|
|
Computer modeling of twin-boundaries in shape memory alloys
Heczko, Martin ; Pokluda, Jaroslav (referee) ; Zelený, Martin (advisor)
This Master‘s thesis is focused on theoretical study of twinning in magnetic shape memory alloys based on Ni2MnGa using ab initio calculations of electronic structure within the projector augmented wave method. In particular, the effect of increasing concentration of manganese at the expense of gallium was studied on total energy and stress profiles along different deformation paths in the (10-1)[101] shear system of non-modulated martensite. Further, this work deals with the effect of the concentration of manganese on the energy of planar fault caused by presence of partial dislocation due to motion of twin boundary. The results show that the shear modulus in studied shear system increases with the increasing concentration of manganese as well as energy barrier and deformation characteristics along shear deformation paths increases, which makes the shear more difficult in Mn-rich alloys. Increasing concentration of manganese also leads to rising the planar fault energy. All these effects can be responsible for lower mobility of twin boundaries in alloys with higher concentration of manganese.
|
|
|
Topological band theory of relativistic spintronics in antiferromagnets
Šmejkal, Libor ; Jungwirth, Tomáš (advisor) ; Kuneš, Jan (referee) ; Shick, Alexander (referee)
Nanoelectronics and spintronics are concerned with writing, transporting, and reading information stored in electronic charge and spin degrees of freedom at the nanoscale. Past few years have shown that two spintronics effects discovered in the 19th century, namely anisotropic magnetoresistance and anomalous Hall effect, can be used also for sensing antiferromagnetism which opened the field of antiferromagnetic spintronics. The more than a century of controversial studies of these effects have shown their relativistic spin-orbit coupling and spin-polarisation symmetry breaking origin. However, a complete understanding of these effects and a fully predictive theory capable of identifying novel suitable antiferromagnetic materials are still lacking. Here, by extending modern symmetry and topology concepts in condensed matter physics, we have further developed the theory of anisotropic magnetoresistance and spontaneous Hall effect. Our approach is based on magnetic symmetry and topology analysis of antiferromagnetic energy bands, Bloch spectral functions, and Berry curvatures calculated from the state-of-the- art first-principle theory. This guided us to the prediction of two novel, previously unanticipated effects: relativistic metal-insulator transition from antiferromagnetic Dirac fermions, and crystal Hall...
|
|
|
Computational prediction of solubility limits in solid solutions
Fikar, Ondřej ; Friák, Martin (referee) ; Zelený, Martin (advisor)
This diploma thesis is focused on a theoretical study of the phase stability of solid solutions in selected aluminium and silver alloys. The ab initio calculations were performed using projected augmented waves method and the thermal dependencies of thermodynamic quantities were acquired using phonon calculations. The main focus of this work is the of aluminium-germanium alloy, while the other examined alloys (Al-Pb, Ag-Ge and Ag-Pb) serve the purpose of comparison of the solid solubility prediction and its reliability in systems with different composition. The temperatures, at which the solid solutions become stable, were evaluated using the energy difference between possible states and this evaluation was carried out for different contributions to the total energy. Also the electronic and phonon densities of states were calculated for all pure elements and solid solutions. The temperatures of solubility were compared to the experimental ones provided by the CALPHAD method and the individual contributions to the total energy were determined and depicted. The results obtained in this work tend to underestimate temperatures of solubility of individual solid solutions by hundreds of Kelvin.
|
|
|
Quantum-mechanical study of magnetic properties of superalloy nanocomposite phase Fe2AlTi
Slávik, Anton ; Miháliková, Ivana ; Friák, Martin ; Všianská, Monika ; Šob, Mojmír
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.
|
|
|
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
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.
|
|
|
Magnetic shape memory alloys - ab initio approach
Heczko, Martin ; Šesták, Petr (referee) ; Zelený, Martin (advisor)
This Bachelor’s thesis is focused on theoretical study of magnetic shape memory alloys based on Ni2MnGa using ab initio calculations of electronic structure within the projector augmented wave method. In particular, the effect of increasing concertation of manganese instead of gallium was studied on total-energy and magnetic moment profiles along the tetragonal deformation path between austenite phase with cubic L21 structure and phase of nonmodulated martensite. Further, the effect of manganese atoms distribution within the gallium sublattice was studied as well as changes of this distribution under applied tetragonal deformation. At last but not at least, the elastic constants for austenitic and martensitic structures of studied alloys were calculated. The results show that the non-modulated martensite stabilized with increasing concentration of manganese, because its total energy decreased. The energetic barrier between austenitic and martensitic structures also decreased, which means the metastable austenite will change to unstable.
