| 1. |
- Razumovskiy, Vsevolod I., et al.
(författare)
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Effect of thermal lattice expansion on the stacking fault energies of fcc Fe and Fe75Mn25 alloy
- 2016
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Ingår i: PHYSICAL REVIEW B. - : American Physical Society. - 2469-9950. ; 93:5
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Tidskriftsartikel (refereegranskat)abstract
- Temperature dependent stacking fault energies in fcc Fe and the Fe75Mn25 random alloy are calculated within density functional theory. The high temperature paramagnetic state of Fe is modeled by the spin wave ( SW) method within a Hamiltonian formalism and by the disordered local moment (DLM) approach in the Green's function technique using the coherent potential approximation (CPA). To determine the stacking fault energy, the supercell approach is used in the case of the SW method, while the axial Ising model is used in both the SW method and CPA-DLM calculations. The SW and CPA-DLM results are in very good agreement with each other, and they also accurately reproduce the existing experimental data. In both cases, fcc Fe and the Fe75Mn25 alloy, the SFE increases with temperature. This increase is almost entirely due to thermal lattice expansion, in contrast to earlier claims connecting such a dependence with magnetic entropy. Additionally, we check the convergence of the SW method with respect to the number of spin waves in the calculations of the phonon spectrum and the vacancy formation energy of paramagnetic fcc Fe.
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| 2. |
- Reyes-Huamantinco, Andrei, et al.
(författare)
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Stacking-fault energy and anti-Invar effect in Fe-Mn alloy from first principles
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:6, s. 060201-
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Tidskriftsartikel (refereegranskat)abstract
- Based on state-of-the-art density-functional-theory methods we calculate the stacking-fault energy of the prototypical high-Mn steel Fe-22.5 at% Mn between 300 and 800 K. We estimate magnetic thermal excitations by considering longitudinal spin fluctuations. Our results demonstrate that the interplay between the magnetic excitations and the thermal lattice expansion is the main factor determining the anti-Invar effect, the hcp-fcc transformation temperature, and the stacking-fault energy, all of which are in good agreement with measurements.
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| 3. |
- Abdoshahi, Neda, et al.
(författare)
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On the energetics of the cubic-to-hexagonal transformations in TiAl plus Mo alloys
- 2022
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 240
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Tidskriftsartikel (refereegranskat)abstract
- Diffusionless transformations allow access to metastable phases and enrich the materials design portfolio. They are well suited for atomistic modeling; nonetheless, they are challenging when involving disordered systems or alloys with complex compositions. This work presents a comprehensive study of transforma-tion energetics between bcc and hcp ordered and disordered phases in the TiAl+Mo model alloy system. By employing two complementary techniques I. VASP-SQS, and II. EMTO-CPA, we can show that chemical disorder flattens the energy landscape but may introduce a small barrier. Unlike that, the energetics of ordered phases are barrier-less and hence would suggest a spontaneous transformation. Finally, we show that Mo stabilizes the bcc phases, leading to a barrier-less transformation hcp -+ bcc for both ordered and disordered states when Mo content exceeds ti 12 at.%.
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| 4. |
- Abdoshahi, Neda, et al.
(författare)
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Structural stability and mechanical properties of TiAl plus Mo alloys : A comprehensive ab initio study
- 2021
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 221, s. 117427-
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Tidskriftsartikel (refereegranskat)abstract
- Titanium aluminides are technologically important intermetallic alloys with also many curious properties interesting from a basic research point of view. When alloyed with Mo, several (meta)stable phases have been reported; their properties are, however, largely unknown due to the alloy processing (e.g. fast cooling) and/or non-existence as a single-phase material. Here we employ first principles calculations to study compositional trends in structural and mechanical properties. We could show that Mo increases the density of all studied phases, leads to their chemical destabilization with the exception of the ordered bcc /3o phase, increases their ductility, and enhances the elastic anisotropy. Discrepancies between two employed ab initio methods (special quasi-random structures vs. coherent potential approximation) in the case of the /3o and B19 phases are rationalized with significant local distortions which may eventually facilitate a spontaneous phase transformation. Predictions of ordering temperatures solely based on the configurational entropy do not yield values in the experimentally expected ranges.
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| 5. |
- Abrikosov, I. A., et al.
(författare)
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Multiscale approach to theoretical simulations of materials for nuclear energy applications : Fe-Cr and Zr-based alloys
- 2013
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Ingår i: Advances in materials for nuclear energy. - : Materials Research Society. - 9781605114910 ; , s. 3-14
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Konferensbidrag (refereegranskat)abstract
- We review basic ideas behind state-of-the-art techniques for first-principles theoretical simulations of the phase stabilities and properties of alloys. We concentrate on methods that allow for an efficient treatment of compositional and thermal disorder effects. In particular, we present novel approach to evaluate free energy for strongly anharmonic systems. Theoretical tools are then employed in studies of two materials systems relevant for nuclear energy applications: Fe-Cr and Zr-based alloys. In particular, we investigate the effect of hydrostatic pressure and multicomponent alloying on the mixing enthalpy of Fe-Cr alloys, and show that in the ferromagnetic state both of them reduce the alloy stability at low Cr concentration. For Zr-Nb alloys, we demonstrate how microscopic parameters calculated from first-principles can be used in higher-level models.
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| 6. |
- Abrikosov, Igor, et al.
(författare)
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Competition between Magnetic Structures in the Fe-Rich FCC FeNi Alloys
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 76:1, s. 014434-
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Tidskriftsartikel (refereegranskat)abstract
- We report on the results of a systematic ab initio study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. %. Calculations are carried out within density-functional theory using two complementary techniques, one based on the exact muffin-tin orbital theory within the coherent potential approximation and another one based on the projector augmented-wave method. We observe that the evolution of the magnetic structure of the alloy with increasing Ni concentration is determined by a competition between a large number of magnetic states, collinear as well as noncollinear, all close in energy. We emphasize a series of transitions between these magnetic structures, in particular we have investigated a competition between disordered local moment configurations, spin spiral states, the double layer antiferromagnetic state, and the ferromagnetic phase, as well as the ferrimagnetic phase with a single spin flipped with respect to all others. We show that the latter should be particularly important for the understanding of the magnetic structure of the Invar alloys.
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| 7. |
- Alling, Björn, 1980-, et al.
