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Publications (10 of 246) Show all publications
Ji, Z.-W., Lu, S., Hu, Q.-m., Kim, D., Yang, R. & Vitos, L. (2018). Mapping deformation mechanisms in lamellar titanium aluminide. Acta Materialia, 144, 835-843
Open this publication in new window or tab >>Mapping deformation mechanisms in lamellar titanium aluminide
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2018 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 144, p. 835-843Article in journal (Refereed) Published
Abstract [en]

Breakdown of Schmid's law is a long-standing problem for exploring the orientation-dependent deformation mechanism in intermetallics. The lack of atomic-level understanding of the selection rules for the plastic deformation modes has seriously limited designing strong and ductile intermetallics for high-temperature applications. Here we put forward a transparent model solely based on first principles simulations for mapping the deformation modes in gamma-TiAl polysynthetic twinned alloys. The model bridges intrinsic energy barriers and different deformation mechanisms and beautifully resolves the complexity of the observed orientation-dependent deformation mechanisms. Using the model, one can elegantly reveal the atomic-level mechanisms behind the unique channeled flow phenomenon in lamellar TiAl alloys.

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2018
Keyword
Stacking fault, Twinning, Slip, First principles
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-347091 (URN)10.1016/j.actamat.2017.11.028 (DOI)000424067100076 ()
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research Carl Tryggers foundation VINNOVA
Available from: 2018-03-26 Created: 2018-03-26 Last updated: 2018-03-26Bibliographically approved
Huang, S., Holmstrom, E., Eriksson, O. & Vitos, L. (2018). Mapping the magnetic transition temperatures for medium- and high-entropy alloys. Intermetallics (Barking), 95, 80-84
Open this publication in new window or tab >>Mapping the magnetic transition temperatures for medium- and high-entropy alloys
2018 (English)In: Intermetallics (Barking), ISSN 0966-9795, E-ISSN 1879-0216, Vol. 95, p. 80-84Article in journal (Refereed) Published
Abstract [en]

Tailorable magnetic state near room temperature is very promising for several technological, including magnetocaloric applications. Here using first-principle alloy theory, we determine the Curie temperature (T-C) of a number of equiatomic medium- and high-entropy alloys with solid solution phases. All calculations are performed at the computed lattice parameters, which are in line with the available experimental data. Theory predicts a large crystal structure dependence of T-C, which explains the experimental observations under specified conditions. The sensitivity of the magnetic state to the crystal lattice is reflected by the magnetic exchange interactions entering the Heisenberg Hamiltonian. The analysis of the effect of composition on T-C allows researchers to explore chemistry-dependent trends and design new multi-component alloys with pre-assigned magnetic properties.

Keyword
Curie temperature, High-entropy alloys, First-principle calculations, Monte-Carlo simulations
National Category
Metallurgy and Metallic Materials Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-351640 (URN)10.1016/j.intermet.2018.01.016 (DOI)000428975100010 ()
Funder
Swedish Research CouncilVINNOVA, 2014-03374Swedish Foundation for Strategic Research Carl Tryggers foundation The Swedish Foundation for International Cooperation in Research and Higher Education (STINT)Swedish Energy Agency
Available from: 2018-06-07 Created: 2018-06-07 Last updated: 2018-06-07Bibliographically approved
Li, X., Schonecker, S., Li, W., Varga, L. K., Irving, D. L. & Vitos, L. (2018). Tensile and shear loading of four fcc high-entropy alloys: A first-principles study. Physical Review B, 97(9), Article ID 094102.
Open this publication in new window or tab >>Tensile and shear loading of four fcc high-entropy alloys: A first-principles study
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2018 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 9, article id 094102Article in journal (Refereed) Published
Abstract [en]

Ab initio density-functional calculations are used to investigate the response of four face-centered-cubic (fcc) high-entropy alloys (HEAs) to tensile and shear loading. The ideal tensile and shear strengths (ITS and ISS) of the HEAs are studied by employing first-principles alloy theory formulated within the exact muffin-tin orbital method in combination with the coherent-potential approximation. We benchmark the computational accuracy against literature data by studying the ITS under uniaxial [110] tensile loading and the ISS for the [11 (2) over tilde](111) shear deformation of pure fcc Ni and Al. For the HEAs, we uncover the alloying effect on the ITS and ISS. Under shear loading, relaxation reduces the ISS by similar to 50% for all considered HEAs. We demonstrate that the dimensionless tensile and shear strengths are significantly overestimated by adopting two widely used empirical models in comparison with our ab initio calculations. In addition, our predicted relationship between the dimensionless shear strength and shear instability are in line with the modified Frenkel model. Using the computed ISS, we derive the half-width of the dislocation core for the present HEAs. Employing the ratio of ITS to ISS, we discuss the intrinsic ductility of HEAs and compare it with a common empirical criterion. We observe a strong linear correlation between the shear instability and the ratio of ITS to ISS, whereas a weak positive correlation is found in the case of the empirical criterion.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-350286 (URN)10.1103/PhysRevB.97.094102 (DOI)000426775200002 ()
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research The Swedish Foundation for International Cooperation in Research and Higher Education (STINT)
Available from: 2018-05-09 Created: 2018-05-09 Last updated: 2018-05-09Bibliographically approved
Li, X., Li, X., Schonecker, S., Li, R., Zhao, J. & Vitos, L. (2018). Understanding the mechanical properties of reduced activation steels. Materials & design, 146, 260-272
Open this publication in new window or tab >>Understanding the mechanical properties of reduced activation steels
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2018 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 146, p. 260-272Article in journal (Refereed) Published
Abstract [en]

