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Rtg difrakce a difuzní rozpyl na Heuslerových slitinách
Cejpek, Petr ; Holý, Václav (advisor)
Recently, Heusler alloys are studied for their interesting magnetic and electronic properties. These properties are strongly dependent on the crystallographic struc- ture. This work deals with Heusler alloys of the A2BC type. We have powder samples and single-crystaline samples for our study as well. An object of interest was a description of crystallographic structure of the samples, site occupation numbers of each type of atoms and their possible occupation disorder. Pow- der difraction and EXAFS have been measured on the powder samples. Clasical single-crystal diffractions has been meassured on the single-crystaline samples. In the case of a modulated structure in the samples, satelite difractions have been measured too. 1
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Rtg difrakce a difuzní rozpyl na Heuslerových slitinách
Cejpek, Petr ; Holý, Václav (advisor) ; Kriegner, Dominik (referee)
Recently, Heusler alloys are studied for their interesting magnetic and electronic properties. These properties are strongly dependent on the crystallographic struc- ture. This work deals with Heusler alloys of the A2BC type. We have powder samples and single-crystaline samples for our study as well. An object of interest was a description of crystallographic structure of the samples, site occupation numbers of each type of atoms and their possible occupation disorder. Pow- der difraction and EXAFS have been measured on the powder samples. Clasical single-crystal diffractions has been meassured on the single-crystaline samples. In the case of a modulated structure in the samples, satelite difractions have been measured too. 1
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Ab initio calculations of the phase stability in Ni-Mn-Ga alloys
Pongrácz, Jakub ; Šesták, Petr (referee) ; Zelený, Martin (advisor)
This thesis deals with theoretical study of magnetic shape memory alloy based on Ni2MnGa with help of ab initio calculations of electronic structure within the projector augmented wave method. In particular the effect of Co- and Cu-doping is studied on total-energy profile along the tetragonal deformation path between austenite phase with cubic L21 structure and phase of nonmodulated martensite. Obtained results are used for estimation of doping influence on martensitic transformation temperature. Transformation temperature increases in the alloy doped by 3.125 at. % of copper instead of gallium whereas transformation temperature decreases in the alloy doped by 3.125 at. % of cobalt instead of nickel. If both type of doping are used the transformation temperature increases slightly because the effect of copper is stronger. Comparison of results from this work with available results obtained with help of the coherent potential approximation shows that a local lattice distortion around doping atom has negligible effect on studied properties.
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Observation of twin boundaries in Ni-Mn-Ga alloys
Čech, Jan ; Jan, Vít (referee) ; Zelený, Martin (advisor)
This bachelor’s thesis is focused on observation of twin boundaries in a magnetic shape memory alloy based on Ni2MnGa with 10M martensitic structure. Theoretical part of the thesis describes materials with shape memory effect, their response on mechanical or magnetic loading, phenomenon of twinning, which is closely connected to the shape memory effect, and various levels of twinning. Experimental part studies different types of twin boundaries in monocrystal of alloy Ni49.2Mn30.4Ga20.4 using optical and electron microscopy. The thesis also deals with detection of structural orientations in different twin variants. A holder of samples was designed and constructed for real time observation of twin-boundary movement by optical microscopy. Deformation by pressure shows, that stress necessary for movement of Type II twin boundaries is ~0.2 MPa, whereas it is ~0.6 MPa for movement of Type I.
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