(författare)
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A unified cluster expansion method applied to the configurational thermodynamics of cubic TiAlN
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 83:10, s. 104203-
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Tidskriftsartikel (refereegranskat)abstract
- We use a study of the cubic Ti1−xAlxN system to illustrate a practical way of combining the major methodologies within alloy theory, the Connolly-Williams cluster expansion and the generalized perturbation method, in order to solve difficult alloy problems. The configurational, concentration dependent, Hamiltonian is separated into a fixed-lattice and a local lattice relaxation part. The effective cluster interactions of the first part is obtained primarily with a GPM-based approach while the later is obtained using cluster expansion. In our case the impact on the isostructural phase diagram of considering short range clustering beyond the mean field approximation, obtained from the mixing enthalpy and entropy of the random alloy, is rather small, especially in the composition region x ≤ 0.66, within reach of thin film growth techniques.
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| 8. |
- Alling, Björn, et al.
(författare)
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Effect of thermal expansion, electronic excitations, and disorder on the Curie temperature of Ni1-xCuxMnSb alloys
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:13
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the importance of thermal effects such as temperature-induced electronic, magnetic and vibrational excitations, as well as structural defects in the first-principles calculations of the magnetic critical temperature of complex alloys using half-Heusler Ni1-xCuxMnSb alloys as a case study. The thermal lattice expansion and one-electron excitations have been accounted for self-consistently in the Curie temperature calculations. In the Ni-rich region, electronic excitations, thermal expansion, and structural defects substantially decrease the calculated Curie temperature. At the same time, some defects are shown to increase T-C in Cu-rich samples.
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| 9. |
- Alling, Björn, 1980-, et al.
(författare)
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Mixing and decomposition thermodynamics of c-Ti1-xAlxN from first-principles calculations
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:045123
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Tidskriftsartikel (refereegranskat)abstract
- Wedescribe an efficient first-principles method that can be used tocalculate mixing enthalpies of transition metal nitrides with B1 structureand substitutional disorder at the metal sublattice. The technique isbased on the density functional theory. The independent sublattice modelis suggested for the treatment of disorder-induced local lattice relaxationeffects. It supplements the description of the substitutional disorder withinthe coherent potential approximation. We demonstrate the excellent accuracy ofthe method by comparison with calculations performed by means ofthe projector augumented wave method on supercells constructed as specialquasirandom structures. At the same time, the efficiency of thetechnique allows for total energy calculations on a very finemesh of concentrations which enables a reliable calculation of thesecond concentration derivative of the alloy total energy. This isa first step towards first-principles predictions of concentrations and temperatureintervals where the alloy decomposition proceeds via the spinodal mechanism.We thus calculate electronic structure, lattice parameter, and mixing enthalpiesof the quasibinary alloy c-Ti1−xAlxN. The lattice parameter follows Vegard'slaw at low fractions of AlN but deviates increasingly withincreasing Al content. We show that the asymmetry of themixing enthalpy and its second concentration derivative is associated withsubstantial variations of the electronic structure with alloy composition. Thephase diagram is constructed within the mean-field approximation.
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| 10. |
- Bergman, Anders, et al.
(författare)
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Magnetic properties of Fe/Co(001) superlattices from first-principles theory
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 74:17
-
Tidskriftsartikel (refereegranskat)abstract
- The magnetic properties of Fe/Co(001) superlattices have been studied using fully-relativistic first-principles theories. The average magnetic moment shows a behavior similar to bulk Fe-Co alloys, i.e., an enhanced magnetic moment for low Co concentrations, as described by the Slater-Pauling curve. The maximum of the magnetization curve, however, is lowered and shifted towards the Fe-rich compositions. The increased average magnetic moment for the Fe-rich superlattices, compared to bulk Fe, is due to an enhancement of the Fe spin moment close to the Fe-Co interface. The orbital moments were found to be of the same size as in bulk. The effect of interface roughness on the magnetic properties was investigated, and it was found that-despite local fluctuations due to the varying coordination-the average magnetic moment is only slightly affected. From a mapping of first-principles interactions onto the screened generalized perturbation method, we calculate the temperatures for when Fe/Co superlattices break up into an alloy configuration. Furthermore, the tetragonal distortion of the superlattice structure was found to only have a minor effect on the magnetic moments. Also, the calculated easy axis of magnetization is in the film plane for all compositions studied. It lies along the [100] direction for Fe-rich superlattices and along the [110] direction for Co-rich compositions. The transition of the easy axis occurs around a Co concentration of 50%.
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| 11. |
- Blanter, M. S., et al.
(författare)
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Ab initio Based O-O Investigation and the Snoek Relaxation in Nb-O
- 2012
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Ingår i: Diffusion and Defect Data Pt.B. - : Trans Tech Publications Inc.. - 9783037853382 ; , s. 63-68
-
Konferensbidrag (refereegranskat)abstract
- It is common knowledge that interstitial-interstitial interaction influence on the Snoek relaxation. We used a computer simulation of this effect in the Nb-O alloy to test the adequacy of various models of the O-O interaction and clarify the mechanism of this effect The energy calculations in the first twelve coordination shells have been performed by the projector augmentedwave (PAW) method as implemented in the Vienna ab initio simulation package (VASP). The energies have been calculated in different ways which vary in the method of determination the energy of non-interacting O-O pairs. The energies calculated on the various variants are similar but in one case there is O-O repulsion in all first twelve coordination shells, whereas in another one can see attraction in four of twelve shells. Internal friction Q -1 was calculated as a sum of the contributions from individual interstitial atoms in different environments, each of which being assumed to be the Debye function. It is assumed that long-range interaction of oxygen atoms affects the distribution of these atoms and the energy of each interstitial atom in the octahedral interstices before a jump and after a jump. The Monte Carlo method is used for simulating short-range order of interstitial atoms and for calculating values of energy changes. Comparison of the calculated temperature and concentration dependence of the Snoek peak with the published data showed that the PAW supercell calculation of the O-O interactions in Nb describes the behavior of the interstitial solid solution adequately. It proves also that the impact of interstitial atom concentration on the Snoek relaxation is connected to the mutual attraction of these atoms.
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| 12. |
- Blanter, M. S., et al.
(författare)
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Interaction of interstitial atoms and configurational contribution to their thermodynamic activity in V, Nb, and Ta
- 2017
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Ingår i: Physics of metals and metallography. - : Maik Nauka/Interperiodica. - 0031-918X .- 1555-6190. ; 118:2, s. 105-112
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Tidskriftsartikel (refereegranskat)abstract
- The pairwise interaction energies of O–O and N–N in bcc metals of group VB, which were calculated earlier using first-principles methods, have been employed to analyze the effect of the interatomic interactions on the configurational contribution to the thermodynamic activity. The strong effect of interstitial- interstitial interaction has been shown. The configurational contribution grows in the row (Nb–N) → (V–N) → (Ta–N) → (Nb–O) → (V–O) → (Ta–O), which is caused by a weakening of the mutual attraction of interstitial atoms in these solid solutions. The strong repulsion that characterizes the majority of coordination shells only weakly affects the thermodynamic activity. The character of the temperature dependence of the configurational contribution is defined by the strength of the mutual attraction of the interstitial atoms, i.e., upon strong attraction, the contribution increases with increasing temperature (Nb–N, V–N, Ta–N, and Nb–O) and, upon weak attraction, it decreases (V–O and Ta–O).
