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Levämäki, H., Tian, Y., Vitos, L. & Ropo, M. (2019). An automated algorithm for reliable equation of state fitting of magnetic systems. Computational materials science, 156, 121-128
Open this publication in new window or tab >>An automated algorithm for reliable equation of state fitting of magnetic systems
2019 (English)In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 156, p. 121-128Article in journal (Refereed) Published
Abstract [en]

In computational physics and materials science ground-state properties are often extracted from an equation of state fit to energy-volume data. Magnetic systems often have multiple magnetic phases present in the energy-volume data, which poses a challenge for the fitting approach because the results are sensitive to the selection of included fitting points. This is because practically all popular equation of state fitting functions, such as Murnaghan and Birch-Murnaghan, assume just one phase and therefore cannot correctly fit magnetic energy-volume data that contains multiple phases. When fitting magnetic energy-volume data it is therefore important to select the range of fitting points in such a way that only points from the one relevant phase are included. We present a simple algorithm that makes the point selection automatically. Selecting fitting points automatically removes human bias and should also be useful for large-scale projects where selecting all fitting points by hand is not feasible.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
EOS, Equation of state fitting, Ground state, Magnetism, Automated algorithms, Computational physics, Equation of state, Fitting functions, Ground state properties, Large-scale projects, Magnetic energies, SIMPLE algorithm, Equations of state
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-236337 (URN)10.1016/j.commatsci.2018.09.026 (DOI)000449375500015 ()2-s2.0-85053772857 (Scopus ID)
Note

QC 20181109

Available from: 2018-11-09 Created: 2018-11-09 Last updated: 2018-11-28Bibliographically approved
Lu, S., Ågren, J. & Vitos, L. (2018). Ab initio study of energetics and structures of heterophase interfaces: From coherent to semicoherent interfaces. Acta Materialia, 156, 20-30
Open this publication in new window or tab >>Ab initio study of energetics and structures of heterophase interfaces: From coherent to semicoherent interfaces
2018 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 156, p. 20-30Article in journal (Refereed) Published
Abstract [en]

Density functional theory calculations have been performed to study the structures and energetics of coherent and semicoherent TiC/Fe interfaces. A systematic method for determining the interfacial energy of the semicoherent interface with misfit dislocation network has been developed. The obtained interfacial energies are used to evaluate the aspect ratio for the plate-like precipitate and a quantitative agreement with the experimental results is reached. Based on the obtained interfacial energies and atomic structure details, we propose two scenarios for heterogeneous nucleation on an edge dislocation, shedding light on the thermodynamics of precipitate nucleation and growth. The present method can be easily applied to any heterophase interfaces between metals and oxides/carbides/nitrides. 

Place, publisher, year, edition, pages
Pergamon Press, 2018
Keywords
Steels, Transition metal carbides, Heterophase interface, Interfacial energy, ab initio calculation
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-234584 (URN)10.1016/j.actamat.2018.06.030 (DOI)000442062800003 ()2-s2.0-85048928839 (Scopus ID)
Funder
The Swedish Foundation for International Cooperation in Research and Higher Education (STINT)Swedish Research CouncilSwedish Foundation for Strategic Research
Note

QC 20180914

Available from: 2018-09-14 Created: 2018-09-14 Last updated: 2018-09-14Bibliographically approved
Lizarraga, R., Holmstrom, E. & Vitos, L. (2018). Alloying effect of tungsten on the structural and magnetic properties of CoCrFeNiW high entropy alloys. Physical Review Materials, 2(9), Article ID 094407.
Open this publication in new window or tab >>Alloying effect of tungsten on the structural and magnetic properties of CoCrFeNiW high entropy alloys
2018 (English)In: Physical Review Materials, ISSN 2475-9953, Vol. 2, no 9, article id 094407Article in journal (Refereed) Published
Abstract [en]

The recent observation of the hexagonal-closed-packed (hcp) phase in CoCrFeNi-based multicomponent alloys has reopened the question of phase stability in these alloys. We investigate the alloying effect of tungsten on the crystal and magnetic structures of (CoCrFeNi)(1-x)W-x high entropy alloys using density functional theory by means of the exact muffin-tin orbital method. The body-centered-cubic (bcc), face-centered-cubic (fcc), and hcp phases are investigated in two magnetic states: ferrimagnetic and paramagnetic. Below 8 at. % W the ground state of (CoCrFeNi)(1-x)W-x is the ferrimagnetic hcp phase and above that, the ferrimagnetic bcc phase is stabilized. Our calculations show that the fcc and hcp phases are energetically very close in the whole range of studied W compositions and because CoCrFeNi and (CoCrFeNi)(0.93)W-0.07 are observed in the fcc phase at room temperature, the hcp-fcc structural phase transition is expected to occur at lower temperatures. The total magnetic moment in bcc is almost double the value calculated for the fcc and hcp structures, which is due to that Cr moments are nearly quenched in bcc but are coupled antiferromagnetically to Fe, Ni, and Co in both hcp and fcc. We calculated also the Curie temperature of these alloys using the mean-field approximation. The calculated value was found to be 155 K for fcc CoCrFeNi, in excellent agreement with experiments, and the addition of W decreases this value. Our results contribute to the development of these relatively unknown corrosion-resistant materials into industrial applications, such as cemented carbides.

