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Ab initio Investigation of Face-centered cubic High-Entropy Alloys
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. (KTH-ITM-MSE-AMP)ORCID iD: 0000-0002-6504-5045
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

High-entropy alloys (HEAs) represent a special group of solid solutions containing five or more principal elements. The new design strategy has attracted extensive attention from the materials science community. The design and development of HEAs with desired properties have become an important subject in materials science and technology. Herein this case, I investigate the basic properties of paramagnetic (PM) HEAs, including the magnetic properties, Curie temperatures, electronic structures, phase stabilities, and elastic properties using the first-principles exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA) for dealing with the chemical and magnetic disorder. To understand and model the mechanical properties of face-centered cubic (fcc) HEAs, I also study the generalized stacking fault energy (GSFE), negative stacking fault energy (SFE) and twinning mechanism of various HEAs. Thesis focuses mainly on AlxCrMnFeCoNi (0 ≤ x ≤ 5, in molar fraction) and related HEAs.Whenever possible, I compare the theoretical predictions to the available experimental data in order to verify the employed ab initio methodology. I make use of the previous theoretical investigations carried out on AlxCrFeCoNi HEAs to reveal and understand the role of Mn in AlxCrMnFeCoNi HEAs. The theoretical lattice constants are found to increase with increasing x, which is in good agreement with the available experimental data. The magnetic transition temperature for the body-centered cubic (bcc) structure strongly decreases with x, whereas that for the fcc structure shows a weak composition dependence. Within their own stability fields, both structures are predicted to be PM at ambient conditions. Upon Al addition, the crystal structure changes from fcc to bcc with a broad two-phase field region, in line with the observations. Bain path calculations suggest that within the duplex region both phases are dynamically stable.Comparison with available experimental data demonstrates that the employed approach describes accurately the elastic moduli of the present HEAs. The elastic parameters exhibit complex composition dependences and the elastic anisotropy is unusually high for both cubic 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 found to be due to the relatively low bulk modulus and C12 elastic constant, which in turn are consistent with the relatively low cohesive energy. Our findings in combination with the experimental data suggest anomalous metallic character for the present HEAs system.The negative SFE of fcc medium-entropy alloys (MEAs) and HEAs originate from the metastable character of the fcc phase. I argue that the common models underlying the experimental measurements of SFE fail in metastable alloys. Considering various metals including concentrated solid solutions, I demonstrate that in contrast to the experimentally measured SFEs, the SFEs obtained by DFT calculations correlate well with the primary deformation mechanisms observed experimentally in these alloys. In the case of negative SFE (or in metastable fcc alloys), the transformation-mediated twinning (TMT) is the predominant mechanism instead of the layer-by-layer twinning mechanism. It provides a continuous avenue for strain accommodation and strain hardening, realizing the joint transformation-induced plasticity and twinning-induced plasticity in the same system, and thus enabling the simultaneous improvement of strength and ductility. For the fcc CrMnFeCoNi HEA, upon Al addition or temperature increase, the intrinsic and extrinsic stacking fault energies increase, whereas the hexagonal close packed (hcp)/fcc interfacial energy stays almost constant.The work and results presented in this thesis give a good background to go further and study the plasticity of fcc HEAs as a function of chemistry and temperature. This is a very challenging task and only a very careful pre-study concerning the phase stability, magnetism, elasticity and GSFE can provide enough information to turn my plan regarding ab initio description of the thermo-plastic deformation mechanisms in fcc HEAs into a successful research. The novel TMT mechanism disclosed for the first time by myself and my colleagues advances our knowledge in plasticity and paves the road to design novel alloys with outstanding mechanical properties using quantum metallurgy.