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| 13. |
- Blanter, M. S., et al.
(författare)
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Interstitial-interstitial interactions in bcc VB group metals : Ab initio calculations
- 2013
-
Ingår i: Journal of Physics and Chemistry of Solids. - : Elsevier BV. - 0022-3697 .- 1879-2553. ; 74:5, s. 716-722
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Tidskriftsartikel (refereegranskat)abstract
- Chemical and strain-induced interactions of oxygen and nitrogen interstitials in bcc V and Ta have been obtained for the first 12 coordination shells in the supercell first-principles calculations by the Viena ab initio simulation package (VASP). It is shown that the chemical interactions are dominating at the first several coordination shells while the strain-induced interactions become important at more distant coordination shells. The latter are in reasonable agreement with the earlier results of a microscopic phenomenological Krivoglaz-Kanzaki-Khachaturyan model (KKKM). The obtained interstitial-interstitial interactions have been used in calculations of the concentration dependence of the oxygen Snoek peak, which is in good agreement with the existing experimental data.
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| 14. |
- Blanter, M. S., et al.
(författare)
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Ordering in V-O and V-N solid solutions : Computer simulation
- 2014
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Ingår i: Bulletin of the Russian Academy of Sciences: Physics. - 1062-8738. ; 78:10, s. 1030-1034
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Tidskriftsartikel (refereegranskat)abstract
- The atomic structures of interstitial solid solutions O and N in V at relatively low concentrations O(N)/V = 1/16 or 1/8 are calculated using the Monte Carlo method. A combined model of long-range interaction between interstitial atoms is employed. The first 12 shells contain ab initio energies and the energies in shells 13–18 are calculated on the basis of a phenomenological model of deformation interaction. The ordered solid solutions are long-period structures with body-centered tetragonal crystal lattices and tetragonality c/a < 1.
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| 15. |
- Blanter, M. S., et al.
(författare)
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Strain-induced interaction of interstitials in IVA group hcp metals
- 2010
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Ingår i: Journal of Physics and Chemistry of Solids. - : Elsevier BV. - 0022-3697 .- 1879-2553. ; 71:10, s. 1416-1420
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Tidskriftsartikel (refereegranskat)abstract
- Strain-induced (elastic) interactions of oxygen, nitrogen and carbon atoms in IVA group metals, alpha-Ti, Zr, and -Hf, are calculated in the framework of the microscopic Krivoglaz-Kanzaki-Khachaturyan theory. The experimental elastic constants, lattice spacing of the host metal, and concentration expansion coefficients are used as the input numerical parameters. The resulting interactions are stronger in a-Ti than in alpha-Zr and alpha-Hf. A comparative analysis of interactions in the hcp IVA group metals with those in bcc and fcc solid solutions reveals the crystal structure effect. In general, the strain-induced interactions of O, N, and C in hcp IVA group metals are weaker than in bcc solid solutions and are stronger than in fcc solid solutions.
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| 16. |
- Bleskov, I., et al.
(författare)
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Impact of local magnetism on stacking fault energies : A first-principles investigation for fcc iron
- 2016
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Ingår i: PHYSICAL REVIEW B. - : American Physical Society. - 2469-9950. ; 93:21
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Tidskriftsartikel (refereegranskat)abstract
- A systematic ab initio study of the influence of local magnetism on the generalized stacking fault energy (GSFE) surface in pure fcc iron at 0 K has been performed. In the calculations we considered ferro- and antiferro-(single- and double-layer) magnetic order of local moments as well as their complete disorder, corresponding to paramagnetic (PM) state. We have shown that local magnetism is one of the most important factors stabilizing austenitic structure in iron (with respect to more stable at 0 K hcp) and that the perturbation of magnetic structure by the formation of stacking fault is a short-range effect. Local magnetism also strongly influences the GSFE surface topology and, therefore, the material's plasticity by reducing the energetic barriers that need to be overcome to form the intrinsic stacking fault (ISF) or return from the ISF structure to fcc. The influence of atomic relaxations on such barriers is moderate and does not exceed 15%. In addition, a methodology to evaluate the PM ISF energy using a superposition of the ISF energies obtained for ordered magnetic structures is proposed to overcome computational impediments arising when dealing with disorder in the PM state. The complications of the proposed methodology together with the ways to overcome them are also discussed.
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| 17. |
- Bligaard, T., et al.
(författare)
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Pareto-optimal alloys
- 2003
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 83:22, s. 4527-4529
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Tidskriftsartikel (refereegranskat)abstract
- Large databases that can be used in the search for new materials with specific properties remain an elusive goal in materials science. The problem is complicated by the fact that the optimal material for a given application is usually a compromise between a number of materials properties and the cost. In this letter we present a database consisting of the lattice parameters, bulk moduli, and heats of formation for over 64 000 ordered metallic alloys, which has been established by direct first-principles density-functional-theory calculations. Furthermore, we use a concept from economic theory, the Pareto-optimal set, to determine optimal alloy solutions for the compromise between low compressibility, high stability, and cost.
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| 18. |
- Bochkarev, A. S., et al.
(författare)
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A single-volume approach for vacancy formation thermodynamics calculations
- 2016
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Ingår i: Europhysics letters. - : EPL ASSOCIATION, EUROPEAN PHYSICAL SOCIETY. - 0295-5075 .- 1286-4854. ; 116:1
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Tidskriftsartikel (refereegranskat)abstract
- The vacancy formation Gibbs free energy, enthalpy and entropy in fcc Al, Ag, Pd, Cu, and bcc Mo are determined by first-principles calculations using the quasi-harmonic approximation to account for vibrational contributions. We show that the Gibbs free energy can be determined with sufficient accuracy in a single-volume approach using the fixed equilibrium volume of the defect-free supercell. Although the partial contributions to the Gibbs free energy, namely, the formation enthalpy and entropy exhibit substantial errors when obtained directly in this approach, they can be computed from the Gibbs free energy using the proper thermodynamic relations. Compared to experimental data, the temperature dependence of the vacancy formation Gibbs free energy is accounted for at low temperatures, while it overestimates the measurements at high temperature, which is attributed to the neglect of anharmonic effects.
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| 19. |
- Bodlos, R., et al.