|
|
|
Ab initio calculation of doping in Ni2MnGa alloy
Janovec, Jozef ; Černý, Miroslav (referee) ; Zelený, Martin (advisor)
Cieľom tejto práce je teoretické štúdium systému Ni-Mn-Ga, vykazujúceho efekt magnetickej tvarovej pamäti. Pri výpočtoch bola použitá metóda Exaktných Mufiin-Tin Orbitálov v kombinácii s aproximáciou koherentného potenciálu vrámci Korringa-Kohn-Rostoker formalizmu. Totálna energia bola spočítaná pomocou metódy úplnej nábojovej hustoty. Skúmaný bol vplyv dopovania zinkom alebo kadmiom na totálne energie pozdĺž tetragonálnej deformačnej dráhy a následne na teplotu martenzitickej premeny TM a Curieho teplotu TC. Taktiež boli skúmané nestechiometrické zliatiny prebytkom Mn na úkor Ga. Výsledky predikujú nárast TM po všetkých pozorovaných substitúciách v podmriežke Ga a pokles TM po substitúciách v podmriežke Mn. Curieho teplotu znižujú všetky študované substitúcie s výnimkou zmeny magnetického usporiadania v martensite nestechiometrickej zliatiny.
|
|
|
Dynamic stability of Ni fcc crystal under isotropic tension
Řehák, Petr ; Černý, M.
Lattice dynamics and stability of fcc crystal of Ni under isotropic (hydrostatic) tensile loading are studied from first principles using supercell method and a harmonic approximation. According to the results, strength of the crystal is determined by occurrence of an instability related to soft phonons with finite wave vector. On the other hand, the critical strains and stresses associated with such instabilities are only slightly lower than those related to the volumetric instability.
|
|
|
Solvent effects on ion pairing and photoionization in water
Pluhařová, Eva ; Jungwirth, Pavel (advisor) ; Nachtigall, Petr (referee) ; Laage, Damien (referee)
Title: Solvent effects on ion pairing and photoionization in water Author: Mgr. et Ing. Eva Pluhařová Department: Physical and Macromoleculer Chemistry Advisor: Prof. Pavel Jungwirth, DSc., IOCB AS CR, v.v.i. Advisor's e-mail address: pavel.jungwirth@uochb.cas.cz Abstract: Various methods of theoretical chemistry, namely classical molecular dynamics simulations with empirical force fields, ab initio molecular dynamics, enhanced sampling methods, and ab initio calculations were used to provide new insight into ion pairing and photoionization in aqueous solutions. Systems mod- eling aqueous solutions of decreasing size were investigated by computational methods of increasing level of sophistication. In a classical molecular dynamics study of concentrated lithium salt solutions, the electronic continuum correction to account for polarization provided qualita- tive improvement over the conventional non-polarizable force fields and enabled molecular interpretation of neutron scattering measurements. The same model- ing approach was also successful in predicting the affinity of halide ions to the solution/oil interface. By combining ab initio molecular dynamics and potential of mean force cal- culations, we designed a reliable computational protocol for calculating the free energy profile for an ion pair...
|
|
|
FIRST PRINCIPLES ANALYSIS OF MECHANICAL STABILITY OF SOLID CRYSTALS
Řehák, Petr ; Buršík, Jiří (referee) ; Šob, Mojmír (referee) ; Černý, Miroslav (advisor)
The aim of the author’s research in the period of his PhD study was the analysis of mechanical stability of cubic crystals under external loading. This work demonstrates several methods used for a study of mechanical stability of fcc crystals (C, Al, Ir, Pt, Au) during isotropic (hydrostatic) tensile loading. Ab initio methods were used for this purpose. Studied crystals were subjected to simulated isotropic tensile deformation and an analysis of elastic stability was performed. This analysis shows that first elastic instability in Al, Pt and Au crystals corresponds to vanishing of the trigonal shear modulus and diamond, Ir remains stable up to the state of maximum isotropic stress. According to the calculated band structure diamond crystal preserves his insulating character up to the onset of instability. Consequently, phonon spectra of all crystals were calculated using the linear response method and their dynamic stability was assessed. Obtained results reveal soft phonon modes in Al, Pt and Ir before an occurrence of elastic instability. Selected short-wavelength instabilities are confirmed using models of microscopic deformation and also using dispersion curves obtained by a supercell method. The observed instabilities lower critical strains related to the volumetric instability up to 40 % whereas the reduction of critical stress is by 20 % at the most.
|
guest ::
