Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Generalized stacking fault energy of carbon-alloyed paramagnetic gamma-Fe
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.ORCID iD: 0000-0001-7724-8299
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
Harbin Inst Technol Weihai, Sch Mat Sci & Engn, Weihai 264209, Peoples R China..ORCID iD: 0000-0002-9081-6518
Show others and affiliations
2019 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 31, no 6, article id 065703Article in journal (Refereed) Published
Abstract [en]

Generalized stacking fault energy (GSFE) is an important parameter for understanding the underlying physics governing the deformation mechanisms in face-centred cubic (fcc) materials. In the present work, we study the long-standing question regarding the influence of C on the GSFE in austenitic steels at paramagnetic state. We calculate the GSFE in both gamma-Fe and Fe-C alloys using the exact muffin-tin orbitals method and the Vienna Ab initio Simulation Package. Our results show that the GSFE is increased by the presence of interstitial C, and the universal scaling law is used to verify the accuracy of the obtained stacking fault energies. The C-driven change of the GSFE is discussed considering the magnetic contributions. The effective energy barriers for stacking fault, twinning and slip formation are employed to disclose the C effect on the deformation modes, and we also demonstrate that the magnetic structures as a function of volume explain the effect of paramagnetism on the C-driven changes of the stacking fault energies as compared to the hypothetical non-magnetic case.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD , 2019. Vol. 31, no 6, article id 065703
Keywords [en]
C-alloyed gamma-Fe, GSFE, paramagnetism, ab initio
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-241183DOI: 10.1088/1361-648X/aaf2faISI: 000454553700001PubMedID: 30524044Scopus ID: 2-s2.0-85059403568OAI: oai:DiVA.org:kth-241183DiVA, id: diva2:1281148
Note

QC 20190121

Available from: 2019-01-21 Created: 2019-01-21 Last updated: 2020-05-12Bibliographically approved
In thesis
1. Carbon Effect on Mechanical Properties in Austenitic Steels - A DFT-based Study
Open this publication in new window or tab >>Carbon Effect on Mechanical Properties in Austenitic Steels - A DFT-based Study
2019 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

To study the effect of carbon interstitials in austenitic steels on plastic deformation mechanisms is the main goal of the present thesis. Using first-principlesmethods, the generalized stacking fault energy (GSFE) of C-alloyed γ-Fe is firstcalculated. The GSFE curve includes several prominent stacking fault energiesthat are fundamental for, e.g, predicting critical twinning stress and twinnability. The C effect was previously investigated in γ-Fe assuming nonmagnetic(NM) state. However, paramagnetic (PM) state with local magnetic momentson each site and total magnetization equal to zero is a more appropriate description for austenites. The Exact Muffin-Tin Orbitals (EMTO) method is capableof modelling the PM state together with the Coherent Potential Approximation (CPA). We also compare the NM GSFEs of C-alloyed γ-Fe obtained fromEMTO and Vienna Ab initio Simulation Package (VASP) to evaluate the performance of EMTO on handling the C-interstitial structure. The EMTO resultsare verified to fit reasonably well with VASP results so the GSFE calculationfor the C-alloyed γ-Fe is further extended to the PM state.The influence of C interstitials on the GSFE for PM γ-Fe is significantly different from what is predicted for NM γ-Fe. Though the GSFE is increased byC addition for both NM and PM γ-Fe, the C-driven change on the GSFE ascompared to pure γ-Fe at the PM state deviates from that at the NM state:paramagnetism significantly weakens the C impact on the intrinsic stacking faultenergy while strengthens it on the unstable stacking fault energy as comparedto the hypothetical NM case. The different behaviours uncovered for the intrinsic and unstable stacking fault energies due to the presence of local magneticmoments is illustrated by the magnetic structures of the Fe-C alloys as a function of volume, which mainly emerged from the suppression effect of C on themagnetic moments of its adjacent Fe neighbours.Using the generalized stacking fault as an approximation for the partial dislocation core, we investigate the minimum energy path (MEP) for C diffusionin the dislocation core (i.e., for various displacement vectors ) for NM γ-Feusing VASP. In contrast to the common assumption of stationary interstitialatoms during the passage of fast-moving dislocations, a pair of partial dislocations moves C atoms forward on the slip plane by one full Burgers vector. Thisdissociated dislocation-mediated transport mechanism for C is a strain inducedprocess, which is present even when the normal thermally activated diffusion isinoperative. Moreover, at the stacking fault ribbon and especially near the partial dislocation core, the in-plane diffusion energy barriers for C are significantlyreduced compared to that in bulk, opening a fast diffusion pathway for C. Themagnetic effect is also indirectly considered for the in-plane C diffusion energybarrier by calculating the MEP in high-spin ferromagnetic (HS-FM) Fe and ferromagnetic (FM) Ni. It is concluded that the presence of magnetic couplingdoes not change this trend. Therefore, contrary to the previously suggestedmechanism based on the reorientation of Mn-C short range order, our resultsreveal that the fast pipe diffusion of C at the dissociated dislocations is primarily responsible for the dynamic strain aging (DSA) in Fe-Mn-C steels and themechanism for DSA-mediated formation of deformation twinning is proposed to understand the strain rate dependence of deformation twinning in the presenceof DSA.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. p. 45
Series
TRITA-ITM-AVL ; 5
National Category
Condensed Matter Physics
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-244853 (URN)978-91-7873-097-1 (ISBN)
Presentation
2019-03-29, KUBEN N111, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2019-03-04 Created: 2019-03-01 Last updated: 2020-05-12Bibliographically approved
2. Defects in Austenitic Steels and Hard Metals - A DFT-based Study
Open this publication in new window or tab >>Defects in Austenitic Steels and Hard Metals - A DFT-based Study
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Materials are never 100% pure due to the limitation of purification method or manufacturing process. Nor are they perfect, especially under deformation. The present work aims to explore different roles played by the defects in austenitic steels and hard metals.