Place, publisher, year, edition, pages
American Physical Society, 2018
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-235876 (URN)10.1103/PhysRevMaterials.2.094407 (DOI)000445518700002 ()
Note

QC 20181009

Available from: 2018-10-09 Created: 2018-10-09 Last updated: 2018-10-09Bibliographically approved
Yang, Z.-b., Sun, J., Lu, S. & Vitos, L. (2018). Assessing elastic property and solid-solution strengthening of binary Ni-Co, Ni-Cr, and ternary Ni-Co-Cr alloys from first-principles theory. Journal of Materials Research, 33(18), 2763-2774
Open this publication in new window or tab >>Assessing elastic property and solid-solution strengthening of binary Ni-Co, Ni-Cr, and ternary Ni-Co-Cr alloys from first-principles theory
2018 (English)In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 33, no 18, p. 2763-2774Article in journal (Refereed) Published
Abstract [en]

The elastic properties and solid-solution strengthening (SSS) of the binary Ni-Co and Ni-Cr, and ternary Ni-Co-Cr alloys were investigated by the first-principles method. The results show that both Co and Cr increase lattice parameters of the binary alloys linearly. However, nonlinearity is found in compositional dependence of lattice parameters in the ternary Ni-Co-Cr alloys, that is, Co increases but decreases the lattice parameter at low and high Cr concentrations, respectively. Co increases the bulk, shear, and Young's moduli (B, G, and E), while Cr increases B but decreases G and E in the binary alloys. In the ternary Ni-Co-Cr alloys, G and E have a similar compositional dependence to those in the binary alloys, except for B. Based on the Labusch model, the SSS parameter of Ni-Cr is larger than that of Ni-Co. The SSS effect increases significantly with Cr addition, especially at low Co concentrations in the ternary Ni-Co-Cr alloys. Meanwhile, it increases mildly with Co addition at low Cr concentrations but decreases with Co addition at high Cr concentrations.

Place, publisher, year, edition, pages
Cambridge University Press, 2018
Keywords
alloy, elastic properties, simulation
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-237124 (URN)10.1557/jmr.2018.174 (DOI)000446676400012 ()2-s2.0-85048827607 (Scopus ID)
Note

QC 20181121

Available from: 2018-11-21 Created: 2018-11-21 Last updated: 2018-11-21Bibliographically approved
Zhang, H., Sun, X., Lu, S., Dong, Z., Ding, X., Wang, Y. & Vitos, L. (2018). Elastic properties of AlxCrMnFeCoNi (0 <= x <= 5) high-entropy alloys from ab initio theory. Acta Materialia, 155, 12-22
Open this publication in new window or tab >>Elastic properties of AlxCrMnFeCoNi (0 <= x <= 5) high-entropy alloys from ab initio theory
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2018 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 155, p. 12-22Article in journal (Refereed) Published
Abstract [en]

Using ab initio calculations, we investigate the elastic properties of paramagnetic AlxCrMnFeCoNi (0 <= x <= 5) high -entropy alloys (HEAs) in both body-centered cubic (bcc) and face-centered cubic (fcc) structures. Comparison with available experimental data demonstrates that the employed approach describes accurately the elastic moduli. The predicted lattice constants increase monotonously with Al addition, whereas the elastic parameters exhibit complex composition dependences. The elastic anisotropy is unusually high for both phases. The brittle/ductile transitions formulated in terms of Cauchy pressure and Pugh ratio become consistent only when the strong elastic anisotropy is accounted for. The negative Cauchy pressure of CrMnFeCoNi is due to the relatively low bulk modulus and C-12 elastic constant, which in turn are consistent with the relatively low cohesive energy. The present findings in combination with the experimental data suggest anomalous metallic character for the HEAs system. 

