding interface states by checking rigorous elasticity-based Born stability criteria. The critical elastic constant is found three-/five-fold softer contributing thus to the reduction of the mechanical stability of Ni3Al polycrystals (experiments showtheir GB-related failure). The tensorial elasto-chemical complexity of interface states associated with the studied GBs exemplifies itself in high sensitivity of elastic constants to the GB composition. As another example we study the impact caused by Si atoms segregating into the atomic layers close to the GB and substituting Al atoms. If wisely exploited, our study paves the way towards solute-controlled design of GB-related interface states with controlled stability and/or tensorial properties.

Martin Friák, Monika Všianská, David Holec and Mojmír Šob,
Quantum-mechanical study of tensorial elastic and high-temperature thermodynamic properties of grain boundary states in
superalloy-phase Ni3Al
Proceedings of the 38th Risø International Symposium on Materials Science,
IOP Conf. Series: Materials Science and Engineering 219 (2017) 012019.

Grain boundaries (GBs), the most important defects in solids and their properties are crucial for many materials properties including (in-)stability. Quantum-mechanical methods can reliably compute properties of GBs and we use them to analyze (tensorial) anisotropic elastic properties of interface states associated with GBs in one of the most important intermetallic compounds for industrial applications, Ni3Al. Selecting the Σ5(210) GBs as a case study because of its significant extra volume, we address the mechanical stability of the GB interface states by checking elasticity-based Born stability criteria. One critically important elastic constant, C55, is found nearly three times smaller at the GB compared with the bulk, contributing thus to the reduction of the mechanical stability of Ni3Al polycrystals. Next, comparing properties of Σ5(210) GB state which is fully relaxed with those of a Σ5(210) GB state when the supercell dimensions are kept equal to those in the bulk we conclude that lateral relaxations have only marginal impact on the studied properties. Having the complete elastic tensor of Σ5(210) GB states we combine Green’s-function based homogenization techniques and an approximative approach to the Debye model to compare thermodynamic properties of a perfect Ni3Al bulk and the Σ5(210) GB states. In particular, significant reduction of the melting temperature (to 79-81% of the bulk value) is predicted for nanometer-size grains.

S. Sandloebes, M. Friák, S. Korte-Kerzel, Z. Pei, J. Neugebauer and D. Raabe,
A rare-earth free magnesium alloy with improved intrinsic ductility,
Scientific Reports 7 (2017) Article number: 10458.

Metals are the backbone of manufacturing owing to their strength and formability. Compared to polymers they have high mass density. There is, however, one exception: magnesium. It has a density of only 1.7 g/cm3, making it the lightest structural material, 4.5 times lighter than steels, 1.7 times lighter than aluminum, and even slightly lighter than carbon fibers. Yet, the widespread use of magnesium is hampered by its intrinsic brittleness. While other metallic alloys have multiple dislocation slip systems, enabling their well-known ductility, the hexagonal lattice of magnesium offers insufficient modes of deformation, rendering it intrinsically brittle. We have developed a quantum-mechanically derived treasure map which screens solid solution combinations with electronic bonding, structure and volume descriptors for similarity to the ductile magnesium-rare earth alloys. Using this insight we synthesized a surprisingly simple, compositionally lean, low-cost and industry-compatible new alloy which is over 4 times more ductile and 40% stronger than pure magnesium. The alloy contains 1 wt.% aluminum and 0.1 wt.% calcium, two inexpensive elements which are compatible with downstream recycling constraints.

Y. Jiraskova, J. Bursik, A. Zemanova, J. Cizek, P. Hruska, O. Zivotsky,
Effect of hydrogen on Fe and Pd alloying and physical properties,
Internationl Journal of Hydrogen Energy 42 (2017) 6885.

Fe95PdH5 and Fe95Pd5 nanopowders were prepared by mechanical alloying (MA) using high energy ball milling. Two ways of the sample preparation were compared in order to examine the effect of hydrogen on MA: (i) A-sample: Pd powder was pre-charged with hydrogen and subsequent MA of Fe and PdH was performed in Ar protective atmosphere; (ii) B-sample: Fe and Pd powders were mechanically alloyed in hydrogen atmosphere. The milling procedure was interrupted and alloying followed by several experimental methods up to the final state achieved after 50 h of milling.

A. Ostapovets, A. Serra,
Slip dislocation and twin nucleation mechanisms in hcp metals
Journal of Materials Science, 52 (2017) 533-540

A new nucleation mechanism is proposed for {1 0 -1 1} deformation twin in hcp materials. The mechanism is based on the results of atomistic computer simulations. It was found that under high shear stress applied on {1 0 -1 1} plane, the core of a slip dislocation can transform to a twin embryo. The transformation and subsequent twin growth are accompanied by nucleation and migration of interfacial defects including disconnections and stacking faults. The paper provides the analysis of the nature of these defects and describes the reactions between them.

