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Li, X., Schönecker, 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
American Physical Society, 2018
Keyword
Ab-Initio, Theoretical Strength, Ideal Strength, Metals, Approximation, Dislocation, Crystals, Aluminum, Molybdenum, Simulation
National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-224677 (URN)10.1103/PhysRevB.97.094102 (DOI)000426775200002 ()2-s2.0-85043990703 (Scopus ID)
Funder
Swedish Research CouncilSwedish Foundation for Strategic Research The Swedish Foundation for International Cooperation in Research and Higher Education (STINT)
Note

QC 20180323

Available from: 2018-03-23 Created: 2018-03-23 Last updated: 2018-03-27Bibliographically 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, 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.

Place, publisher, year, edition, pages
Elsevier, 2017
Keyword
High-entropy alloys, Lattice parameter, Elastic constant, Local lattice distortion, Alloying effect
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-200188 (URN)10.1016/j.matdes.2016.11.079 (DOI)000390650800031 ()2-s2.0-85006056575 (Scopus ID)
Note

QC 20170123

Available from: 2017-01-23 Created: 2017-01-23 Last updated: 2017-06-30Bibliographically 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:kth:diva-215810 (URN)10.1103/PhysRevB.96.125156 (DOI)000412028700002 ()2-s2.0-85030129223 (Scopus ID)
Note

QC 20171018

Available from: 2017-10-18 Created: 2017-10-18 Last updated: 2017-11-29Bibliographically approved
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, 2017
Keyword
The Bain path, The Exact Muffin-Tin Orbital (EMTO) method, Transition metals, Crystal symmetry, Tin, Bain paths, Body-centred cubic, Exact muffin-tin orbitals methods, Face-centred cubic, Tetragonal crystals, Theoretical calculations, Total energy calculation, Spheres
National Category
Philosophy
Identifiers
urn:nbn:se:kth:diva-207318 (URN)10.1080/14786435.2017.1293862 (DOI)000399958500005 ()2-s2.0-85014505795 (Scopus ID)
Note

QC 20170609

Available from: 2017-06-09 Created: 2017-06-09 Last updated: 2017-11-10Bibliographically 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.

Place, publisher, year, edition, pages
American Physical Society, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-201227 (URN)10.1103/PhysRevB.95.024203 (DOI)000391852800001 ()2-s2.0-85010427119 (Scopus ID)
Note

QC 20172222

Available from: 2017-02-20 Created: 2017-02-20 Last updated: 2017-11-29Bibliographically approved
Dong, Z., Schönecker, S., Chen, D., Li, W., Long, M. & Vitos, L. (2017). Elastic properties of paramagnetic austenitic steel at finite temperature: Longitudinal spin fluctuations in multicomponent alloys. Physical Review B, 96(17), Article ID 174415.
Open this publication in new window or tab >>Elastic properties of paramagnetic austenitic steel at finite temperature: Longitudinal spin fluctuations in multicomponent alloys
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2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 17, article id 174415Article in journal (Refereed) Published
Abstract [en]

We propose a first-principles framework for longitudinal spin fluctuations (LSFs) in disordered paramagnetic (PM) multicomponent alloy systems and apply it to investigate the influence of LSFs on the temperature dependence of two elastic constants of PM austenitic stainless steel Fe15Cr15Ni. The magnetic model considers individual fluctuating moments in a static PM medium with first-principles-derived LSF energetics in conjunction with describing chemical disorder and randomness of the transverse magnetic component in the single-site alloy formalism and disordered local moment (DLM) picture. A temperature-sensitive mean magnetic moment is adopted to accurately represent the LSF state in the elastic-constant calculations. We make evident that magnetic interactions between an LSF impurity and the PM medium are weak in the present steel alloy. This allows gaining accurate LSF energetics and mean magnetic moments already through a perturbation from the static DLM moments instead of a tedious self-consistent procedure. We find that LSFs systematically lower the cubic shear elastic constants c' and c(44) by similar to 6 GPa in the temperature interval 300-1600 K, whereas the predominant mechanism for the softening of both elastic constants with temperature is the magneto-volume coupling due to thermal lattice expansion. We find that non-negligible local magnetic moments of Cr and Ni are thermally induced by LSFs, but they exert only a small influence on the elastic properties. The proposed framework exhibits high flexibility in accurately accounting for finite-temperature magnetism and its impact on the mechanical properties of PM multicomponent alloys.

Place, publisher, year, edition, pages
American Physical Society, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-218210 (URN)10.1103/PhysRevB.96.174415 (DOI)000414953200003 ()
Note

QC 20171128

Available from: 2017-11-28 Created: 2017-11-28 Last updated: 2018-03-12Bibliographically approved
Tian, F., Wang, Y. & Vitos, L. (2017). Impact of aluminum doping on the thermo-physical properties of refractory medium-entropy alloys. Journal of Applied Physics, 121(1), Article ID 015105.
Open this publication in new window or tab >>Impact of aluminum doping on the thermo-physical properties of refractory medium-entropy alloys
2017 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 121, no 1, article id 015105Article in journal (Refereed) Published
Abstract [en]

We investigate the elastic moduli, ideal tensile strength, and thermodynamic properties of TiVNb and AlTiVNb refractory medium-entropy alloys (HEAs) by using ab initio alloy theories: the coherent potential approximation (CPA), the special quasi-random supercell (SQS), and a 432-atom supercell (SC). We find that with increasing number of alloy components, the SQS elastic constants become sensitive to the supercell size. The predicted elastic moduli are consistent with the available experiments. Aluminum doping decreases the stability of the body centered cubic phase. The ideal tensile strength calculation indicates that adding equiatomic Al to TiVNb random solid solution increases the intrinsic strength (ideal strain increase from 9.6% to 11.8%) and decreases the intrinsic strength (from 9.6 to 5.7GPa). Based on the equation of states calculated by the CPA and SC methods, the thermodynamic properties obtained by the two ab initio methods are assessed. The L2(1) AlTiVNb (Ti-Al-V-Nb) alloy is predicted to be thermodynamically and dynamically stable with respect to the solid solution.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-202436 (URN)10.1063/1.4973489 (DOI)000392839400042 ()2-s2.0-85008951918 (Scopus ID)
Note

