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Ab initio atomistic simulation of metals and multicomponent alloys
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Ab initio theory provides a powerful tool to understand and predict the behavior of materials. This thesis contains both of these aspects. First we use ab initio alloy theory to investigate a new kind of complex alloy (high-entropy alloy). Second we introduce a novel potential (interlayer potential), which can be extracted from ab inito total energy calculations using the Chen-Möbius inversion method.

High-entropy alloys (HEAs) are composed of four or more metallic elements with nearly equimolar composition. In spite of the large number of components, most of the HEAs have a simple solid-solution phase rather than forming complex intermetallic structures. Extensive experiments have reported the unique microstructures and special properties of HEAs. Single-phase HEAs may be divided into three types, i.e. the

3d-HEAs adopting the face centered cubic (fcc) phase, the refractory-HEAs with a body centered cubic (bcc) phase, and the HEAs with the duplex fcc-bcc structure. We employ the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA) to investigate the electronic structure, the equilibrium volume and the elastic properties of these three-type HEAs.

First we compare the CPA with the super cell technique (SC) to assess the performance of the EMTO-CPA method. As typical fcc 3d-HEAs, we consider the CuNiCoFeCrTix systems in the paramagnetic state. Starting from the calculated electronic structure, we give an explanation for the observed magnetic states. Furthermore, we provide a theoretical prediction for the elastic parameters and polycrystalline elastic moduli for CuNiCoFeCrTix (x= 0.0−0.5, 1.0) and NiCoTeCrTi. A detailed comparison between the theoretical results and the available experimental data demonstrates that ab initio theory can properly describe the fundamental properties of this important class of engineering alloys.

Refractory-HEAs are composed of Ti, Zr, Hf, V, Nb, Ta, Mo, and W. These HEAs have a simple bcc structure. Taking the TiZrNbMoVx and TiZrVNb HEAs as examples, we provide a detailed investigation of the effect of alloying elements on the elastic parameters and the elastic isotropy. Our results indicate that vanadium enhances the anisotropy and ductility of TiZrNbMoVx. As an application of the present theoretical database, we verify the often quoted correlation between the valence charge concentration (VEC) and the micro-mechanical properties in the case of multi-component alloys. Furthermore, we predict that the present HEAs become elastically isotropic for VEC ≃ 4.72.

With increase of the aluminum content, phase transformations (fcc→(fcc+bcc)→bcc) occur in NiCoFeCrAlx HEAs. Our ab initio results predict that at room temperature the paramagnetic NiCoFeCrAlx HEAs adopt the fcc structure for x ≤ 0.60 and the bcc structure for x ≥ 1.23, with an fcc-bcc duplex region in between the two pure phases. The calculated single- and polycrystal elastic parameters exhibit strong composition and crystal structure dependence. Based on the present theoretical findings, it is concluded that alloys around the equimolar NiCoFeCrAl composition have superior mechanical performance as compared to the single-phase regions.

Many modern materials and material systems are layered. The properties related to layers are connected to interactions between atomic layers. We introduce the interlayer potential (ILP), a novel model potential which fully describes the interaction between layers. The ILPs are different from the usual interatomic potentials which present inter- action between atoms. We use the Chen-Möbius inversion method to extract the ILPs from ab initio total energy calculations. The so obtained ILPs can be employed to investigate several physical parameters connected with the particular set of atomic layers, e.g. surface energy, stacking fault energy, elastic parameters, etc.

As an application, we adopt the supercell method and the axial interaction model in connection with the ILPs to calculate the stacking fault energy along the fcc ⟨111⟩ direction, including the intrinsic stacking fault energy, extrinsic stacking fault energy and twin stacking fault energy as well as the interactions between the intrinsic stacking faults. We find that the data derived from ILPs are consistent with those obtained in direct ab initio calculations. Along the fcc ⟨111⟩ direction, we study the surface energy and surface relaxation using the ILPs. The phonon dispersions are also described.