Reduced activation ferritic/martensitic (RAFM) steels are structural materials with potential application in Generation-IV fission and fusion reactors. We use density-functional theory to scrutinize the micro-mechanical properties of the main alloy phases of three RAFM steels based on the body-centered cubic FeCrWVMn solid solution. We assess the lattice parameters and elastic properties of ferromagnetic alpha-Fe and Fe91Cr9, which are the main building blocks of the RAFM steels, and present a detailed analysis of the calculated alloying effects of V, Cr, Mn, and W on the mechanical properties of Fe91Cr9. The composition dependence of the elastic parameters is decomposed into electronic and volumetric contributions and studied for alloying levels that cover the typical intervals in RAFM steels. A linear superposition of the individual solute effects on the properties of Fe91Cr9 is shown to provide an excellent approximation for the ab initio values obtained for the RAFM steels. The intrinsic ductility is evaluated through Rice's phenomenological theory using the surface and unstable stacking fault energies, and the predictions are contrasted with those obtained by empirical criteria. Alloying with V or W is found to enhance the ductility, whereas additional Cr or Mn turns the RAFM base alloys more brittle.

Keyword
Reduced activation ferritic/martensitic steels, Elastic properties, Ductility
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:uu:diva-351615 (URN)10.1016/j.matdes.2018.03.009 (DOI)000428802500026 ()
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research
Available from: 2018-06-13 Created: 2018-06-13 Last updated: 2018-06-13Bibliographically approved
Tian, L.-Y., Lizarraga, R., Larsson, H., Holmstrom, E. & Vitos, L. (2017). A first principles study of the stacking fault energies for fcc Co-based binary alloys. Acta Materialia, 136, 215-223
Open this publication in new window or tab >>A first principles study of the stacking fault energies for fcc Co-based binary alloys
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2017 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 136, p. 215-223Article in journal (Refereed) Published
Abstract [en]

The stacking fault energy is closely related to structural phase transformations and can help to understand plastic deformation mechanisms in materials. Here we perform first principles calculations of the stacking fault energy in the face centered cubic (fcc) Cobalt-based binary alloys Co1-x M-x, where M = Cr, Fe, Ni, Mo, Ru, Rh, Pd and W. We investigate the concentration range between 0 and 30 at.% of the alloying element. The results are discussed in connection to the phase transition between the low temperature hexagonal close packed (hcp) and the fcc structures observed in Co and its alloys. By analyzing the stacking fault energies, we show that alloying Co with Cr, Ru, and Rh promotes the hcp phase formation while Fe, Ni and Pd favor the fcc phase instead. The effect of Mo and W on the phase transition differs from the other elements, that is, for concentrations below 10% the intrinsic stacking fault energy is lower than that for pure fcc Co and the energy barrier is higher, whereas above 10% the situation reverses. We carry out also thermodynamic calculations using the ThermoCalc software. The trends of the ab initio stacking fault energy are found to agree well with those of the molar Gibbs energy differences and the phase transition temperature in the binary phase diagrams and give a solid support for the phase stability of these alloys.