The first focus is iron-manganese (Fe-Mn) based twinning induced plasticity(TWIP) steels which are a category of austenitic materials showing a good combination of high strength and ductility. The planar fault is fundamental for the TWIP mechanism. First, the γ-surface of pure γ-Fe (fcc-Fe) is calculated for different magnetic states. Next, the effects of alloying elements, including Mn,interstitial carbon (C) and nitrogen (N), are addressed. The γ-surface includes several prominent stacking fault energies that are fundamental for, e.g, predicting critical twinning stress and twinnability. The present work compares the γ-surface obtained at different magnetic states, including nonmagnetic (NM), paramagnetic(PM), antiferromagnetic single-layer (AFMI) and double-layer (AFMD) states. The local magnetism significantly influences the γ-surface. In addition, the existing antiferromagnetic (AFM) order results in two different deformation paths inγ-Fe, leading to the generations of superlattice intrinsic stacking fault (SISF) and complex stacking fault (CSF), respectively. The intrinsic stacking fault energy corresponding to SISF is relatively low while its corresponding unstable stacking fault energy is relatively high. The magnetic structures are investigated in the unstable stacking fault and the intrinsic stacking fault configurations via Monte Carlo (MC) simulations. The MC results show that only SISF configuration is favourable, and the two distinctive unstable stacking fault configurations may coexist.

The Mn effect on the γ-surface of γ-Fe is studied at AFMI state and the crystal tetragonality is considered. The comparison with experimentally measured stacking fault energy (SFE) dependence on Mn composition shows that the AFMI results reproduce better the experimental trend in high-Mn Fe-Mn alloys than the PM results. Further, the interstitial alloying effects of C and N on the γ-surface of γ-Fe are investigated and no remarkable difference is observed betweenthe C and N impacts. The interaction between dislocation and interstitial atoms, which is fundamental to understand the phenomenon like dynamic strain ageing (DSA), is studied using the generalized stacking fault as an approximation of the partial dislocation core. The minimum migration energy path (MEP) and migration energy surface (MES) of C in the dislocation core of AFMD γ-Fe are calculated. In contrast to the common assumption that the interstitial atoms are stationary during the passage of fast-moving dislocations, the present work suggests that a pair of dislocation partials are capable of moving C atoms forward on the slip plane by one full Burgers vector. Moreover, at the stacking fault ribbon and especially near the dislocation core, the in-plane diffusion energy barriers of C are significantly reduced compared to that in the bulk, rendering a fast diffusion channel for C. The proposed mechanisms for C transport and diffusion are not decided by local magnetic order and can be used to explain the strain rate dependent formation kinetics of twinning or hexagonal close-packed (hcp) martensite in C-alloyed TWIP steels or high entropy alloys. Similarly, the ab initio results show that the diffusion energy barrier of N in the dislocation core is approximately 14.9% of that in the bulk. According to experimental observations, carbon promotes while N suppresses the DSA. However, the different C and N effects on the DSA cannot be understood from current thermodynamic investigations.

The defects in the binder phase of hard metals (cemented carbides) are another important topic in this thesis. The interstitial tungsten (W) and C defects in hard metals come from the sintering process during industrial manufacturing. The cemented carbides are composite materials made of tungsten carbide (WC) grains glued together by a binder phase. Typically, the binder phase consists of ductile cobalt (Co) and some amount of dissolved W and C. The measurement ofthe magnetic saturation is one method employed for quality control of cemented carbides. Despite the great success of Co, a substitute of Co is needed due to its rising price and health threats. The substitution of a material in production processes can be complex. Ideally, manufacturing processes and quality controls should be used as usual or at least new ones have to be devised in a simpleway. The present work selects 85Ni-15Fe (85 at.% of Ni and 15 at.% of Fe) to demonstrate the relation between the magnetic saturation and the components of the binder phase of cemented carbides using ab initio method, which providesa non-destructive quality control method in cemented carbides.