Our conclusions are as follows

  • the EMTO-CPAab initioalloy theory can be used to understand and predict the fundamental properties of multicomponent alloys.
  • the interlayer potentials based on the Chen-Möbius inversion method may provide a new way to investigate the properties related to layers in layered materials,
  • the EMTO-CPA alloy theory combined with the Chen-Möbius inversion method offers a powerful technique to study the properties of complex alloys.
Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , viii, 80 p.
Keyword [en]
high entropy alloys, ab inito
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-133237ISBN: 978-91-7501-899-7 (print)OAI: oai:DiVA.org:kth-133237DiVA: diva2:660286
Public defence
2013-11-29, B2, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20131108

Available from: 2013-11-08 Created: 2013-10-29 Last updated: 2013-11-08Bibliographically approved
List of papers
1. Empirical design of high-entropy alloys with optimal properties
Open this publication in new window or tab >>Empirical design of high-entropy alloys with optimal properties
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(English)Manuscript (preprint) (Other academic)
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-133319 (URN)
Note

QS 2013

Available from: 2013-10-30 Created: 2013-10-30 Last updated: 2013-11-08Bibliographically approved
2. Ab initio investigation of high-entropy alloys of 3d elements
Open this publication in new window or tab >>Ab initio investigation of high-entropy alloys of 3d elements
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2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 7, 075144- p.Article in journal (Refereed) Published
Abstract [en]

Single-phase high-entropy alloys are investigated using the exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA). Choosing the paramagnetic face-centered-cubic NiCoFeCr alloy as an example, we compare the CPA results with those obtained using the supercell (SC) method. For the equilibrium Wigner-Seitz radius and elastic properties, the single-site mean-field approximation turns out to yield consistent results with the SC approach. Next, we employ the EMTO-CPA method to study the bulk properties of CuNiCoFeCrTix (x = 0.0-0.5,1.0) and NiCoFeCrTi high-entropy alloys. A detailed comparison between the theoretical results and the available experimental data demonstrates that ab initio theory can properly describe the fundamental properties of this important class of engineering alloys. Theory predicts NiCoFeCr and CuNiCoFeCr to be more isotropic and less ductile than the Ti-containing single-phase alloys (CuNiCoFeCrTix with x greater than or similar to 0.4 and NiCoFeCrTi).

Keyword
Coherent-Potential-Approximation, Solid-Solution Phase, Multicomponent Alloys, Multiprincipal Elements, Annealing Treatment, Microstructure, Energy, Metals, Iron, Transition
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-119732 (URN)10.1103/PhysRevB.87.075144 (DOI)000315375200003 ()2-s2.0-84874531989 (Scopus ID)
Funder
Swedish Research CouncilEU, European Research Council
Note

QC 20130322

Available from: 2013-03-22 Created: 2013-03-21 Last updated: 2017-12-06Bibliographically approved
3. Ab initio design of elastically isotropic TiZrNbMoVx high-entropy alloys
Open this publication in new window or tab >>Ab initio design of elastically isotropic TiZrNbMoVx high-entropy alloys
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2014 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 599, 19-25 p.Article in journal (Refereed) Published
Abstract [en]

The TiZrVNb and TiZrNbMoVx (x = 0-1.5) high-entropy alloys (HEAs) are single-phase solid solutions having the body centered cubic crystallographic structure. Here we use the ab initio exact muffin-tin orbitals method in combination with the coherent potential approximation to study the equilibrium bulk properties of the above refractory HEAs. We provide a detailed investigation of the effect of alloying elements on the electronic structure and elastic parameters. Our results indicate that vanadium enhances the anisotropy of TiZrNbMoVx. As an application of the present theoretical database, we verify the often quoted correlation between the valence electron concentration (VEC) and the micro-mechanical properties in the case of multi-component alloys. Furthermore, we predict that the present HEAs become elastically isotropic for VEC similar to 4.72.

Keyword
High entropy alloys, Density functional theory, Coherent potential approximation, Equilibrium volume, Elastic isotropy
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-133324 (URN)10.1016/j.jallcom.2014.01.237 (DOI)000333411100003 ()2-s2.0-84896756836 (Scopus ID)
Funder
Swedish Research CouncilEU, European Research Council
Note

QC 20140508. Updated from manuscript to article in journal.