Keyword
First principles calculations, Stacking fault energies, Cobalt-based alloys, Thermodynamic calculations
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-333947 (URN)10.1016/j.actamat.2017.07.010 (DOI)000407665300019 ()
Available from: 2017-12-13 Created: 2017-12-13 Last updated: 2017-12-13Bibliographically approved
Tian, L.-Y., Wang, G., Harris, J. S., Irving, D. L., Zhao, J. & Vitos, L. (2017). Alloying effect on the elastic properties of refractory high-entropy alloys. Materials & design, 114, 243-252
Open this publication in new window or tab >>Alloying effect on the elastic properties of refractory high-entropy alloys
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2017 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 114, p. 243-252Article in journal (Refereed) Published
Abstract [en]

Ab initio total energy calculations are used to determine the elastic properties of TiZrVNb, TiZrNbMo and TiZrVNbMo high-entropy alloys in the body centered cubic (bcc) crystallographic phase. Calculations are performed using the Vienna Ab initio Simulation Package and the Exact Muffin-Tin Orbitals methods, and the compositional disorder is treated within the frameworks of the special quasi-random structures technique and the coherent potential approximation, respectively. Special emphasis is given to the effect of local lattice distortion and trends against composition. Significant distortion can be observed in the relaxed cells, which result in an overlap of the first and second nearest neighbor (NN) shells represented in the histograms. When going from the four-component alloys TiZrVNb and TiZrNbMo to the five-component TiZrVNbMo, the changes in the elastic parameters follow the expected trends, except that of C-44 which decreases upon adding equiatomic Mo to TiZrVNb despite of the large shear elastic constant of elemental Mo. Although the rule of mixtures turns out to be a useful tool to estimate the elastic properties of the present HEAs, to capture the more delicate alloying effects one needs to resort to ab initio results.

Keyword
High-entropy alloys, Lattice parameter, Elastic constant, Local lattice distortion, Alloying effect
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-313929 (URN)10.1016/j.matdes.2016.11.079 (DOI)000390650800031 ()
Available from: 2017-04-05 Created: 2017-04-05 Last updated: 2017-11-29Bibliographically approved
Ostlin, A., Vitos, L. & Chioncel, L. (2017). Analytic continuation-free Green's function approach to correlated electronic structure calculations. Physical Review B, 96(12), Article ID 125156.
Open this publication in new window or tab >>Analytic continuation-free Green's function approach to correlated electronic structure calculations
2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 12, article id 125156Article in journal (Refereed) Published
Abstract [en]

We present a charge self-consistent scheme combining density functional and dynamical mean field theory, which uses Green's functions of multiple-scattering type. In this implementation, the many-body effects are incorporated into the Kohn-Sham iterative scheme without the need for the numerically ill-posed analytic continuation of the Green's function and of the self-energy, which was previously a bottleneck in multiple-scattering-type Green's function approaches. This is achieved by producing the Kohn-Sham Hamiltonian in the subspace of correlated partial waves and allows to formulate the Green's function directly on theMatsubara axis. The spectral moments of the Matsubara Green's function enable us to put together the real-space charge density, therefore, the charge self-consistency can be achieved. Our results for the spectral functions (density of states) and equation-of-state curves for transition-metal elements Fe, Ni, and FeAl compound agree very well with those of Hamiltonian-based LDA+DMFT implementations. The current implementation improves on numerical accuracy, compared to previous implementations where analytic continuation was required at each Kohn-Sham self-consistent step. A minimal effort aside from the multiple-scattering formulation is required, and the method can be generalized in several ways that are interesting for applications to real materials.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-337091 (URN)10.1103/PhysRevB.96.125156 (DOI)000412028700002 ()
Funder
Swedish Foundation for Strategic Research Swedish Research CouncilThe Swedish Foundation for International Cooperation in Research and Higher Education (STINT)
Available from: 2018-01-26 Created: 2018-01-26 Last updated: 2018-01-26
Al-Zoubi, N., Schönecker, S., Johansson, B. & Vitos, L. (2017). Assessing the Exact Muffin-Tin Orbitals method for the Bain path of metals. Philosophical Magazine, 97(15), 1243-1264
Open this publication in new window or tab >>Assessing the Exact Muffin-Tin Orbitals method for the Bain path of metals
2017 (English)In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 97, no 15, p. 1243-1264Article in journal (Refereed) Published
Abstract [en]

We scrutinise the muffin-tin approximation and the screening within the framework of the Exact Muffin-Tin Orbitals method in the case of cubic and tetragonal crystal symmetries. Systematic total energy calculations are carried out for the Bain path including the body-centred cubic and face-centred cubic structures for a set of simple and transition metals. The present converged results in terms of potential sphere radius (S) and hard sphere radius (b) are in good agreement with previous theoretical calculations. We demonstrate that for all structures considered here, potential sphere radii around and slightly larger than the average Wigner-Seitz radius (w) yield accurate total energy results whereas S values smaller than w give large errors. It is shown that for converged total energies hard spheres with radii b = 0.7-0.8w should be used for an efficient screening within real space clusters consisting typically of 70-90 lattice sites. The less efficient convergence of the total energy in the case of small hard spheres is ascribed to the delocalisation of the screened spherical waves, which leads to inaccurate interstitial overlap matrix. The above conclusions are not significantly affected by the volume of the system.

