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The pseudoelasticity and the shape memory effect in CoNiAl alloys
Kopeček, Jaromír ; Jarošová, Markéta ; Jurek, Karel ; Heczko, Oleg
The cobalt alloys (close to the CoNiAl stoichiometry) are the less known shape memory alloys. Such behavior is consequence of the martensitic transformation. The pseudoelasticity is caused by the stress-induced martensitic transformation above the equilibrium martensite start temperature from high temperature cubic phase (austenite) to lower symmetry phase(martensite). In CoNiAl the pseudoelastic behavior can be obtained by the high temperature annealing. In presented work the effect of the annealing temperature on both pseudoelastic behavior and microstructure was investigated.
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Qantum-mechanical study of structural stability of Ni4N allotropes
Hemzalová, P. ; Friák, Martin ; Šob, Mojmír ; Neugebauer, J.
Parameter-free density functional theory (DFT) calculations of Ni4N in eight crystallographic phases were performed using the pseudopotential approach implemented in the VASP code; the exchange-correlation energy was evaluated within the generalized gradient approximation (GGA). In agreement with experiments, the cubic structure with Pearson symbol cP5, space group Pm-3m (221), has been found to be the most stable. It is also the only thermodynamically stable structure at T=0 K with respect to decomposition into elemental Ni crystal and N2 gas phase. We determine structural, thermodynamic, electronic, magnetic and elastic properties of all eight Ni4N allotropes studied. The thermodynamic stability and bulk modulus is found to be anti-correlated. For the cubic allotropes, we predict a complete set of single-crystalline elastic constants, directional dependence of the single-crystalline Young modulus and homogenized polycrystalline elastic moduli.
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Study of Influence of Segregated Impurities on Magnetism of Grain Boundaries and Free Surfaces in FCC Nickel and Cobalt
Všianská, Monika ; Vémolová, H. ; Šob, Mojmír
We present an ab initio study of segregation of 12 nonmagnetic sp impurities (Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb and Te) at the Sigma5(210) grain boundary (GB) and (210) free surface (FS) in ferromagnetic fcc nickel and cobalt. We analyze their effect on structure, magnetic and mechanical properties. We determine the preferred segregation sites of the impurity atoms, their segregation enthalpies and strengthening/embrittling energies with their decomposition into the chemical and mechanical components. In this contribution, we focused on the influence of segregated impurities on the magnetic moments of neighbouring atoms, the changes in the density of states and why the magnetically dead layers may be present in nickel but not in the cobalt.
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Strength and magnetism of Mn-Pt nanocomposites
Káňa, Tomáš ; Šob, Mojmír
Using ab initio electronic structure calculations, we have analysed Mn atoms dissolved in a platinum matrix, forming thus intermetallics MnPt3, MnPt7 and MnPt15 that can be considered as prototypes of natural Mn–Pt nanocomposites. On the whole, manganese addition makes the resulting Mn–Pt compound softer but increases its resistance to shape deformation. With increasing Mn content, both tensile and compressive theoretical strength is enhanced. In agreement with experiment, our calculations confirm the antiferromagnetic (AFM) ground state of the MnPt7 structure with spins directions altering along the [100] crystallographic direction (AFM [100]). In addition, we have proposed and studied three prototypes of linear MnPt15 nanocomposites consisting of Mn nanochains in the Pt matrix. Again, AFM [100] ordering was found in the ground state of MnPt15.
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Investigating ground state of nickel nitrides NiN and Ni2N with the help of quantum-mechanical calculations
Elstnerová, P. ; Friák, Martin ; Šob, Mojmír ; Neugebauer, J.
We have employed quantum mechanical calculations to identify ground-state structures of nickel nitrides NiN and Ni2N for which experimental data are lacking. In total 19 crystalline phases have been calculated for which not only thermodynamic but also structural and selected elastic properties have been determined. Employing density functional theory (DFT) methods, the total energies were calculated by means of a pseudopotential approach implemented in the VASP code and selected states were benchmarked by the full-potential linearized augmented plane wave (FP-LAPW) method implemented in the WIEN2k code. For the exchange-correlation energy the generalized gradient approximation (GGA) has been used.
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Effect of segregated nonmagnetic sp-impurities on the properties of grain boundaries and surfaces in nickel
Všianská, Monika ; Šob, Mojmír
We present an ab initio study of segregation of 12 nonmagnetic sp impurities (Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb and Te) at the Sigma5(210) grain boundary (GB) and (210) free surface (FS) in fcc ferromagnetic nickel and analyze their effect on structure, magnetic and mechanical properties. We determine the preferred segregation sites of the impurity atoms, their segregation enthalpies and strengthening/embrittling energies with their decomposition into the chemical and mechanical components. We find that the value of mechanical component is dependent on the type of the stable segregation position and is nearly constant in each period. On the other hand the chemical component changes strongly within each period and is determined by the electronegativity of impurity atoms. Magnetically dead layers found at the impurity segregated GB and FS are caused by a strong hybridization of sp states of the impurities with the d states of nickel.
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Structure and magnetism of clean and impurity-decorated grain boundaries in nickel from first principles
Všianská, Monika ; Šob, Mojmír
We present a detailed theoretical study of segregation of sp-elements from the 3rd-5th period (Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb and Te) at the Sigma5(210) grain boundary (GB) in fcc FM Ni. Whereas there is a slight enhancement of magnetization at the clean GB and FS with respect to bulk nickel (3–7% and 24%, respectively), the studied impurities entirely kill or strongly reduce ferromagnetism at the GB so that magnetically dead layers are formed. We determine the preferred segregation sites at the GB for the impurities studied, their segregation enthalpies and strengthening/embrittling energies. We find interstitially segregated Si and P and substitutionally segregated Al as a GB cohesion enhancer, and interstitially segregated S, Ge, As, Se and substitutionally segregated Ga, In, Sn, Sb and Te as GB embrittlers in Ni. As there is very little experimental information on GB segregation in nickel most of the present results are theoretical predictions which may motivate future experimental work.
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