Available from: 2013-10-30 Created: 2013-10-30 Last updated: 2017-12-06Bibliographically approved
4. Structural stability of NiCoFeCrAlx high-entropy alloy from ab initio theory
Open this publication in new window or tab >>Structural stability of NiCoFeCrAlx high-entropy alloy from ab initio theory
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2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 8, 085128- p.Article in journal (Refereed) Published
Abstract [en]

First-principles alloy theory predicts that at room temperature the paramagnetic NiCoFeCrAlx high entropy alloys adopt the face centered cubic (fcc) structure for x less than or similar to 0.60 and the body centered cubic (bcc) structure for x greater than or similar to 1.23, with an fcc-bcc duplex region in between the two pure phases. The calculated single- and polycrystal elastic parameters exhibit strong composition and crystal structure dependence. Based on the present theoretical findings, it is concluded that alloys around the equimolar NiCoFeCrAl composition have superior mechanical performance as compared to the single-phase regions.

Keyword
Coherent-Potential Approximation, 1st-Principles Theory, Microstructure, Metals, Iron, Magnetism, Energy, Model
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-129116 (URN)10.1103/PhysRevB.88.085128 (DOI)000323712400004 ()2-s2.0-84884509314 (Scopus ID)
Funder
Swedish Research CouncilEU, European Research Council
Note

QC 20130920

Available from: 2013-09-20 Created: 2013-09-19 Last updated: 2017-12-06Bibliographically approved
5. A novel potential: the interlayer potential for the fcc (111) plane family
Open this publication in new window or tab >>A novel potential: the interlayer potential for the fcc (111) plane family
2012 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 24, no 4, 045001- p.Article in journal (Refereed) Published
Abstract [en]

We propose a novel interlayer potential, which is different from usual interatomic potentials. The interlayer potential represents the interaction between atomic layers in a layered material. Based on the Chen-Mobius inversion method in combination with ab initio calculations, the interlayer interactions are obtained for the face centered cubic (fcc) (111) planes. In order to check the validity of our interlayer potential, we calculate the intrinsic stacking fault energy (gamma(sf)) and the surface energy (gamma(s)) of five metals: Al, Ni, Cu, Ag and Au. The predicted gamma(sf) and gamma(s) values are compared with the theoretical results obtained from direct calculations and also with the available experimental data. Using the interlayer potentials, we also investigate the phonon dispersion and phonon density of state in the fcc (111) plane family of the considered metals.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-91130 (URN)10.1088/0953-8984/24/4/045001 (DOI)000299326100002 ()2-s2.0-84855645725 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20120309

Available from: 2012-03-09 Created: 2012-03-08 Last updated: 2017-12-07Bibliographically approved
6. Interlayer potentials for fcc (111) planes of Pd-Ag random alloys
Open this publication in new window or tab >>Interlayer potentials for fcc (111) planes of Pd-Ag random alloys
2012 (English)In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 63, 20-27 p.Article in journal (Refereed) Published
Abstract [en]

Using ab initio alloy theory in combination with the Chen-Mobius inversion method, we obtain the interaction potentials between layers of the face centered cubic (fcc) (111) plane family for elemental Pd and Ag metals and for Pd0.5Ag0.5 random alloy. Based on the interlayer potentials, we investigate properties related to the fcc (111) planes, such as the stacking fault energy, shear elastic constant, surface energy, surface relaxations, and phonon dispersion along the fcc [111] direction. The results agree well with those obtained by fully self-consistent ab initio calculations and also with the available experimental data, indicating that the properties related to the close-packed planes are well described by the interlayer potentials.

Keyword
Pd-Ag alloys, Stacking fault energy, Shear elastic constant, Surface relaxations, Phonon dispersion
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-102896 (URN)10.1016/j.commatsci.2012.05.051 (DOI)000307827500003 ()2-s2.0-84862754985 (Scopus ID)
Funder
Swedish Research CouncilEU, European Research Council
Note

QC 20121004

Available from: 2012-10-04 Created: 2012-09-28 Last updated: 2017-12-07Bibliographically approved

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