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Quantum-mechanical study of magnetic properties of superalloy nanocomposite phase Fe2AlTi
Slávik, Anton; Miháliková, Ivana; Friák, Martin; Všianská, Monika; Šob, Mojmír
2018 - anglický
The L21-structure Fe2AlTi intermetallic compound is one of the two phases identified in Fe-Al-Ti superalloy nanocomposites. Experimental data related to low-temperature magnetic properties of this Heusler compound indicate that magnetic moment is about 0.1 Bohr magneton per formula unit. In contrast, previous quantum-mechanical calculations predicted Fe2AlTi to have much higher magnetic moment, 0.9 Bohr magneton per formula unit. In order to solve this discrepancy between the theory and experiment we have performed a series of quantum-mechanical fix-spin-moment calculations and compared our results with those for non-magnetic state. It turns out that the total energy of the non-magnetic state is only by 10.73 meV/atom higher than that of the magnetic state. When applying Boltzmann statistics to this very small energy difference we predict that the non-magnetic state appears at non-zero temperatures with significant probabilities (for instance, 22.36 % at T = 100 K) and reduces the overall magnetic moment. As another mechanism lowering the magnetization we studied selected shape deformations, in particular trigonal shearing. Fe2AlTi exhibits a compression-tension asymmetry with respect to these strains and, for example, the strain 0.08 destabilizes the spin-polarized state, leaving the non-magnetic state as the only stable one.
Klíčová slova:
Ab initio calculations; Fe-Al based superalloys; Fixed-spin-moment; Nanocomposites
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Quantum-mechanical study of magnetic properties of superalloy nanocomposite phase Fe2AlTi
The L21-structure Fe2AlTi intermetallic compound is one of the two phases identified in Fe-Al-Ti superalloy nanocomposites. Experimental data related to low-temperature magnetic properties of this ...
DISPLACEMENT FIELD ANALYSIS IN WEDGE SPLITTING TEST\nSPECIMEN BY ODM METHOD
Seitl, S.; Miarka, P.; Růžička, V.; Malíková, Lucie; Merta, I.
2018 - anglický
Multi-parameter fracture mechanics is nowadays quite extensively applied when cracked\nstructures/specimens are investigated. The reason is that it has been shown that it can be\nhelpful and bring more accurate results when fracture processes happen in a larger\nregion around the crack tip, what can be typical for material like concrete or other\nmaterials with quasi-brittle behaviour. Various relative crack length configurations have\nbeen chosen in order to investigate the importance of the higher-order terms of the\nWilliams power expansion on the crack-tip stress field distribution in Wedge splitting\ntest specimen. The higher-order terms are calculated by means of the over-deterministic\nmethod from displacements of nodes around the crack tip obtained by a finite element\nanalysis in different radial distances from the crack tip. The effect of the constraint level\nis investigated. Although the third and higher terms of the Williams series are very often\nneglected, their influence on the opening stress values is investigated.
Klíčová slova:
ODM, WST; WST; constraint; Williams series; FEM; stress intensity factor; displacement
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
DISPLACEMENT FIELD ANALYSIS IN WEDGE SPLITTING TEST\nSPECIMEN BY ODM METHOD
Multi-parameter fracture mechanics is nowadays quite extensively applied when cracked\nstructures/specimens are investigated. The reason is that it has been shown that it can be\nhelpful and bring ...
MICROSTRUCTURE AND COMPOSITION OF FINE PARTICLES RELEASED BY CAR BRAKING
Švábenská, Eva; Roupcová, Pavla; Pizúrová, Naděžda; Schneeweiss, Oldřich
2018 - anglický
Vehicular traffic is connected with large volume of fine particles released during brake processes of cars. Our research is focused on the phase, structure and chemical analysis of the fine particles taken from some car brake parts by their services. The information on structure and phase composition was obtained by X-Ray Powder Diffraction, Mossbauer Spectroscopy, scanning electron microscopy with EDX and transmission electron microscopy. The results of the wear debris analysis are compared with original brake materials components. Most of recognized particles are based mainly on iron oxides. Wear brake particles are discussed in the relation to the potential risk to the environment and human health.
Klíčová slova:
Nanoparticles; wear debris; environment
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
MICROSTRUCTURE AND COMPOSITION OF FINE PARTICLES RELEASED BY CAR BRAKING
Vehicular traffic is connected with large volume of fine particles released during brake processes of cars. Our research is focused on the phase, structure and chemical analysis of the fine particles ...