National Category
Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-232868 (URN)10.1016/j.actamat.2018.05.050 (DOI)000439675000002 ()2-s2.0-85048515476 (Scopus ID)
Note

QC 20180810

Available from: 2018-08-10 Created: 2018-08-10 Last updated: 2018-10-30Bibliographically approved
Li, X., Irving, D. L. & Vitos, L. (2018). First-principles investigation of the micromechanical properties of fcc-hcp polymorphic high-entropy alloys. Scientific Reports, 8, Article ID 11196.
Open this publication in new window or tab >>First-principles investigation of the micromechanical properties of fcc-hcp polymorphic high-entropy alloys
2018 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, article id 11196Article in journal (Refereed) Published
Abstract [en]

High-entropy alloys offer a promising alternative in several high-technology applications concerning functional, safety and health aspects. Many of these new alloys compete with traditional structural materials in terms of mechanical characteristics. Understanding and controlling their properties are of the outmost importance in order to find the best single-or multiphase solutions for specific uses. Here, we employ first-principles alloy theory to address the micro-mechanical properties of five polymorphic high-entropy alloys in their face-centered cubic (fcc) and hexagonal close-packed (hcp) phases. Using the calculated elastic parameters, we analyze the mechanical stability, elastic anisotropy, and reveal a strong correlation between the polycrystalline moduli and the average valence electron concentration. We investigate the ideal shear strength of two selected alloys under shear loading and show that the hcp phase possesses more than two times larger intrinsic strength than that of the fcc phase. The derived half-width of the dislocation core predicts a smaller Peierls barrier in the fcc phase confirming its increased ductility compared to the hcp one. The present theoretical findings explain a series of important observations made on dual-phase alloys and provide an atomic-level knowledge for an intelligent design of further high-entropy materials.

Place, publisher, year, edition, pages
Nature Publishing Group, 2018
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-232883 (URN)10.1038/s41598-018-29588-z (DOI)000439686700019 ()30046064 (PubMedID)2-s2.0-850506808042-s2.0-85050680804 (Scopus ID)
Note

QC 20180809

Available from: 2018-08-09 Created: 2018-08-09 Last updated: 2018-12-06Bibliographically approved
Levamäki, H., Tian, L., Kokko, K. & Vitos, L. (2018). Gradient-level and nonlocal density functional descriptions of Cu-Au intermetallic compounds. European Physical Journal B: Condensed Matter Physics, 91(6), Article ID 128.
Open this publication in new window or tab >>Gradient-level and nonlocal density functional descriptions of Cu-Au intermetallic compounds
2018 (English)In: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 91, no 6, article id 128Article in journal (Refereed) Published
Abstract [en]

We use three gradient level and two nonlocal density functional approximations to study the thermodynamic properties of Cu-Au compounds. It is found that a well-designed gradient level approximation (quasi non-uniform approximation, QNA) reproduces the experimental equilibrium volumes and the formation energies of L12 and L10 phases. On the other hand, QNA predicts a non-existent beta(2) phase, which can be remedied only when employing the nonlocal hybrid-level Heyd-Scuseria-Ernzerhof (HSE06) or Perdew-Burke-Ernzerhof (PBE0) approximations. Gradient-level approximations lead to similar electronic structures for the Cu-Au compounds whereas hybrids shift the d-band towards negative energies and account for the complex d-d hybridization more accurately.

Place, publisher, year, edition, pages
SPRINGER, 2018
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-232252 (URN)10.1140/epjb/o2018-90166-9 (DOI)000436369100003 ()2-s2.0-85048883933 (Scopus ID)
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research The Swedish Foundation for International Cooperation in Research and Higher Education (STINT)Carl Tryggers foundation VINNOVASwedish Energy Agency
Note

QC 20180720

Available from: 2018-07-20 Created: 2018-07-20 Last updated: 2018-10-16Bibliographically approved
Holmström, E., Lizarraga, R., Linder, D., Salmasi, A., Wang, W., Kaplan, B., . . . Vitos, L. (2018). High entropy alloys: Substituting for cobalt in cutting edge technology. Applied Materials Today, 12, 322-329
Open this publication in new window or tab >>High entropy alloys: Substituting for cobalt in cutting edge technology
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2018 (English)In: Applied Materials Today, ISSN 2352-9407, Vol. 12, p. 322-329Article in journal (Refereed) Published
Abstract [en]