Place, publisher, year, edition, pages
TAYLOR & FRANCIS LTD, 2017
Keyword
The Bain path, Transition metals, The Exact Muffin-Tin Orbital (EMTO) method
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-322885 (URN)10.1080/14786435.2017.1293862 (DOI)000399958500005 ()
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research The Swedish Foundation for International Cooperation in Research and Higher Education (STINT)Carl Tryggers foundation
Available from: 2017-05-31 Created: 2017-05-31 Last updated: 2017-05-31Bibliographically approved
Tian, L.-Y., Ye, L.-H., Hu, Q.-M., Lu, S., Zhao, J. & Vitos, L. (2017). CPA descriptions of random Cu-Au alloys in comparison with SQS approach. Computational materials science, 128, 302-309
Open this publication in new window or tab >>CPA descriptions of random Cu-Au alloys in comparison with SQS approach
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2017 (English)In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 128, p. 302-309Article in journal (Refereed) Published
Abstract [en]

The lattice constant, formation enthalpy, and elastic parameters of Cu1-xAux (0 <= x <= 1) alloys in the face centered cubic crystallographic phase are investigated by using the first-principles exact muffin-tin orbitals and plane-wave pseudopotential methods in order to explore the effect of alloying with special focus on the impact of local lattice distortion (LLD) on the above properties. The compositional disorder is treated within the framework of the coherent potential approximation (CPA) and the special quasi-random structure (SQS) scheme. Calculations based on SQS and CPA show that, while LLD lowers significantly the formation enthalpy of Cu1-xAux due to the large size mismatch between Cu and Au atoms, it has negligible influence on the lattice constants and elastic parameters. These findings confirm the reliability of CPA for computing the enthalpy changes upon isotropic and unisotropic lattice distortions in disordered alloys with sizable atomic size differences.

Keyword
Local lattice distortion, Formation enthalpy, Elastic constant, Special quasi-random structure (SQS), Cu-Au alloys
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-314399 (URN)10.1016/j.commatsci.2016.11.045 (DOI)000391022600033 ()
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research Carl Tryggers foundation The Swedish Foundation for International Cooperation in Research and Higher Education (STINT)
Available from: 2017-02-06 Created: 2017-02-02 Last updated: 2017-11-29Bibliographically approved
Li, X., Schönecker, S., Zhao, J., Vitos, L. & Johansson, B. (2017). Elastic anharmonicity of bcc Fe and Fe-based random alloys from first-principles calculations. Physical Review B, 95(2), Article ID 024203.
Open this publication in new window or tab >>Elastic anharmonicity of bcc Fe and Fe-based random alloys from first-principles calculations
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 95, no 2, article id 024203Article in journal (Refereed) Published
Abstract [en]

We systematically investigate elastic anharmonic behavior in ferromagnetic body-centered cubic (bcc) Fe and Fe1-xMx (M = Al, V, Cr, Co, or Ni) random alloys by means of density-functional simulations. To benchmark computational accuracy, three ab initio codes are used to obtain the complete set of second-and third-order elastic constants (TOECs) for bcc Fe. The TOECs of Fe1-xMx alloys are studied employing the first-principles alloy theory formulated within the exact muffin-tin orbital method in combination with the coherent-potential approximation. It is found that the alloying effects on C-111, C-112, and C-123, which are governed by normal strains only, are more pronounced than those on C-144, C-166, and C-456, which involve shear strains. Remarkably, the magnitudes of all TOECs but C-123 decrease upon alloying with Al, V, Cr, Co, or Ni. Using the computed TOECs, we study compositional effects on the pressure derivatives of the effective elastic constants (dB(ij)/dP), bulk (dK/dP), and shear moduli (dG/dP) and derive longitudinal acoustic nonlinearity parameters (beta). Our predictions show that the pressure derivatives of K and G decrease with x for all solute elements and reveal a strong correlation between the compositional trends on dK/dP and dG/dP arising from the fact that alloying predominantly altersdB(11)/dP. The sensitivity of dB(11)/dP to composition is attributed to intrinsic alloying effects as opposed to lattice parameter changes accompanying solute addition. For Fe and the considered Fe-based alloys, beta along high-symmetry directions orders as beta[111] > beta[100] > beta[110], and alloying increases the directional anisotropy of beta but reduces its magnitude.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-315812 (URN)10.1103/PhysRevB.95.024203 (DOI)000391852800001 ()
Funder
Carl Tryggers foundation Swedish Research CouncilSwedish Foundation for Strategic Research The Swedish Foundation for International Cooperation in Research and Higher Education (STINT)
Available from: 2017-02-22 Created: 2017-02-22 Last updated: 2017-11-29Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0003-2832-3293

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