First-principles study of interface energies in Fe-Al-based superalloy nanocomposites
Miháliková, Ivana; Slávik, Anton; Friák, Martin; Všianská, Monika; Koutná, N.; Holec, David; Šob, Mojmír
2018 - anglický
Fe-Al-based nanocomposites with a superalloy-type of microstructure constitute a very promising class of materials. They possess a great potential as an alternative to the currently used steel grades in high temperature applications. Intermetallics-containing nanocomposites, such as those with the Fe3Al compound being one of the phases, may open a way towards future automotive and energy-conversion technologies with lower fuel consumption and reduced environmental impact. We employ quantum-mechanical calculations to analyze relations between ordering tendencies of Al atoms in the disordered Fe-18.75at.%Al phase on one hand and thermodynamic, structural and magnetic properties of Fe-Al-based nanocomposites on the other. When comparing supercells modeling disordered Fe-Al phase with different atomic distribution of atoms we find out that the supercell without 1st and 2nd nearest neighbor Al-Al pairs has a lower energy than that mimicking a perfect disorder (a special quasi-random structure, SQS). Further, coherent interfaces with (001), (110) and (1-10) crystallographic orientations between Fe3Al compound and SQS Fe-Al phase have higher energies than those exhibiting atomic distribution without 1st and 2nd nearest neighbor Al-Al pairs.
Klíčová slova:
Ab initio calculations; Fe-Al based superalloys; Interface energies; Nanocomposites
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
First-principles study of interface energies in Fe-Al-based superalloy nanocomposites
Fe-Al-based nanocomposites with a superalloy-type of microstructure constitute a very promising class of materials. They possess a great potential as an alternative to the currently used steel grades ...
Estimation of equilibrium hydrogen pressure - A new method
Čermák, Jiří; Král, Lubomír
2018 - anglický
A new method is proposed to estimation of hydrogen pressure in equilibrium with hydride phase in a hydrogen\nstorage material. It is applicable both for hydrogen absorption and desorption in cases where the hydride phase\nis formed by nucleation and growth mechanism. The proposed method saves considerably the experimental\ntime replacing the conventional time consuming measurement of pressure-composition isotherms, the so\ncalled PCT curves. The proposed evaluation procedure is illustrated using hydrogen chemi-sorption at\ntemperatures 623 K, 573 K and 523 K in chosen hydrogen storage alloys Mg-Si-C, Mg-Li-C and Mg-Na-C.
Klíčová slova:
hydrogen storage; magnesium; new method
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Estimation of equilibrium hydrogen pressure - A new method
A new method is proposed to estimation of hydrogen pressure in equilibrium with hydride phase in a hydrogen\nstorage material. It is applicable both for hydrogen absorption and desorption in cases ...
HYDROGEN STORAGE PROPERTIES OF GRAPHENE OXIDE MATERIALS PREPARED BY DIFFERENT WAYS
Král, Lubomír; Čermák, Jiří; Bytesnikova, Z.
2018 - anglický
Graphene-based materials show unique properties. These single layered materials consist of 2D structure of carbon atoms, belong to the strongest known materials, that are very mechanically flexible, optically transparent and that are excellent electrical and thermal conductors. Recently, several studies on these types of materials have highlighted the potential of this material for hydrogen storage (HS) and raised new hopes for the development of an effective solid-state HS media. In the present paper, the structure and HS properties of graphene oxide (GO) and chemically reduced graphene oxide (rGO) produced by different procedures were studied. Hydrogen sorption characteristics of GO and rGO were measured using the Sieverts-type gas sorption analyzer PCT-Pro Setaram Instrumentation. The study of HS was carried out at temperature range from 198 K to 423 K under hydrogen pressure from 1x10(-4) to 4 MPa. \n\nFor the HS point of view, the advantage of GO or rGO compared to graphene, is the presence of multiple chemical groups that can be used for introducing modifiers and their superior spreading on the materials surface. The suitably functionalized GO or rGO materials could potentially exhibit outstanding HS properties.
Klíčová slova:
Graphene oxide; hydrogen storage; adsorption; adsorption
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
HYDROGEN STORAGE PROPERTIES OF GRAPHENE OXIDE MATERIALS PREPARED BY DIFFERENT WAYS
Graphene-based materials show unique properties. These single layered materials consist of 2D structure of carbon atoms, belong to the strongest known materials, that are very mechanically flexible, ...
Estimation of mechanical parameters of thin films using finite element analysis
Tinoco Navaro, Hector Andres; Holzer, Jakub; Pikálek, Tomáš; Buchta, Zdeněk; Lazar, Josef; Chlupová, Alice; Kruml, Tomáš; Hutař, Pavel
2018 - anglický
This study shows a methodology to estimate mechanical parameters of thin films by means of a bulge\ntest and a numerical approach. The methodology is based on the combination of finite element analysis with a\nclassical analytical method. Finite element modelling was conducted for monolayer (Si3N4) membranes of 2x2mm\nwith the aim to approximate both the load-deflection curves experimentally measured and the classical loaddeflection\nanalytical model. Error functions were constructed and minimized to delimit a coupled solution space\nbetween Young’s modulus and Poison’s ratio. In a traditional bulge test analysis only one of the elastic properties\ncan be determined due to that there is not unique solution in the estimations of these parameters. However, both\nelastic parameters were determined through the proposed numerical procedure which compares the deformed\nsurfaces for a specific set of optimal elastic parameters computed. Results shows that the estimated elastic\nproperties agree with corresponding values determined by other methods in the literature
Klíčová slova:
bulge test; thin film; finite element analysis; elastic properties
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Estimation of mechanical parameters of thin films using finite element analysis
This study shows a methodology to estimate mechanical parameters of thin films by means of a bulge\ntest and a numerical approach. The methodology is based on the combination of finite element ...