(författare)
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Ab initio investigation of the atomic volume, thermal expansion, and formation energy of WTi solid solutions
- 2021
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Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 5:4
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Tidskriftsartikel (refereegranskat)abstract
- WTi is used as an adhesive layer in integrated circuit devices. The temperature dependent mechanical properties of WTi are still largely unexplored. In this paper we investigate WTi solid solutions with density functional theory calculations to determine the temperature and concentration dependent behavior of volume and coefficient of thermal expansion. The coefficient of thermal expansion is analyzed in terms of the bulk modulus, heat capacity, and Gruneisen parameter. Furthermore, we gain insight into the bonding of the system via investigation of the electronic structure, phonon density of states, and analysis of the formation energy. Low Ti concentrations lead to strong W-Ti bonding, as manifested in additional high frequency peaks in the phonon density of states. As a consequence, deviations from Vegard's law are found at low Ti concentrations, with a minimum of the lattice constant at about 15 at.% Ti. The CTE as a function of Ti concentration shows a negative trend at low temperatures and Ti concentrations, which is related to a strong decrease of heat capacity. Finally we show that the Debye-Gruneisen model yields results for WTi comparable to the quasiharmonic approach at a fraction of the computational cost.
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| 20. |
- Bogdanov, V. I., et al.
(författare)
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Chemical and deformational interactions in solid solution of carbon in nickel
- 2012
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Ingår i: Physics of metals and metallography. - 0031-918X .- 1555-6190. ; 113:9, s. 831-835
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Tidskriftsartikel (refereegranskat)abstract
- A first-principles stuy of ordering phenomena in hcp interstitial solid solutions of oxygen and nitrogen in Ti, Zr and Hf has demonstrated that the dominant contributions to the interaction energy of interstitial atoms are of chemical nature; thus, it is necessary to modify the previously established concepts about the priority role of deformational interactions in interstitial solutions. We have continued studies of the role of chemical and deformational interactions of interstitial atoms by the example of solid solutions of carbon in nickel. The results obtained also confirm a significant role of chemical interactions between carbon atoms in these solid solutions. The results were compared with the experimental data on the enthalpy of carbon dissolution in nickel and on the coefficient of solutal expansion of the lattice.
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| 21. |
- Bogdanov, V. I., et al.
(författare)
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First-principles investigations of interatomic interactions in Ni3Al alloyed by interstitial and substitutional impurities
- 2013
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Ingår i: Physics of metals and metallography. - 0031-918X .- 1555-6190. ; 114:3, s. 191-196
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Tidskriftsartikel (refereegranskat)abstract
- First-principles calculations of the total energy of interstitial and substitutional solid solutions in intermetallic compound Ni3Al were performed based on methods using Vienna ab-initio simulation package (VASP). The results of the calculations for interstitial solutions of carbon in Ni3Al confirmed the priority role of chemical interactions over deformational ones for the nearest neighbors. We attempted to use first-principles methods of calculation of the deformation interaction and continuum approaching in the theory of solutions to calculate coefficients of the concentration changes of the lattice spacing. Comparison of the calculation results with experimental data of substitutional impurities in Ni3Al has shown that the proposed method can aid in the study of the distribution of impurity atoms on the sublattices of the ordered phases, intermetallic compounds. We have proposed a method of calculating the partial molar volume of impurity in interstitial solid solutions.
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| 22. |
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| 23. |
- Breidi, A., et al.
(författare)
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First-principles modeling of energetic and mechanical properties of Ni-Cr, Ni-Re and Cr-Re random alloys
- 2016
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Ingår i: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 117, s. 45-53
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Tidskriftsartikel (refereegranskat)abstract
- We apply the exact-muffin-tin-orbitals (EMTO) method to investigate structural properties, formation enthalpies, mechanical stability and polycrystalline moduli in Ni-Re, Ni-Cr and Cr-Re disordered fcc, bcc and hcp phases. Substitutional disorder is treated by using the coherent potential approximation (CPA). We predict the alloy lattice parameters in good agreement with the experiment. We find a continuous softening, as a function of Cr composition, of the tetragonal shear modulus C' in fcc Ni-Cr phase indicating mechanical instability in Cr-rich Ni-Cr alloys. On the other hand, we show that the mechanical stability of fcc Ni-Re alloys persists through the whole composition range. We observe an intriguing behaviour of the Young's modulus vs. the intrinsic ductility curve in Ni-rich Ni-Re fcc phase.
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| 24. |
- Breidi, A., et al.
(författare)
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Ideal compressive strength of fcc Co, Ni, and Ni-rich alloys along the (001) direction : A first-principles study
- 2016
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 93:14
-
Tidskriftsartikel (refereegranskat)abstract
- We perform density functional theory based first-principles calculations to identify promising alloying elements (X) capable of enhancing the compressive uniaxial theoretical (ideal) strength of the fcc Ni-matrix along the 001 direction. The alloying element belongs to a wide range of 3d,4d, and 5d series with nominal composition of 6.25 at. %. Additionally, a full elastic study is carried to investigate the ideal strength of fcc Ni and fcc Co. Our results indicate that the most desirable alloying elements are those with half d-band filling, namely, Os, Ir, Re, and Ru.
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| 25. |
- Burkert, Till, et al.
(författare)
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Magnetic anisotropy of L1(0) FePt and Fe1-xMnxPt
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:13, s. 134411-
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Tidskriftsartikel (refereegranskat)abstract
- The uniaxial magnetic anisotropy energy (MAE) of L1(0) FePt and Fe1-xMnxPt, x=0-0.25, was studied from first principles using two fully relativistic computational methods, the full-potential linear muffin-tin orbitals method and the exact muffin-tin orbitals method. It was found that the large MAE of 2.8 meV/f.u. is caused by a delicate interaction between the Fe and Pt atoms, where the large spin-orbit coupling of the Pt site and the hybridization between Fe 3d and Pt 5d states is crucial. The effect of random order on the MAE was modeled by mutual alloying of the sublattices within the coherent potential approximation (CPA), and a strong dependence of the MAE on the degree of chemical long-range order was found. The alloying of FePt with Mn was investigated with the virtual crystal approximation and the CPA as well as supercell calculations. The MAE increases up to 33% within the concentration range studied here, an effect that is attributed to band filling. Furthermore, the dependence of the MAE on the structural properties was studied.
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| 26. |
- Christoffersen, E., et al.