Oldřich Schneeweiss, Martin Friák, Marie Dudová, David Holec, Mojmír Šob, Dominik Kriegner, Václav Holý, Přemysl Beran, Easo P. George, Jörg Neugebauer, and Antonín Dlouhý,
Magnetic properties of the CrMnFeCoNi high-entropy alloy,
Physical Review B 96 (2017) 014437-1 - 014437-13.

We present experimental data showing that the equiatomic CrMnFeCoNi high-entropy alloy undergoes two magnetic transformations at temperatures below 100 K while maintaining its fcc structure down to 3 K. The first transition, paramagnetic to spin glass, was detected at 93 K and the second transition of the ferromagnetic type occurred at 38 K. Field-assisted cooling below 38 K resulted in a systematic vertical shift of the hysteresis curves. Strength and direction of the associated magnetization bias was proportional to the strength and direction of the cooling field and shows a linear dependence with a slope of 0.006 +/- 0.001 emu/T. The local magnetic moments of individual atoms in the CrMnFeCoNi quinary fcc random solid solution were investigated by ab initio (electronic density functional theory) calculations. Results of the numerical analysis suggest that, irrespective of the initial configuration of local magnetic moments, the magnetic moments associated with Cr atoms align antiferromagnetically with respect to a cumulative magnetic moment of their first coordination shell. The ab initio calculations further showed that the magnetic moments of Fe and Mn atoms remain strong (between 1.5 and 2 Bohr magneton), while the local moments of Ni atoms effectively vanish. These results indicate that interactions of Mn- and/or Fe-located moments with the surrounding magnetic structure account for the observed macroscopic magnetization bias.

D1 PUBLICATION (IF = 31.083):
P. Lejček, M. Šob, V. Paidar,
Interfacial segregation and grain boundary embrittlement: An overview and critical assessment of experimental data and calculated results,
Progress in Materials Science 87 (2017) 83–139.

One of the most dangerous technical failures of materials is intergranular brittle fracture (temper embrittlement) as it proceeds very quickly and its appearance is often hardly predictable. It is known that this phenomenon is closely related to the chemistry of grain boundaries and to the difference of the segregation energies of the grain boundaries and the free surfaces (Rice–Wang model). To elucidate the effect of individual solutes on embrittlement of various materials such as steels and nickel-base superalloys, grain boundary and surface segregation was extensively studied in many laboratories. As a result, numerous data on surface and grain boundary segregation have been gathered in literature. They were obtained in two main ways, by computer simulations and from experiments. Consequently, these results are frequently applied to quantify the embrittling potency of individual solutes. Unfortunately, the values of the segregation energy of a solute at grain boundaries as well as at the surfaces obtained by various authors sometimes differ by more than one order of magnitude: such a difference is unacceptable as it cannot provide us with representative view on the problem of material temper embrittlement. In some cases it seems that these values do not properly reflect physical reality or are incorrectly interpreted. Due to the above mentioned large scatter of the segregation and embrittlement data a critical assessment of the literature results is highly needed which would enable the reader to avoid both the well known and less well known pitfalls in this field. Here we summarize the available data on interfacial segregation and embrittlement of various solutes in nickel and bcc iron and critically discuss their reliability, assessing also limitations of individual approaches employed to determine the values of segregation and strengthening/embrittling energies, such as density functional theory, Monte Carlo method, molecular statics and dynamics and tight binding on the theoretical side, and Auger electron spectroscopy, 3D tomographic atom probe, and electron microscopy techniques on the experimental side. We show that experimental methods have serious limitations which can be overcome by accepting reasonable assumptions and models. On the other hand, the theoretical approaches are limited by the size of the computational repeat cell used for the calculations of the segregation energy. In both cases, a careful critical analysis of the available segregation energy and/or enthalpy reflecting physical reality allows to assess the reliability of these values and their applicability in analysis of intergranular brittle fracture in steels and nickel-base alloys.

CHLUPOVÁ A, HECZKO M, OBRTLÍK K, POLÁK J, ROUPCOVÁ P, BERAN P, KRUML T: Mechanical properties of high niobium TiAl alloys doped with Mo and C. Mater. Des. 99 (2016) 284–292.

Z. Strelcova, P. Kulhanek, M. Friak, H.O. Fabritius, M. Petrov, J. Neugebauer, J. Koca,
The structure and dynamics of chitin nanofibrils in an aqueous environment revealed by molecular dynamics simulations, RSC Advances 6 (2016) 30710-30721.

Chitin is one of the most abundant structural biomolecules in nature, where it occurs in the form of nanofibrils that are the smallest building blocks for many biological structural materials. Despite this fact, little is known about the structural properties of these nanofibrils. Here, we present a theoretical study of a single chitin molecule and 10 α-chitin nanofibrils with different numbers of chains in an aqueous environment that mimics the conditions in natural systems during self-assembly.