Cemented carbide, also known as hard metal, is one of the most outstanding composite engineering materials since its commercial introduction in the 1920s. The unique combination of strength, hardness and toughness makes cemented carbides highly versatile materials for the most demanding engineering applications. In their simplest form, these materials are composites of tungsten carbide (WC) grains that are cemented with a ductile metallic binder phase, typically cobalt. However, despite the superiority of Co as binder material, there is a long-standing need to find alternative binders due to serious health concerns that have haunted the industry for nearly 80 years. In the present study, we develop a new cemented carbide with a high entropy alloy binder phase (CoCrFeNi) from raw materials to a fully functional, coated and gradient-sintered cutting tool insert. The new hard metal with reduced Co content is designed by using first principles theory and the CALPHAD method. The cutting tool was made by pressing the new hard metal in a standard geometry, sintered to have a thin binder phase enriched surface zone, free from cubic carbides and coated with protective layers of Ti(C,N) and Al2O3. The resulting cutting insert was tested in a real machining operation and compared to a state-of-the-art reference that had Co as binder phase. The cutting tool made of the newly developed cemented carbide has an exceptionally high resistance against plastic deformation at all tested cutting speeds in the machining test, outperforming the reference insert, which shows a linear increase in edge depression when the cutting speed is increased. This result opens up the possibility to utilize the unique properties of high entropy alloys for industrial applications, in particular, as binder phase in new cemented carbides.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
High entropy alloys, Cemented carbides, Cobalt binder, Alternative binders, Density functional theory, Calphad
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-235109 (URN)10.1016/j.apmt.2018.07.001 (DOI)000443213700028 ()2-s2.0-85049613452 (Scopus ID)
Funder
VINNOVA, 2016-00805Swedish Research CouncilSwedish Foundation for Strategic Research The Swedish Foundation for International Cooperation in Research and Higher Education (STINT)Carl Tryggers foundation
Note

QC 20180919

Available from: 2018-09-19 Created: 2018-09-19 Last updated: 2018-11-13Bibliographically approved
Landa, A., Soederlind, P., Naumov, I. I., Klepeis, J. E. & Vitos, L. (2018). Kohn Anomaly and Phase Stability in Group VB Transition Metals. Computation, 6(2), Article ID 29.
Open this publication in new window or tab >>Kohn Anomaly and Phase Stability in Group VB Transition Metals
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2018 (English)In: Computation, E-ISSN 2079-3197, Vol. 6, no 2, article id 29Article, review/survey (Refereed) Published
Abstract [en]

In the periodic table, only a few pure metals exhibit lattice or magnetic instabilities associated with Fermi surface nesting, the classical examples being alpha-U and Cr. Whereas alpha-U displays a strong Kohn anomaly in the phonon spectrum that ultimately leads to the formation of charge density waves (CDWs), Cr is known for its nesting-induced spin density waves (SDWs). Recently, it has become clear that a pronounced Kohn anomaly and the corresponding softening in the elastic constants is also the key factor that controls structural transformations and mechanical properties in compressed group VB metals-materials with relatively high superconducting critical temperatures. This article reviews the current understanding of the structural and mechanical behavior of these metals under pressure with an introduction to the concept of the Kohn anomaly and how it is related to the important concept of Peierls instability. We review both experimental and theoretical results showing different manifestations of the Kohn anomaly in the transverse acoustic phonon mode TA (xi 00) in V, Nb, and Ta. Specifically, in V the anomaly triggers a structural transition to a rhombohedral phase, whereas in Nb and Ta it leads to an anomalous reduction in yield strength.

Place, publisher, year, edition, pages
MDPI, 2018
Keywords
Kohn anomaly, Fermi surface nesting, phonon softening
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-228420 (URN)10.3390/computation6020029 (DOI)000431867700003 ()
Note

QC 20180528

Available from: 2018-05-28 Created: 2018-05-28 Last updated: 2018-05-28Bibliographically approved
Levamaki, H., Nagy, A., Vilja, I., Kokko, K. & Vitos, L. (2018). Kullback-Leibler and relative Fisher information as descriptors of locality. International Journal of Quantum Chemistry, 118(12), Article ID e25557.
Open this publication in new window or tab >>Kullback-Leibler and relative Fisher information as descriptors of locality
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2018 (English)In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 118, no 12, article id e25557Article in journal (Refereed) Published
Abstract [en]

Kullback-Leibler and relative Fisher information functionals are applied in studying deviation from local density approximation. The reduced density gradient s and the local kinetic energy parameter alpha are key ingredients of these new locality descriptors. The relative Kullback-Leibler information density contains extra knowledge as it is negative where the given probability density is smaller than the reference density. The relative Fisher information incorporates the highest order deviations from the uniform electron gas approximation.

Place, publisher, year, edition, pages
WILEY, 2018
Keywords
density functional theory, descriptors of locality, Kullback-Leibler information, relative Fisher information
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-228414 (URN)10.1002/qua.25557 (DOI)000432000700004 ()2-s2.0-85037672111 (Scopus ID)
Note

QC 20180528

Available from: 2018-05-28 Created: 2018-05-28 Last updated: 2018-05-28Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0003-2832-3293

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