(författare)
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Anode materials for low-temperature fuel cells : A density functional theory study
- 2001
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Ingår i: Journal of Catalysis. - : Elsevier BV. - 0021-9517 .- 1090-2694. ; 199:1, s. 123-131
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Tidskriftsartikel (refereegranskat)abstract
- Based on density functional calculations, we discuss the effect of alloying Pt with other metals for use as anode catalyst materials in low-temperature fuel cells. We discuss why a few parts per million of CO in the H-2 fuel can poison Pt surfaces and how this problem can be alleviated by alloying, and an extensive data base of the effect of alloying on the reactivity that includes all binary combinations of the transition metals to the right in the periodic table is given. We also discuss the effect of surface segregation and give a calculated data base of segregation energies of binary transition metal alloys. Based on extensive Monte Carlo simulations we show that while the adsorbate-free surface of a Ru0.5Pt0.5 alloy has no Ru in the first layer, the presence of CO can move some Ru to the surface, but all these Ru atoms are covered by CO.
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| 27. |
- Dehghani, Mohammad, et al.
(författare)
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Stability and ordering of bcc and hcp TiAl plus Mo phases : An ab initio study
- 2022
-
Ingår i: Computational materials science. - : Elsevier BV. - 0927-0256 .- 1879-0801. ; 205
-
Tidskriftsartikel (refereegranskat)abstract
- Atomic ordering in bcc and hcp TiAl+Mo alloys near equiatomic TiAl composition is investigated by different ab initio tools. We show that small addition of Mo, about 5 at. %, is enough to make bcc alloys with more than 50 at. % of Ti stable with respect to the hcp alloys. Moreover, such alloying also leads to stabilizing the B2 ordered structure with respect to its B2(2) modification, which is the bcc-based ground state structure of binary TiAl. The site preference of Mo in the B2 and B19 ordered alloys is investigated by different methods: in the dilute limit, using the transfer energy formalism; in concentrated alloys, from the total energies of disordered and partially ordered alloys in the mean-field coherent potential approximation; and from Monte Carlo simulations. These methods produce consistent results for the B2 phase predicting a strong preference of Mo to Al sublattice. The site preference of Mo in the B19 phases varies from a weak preference for Al sites in the single impurity calculations to a quite strong preference for Ti sites in the mean-field approximation and finally to a strong Al preference in Monte Carlo simulations. Mo alloying dramatically increases the order-disorder transition temperatures in bcc and hcp Al-deficient Ti0.5Al0.5-xMox alloys.
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| 28. |
- Delczeg-Czirjak, Erna K., et al.
(författare)
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The effect of longitudinal spin-fluctuations on high temperature properties of Co3Mn2Ge
- 2023
-
Ingår i: Scripta Materialia. - : Elsevier BV. - 1359-6462 .- 1872-8456. ; 226
-
Tidskriftsartikel (refereegranskat)abstract
- It is demonstrated that thermally induced longitudinal spin fluctuations (LSF) play an important role in itinerant Co3Mn2Ge at an elevated temperature. The effect of LSF is taken into account during ab initio calculations via a simple model for the corresponding entropy contribution. We show that the magnetic entropy leads to the appearance of a medium size local moment on Co atoms. As a consequence, this leads to a renormalization of the magnetic exchange interactions with a quite substantial impact upon the calculated Curie temperature. Taking LSF into account, the calculated Curie temperature can be brought to be in good agreement with the experimental value.
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| 29. |
- Dmitriev, V. V., et al.
(författare)
-
Ab initio based investigation of interstitial interactions and Snoek relaxation in Nb-O
- 2012
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Ingår i: Journal of Physics and Chemistry of Solids. - : Elsevier BV. - 0022-3697 .- 1879-2553. ; 73:2, s. 182-187
-
Tidskriftsartikel (refereegranskat)abstract
- Chemical and strain-induced effective pair interactions of interstitial oxygen atoms in bcc Nb have been determined in supercell first-principles calculations using Vienna ab initio simulation package (VASP). The strain-induced interactions are in reasonable agreement with those obtained earlier within a phenomenological microscopic Krivoglaz-Kanzaki-Khachaturyan model (KKKM). At the same time, the chemical interactions, which have been considered to be small in earlier theoretical considerations, turned out to be dominating at the first several coordination shells. The obtained interactions have been used in calculations of the concentration- and temperature-dependence of the internal friction Snoek peak. The theoretical results are found to be in good agreement with the existing experimental data.
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| 30. |
- Dmitriev, V. V., et al.
(författare)
-
Interaction of interstitial nitrogen atoms in Nb : Ab initio calculations
- 2012
-
Ingår i: Bulletin of the Russian Academy of Sciences: Physics. - 1062-8738. ; 76:1, s. 1-6
-
Tidskriftsartikel (refereegranskat)abstract
- Ab initio calculations of pair nitrogen interstitials interaction in the first 12 coordination shells of a Nb crystal lattice are performed using the Vienna ab initio simulation package (VASP), and chemical and strain-induced contributions are analyzed. It is shown that rapidly decreasing chemical repulsion prevails in the nearest coordination shells, whereas strain-induced (elastic) interaction makes the main contribution in more distant shells.
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| 31. |
- Duong, T. C., et al.
(författare)
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A hierarchical computational thermodynamic and kinetic approach to discontinuous precipitation in the U-Nb system
- 2015
-
Ingår i: PTM 2015 - Proceedings of the International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015. - : International Conference on Solid-Solid Phase Transformations in Inorganic Materials. - 9780692437360 ; , s. 887-894
-
Konferensbidrag (refereegranskat)abstract
- U-Nb alloys decompose via discontinuous precipitation (DP) over a broad range of aging conditions, adversely affecting their properties. The growth kinetics, lamellar spacing, and Nb partitioning have been measured, but the thermodynamic and kinetic factors underlying these specific transformation characteristics and reaction paths, vis-a-vis the monotectoid reaction, are not fully resolved. In this work, a hierarchical computational thermodynamic and kinetic approach was carried out to investigate DP. The hierarchical approach started with density-functional theory (DFT) investigations of ground-state formation energies of bcc-based U-Nb alloys. The estimated energetic data was then utilized as an imposed first-principles-based constraint to improve the consistency of the CALPHAD thermodynamic and, subsequently, kinetic assessments of U-Nb. Phasefield simulations were then carried out to study DP's microstructure evolution using the assessed CALPHAD thermodynamic and kinetic representations. Good agreement with experiments on different physical/length scales was achieved, which validates the present theoretical contributions to a better understanding of DP in U-Nb alloys.
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| 32. |
- Duong, Thien C., et al.