K. Castkova, H. Hadraba, A. Matousek, P. Roupcova, Z. Chlup, L. Novotna, J. Cihlar: Synthesis of Ca,Y-zirconia/hydroxyapatite nanoparticles and composites, Journal of the European Ceramic Society 36 (2016) 2903-2912.

Lunacek J., Zivotsky O., Jiraskova Y., Bursik J., Janos P.: Thermally stimulated iron oxide transformations and magnetic behaviour of cerium dioxide/iron oxide reactive sorbents.
Mat. Charact. 120 (2016) 295-303.

The paper was devoted to detailed study of the magnetically separable sorbents based on a cerium dioxide/iron oxide composite. An identification of iron oxides, their transformations Fe3O4 → gamma-Fe2O3 (epsilon-Fe2O3) → alpha-Fe2O3 and quantitative estimation of the grain size distributions in dependence on calcination temperatures have contributed to detail description of magnetic properties.

Stella Vallejos, Naděžda Pizúrová, Isabel Gràcia, Carlos Sotelo-Vazquez,
Jan Čechal, Chris Blackman, Ivan Parkin, and Carles Cané:
ZnO Rods with Exposed {100} Facets Grown via a Self-Catalyzed
Vapor-Solid Mechanism and Their Photocatalytic and Gas Sensing Properties.
ACS Appl. Mater. Interfaces, 8 (2016) pp 33335-33342.

Chemické, elektrické a optické vlastnosti nanostruktur na bázi ZnO činí z tohoto materiálu ideálního adepta pro aplikace v optoelektronice, fotovoltaice, fotokatalýze, nebo pro detekci plynů. Metodou AACVD (aerosol – assisted chemical vapor deposition) se podařilo vypěstovat monokrystalické nanotyčinky ZnO s definovaným typem povrchů (fazet) a výbornými sledovanými vlastnostmi. Získané kolumnární krystaly vykazovaly šestnáctkrát vyšší fotokatalytickou aktivitu než komerční fotokatalytická skla a fotoaktivovaná detekce plynů při pokojové teplotě (demonstrována na redukčních plynech, jako jsou CO a C2H6O, vykázala lepší selektivitu vzhledem k srovnávacím testům.
Tyto výsledky přispějí například k ochraně životního prostředí (rozklad rizikových látek fotokatalýzou) a bezpečnosti (přesnější detekce nebezpečných plynů).

M.M. Vasić, P. Roupcová, N. Pizúrová, S, Stevanovic, V.A. Blagojević, T. Žák, D.M. Minić:
Thermally Induced Structural Transformations of Fe40Ni40P14B6 Amorphous Alloy

Thermal stability and thermally induced structural transformations of Fe40Ni40P14B6 amorphous alloy were examined under non-isothermal and isothermal conditions. Formation of metastable alpha-(Fe,Ni), and stable gamma-(Fe,Ni) and (Fe,Ni)(3)(P,B) crystalline phases as the main crystallization products was observed, while the presence of small amounts of other crystalline phases like Fe23B6 and Fe2NiB was indicated by electron diffraction in HRTEM. Thermomagnetic curve indicated that Fe content in different crystalline phases is very different, resulting in markedly different Curie temperatures after crystallization. Transmission electron microscopy and atomic force microscopy study suggested multiple-layered platelet-shaped morphology, both on the surface and in the bulk of the crystallized alloy sample. The thermal treatment heating rate and maximum temperature affected surface roughness and grain size inhomogeneity.

Zivotsky O., Titov A., Jiraskova Y., Bursik J., Hendrych A., Hrabovska K., Tsepelev VS.:
Surface and bulk magnetic anisotropy in bilayered CoSiB/FeNbCuSiB and FeNbSiB/FeSiB ribbons,
J. Alloys Comp. 681 (2016) 402-411.

The study was devoted to the surface and bulk magnetic anisotropy of the bilayered Co72.5Si12.5B15/Fe73.5Nb3Cu1Si13.5B9 and Fe74.5Nb3Si13.5B9/Fe77.5Si7.5B15 ribbons prepared by modernized planar flow casting technology. The thickness of ribbons was about 36 mm while the interlayer thickness reaches typically a few μm. A bending of ribbons has led to changes in both surface and bulk anisotropies which, completed with microstructure analysis, have given the basic complex data prospectively usable for sensor applications.

I. Turek
Static transport properties of random alloys: Vertex corrections in conserving approximations,
Phys. Rev. B 93 (2016) 245114.

