Mgr. Martin Friák, Ph.D.
Position: head
Group: Electrical and Magnetic Properties Group
Rooms: 310a
Phone numbers: +420 532 290 400
Reseacher ID: F - 9741 - 2014
Theory-guided design of novel Fe-Al-based superalloys

Nikola Koutná, David Holec, Martin Friák, Paul H. Mayrhofer, Mojmír Šob,
Stability and elasticity of metastable solid solutions and superlattices
in the MoN–TaN system: First-principles calculations,
Materials & Design 144 (2018) 310–322.

In order to develop design rules for novel nitride-based coatings, we investigate trends in thermodynamic, structural, elastic, and electronic properties of Mo1−xTaxN single-phase alloys together with (MoN)1−x/(TaN)x superlattices. Our calculations predict that hexagonal Mo1−xTaxN are the overall most stable ones, followed by the disordered cubic solid solutions and superlattices. The disordered cubic systems are energetically clearly favoured over their ordered counterparts. To explain this unexpected phenomenon, we perform an in-depth structural analysis of bond-lengths and angles, revealing that the disordered phase is structurally between the NaCl-type and the hexagonal NiAs-type modifications. Similarly, the bi-axial coherency stresses in MoN/TaN break the cubic symmetry beyond simple tetragonal distortions, leading to a new tetragonal ζ-phase (P4/nmm, #129). Both ζ-MoN and ζ-TaN have lower formation energy than their cubic counterparts. Unlike the cubic TaN, the ζ-TaN is also dynamically stable. The hexagonal alloys are predicted to be extremely hard, though, much less ductile than the cubic polymorphs and superlattices.

Martin Friák, David Holec, Mojmír Šob,
An ab initio study of mechanical and dynamical stability of MoSi2,
Journal of Alloys and Compounds 746 (2018) 720-728.

We present a quantum-mechanical study of changes in the electronic structure, total energy, elastic properties, phonon spectra and structure of molybdenum disilicide (MoSi2) with tetragonal C11b structure due to uniaxial strains along the [001] direction, biaxial (epitaxial) loads within the (001) plane as well as triaxial (volumetric) strains/stresses. Total energies and optimized structural parameters are computed by the Vienna Ab initio Simulation Package (VASP) using the local density approximation (LDA). When simulating extreme loading conditions that may be relevant for highly-strained regions we predict a semi-metal to metal phase transition that is connected with the onset of mechanical instability for higher values of triaxial loads as well as many other multi-axial loading conditions. The instability is assessed by both rigorous Born-Huang criteria connected with elastic stiffness coefficients as well as by phonon spectra computed for all three straining modes. The values of theoretical tensile strength of MoSi2 for uniaxial, biaxial and triaxial loading corresponding to the first phonon instability amount to 30, 41 and 35 GPa, respectively. We show that the semi-metal to metal transition is connected with the softening of acoustic phonons at the Gamma point rather than with the instability of other phonon modes.


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.

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.


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.

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Last update
19. 06. 2018