(författare)
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Revisiting thermodynamics and kinetic diffusivities of uranium-niobium with Bayesian uncertainty analysis
- 2016
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Ingår i: CALPHAD-COMPUTER COUPLING OF PHASE DIAGRAMS AND THERMOCHEMISTRY. - : Elsevier. - 0364-5916. ; 55, s. 219-230
-
Tidskriftsartikel (refereegranskat)abstract
- In this work, thermodynamic and kinetic diffusivities of uranium-niobium (U-Nb) are re-assessed by means of the CALPHAD (CALculation of PHAse Diagram) methodology. In order to improve the consistency and reliability of the assessments, first-principles calculations are coupled with CALPHAD. In particular, heats of formation of gamma-U-Nb are estimated and verified using various density-functional theory (DFT) approaches. These thermochemistry data are then used as constraints to guide the thermodynamic optimization process in such a way that the mutual-consistency between first-principles calculations and CALPHAD assessment is satisfactory. In addition, long-term aging experiments are conducted in order to generate new phase equilibria data at the gamma(2)/alpha + gamma(2) boundary. These data are meant to verify the thermodynamic model. Assessment results are generally in good agreement with experiments and previous calculations, without showing the artifacts that were observed in previous modeling. The mutual-consistent thermodynamic description is then used to evaluate atomic mobility and diffusivity of gamma-U-Nb. Finally, Bayesian analysis is conducted to evaluate the uncertainty of the thermodynamic model and its impact on the system's phase stability.
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| 33. |
- Ehteshami, Hossein
(författare)
-
Finite temperature properties of elements and alloy phases from first principles
- 2018
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- First principles calculations are usually concerned with properties calculated at temperature 0 K. However, the industrially important materials are functioning at finite temperatures. To fill such a gap a first-principles based modeling of free energy has been developed in this thesis and finite temperature properties of different phases of Fe and Mn have been calculated and contrasted with available experimental data.In particular, using partitioning of the Helmholtz free energy, thermophysical properties of paramagnetic Fe have been reported. The heat capacity, lattice constant, thermal expansion and elastic moduli of γ- and δ-Fe show a good agreement with available experimental data. In the case of α-Fe, we observe a good agreement for elastic moduli and thermal expansion with experiments but the heat capacity is not well-reproduced in the calculations because of the large contribution of magnetic short-range which our models are not capable of capturing.α- and β-Mn theoretically pose a challenge for direct simulations of thermodynamic properties because of the complexity of magnetic and crystal structure. The partitioning of free energy has been used and thermodynamics of these phases have been derived. The obtained results show a good agreement with experimental data suggesting that, despite the complexities of these phases, a rather simple approach can well describe their finite temperature properties. High temperature phases of Mn, γ and δ, are also theoretically challenging problems. Employing a similar approach to Fe, thermophysical properties of these high symmetry phases of Mn have been reported which also show good agreement with available experimental data.The point defect and metal-self diffusion in titanium carbide (TiC), a refractory material, have been investigated in the present work. The common picture of metal-vacancy exchange mechanism for metal self-diffusion was shown to be unable to explain the experimentally observed values of activation energy. Several new clusters of point defects such as vacancies and interstitials have been found and reported which are energetically lower that a single metal vacancy. In a subsequent study, we showed that some of these clusters can be considered as mediators of metal self-diffusion in TiC.Evaluation of structural properties of Ti(O,C), a solid solution of TiC and β-TiO, from supercell approach is an extremely difficult task. For a dilute concentration of O, we show the complexity of describing an impurity of O in TiC using supercell approach. A single-site method such as the exact muffin-tin orbital method in the coherent potential approximation (EMTO-CPA) is a good alternative to supercell modeling of Ti(O,C). However, a study of Ti(O,C) using EMTO-CPA requires a further development of the technique regarding the partitioning of space. The shape module of EMTO has been modified for this purpose. With the help of the modified module, Ti(O,C) have been studied using EMTO-CPA. The results for the divacancy concentration and corresponding lattice parameter variations show good agreement with experimental data.
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| 34. |
- Ehteshami, Hossein, et al.
(författare)
-
High-temperature thermophysical properties of gamma- and delta-Mn from first principles
- 2018
-
Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 2:3
-
Tidskriftsartikel (refereegranskat)abstract
- Thermophysical properties of gamma-and delta-Mn phases have been investigated using first-principles calculations in their thermodynamically stable temperature range. An adiabatic approximation is used for partitioning of the Helmholtz free energy into electronic, magnetic, and vibrational contributions from the corresponding temperature induced excitations, where the fastest degree of freedom has been included in the slower ones. Namely, electronic excitations (on a one-electron level) have been included directly in the first-principles calculations at the corresponding temperatures. Magnetic excitations in the paramagnetic state then have been taken into consideration in the two opposite limits: localized, considering only transverse spin fluctuations (TSF), and itinerant, allowing for the full coupling of transverse and longitudinal spin fluctuations (LSF). Magnetic contribution to the free energy has been included in the calculations of the vibrational one, which has been obtained within the Debye-Gruneisen model. The calculated thermophysical properties such as lattice constance, thermal lattice expansion, and heat capacity are in good agreement with available experimental data, especially in the case when the itinerant magnetic model is chosen. We also present our results for elastic properties at high temperatures.
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| 35. |
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| 36. |
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| 37. |
- Ekholm, Marcus, et al.
(författare)
-
Influence of the Magnetic State on the Chemical Order-Disorder Transition Temperature in Fe-Ni Permalloy
- 2010
-
Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 105:16, s. 167208-
-
Tidskriftsartikel (refereegranskat)abstract
- In magnetic alloys, the effect of finite temperature magnetic excitations on phase stability below the Curie temperature is poorly investigated, although many systems undergo phase transitions in this temperature range. We consider random Ni-rich Fe-Ni alloys, which undergo chemical order-disorder transition approximately 100 K below their Curie temperature, to demonstrate from ab initio calculations that deviations of the global magnetic state from ideal ferromagnetic order due to temperature induced magnetization reduction have a crucial effect on the chemical transition temperature. We propose a scheme where the magnetic state is described by partially disordered local magnetic moments, which in combination with Heisenberg Monte Carlo simulations of the magnetization allows us to reproduce the transition temperature in good agreement with experimental data.
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| 38. |
- Engelke, M., et al.