CALIBRATION FUNCTIONS FOR EDGE CRACKS UNDER SELECTED BENDING LOADS.\n\n
Seitl, Stanislav; Miarka, Petr
2018 - anglický
Fatigue cracks are found during the regular structural inspections. To precisely\ndescribe/suggest of fatigue cracks propagation throughout structure and for designed\nservice life, the knowledge of calibration functions is important. The cracks usually\npropagate from the edge or the surface of the structural element. The theoretical model\nof fatigue crack propagation is based on linear fracture mechanics (Paris law). Steel\nstructural elements are subjected to various bending load (three- and four-point bending,\npure bending etc.). The calibration functions for the edge cracks are calculated for\nvarious load and appropriate polynomial function independent on the distance are\nproposed for 3PB and 4PB load.
Klíčová slova:
Fracture mechanics; calibration curve; stress intensity factor; Paris-Erdogan Law; edge
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
CALIBRATION FUNCTIONS FOR EDGE CRACKS UNDER SELECTED BENDING LOADS.\n\n
Fatigue cracks are found during the regular structural inspections. To precisely\ndescribe/suggest of fatigue cracks propagation throughout structure and for designed\nservice life, the knowledge of ...
Evaluation of thin discontinuities in planar conducting materials using the diffraction of electromagnetic field
Savin, A.; Nový, F.; Fintová, Stanislava; Steigmann, R.
2017 - anglický
The current stage of nondestructive evaluation techniques imposes the development of new electromagnetic (EM) methods that are based on high spatial resolution and increased sensitivity. In order to achieve high performance, the work frequencies must be either radifrequencies or microwaves. At these frequencies, at the dielectric/conductor interface, plasmon polaritons can appear, propagating between conductive regions as evanescent waves. In order to use the evanescent wave that can appear even if the slits width is much smaller that the wavwelength of incident EM wave, a sensor with metamaterial (MM) is used. The study of the EM field diffraction against the edge of long thin discontinuity placed under the inspected surface of a conductive plate has been performed using the geometrical optics principles. This type of sensor having the reception coils shielded by a conductive screen with a circular aperture placed in the front of reception coil of emission reception sensor has been developed and 'transported' information for obtaining of magnified image of the conductive structures inspected. This work presents a sensor, using MM conical Swiss roll type that allows the propagation of evanescent waves and the electromagnetic images are magnified. The test method can be successfully applied in a variety of applications of maxim importance such as defect/damage detection in materials used in automotive and aviation technologies. Applying this testing method, spatial resolution can be improved.
Klíčová slova:
Electromagnetic fields; Electromagnetic wave reflection; Electromagnetic waves; Geometrical optics; Image resolution; Nondestructive examination; Testing
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Evaluation of thin discontinuities in planar conducting materials using the diffraction of electromagnetic field
The current stage of nondestructive evaluation techniques imposes the development of new electromagnetic (EM) methods that are based on high spatial resolution and increased sensitivity. In order to ...
Effect of the load eccentricity on fracture behaviour of cementitious materials subjected to the modified compact tension test
Seitl, Stanislav; Ríos, J. D.; Cifuentes, H.; Veselý, V.
2017 - anglický
Fracture properties of quasi-brittle cementitious composites are typically determined from the load–displacement response recorded during a fracture test by using the work-of-fracture method or possibly other relevant fracture models. Our contribution is focused on a set of experimental tests which are used to study the fracture behaviour on notched dog-bone-shaped specimens made of cementitious materials. These specimens are subjected to modified compact tension (ModCT) test under a specific range of eccentricity of the tensile load. This type of test generates a stress state in the specimen ligament which combines a direct tension with a defined level of bending due to eccentricity of the tensile load. Several values of relative notch length are also considered. While the crack propagates, a variety of stress states, resulting in variations in the crack-tip stress and deformation constraint, appears in the ligament zone because of the changes in the eccentricity of the applied load, which influences the fracture behaviour of the investigated specimens. The K-calibration, T-stress, CMOD and COD curves for ModCT specimens are introduced and variations of these curves with varying load eccentricity are discussed.
Klíčová slova:
constraint; fracture; load accentricity
Plné texty jsou dostupné na jednotlivých ústavech Akademie věd ČR.
Effect of the load eccentricity on fracture behaviour of cementitious materials subjected to the modified compact tension test
Fracture properties of quasi-brittle cementitious composites are typically determined from the load–displacement response recorded during a fracture test by using the work-of-fracture method or ...
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