Description in Czech (see English below)
Takzvané vrcholové opravy hrají významnou úlohu v moderních teoretických přístupech k transportním vlastnostem náhodných slitin. V publikované práci byla podána teoretická analýza dokazující, že běžné numerické výpočty těchto vrcholových oprav vedou k inverzi singulární matice. Dále byl navržen jednoduchý postup k odstranění této singularity; jeho efektivita byla úspěšně testována pro zbytkovou rezistivitu vybraných náhodných slitin, včetně systémů spinově polarizovaných a relativistických, se zvláštním zřetelem k roli různých symetrií (inverze prostoru a času).

The so-called vertex corrections play an important role in modern theoretical approaches to transport properties of random alloys. In the published work, a theoretical analysis has been given which proves that current numerical evaluations of the vertex corrections lead to the inversion of a singular matrix. A simple procedure removing this singularity has been proposed and its efficiency has been successfully tested for the residual resistivity of selected random alloys, including spin-polarized and relativistic systems, with particular attention paid to the role of different symmetries (inversion of space and time).

J. Kudrnovsky, V. Drchal, I. Turek:
Exchange and spin-orbit induced phenomena in diluted (Ga,Mn)As from first principles,
Phys. Rev. B 94 (2016) 054428.

Description in Czech (see English below)
Manganem dopovaný GaAs představuje jeden z nejdůkladněji studovaných zředěných feromagnetických polovodičů. V práci byly jeho vybrané magnetické vlastnosti způsobené výměnnou interakcí (Curieova teplota, spinová tuhost) a spin-orbitální vazbou (anomální Hallův jev, anizotropní magnetorezistence, Gilbertovo tlumení) studovány z prvních principů. Byla získána poměrně dobrá shoda s existujícími experimenty na vzorcích obsahujících minimální množství kompenzujících strukturních poruch. Studium závislosti Gilbertova tlumení na koncentraci manganu odhalilo, že na rozdíl od tlumení v kovových slitinách, kde se primárně uplatňuje zejména hustota elektronových stavů na Fermiho energii, je tlumení v (Ga,Mn)As silně ovlivněno i celkovým magnetickým momentem systému.

Mn-doped GaAs represents one of the best investigated diluted ferromagnetic semiconductors. In this work, its magnetic properties driven by exchange interactions (Curie temperature, spin stiffness) and spin-orbit coupling (anomalous Hall effect, anisotropic magnetoresistance, Gilbert damping) have been studied from first principles. Good agreement with existing experiments on well-annealed samples containing only a small amount of compensating defects has been obtained. The study of the dependence of the Gilbert damping on Mn concentration revealed that—in contrast to metallic alloys where the damping is related mainly to the density of electron states at the Fermi energy—the damping in (Ga,Mn)As is also strongly influenced by the total magnetic moment of the system.

I. Turek, J. Kudrnovsky, V. Drchal,
Coherence and stiffness of spin waves in diluted ferromagnets.
Physical Review B 94 (2016) 174447.

Description (see English below)
Tuhost spinových vln představuje důležitou charakteristiku feromagnetů, která je těsně spjata např. s teplotní závislostí magnetizace při nízkých teplotách. V publikované práci byla navržena účinná numerická technika k jejímu výpočtu pro náhodné zředěné feromagnety. Rozpracovaný postup využívá koherentního charakteru nízkoenergetických spinových vln v neuspořádaných feromagnetech a z něj plynoucích interferenčních jevů. Vyvinutý formalismus byl použit ke studiu manganem dopovaných polovodičů GaAs, Ga(As,P) a GaN, které jsou významné pro spintroniku.

The spin-wave stiffness represents an important property of ferromagnets, related closely, e.g., to the temperature dependence of magnetization at low temperatures. In the published paper, an efficient numerical technique has been presented for its evaluation for random diluted ferromagnets. The developed procedure rests on the coherent nature of low-energy spin waves in disordered ferromagnets and on accompanying interference phenomena. The developed formalism has been used for investigation of manganese-doped GaAs, Ga(As,P) and GaN semiconductors which are relevant for spintronics.

A.R. Ćosović, T. Žák, S.B. Glisić, M.D. Sokić, S.S. Lazarević, V.R. Ćosović, A.M. Orlović:
Synthesis of nano-crystalline NiFe2O4 powders in subcritical and supercritical ethanol

Nano-crystalline nickel ferrite has a broad range of applications due to its favourable magnetic properties. Those characteristics can be significantly influenced by the synthesis pathway including methods conducted in the presence of supercritical alcohols. Nano-crystalline NiFe2O4 powders were obtained in the reaction under subcritical and supercritical conditions of ethanol. Both high pressure synthesis routes resulted in powders with smaller primary particles and higher mesoporosity than co-precipitation method. Upon the annealing treatment the average crystallite size increased while material structure remained uniform, resulting in significantly enhanced magnetic properties, such as coercivity and remanence. It was found that synthesis under supercritical conditions provides higher conversion but also material with larger average crystallite size upon annealing. The time and temperature of annealing stage significantly influenced the morphology and magnetic properties of the obtained powders.

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19. 09. 2017