(författare)
-
Near-surface microstructure of Ni-23 at. % Pt : Grazing incidence diffraction and first-principles calculations
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 81:5, s. 054205-
-
Tidskriftsartikel (refereegranskat)abstract
- Diffuse x-ray scattering under grazing incidence was measured of a Ni-23.2 at. % Pt(110) surface at 923 K. The modulation of in-plane and out-of-plane scattering is characterized by the maxima in short-range order scattering located at positions of the X type (as for the bulk microstructure). The Warren-Cowley short-range order parameters from diffuse scattering are in good agreement with theoretical simulations based on bulk effective pair interaction parameters from first-principles theory. The nearest-neighbor interaction parameter is dominant also when determined from short-range order scattering. Its value is larger than for the bulk microstructure of Pt-rich alloys, what might reflect the difference in composition and is not compellingly due to the near-surface microstructure. Allowing for the presence of a tetragonal site occupation, indications for a segregation profile are weak in diffuse scattering and hardly resolvable for the Warren-Cowley short-range order parameters. This is consistent with the findings for layer-resolved short-range order parameters from theory. Theoretical simulations also demonstrated that the strong Ni enrichment of the surface layer has the same origin as the segregation reversal for the (110) surface in the case of Ni-50 at. % Pt.
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| 39. |
- Forslund, Axel, 1991-
(författare)
-
Ab initio and phenomenological modeling of materials related to CVD hard coatings
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis describes the work based on two different tools in computational materials science: a first-principles approach, namely that of density functional theory, and the CALPHAD approach.These two methods were used in this work to calculate properties of materials related to hard coatings, in particular coatings produced with chemical vapor deposition for the purpose of wear protection in cutting tools. Several parts of the work is also, in many aspects, of a general character. In a few cases, the material investigations were performed on simpler demonstration systems, with the intention of further application on more involved material systems.A variety of different methods and specific applications are included in this thesis. The reaction-diffusion in Ni-base superalloys deposited by vapor deposition methods was simulated with a continuum approach with CALPHAD thermodynamic and kinetic data. CALPHAD models were also used to predict the stable phases for TiN deposition on a CoCrFeNi substrate. Surfaces and segregation energies were investigated in a random alloy, pseudobinary (Al,Ti)N system. This system was also the subject of calculations of formation energies of structural vacancies, and the configurational dependence of these properties was investigated.Further, surface free energies including all relevant thermal excitations were calculated for TiN(001) and several W surfaces in a newly developed methodology including machine-learning interatomic potentials. For W, the temperature dependence of the surface anisotropy was obtained, which was shown to be decreasing with temperature, with a surface free energy approaching experimental values at the melting temperature.
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| 40. |
- Forslund, Axel, et al.
(författare)
-
Ab initio simulations of the surface free energy of TiN(001)
- 2021
-
Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 103:19
-
Tidskriftsartikel (refereegranskat)abstract
- The temperature dependence of the surface free energy of the industrially important TiN(001) system has been investigated by means of an extended two-stage upsampled thermodynamic integration using Langevin dynamics (TU-TILD) methodology, to include the fully anharmonic vibrational contribution, as obtained from ab initio molecular dynamics (AIMD). Inclusion of the fully anharmonic behavior is crucial, since the standard low-temperature quasiharmonic approximation exhibits a severe divergence in the surface free energy due to a high-temperature dynamical instability. The anharmonic vibrations compensate for the quasiharmonic divergence and lead to a modest overall temperature effect on the TiN(001) surface free energy, changing it from around 78 meV angstrom(-2) at 0 K to 73 meV angstrom(-2) at 3000 K. The statistical convergence of the molecular dynamics is facilitated by the use of machine-learning potentials, specifically moment tensor potentials, fitted for TiN(001) at finite temperature. The surface free energy obtained directly from the fitted machine-learning potentials is close to that obtained from the full AIMD simulations.
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| 41. |
- Forslund, Axel, et al.
(författare)
-
Ab initio surface free energies of tungsten with full account of thermal excitations
- 2022
-
Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 105:4
-
Tidskriftsartikel (refereegranskat)abstract
- The surface free energies of seven different facets of tungsten (W) are obtained up to the melting point with full account of all the relevant thermal excitations; in particular, thermal atomic vibrations, electronic excitations, and their mutual coupling. The latter is done using ab initio molecular dynamics simulations coupled with the thermodynamic integration technique. In this way, the calculations contain almost no error but the one related to the used exchange-correlation functional, which makes the results truly first principles. The obtained results are compared with previous quasiharmonic calculations for the surface free energies of W and experimental data. The anharmonic contribution is, as expected, important for open surfaces at high temperatures, which leads to a temperature dependence of the surface energy anisotropy. The calculated Wulff shapes and surface energies are in excellent agreement with experimental data close to the melting point, where the crystalline structure of the surface layers is destroyed by a dramatic mobility of the atoms there.
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| 42. |
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| 43. |
- Forslund, Axel, et al.
(författare)
-
Structural vacancies in (Ti,Al)N : An ab initio study
- 2022
-
Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 6:1
-
Tidskriftsartikel (refereegranskat)abstract
- The zero K formation energies of metal and nitrogen vacancies in several (Ti,Al)N alloys and at the (001) (Ti,Al)N/AlN interface are obtained in ab initio supercell calculations. The dependence of the formation energies of metal vacancies on their local environment and type are analyzed and explained in terms of effective cluster interactions for unrelaxed calculations. The common trend for all investigated types of vacancies is that their formation energy increases with the number of Al nearest neighbors if local lattice relaxations are not allowed. However, local lattice relaxations produce a dramatic effect especially in the case of metal vacancies leading to a complicated nonlinear dependence on the local environment indicating the existence of strong multisite strain-induced interactions.
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| 44. |
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| 45. |
- Forslund, Axel, et al.
(författare)
-
Surface energetics of AlxTi1-xN alloys
- 2020
-
Ingår i: Computational materials science. - : Elsevier BV. - 0927-0256 .- 1879-0801. ; 183
-
Tidskriftsartikel (refereegranskat)abstract
- The (100), (110) and (111) surface energies of random AlxTi1-xN alloys with homogeneous concentration profile are determined in first-principles calculations. The (100) surface has the lowest energy of 1.25 J/m(2) in the case of TiN and 1.32 J/m(2) for cubic AlN and exhibit very little concentration dependence. The (110) and (111) surfaces have much higher energy for all the compositions. The segregation energies of Ti and Al were obtained for the (100) surface of pure TiN and cubic AlN as well as Al0.5Ti0.5N and Al0.9Ti0.1 N random alloys. In the latter case, we have used two different methods: direct averaging of the substitution energies with respect to the local environment of the substitution site and the cluster expansion technique (CLE). We find that the segregation of Ti is favorable in the whole concentration range of random AlxTi1-xN alloys. However, it is weak in almost the whole concentration range except in the Al-rich alloys: Al0.9Ti0.1N -/+ and cubic AlN. The strengthening of the surface segregation of Ti in the latter case is related to the sharp increase in phase separation tendency in AlxTi1-xN alloys at compositions close to pure AlN. The increased tendency for Ti segregation close to pure AlN helps explain the formation of a lamella structure in industrially important AlxTi1-xN coatings. As for nitrogen vacancies, their segregation energies are relatively small in the whole concentration range, but for pure AlN where they exhibit strong preference for the surface. The latter is obviously connected with the fact that cubic AlN has a high degree of ionic bonding and a vacancy creation on the nitrogen sublattice is highly energetically unfavorable, especially in the bulk due to a larger number of Al-N ionic bonds.
