| 1. |
- Hu, Q. M., et al.
(författare)
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Hardness and elastic properties of covalent/ionic solid solutions from first-principles theory
- 2008
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 103:8
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Tidskriftsartikel (refereegranskat)abstract
- Most of the engineering materials are alloys (solid solutions) and inevitably contain some impurities or defects such as vacancies. However, theoretical predictions of the hardness of this kind of materials have rarely been addressed in literature. In this paper, a hardness formula for multicomponent covalent solid solution is proposed based on the work of Simunek and Vackar [Phys. Rev. Lett. 96, 085501 (2006)]. With this formula, the composition dependence of the hardness is investigated for titanium nitrogencarbide (TiN1-xCx), off-stoichiometric transition-metal nitrides (TiN1-x and VN1-x), and B-doped semiconductors. The predicted hardness is in good agreement with experiments. To investigate the most frequently quoted correlation between hardness and elastic modulus, the elastic moduli of the systems involved in this paper have also been calculated. The results show that the elastic moduli cannot be used for rigorous predictions of the hardness of the solid solutions.
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| 2. |
- Hu, Qing-Miao, et al.
(författare)
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Predicting hardness of covalent/ionic solid solution from first-principles theory
- 2007
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 91:12, s. 121918-
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Tidskriftsartikel (refereegranskat)abstract
- We introduce a hardness formula for the multicomponent covalent and ionic solid solutions. This expression is tested on nitride spinel materials A3N4 (A=C,Si,Ge) and applied to titanium nitrogen carbide (TiN1-xCx with 0<=x<=1), off-stoichiometric transition-metal nitride (TiN1-x and VN1-x with x<=0.25), and B-doped semiconductors (C1-xBx, Si1-xBx, and Ge1-xBx with x<=0.1). In all cases, the theoretical hardness is in good agreement with experiments.
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| 3. |
- Kádas, Krisztina, et al.
(författare)
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Magnetism-driven anomalous surface alloying between Cu and Cr
- 2009
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 94:17
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Tidskriftsartikel (refereegranskat)abstract
- Cu-Cr contact materials are widely used as medium- and high-voltage vacuum interrupters. The microstructure of these materials is critical in their performance: the finer structure they have, the better are their physical properties. A solid solution of Cu and Cr could significantly increase the performance of such contact materials. However, Cu and Cr are practically immiscible in the bulk phase. Based on first principles density functional theory we show here that the solubility of Cr in Cu is dramatically increased on Cu surfaces already at room temperature and Cu-Cr alloys are formed on both the Cu(111) and Cu(100) surfaces. We demonstrate that the origin of this phenomenon is the unique magnetic properties of Cr atoms near surfaces.
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| 4. |
- Kádas, Krisztina, et al.
(författare)
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Stability of body-centered cubic iron-magnesium alloys in the Earth's inner core
- 2009
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 106:37, s. 15560-15562
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Tidskriftsartikel (refereegranskat)abstract
- The composition and the structure of the Earth's solid inner core are still unknown. Iron is accepted to be the main component of the core. Lately, the body-centered cubic (bcc) phase of iron was suggested to be present in the inner core, although its stability at core conditions is still in discussion. The higher density of pure iron compared with that of the Earth's core indicates the presence of light element(s) in this region, which could be responsible for the stability of the bcc phase. However, so far, none of the proposed composition models were in full agreement with seismic observations. The solubility of magnesium in hexagonal Fe has been found to increase significantly with increasing pressure, suggesting that Mg can also be an important element in the core. Here, we report a first-principles density functional study of bcc Fe-Mg alloys at core pressures and temperatures. We show that at core conditions, 5-10 atomic percent Mg stabilizes the bcc Fe both dynamically and thermodynamically. Our calculated density, elastic moduli, and sound velocities of bcc Fe-Mg alloys are consistent with those obtained from seismology, indicating that the bcc-structured Fe-Mg alloy is a possible model for the Earth's inner core.
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| 5. |
- Kádas, Krisztina, et al.
(författare)
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Structural stability of beta-beryllium
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:3
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Tidskriftsartikel (refereegranskat)abstract
- Using density-functional theory formulated within the framework of the exact muffin-tin orbitals method, we investigate the stability of the body-centered-cubic phase of Be (beta-Be). The elastic constants and Debye temperature of beta-Be are calculated over a wide volume range and compared to those obtained for the low-temperature hexagonal phase (alpha-Be). A significant difference in the anisotropy of the bcc and hcp structures is found. In line with experiments, we predict that the hcp -> bcc phase transition occurs at 240 GPa at 0 K and 239 GPa at ambient temperature. We find that the cubic shear constant C '=(C-11-C-12)/2 rapidly decreases for volumes above similar to 1.05V(0), where V-0 is the zero temperature equilibrium volume for beta-Be. At 1.17V(0), the stability condition C-'> 0 is violated and the bcc phase becomes mechanically unstable. We demonstrate that at 0 K, the softening of beta-Be near its experimental volume of 1.063V(0) is related to an electronic topological transition due to the increased number of occupied s states near the Fermi level compared to that at V-0. This softening turns out to be important for the stability of the bcc phase before melting. The disclosed electronic topological transition is found to be present in other analogous hexagonal metals as well.