Abstract [sv]

Högentropi-legeringar (HEA) representerar en speciell grupp av fasta lösningar som innehåller fem eller flera huvudelement. Den helt nya designstrategin och de utmärkta egenskaperna hos HEA har lockat stor uppmärksamhet från materialvetenskapssamhället. Formgivning och utveckling av HEA med önskade egenskaper har blivit ett viktigt ämne inom materialvetenskap och teknik. För att förstå de grundläggande egenskaperna hos HEA, undersöker vi de magnetiska egenskaperna, Curietemperaturen, den elektronstrukturen, fasstabiliteten och elastiska egenskaper hos paramagnetiska rymdcentrerat kubisk (bcc) och ytcentrerat kubisk (fcc) AlxCrMnFeCoNi (0 ≤ x ≤ 5) högentropi-legeringar baserat på den första princips EMTO-metoden.De teoretiska gitterkonstanterna ökar med ökande x, vilket är i god överensstämmelse med experimentella data. Den magnetiska övergångstemperaturen för bcc-strukturen minskar kraftigt med x, medan för fcc-strukturen uppvisar den övergångstemperaturen svag kompositionberoende. Inom sina egna stabilitetsfält förutsägas båda strukturerna vara paramagnetiska under normala omgivningsförhållanden. Vid tillägg av Al ändras kristallstrukturen från fcc till bcc med en bred tvåfasfältregion, vilket är i linje med observationer. Bain-väg beräkningar stöder att båda faserna är dynamiskt stabila inom den duplexregionen.Jämförelse med tillgängliga experimentella data visar att den här använda metoden beskriver den elastiska modulen med god noggrannhet. De elastiska parametrarna uppvisar komplexa kompositionberoende, även om de förutsagda gitterkonstanterna ökar monotont med Al tillsats. Den elastiska anisotropin är ovanligt hög för båda faser. De spröda / duktila övergångarna formulerade i form av Cauchy tryck och Pugh förhållandet blir konsekventa endast när man tar den starka elastiska anisotropin med i beräkningen. Det negativa Cauchy trycket hos CrMnFeCoNi beror på de relativt låga bulkmodulen och C12 elastiska konstanten, som i sin tur är konsekventa med den relativt låga kohesionenergin. Denna resultaten i kombination med experimentella data antyder anomalös metallisk karaktär för HEA system.För att förstå och modellera de mekaniska egenskaperna hos fcc HEA studerar jag också generaliserad staplingsfelenergi (GSFE), negativ staplingsfelenergi (SFE) och tvillingmekanism för olika HEA. Denna avhandling fokuserar huvudsakligen på AlxCrMnFeCoNi och relaterade HEA.

Place, publisher, year, edition, pages
KTH: KTH Royal Institute of Technology, 2020. , p. 80
Series
TRITA-ITM-AVL ; 2020:16
Keywords [en]
ab initio; high-entropy alloys; twinning; martensitic transformation; elastic properties; phase stability
National Category
Other Materials Engineering
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-273286ISBN: 978-91-7873-495-5 (print)OAI: oai:DiVA.org:kth-273286DiVA, id: diva2:1429801
Public defence
2020-06-04, https://kth-se.zoom.us/webinar/register/WN_ZGfpgdfbTaWDGBGKrV9tcQ, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
The Swedish Foundation for International Cooperation in Research and Higher Education (STINT)VinnovaAvailable from: 2020-05-12 Created: 2020-05-12 Last updated: 2020-05-27Bibliographically approved
List of papers
1. Transformation-mediated twinning
Open this publication in new window or tab >>Transformation-mediated twinning
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Twinning plays a critical role in plasticity and strengthening of metals and alloys. Inface-centered cubic materials, mechanical twinning is considered to be realized by themovement of twinning partial dislocations on consecutive close-packed atomic planes.Here, we report an unrivaled twinning mechanism which is accomplished through a precedingaustenite to hexagonal martensite phase transformation. This transformationmediatedtwinning operates predominantly in metastable face-centered cubic materialswith small to intermediate negative stacking fault energy. It provides a continuous avenuefor strain accommodation and strain hardening, realizing the joint transformationinducedplasticity and twinning-induced plasticity in the same system, and thus enablingthe simultaneous improvement of strength and ductility. The disclosed plasticdeformation mechanism explains the behavior of a large number of known alloys andgreatly expands the composition territory for designing novel alloys with outstandingmechanical properties.