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| 46. |
- Garmroudi, Fabian, et al.
(författare)
-
Anderson transition in stoichiometric Fe2VAl : high thermoelectric performance from impurity bands
- 2022
-
Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 13:1
-
Tidskriftsartikel (refereegranskat)abstract
- The mathematical conditions for the best thermoelectric is well known but never realised in real materials. Here, the authors propose the Anderson transition in a narrow impurity band as a physical realisation of this seemingly unrealisable scenario. Discovered more than 200 years ago in 1821, thermoelectricity is nowadays of global interest as it enables direct interconversion of thermal and electrical energy via the Seebeck/Peltier effect. In their seminal work, Mahan and Sofo mathematically derived the conditions for 'the best thermoelectric'-a delta-distribution-shaped electronic transport function, where charge carriers contribute to transport only in an infinitely narrow energy interval. So far, however, only approximations to this concept were expected to exist in nature. Here, we propose the Anderson transition in a narrow impurity band as a physical realisation of this seemingly unrealisable scenario. An innovative approach of continuous disorder tuning allows us to drive the Anderson transition within a single sample: variable amounts of antisite defects are introduced in a controlled fashion by thermal quenching from high temperatures. Consequently, we obtain a significant enhancement and dramatic change of the thermoelectric properties from p-type to n-type in stoichiometric Fe2VAl, which we assign to a narrow region of delocalised electrons in the energy spectrum near the Fermi energy. Based on our electronic transport and magnetisation experiments, supported by Monte-Carlo and density functional theory calculations, we present a novel strategy to enhance the performance of thermoelectric materials.
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| 47. |
- Garmroudi, F., et al.
(författare)
-
Unveiling the structure-property relationship in metastable Heusler compounds by systematic disorder tuning
- 2023
-
Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:1
-
Tidskriftsartikel (refereegranskat)abstract
- Heusler compounds represent a unique class of materials that exhibit a wide range of fascinating and tuneable properties such as exotic magnetic phases, superconductivity, band topology, or thermoelectricity. An exceptional, but for Heusler compounds common, feature is that they are prone to antisite defects and disorder. In this regard, the Fe2VAl Heusler compound has been a particularly interesting and disputed candidate. Even though various theoretical scenarios for the interplay of physical properties and disorder have been proposed, the metastable disordered A2 phase hitherto precluded experimental investigation in bulk samples. Here, we report experimental results on disorder-tuned Fe2VAl0.9Si0.1 alloys all the way toward the A2 phase, which we realized via rapidly quenching our samples from high temperatures. We measured the thermoelectric properties of these materials in a wide temperature range (4 to 700 K); they suggest a gradual semimetal/narrow-gap semiconductor -> metal transition upon increasing the disorder. We also find a large anomalous Hall effect in the disordered A2 phase, arising from the side-jump scattering of charge carriers at the antisite magnetic moments. This is corroborated by measurements of the temperature-and field-dependent magnetization, which increases dramatically up to approximate to 2.5 mu B/f.u. as compared to the ordered compound (<0.1 mu B/f.u.). This study provides an experimental realization of the metastable A2 structure in bulk Fe2VAl-based alloys and grants insight into the structure-property relationship of these materials. Our work confirms that temperature-induced antisite disorder, occurring during thermal heat treatment, can be a precisely tuneable parameter in the family of Heusler compounds.
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| 48. |
- Glazyrin, K., et al.
(författare)
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Importance of Correlation Effects in hcp Iron Revealed by a Pressure-Induced Electronic Topological Transition
- 2013
-
Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 110:11, s. 117206-
-
Tidskriftsartikel (refereegranskat)abstract
- We discover that hcp phases of Fe and Fe0.9Ni0.1 undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio, and Mossbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe.
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| 49. |
- Gorbatov, O. I., et al.
(författare)
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Ab initio modeling of decomposition in iron based alloys
- 2016
-
Ingår i: Physics of metals and metallography. - : Springer. - 0031-918X .- 1555-6190. ; 117:13, s. 1293-1327
-
Tidskriftsartikel (refereegranskat)abstract
- This paper reviews recent progress in the field of ab initio based simulations of structure and properties of Fe-based alloys. We focus on thermodynamics of these alloys, their decomposition kinetics, and microstructure formation taking into account disorder of magnetic moments with temperature. We review modern theoretical tools which allow a consistent description of the electronic structure and energetics of random alloys with local magnetic moments that become totally or partially disordered when temperature increases. This approach gives a basis for an accurate finite-temperature description of alloys by calculating all the relevant contributions to the Gibbs energy from first-principles, including a configurational part as well as terms due to electronic, vibrational, and magnetic excitations. Applications of these theoretical approaches to the calculations of thermodynamics parameters at elevated temperatures (solution energies and effective interatomic interactions) are discussed including atomistic modeling of decomposition/clustering in Fe-based alloys. It provides a solid basis for understanding experimental data and for developing new steels for modern applications. The precipitation in Fe-Cu based alloys, the decomposition in Fe-Cr, and the short-range order formation in iron alloys with s-p elements are considered as examples.
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| 50. |
- Gorbatov, O. I., et al.
(författare)
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Dependence of vacancy-solute interactions on magnetic state in dilute iron-based alloys
- 2011
-
Ingår i: Solid-Solid Phase Transformations in Inorganic Materials. - Zurich-Durnten, Switzerland : TRANS TECH PUBLICATIONS. ; , s. 979-984
-
Konferensbidrag (refereegranskat)abstract
- Vacancy-solute interactions play a crucial role in diffusion-controlled phase transformations, such as ordering or decomposition, which occur in alloys under heat treatment or under irradiation. The knowledge of these interactions is important for predicting long-term behavior of nuclear materials (such as reactor steels and nuclear-waste containers) under irradiation, as well as for advancing our general understanding of kinetic processes in alloys. Using first-principles calculations based on density functional theory and employing the locally self-consistent Green’s function technique, we develop a database of vacancy-solute interactions in dilute alloys of bcc Fe with 3p (Al, Si, P, S), 3d (Ti – Cu), and 4d (Nb – Ag) elements. Interactions within the first two coordination shells have been computed in the ferromagnetic state as well as in the paramagnetic (disordered local moment) state of the iron matrix. Magnetism is found to have a very strong effect on the vacancy-solute interactions.
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