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Anisotropic Elastic and Acoustic Properties of Bulk Graphene Nanoplatelets Consolidated by Spark Plasma Sintering

100%
Koller M., Seiner H., Landa M., Nieto A., Agarwal A.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
670-674
EN
Elastic anisotropy and acoustic attenuation in bulk material consisting of consolidated graphene nanoplatelets are studied. The material was prepared by spark plasma sintering, and exhibits highly anisotropic microstructure with the graphene nanoplatelets oriented perpendicular to the spark plasma sintering compression axis. The complete tensor of elastic constants is obtained using a combination of two ultrasonic methods: the through-transmission method and the resonant ultrasound spectroscopy. It is shown that the examined material exhibits very strong anisotropy both in the elasticity (the Young moduli in directions parallel to the graphene nanoplatelets and perpendicular to them differ by more than 20 times) and in the attenuation, where the dissipative effect of the internal friction in the graphene nanoplatelets combines with strong scattering losses due to the porosity. The results are compared with those obtained for ceramic-matrix/graphene nanoplatelet composites by the same ultrasonic methods.
2
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Effect of Compressive Load on Magnetic Shape Memory Effect in Ni-Mn-Ga Single Crystal

86%
Richterová K., Drahokoupil J., Kopecký V., Dragounová K., Landa M., Heczko O.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
704-708
EN
We investigated the effect of large compressive stress on magnetic shape memory effect in modulated 10M martensite of Ni-Mn-Ga Heusler alloy. The single crystalline sample deformed approximately elastically up to highest load 540 MPa. Structural and microstructural changes of a single crystal were analysed by X-ray diffraction 2D scan mapping in 2θ and ω. Although the crystal structure, lattice cell parameters, and twinned microstructure (a/b and modulation twinning) exhibited only small changes after the loading, the volume of the sample undergoing the magnetically induced reorientation decreased sharply while magnetic field necessary for the reorientation gradually increased. The extrapolation suggested that no magnetically induced reorientation might occur after compression of about 1 GPa.
3
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Characterization of Superelastic NiTi Alloys by Nanoindentation: Experiments and Simulations

73%
Frost M., Kruisová A., Sháněl V., Sedlák P., Haušild P., Kabla M., Shilo D., Landa M.
Acta Physica Polonica A
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2015
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vol. 128
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issue 4
664-669
EN
NiTi-based shape memory alloys are metallic materials exhibiting remarkable response to mechanical and/or thermal loading, e.g. superelasticity, pseudoplasticity or one-way shape memory effect. They can be engineered into structures of micro-size dimensions, hence, they appear promising for application in micro-electromechanical systems. For their efficient utilization, appropriate characterization is important. Due to relative simplicity, indentation represents a very standard and popular technique for basic characterization of metallic materials providing information about stiffness and hardness. Moreover, it can be used for identification of other events in the material. This study aims to verify applicability of the recently developed constitutive model for NiTi-based shape memory alloy in simulations of nanoindentation tests. The model is fitted to a particular sample material using experimental data, and a series of simulations mimicking tests at various temperatures is performed. Since the model also captures two-stage martensitic transformation via the intermediate R-phase, its influence on the simulations is investigated as well. It is confirmed that spherical indentation is a suitable method for simple and fast detection whether the material is in superelastic or pseudoplastic regime.
4
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Interrelation of Microstructure and Corrosion Resistance in Biodegradable Magnesium Alloys with Aluminum, Lithium and Rare Earth Additions

73%
Minárik P., Landa M., Lesná I., Zemková M., Jablonská E., Hadzima B., Janeček M., Král R.
Acta Physica Polonica A
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2015
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vol. 128
|
issue 4
491-496
EN
The influence of equal channel angular pressing and rotary swaging on the microstructure and corrosion resistance was investigated in three magnesium alloys with the addition of aluminum, lithium and rare earth elements - AE21, AE42 and LAE442. The processing resulted in grain refinement in all cases; nevertheless, the effect on the corrosion resistance was ambiguous. A continuous increase of the polarization resistance during the gradual equal channel angular pressing was observed in the AE42 and LAE442 alloys, whereas there was almost no effect in the AE21 alloy. The rotary swaging of AE42 resulted in a decrease of polarization resistance. The increase of polarization resistance in the alloys with the higher concentration of alloying elements was caused by the combined effect of grain refinement and better dispersion of particles in the matrix thanks to shear deformation during equal channel angular pressing. In the AE42 alloy, the increase of the corrosion resistance after equal channel angular pressing was also proven by chemical analysis of the solution and acoustic emission detection. This beneficial effect was not observed when the concentration of alloying elements was lower or when the deformation mode was different from shearing.
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