Keywords
Twinning; martensitic transformation; stacking fault; metastable alloy
National Category
Other Materials Engineering
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-273282 (URN)
Note

QC 20200513

Available from: 2020-05-12 Created: 2020-05-12 Last updated: 2020-05-13Bibliographically approved
2. Phase selection rule for Al-doped CrMnFeCoNi high-entropy alloys from first-principles
Open this publication in new window or tab >>Phase selection rule for Al-doped CrMnFeCoNi high-entropy alloys from first-principles
Show others...
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
Keywords
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: 2020-05-12Bibliographically approved
3. Elastic properties of AlxCrMnFeCoNi (0 <= x <= 5) high-entropy alloys from ab initio theory
Open this publication in new window or tab >>Elastic properties of AlxCrMnFeCoNi (0 <= x <= 5) high-entropy alloys from ab initio theory
Show others...
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: 2020-05-12Bibliographically approved
4. Generalized stacking fault energy of al-doped CrMnFeCoNi high-entropy alloy
Open this publication in new window or tab >>Generalized stacking fault energy of al-doped CrMnFeCoNi high-entropy alloy
Show others...
2020 (English)In: Nanomaterials, ISSN 2079-4991, Vol. 10, no 1, article id 59Article in journal (Refereed) Published
Abstract [en]

Using first-principles methods, we investigate the effect of Al on the generalized stacking fault energy of face-centered cubic (fcc) CrMnFeCoNi high-entropy alloy as a function of temperature. Upon Al addition or temperature increase, the intrinsic and extrinsic stacking fault energies increase, whereas the unstable stacking fault and unstable twinning fault energies decrease monotonously. The thermodynamic expression for the intrinsic stacking fault energy in combination with the theoretical Gibbs energy difference between the hexagonal close packed (hcp) and fcc lattices allows one to determine the so-called hcp-fcc interfacial energy. The results show that the interfacial energy is small and only weakly dependent on temperature and Al content. Two parameters are adopted to measure the nano-twinning ability of the present high-entropy alloys (HEAs). Both measures indicate that the twinability decreases with increasing temperature or Al content. The present study provides systematic theoretical plasticity parameters for modeling and designing high entropy alloys with specific mechanical properties.

Place, publisher, year, edition, pages
MDPI AG, 2020
Keywords
First-principles, Generalized stacking fault energy, High-entropy alloys, Interfacial energy
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-267999 (URN)10.3390/nano10010059 (DOI)000516825600059 ()2-s2.0-85077398336 (Scopus ID)
Note

QC 20200329

Available from: 2020-03-29 Created: 2020-03-29 Last updated: 2020-05-12Bibliographically approved
5. Can experiment determine the stacking fault energy of metastable alloys?
Open this publication in new window or tab >>Can experiment determine the stacking fault energy of metastable alloys?
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Stacking fault energy (SFE) plays an important role in deformation mechanisms and mechanical properties of face-centered cubic (fcc) metals and alloys. In metastable fccalloys, the SFEs determined from density functional theory (DFT) calculations andexperimental methods often have opposite signs. Here, we show that the negative SFE byDFT reflects the thermodynamic instability of the fcc phase relative to the hexagonalclose-packed one; while the experimentally determined SFEs are restricted to be positiveby the models behind the indirect measurements. We argue that the common modelsunderlying the experimental measurements of SFE fail in metastable alloys. In variousconcentrated solid solutions, we demonstrate that the SFEs obtained by DFT calculationscorrelate well with the primary deformation mechanisms observed experimentally,showing a better resolution than the experimentally measured SFEs. Furthermore, webelieve that the negative SFE is important for understanding the abnormal behaviors ofpartial dislocations in metastable alloys under deformation. The present work advancesthe fundamental understanding of SFE and its relation to plastic deformations, and shedslight on future alloy design by physical metallurgy.

Keywords
metastable alloy, stacking fault energy, twinning, martensitic transformation
National Category
Other Materials Engineering
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-273281 (URN)
Note

QC 20200513

Available from: 2020-05-12 Created: 2020-05-12 Last updated: 2020-05-13Bibliographically approved

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