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| 6. |
- Kádas, Krisztina, et al.
(författare)
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Temperature-dependent elastic properties of alpha-beryllium from first principles
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 76:23
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Tidskriftsartikel (refereegranskat)abstract
- Using density functional theory formulated within the framework of the exact muffin-tin orbitals method, we investigate the temperature dependence of the structural parameters and the elastic properties of the hexagonal closed-packed phase of Be (alpha-Be). We find that the elastic constants follow a normal behavior with temperature: decrease with increasing temperature with a slightly increasing slope. Up to the melting point, the monocrystalline elastic constants decrease by an average of 16% and the polycrystalline elastic constants by 10%. These trends contradict the large temperature factor observed in high-temperature direct pulse ultrasonic experiments. At the same time, the low-temperature pulse echo measurements confirm the present theoretical findings. Our results call for further accurate experimental studies on the elastic properties of alpha-Be at high temperatures.
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| 7. |
- Kádas, Krisztina, et al.
(författare)
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Theoretical prediction of the elastic properties of body-centered cubic Fe-Ni-Mg alloys under extreme conditions
- 2012
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Ingår i: Philosophical Magazine. - : Informa UK Limited. - 1478-6435 .- 1478-6443. ; 92:7, s. 888-898
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Tidskriftsartikel (refereegranskat)abstract
- Using density functional theory formulated within the framework of the exact muffin-tin orbital method, we investigate the elastic properties of the body-centered cubic Fe0.85Ni0.1Mg0.05 alloy in the conditions at the Earth's inner core. We demonstrate that in this system, the chemical stabilization effect of Mg is significantly larger than that of Ni. We show that the elastic properties of Fe(0.85)Ni(0.1)Mg(0.0)5 are in good agreement with those of the Earth's inner core, as given by seismic observations. We find that the excellent mechanical properties of Fe0.85Ni0.1Mg0.05 are primarily due to Mg.
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| 8. |
- Kadas, Krisztina, et al.
(författare)
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Thermo-physical properties of body-centered cubic iron-magnesium alloys under extreme conditions
- 2011
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Ingår i: Solid State Communications. - : Elsevier BV. - 0038-1098 .- 1879-2766. ; 151:3, s. 203-207
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Tidskriftsartikel (refereegranskat)abstract
- Using density functional theory formulated within the framework of the exact muffin-tin orbitals method, we investigate the thermo-physical properties of body-centered cubic (bcc) iron-magnesium alloys, containing 5 and 10 atomic % Mg, under extreme conditions, at high pressure and high temperature. The temperature effect is taken into account via the Fermi-Dirac distribution of the electrons. We find that at high pressures pure bcc iron is dynamically unstable at any temperature, having a negative tetragonal shear modulus (C'). Magnesium alloying significantly increases C' of Fe, and bcc Fe-Mg alloys become dynamically stable at high temperature. The electronic structure origin of the stabilization effect of Mg is discussed in detail. We show that the thermo-physical properties of a bcc Fe-Mg alloy with 5% Mg agree well with those of the Earth's inner core as provided by seismic observations. (C) 2010 Elsevier Ltd. All rights reserved.
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| 9. |
- Lee, J. -Y, et al.
(författare)
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The surface energy and stress of metals
- 2018
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Ingår i: Surface Science. - : Elsevier. - 0039-6028 .- 1879-2758. ; 674, s. 51-68
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Tidskriftsartikel (refereegranskat)abstract
- We investigated surface properties of metals by performing first-principles calculations. A systematic database was established for the surface relaxation, surface energy (γ), and surface stress (τ) for metallic elements in the periodic table. The surfaces were modeled by multi-layered slab structures along the direction of low-index surfaces. The surface energy γ of simple metals decreases as the atomic number increases in a given group, while the surface stress τ has its minimum in the middle. The transition metal series show parabolic trends for both γ and τ with a dip in the middle. The dip occurs at half-band filling due to a long-range Friedel oscillation of the surface charge density, which induces a strong stability to the Peierls-like transition. In addition, due to magnetic effects, the dips in the 3d metal series are shallower and deeper for γ and τ, respectively, than those of the 4d and 5d metals. The surface stress of the transition metals is typically positive, only Cr and Mn have a negative τ for the (100) surface facet, indicating that they are under compression. The light actinides have an increasing γ trend according to the atomic number. The present work provides a useful and consistent database for the theoretical modelling of surface phenomena.
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| 10. |
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