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The effect of impurities on the interface cohesion in multilayers in transition metal nitrides
Češka, Jakub ; Zelený, Martin (referee) ; Černý, Miroslav (advisor)
This work deals with the study of transtition metal nitride multilayers using first-principles calculations. Objects of this study are three particular systems AlN / TiN, AlN / VN and TiN / VN. Studied systems are in the B1 structure with an interface along the (001) plane. The main goal is to unravel the effect of impurity on cohesion in these multilayers. The impurity in question is a substitutional O atom replacing N in the lattice. Preferred positions of these substitutions are predicted for three different concentrations of substitution impurity. These predictions are based on the energy balance of substitutions in different positions. Resulting preferred positions within the multilayer may differ depending on the oxygen concentration. In most cases, the preferred position is at the interface between the two nitrides. For such systems with oxygen impurity in the preferred position a cleavage energy along several (001) planes is calculated. The effect of the impurity on the value of cleavage energy depends on its concentration. In the case of AlN / TiN multilayer, a suitable concentration of the impurity may increase the cleavage energy of the weakest link in multilayer compared to clean multilayer. In other cases the presence of impurity either causes a decrease in the cleavage energy or does not significantly affect its value.
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Study of inter-atomic interactions in advanced materials with help of ab initio calculations
Janovec, Jozef ; Šob, Mojmír (referee) ; Zelený, Martin (advisor)
Pomocou COHP analýzy sme študovali chemickú väzbu v diboridoch prechodných kovov ako aj v zliatine Ni2MnGa. Elektrónová štruktúra študovaných materiálov bola vypočítaná použitím výpočtov z prvých princípov pomocou metódy PAW. V prípade diboridov tranzitných kovov z výsledkov vyplýva, že sila väzby bór-bór je silne závislá od transferu elektrónov na atómy bóru. Zvyšujúci sa počet valenčných elektrónov v kove spôsobuje destabilizáciu alfa štruktúry kvôli vzájomnej interakcii elektrónov prislúchajúcich jednému atómu (on-site interakcie) v blízkosti Fermiho hladiny. Pre zliatinu s tvarovou pamäťou Ni2MnGa bola použitá metóda DFT+ U upravujúca popis lokalizácie elektrónov. V prípade použitia parametra U na Ni dochádza k destabilizácii kubického austenitu a k stabilizácii tetragonálneho nemodulovaného martenzitu. Naopak, zvýšenie lokalizácie elektrónov mangánu martenzit destabilizuje. Analýza väzieb ukázala, že najsilnejšou väzbou je Ni-Ga s kovalentným charakterom. Zvýšená lokalizácia valenčných elektrónov Mn zvyšuje podiel kovalentnosti Mn-Ni väzby a kovový charakter Ni-Ni väzby. Vplyvom zvýšenej lokalizácie Ni elektrónov sa Mn-Ni väzba stáva viac kovovou.
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Computer modeling of high-entropy alloys
Papež, Pavel ; Jan, Vít (referee) ; Zelený, Martin (advisor)
This Master’s thesis is focused on theoretical study of the high entropy alloy CoCrNi using ab initio calculations. The focus was on the effect of short range order on the relative stability of FCC and HCP structures and the value of stacking fault energy.The results show increase of stability in both types of structures wtih decreasing number of Cr-Cr nearest neighbours. The effect of the number of Cr-Cr nearest neighbours on the stacking fault energy previously shown in literature was not observed. However the strong dependency was found on the change of short range order caused by the shift of (1 1 1) planes after the transformation from the FCC to HCP structure. The effect of interstitial atoms C a N was also studied. Both these interstitials stabilise FCC structure and thus cause the increase of stacking fault energy. Both interstitials prefer octahedral positions with higher amount of Cr in their nearest neighbour shell.
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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.
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Strucure and mechnical properties of cold sprayed Ti-6Al-4V layer
Sabela, Jakub ; Zelený, Martin (referee) ; Řehořek, Lukáš (advisor)
Diploma thesis deals in the first part with recent knowledge of cold spray, its mechanism and parameters of deposition, advanced coatings made by cold spray and their applications, knowledge of Ti-6Al-4V coatings and their heat treatment. Deposit of Ti- 6Al-4V powder was made by cold spray process. In experimental part, microstructure and mechanical properties of supplied and its heat treated material were observed and examined. Mechanical properties and microstructure remained unchanged by annealing at 600 °C as in the case of supplied material. Recrystallization occured in microstructure of and phases by annealing at 800 °C. Grains were emerged in microstructure and mechanical properties were decreased. Mechanical properties were improved by annealing at 900 °C due to quenching. Microstructure consists of and ’ phases. Mechanical properties were the worst for annealed material at 1000 °C because of coarsed grains. Material which was annealed at 800 °C, quenched and precipitation hardened had the best microstructure and mechanical properties.
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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.
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Socioeconomic passportization of immovable cultural heritage
Kouba, Vojtěch ; Lindner, Milan ; Vořechovský, Jan ; Dostál, Petr ; Pátek, Zdeněk ; Dianová, Markéta ; Titelbachová, Šárka ; Tyslová, Irena ; Straka, Michal ; Zelený, Martin ; Kubaš, Mario ; Hanzlík, Jan ; Vančurová, Alena ; Vond, Zdeněk
Předkládaná metodika je výsledkem aplikovaného výzkumu a je zaměřena na ekonomiku památek, resp. nemovitých objektů prohlášených kulturními památkami České republiky. Jejím cílem je vytvoření pasportů ekonomických hodnot pro jednotlivé památkové objekty, v případě využití metodiky orgány veřejné správy i pro soubory památek určitého druhu či soubory památek na vymezeném území.
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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.
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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.
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