QC 20170306

Available from: 2017-03-06 Created: 2017-03-06 Last updated: 2017-11-29Bibliographically approved
Song, H., Tian, F., Hu, Q.-M., Vitos, L., Wang, Y., Shen, J. & Chen, N. (2017). Local lattice distortion in high-entropy alloys. PHYSICAL REVIEW MATERIALS, 1(2), Article ID 023404.
Open this publication in new window or tab >>Local lattice distortion in high-entropy alloys
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2017 (English)In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 1, no 2, article id 023404Article in journal (Refereed) Published
Abstract [en]

The severe local lattice distortion, induced mainly by the large atomic size mismatch of the alloy components, is one of the four core effects responsible for the unprecedented mechanical behaviors of high-entropy alloys (HEAs). In this work, we propose a supercell model, in which every lattice site has similar local atomic environment, to describe the random distributions of the atomic species in HEAs. Using these supercells in combination with ab initio calculations, we investigate the local lattice distortion of refractory HEAs with body-centered-cubic structure and 3d HEAs with face-centered-cubic structure. Our results demonstrate that the local lattice distortion of the refractory HEAs is much more significant than that of the 3d HEAs. We show that the atomic size mismatch evaluated with the empirical atomic radii is not accurate enough to describe the local lattice distortion. Both the lattice distortion energy and the mixing entropy contribute significantly to the thermodynamic stability of HEAs. However the local lattice distortion has negligible effect on the equilibrium lattice parameter and bulk modulus.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-220298 (URN)10.1103/PhysRevMaterials.1.023404 (DOI)000416557400001 ()
Note

QC 20171222

Available from: 2017-12-22 Created: 2017-12-22 Last updated: 2017-12-22Bibliographically approved
Dong, Z., Li, W., Chen, D., Schönecker, S., Long, M. & Vitos, L. (2017). Longitudinal spin fluctuation contribution to thermal lattice expansion of paramagnetic Fe. Physical Review B. Condensed Matter and Materials Physics, 95(5), Article ID 054426.
Open this publication in new window or tab >>Longitudinal spin fluctuation contribution to thermal lattice expansion of paramagnetic Fe
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2017 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 95, no 5, article id 054426Article in journal (Refereed) Published
Abstract [en]

Using an efficient first-principles computational scheme for paramagnetic body-centered cubic (bcc) and face-centered cubic (fcc) Fe, we investigate the impact of thermal longitudinal spin fluctuations (LSFs) on the thermal lattice expansion. The equilibrium physical parameters are derived from the self-consistent Helmholtz free energy, in which the LSFs are considered within the adiabatic approximation and the anharmonic lattice vibration effect is included using the Debye-Grüneisen model taking into account the interplay between thermal, magnetic, and elastic degrees of freedom. Thermal LSFs are energetically more favorable in the fcc phase than in the bcc one giving a sizable contribution to the linear thermal expansion of γ-Fe. The present scheme leads to accurate temperature-dependent equilibrium Wigner-Seitz radius, bulk modulus, and Debye temperature within the stability fields of the two phases and demonstrates the importance of thermal spin fluctuations in paramagnetic Fe.

Place, publisher, year, edition, pages
American Physical Society, 2017
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-207999 (URN)10.1103/PhysRevB.95.054426 (DOI)000394655800005 ()2-s2.0-85014515470 (Scopus ID)
Note

QC 2017-06-08

Available from: 2017-06-08 Created: 2017-06-08 Last updated: 2017-11-10Bibliographically approved
Sun, X., Zhang, H., Lu, S., Ding, X., Wang, Y. & Vitos, L. (2017). Phase selection rule for Al-doped CrMnFeCoNi high-entropy alloys from first-principles. Acta Materialia, 140, 366-374
Open this publication in new window or tab >>Phase selection rule for Al-doped CrMnFeCoNi high-entropy alloys from first-principles
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2017 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 140, p. 366-374Article in journal (Refereed) Published
Abstract [en]

Using ab initio alloy theory, we investigate the lattice stability of paramagnetic AlxCrMnFeCoNi (0 <= x <= 5) high-entropy alloys considering the competing body-centered cubic (bcc) and face-centered cubic (fcc) crystal structures. The theoretical lattice constants increase with increasing x, in good agreement with experimental data. Upon Al addition, the crystal structure changes from fcc to bcc with a broad two-phase field region, in line with observations. The magnetic transition temperature for the bcc structure strongly decreases with x, whereas that for the fee structure shows weak composition dependence. Within their own stability fields, both structures are predicted to be paramagnetic at ambient conditions. Bain path calculations support that within the duplex region both phases are dynamically stable. As compared to AlxCrFeCoNi, equiatomic Mn addition is found to shrink the stability range of the fcc phase and delay the appearance of the bcc phase in terms of Al content, thus favoring the duplex region in 3d-metals based high-entropy alloys.

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2017
Keyword
High-entropy alloys, Phase stability, Ab initio calculation
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-217412 (URN)10.1016/j.actamat.2017.08.045 (DOI)000413879800037 ()2-s2.0-85028725768 (Scopus ID)
Note

QC 20171121

Available from: 2017-11-21 Created: 2017-11-21 Last updated: 2017-11-21Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0003-2832-3293

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