Abstract [sv]

Järn-Mangan (Fe-Mn)-baserade twinning induced plasticity (TWIP) stål är en kategori av austenitiska material som har en kombination av hög hållfasthet och god duktilitet. För att få en mer tydlig bild av olika roller som spelas av defekter i Fe-Mn-baserade legeringar med ytcentrerad kubisk struktur (fcc), specifikt deras effekter på mekaniska egenskaper och magnetiska strukturer, beräknas först γ- ytan för rent γ-Fe vid olika magnetiska tillstånd. Därefter behandlas effekterna av legeringsämnet mangan och de interstitiella legeringsämnena kol (C) och kväve (N).

γ-ytan innehåller flera framstående staplingsfelenergier som är grundläggande för att, till exempel, förutsäga kritisk spänning för tvilllingbildning och twinnability . Vi jämför γ-ytan som erhålls vid olika magnetiska tillstånd, inklusive ickemagnetisk (NM), paramagnetisk (PM), antiferromagnetiskt enkelskikt (AFMI) och dubbelskikt (AFMD). Det har visat sig att det lokala magnetiska momentet väsentligt påverkar γ-ytan. Dessutom resulterar den befintliga antiferromagnetiaka (AFM) ordningen i två olika deformationsvägar i γ-Fe. De två olika deformationsvägarna leder till generering av intrinsiska supergitterstaplingsfel (SISF) respektive komplexa staplingsfel (CSF), där den intrinsiska staplingsfelenergin motsvarande SISF är relativt lägre medan den motsvarande instabila staplingsfelenergin är relativt högre. Vi undersöker sedan de magnetiska strukturerna nära det instabila staplingsfelet och det intrinsiska staplingsfelet med Monte Carlo (MC) simuleringar. Vi fann att närvaron av stapelfel ändrar fördelningen av magnetiska moment för på bulkmaterial. Och i verkligheten är det bara SISF som är gynnsamma med avseende på CSF, men de två distinkta instabila stapelfelkonfigurationerna kan samexistera.

Effekten av Mn på γ-ytan för γ-Fe studeras vidare i AFMI-tillståndet och kristall-tetragonaliteten beaktas. Jämförelsen med experimentellt uppmätt staplingsfelsenergi (SFE) beroende av Mn-innehållet visar att AFMI-resultaten reproducerar den trend som observeras experimentellt i hög-Mn Fe-Mn-legeringar bättre än PM-resultaten. Vidare undersöks effekterna av de interstitiella legeringsämnena C och N på γ-ytan för γ-Fe och ingen märkbar skillnad observeras mellan C och N-tillsats. Vi beräknar lägsta migrationsenergiban (MEP) och migrationsenergi yta (MES) för C i dislokationskärnan i AFMD γ-Fe. Vi föreslår att ett par partiella förflyttningar kan flytta C-atomer framåt på glidplanet med en hel Burgers-vektor. Vid staplingsfelsbandet och speciellt nära den partiella dislokationskärnan minskas dessutom energibarriärerna för C-diffusion betydligt jämfört med den i bulk, vilket ger en snabbdiffusionskanal för C. På liknande sätt visar inte N-diffusionsbeteendet någon signifikant skillnad jämfört med C, vilket indikerar att orsaken till att N undertrycker dynamic strain aging (DSA) medan C främjar DSA inte kan förstås fullständigt från nuvarande termodynamisk forskning.

Slutligen presenterar vi också en teoretisk studie där vi använder de beräknade effekterna av inlöst C och volfram (W) på det totala magnetiska momentet för en FeNi-bindefas i hårdmetall för att framgångsrikt beräkna W-koncentration i bindefasen vilket möjliggör icke-förstörande kvalitetskontroll.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2020. p. 83
Series
TRITA-ITM-AVL ; 2020:15
Keywords
defects, austenitic steels, cemented carbides, ab-initio
National Category
Condensed Matter Physics
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-273258 (URN)978-91-7873-497-9 (ISBN)
Public defence
2020-06-05, https://kth-se.zoom.us/webinar/register/WN_uqGMc4x8QwWk4TPfSb5TFA, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish National Infrastructure for Computing (SNIC)Vinnova
Available from: 2020-05-12 Created: 2020-05-12 Last updated: 2020-05-27Bibliographically approved

Open Access in DiVA

No full text in DiVA

Other links

Publisher's full textPubMedScopus

Authority records BETA

Xie, RuiwenLi, WeiLu, SongVitos, Levente

Search in DiVA

By author/editor
Xie, RuiwenLi, WeiLu, SongSong, YanVitos, Levente
By organisation
Applied Material PhysicsApplied Physics
In the same journal
Journal of Physics: Condensed Matter
Materials Engineering

Search outside of DiVA

GoogleGoogle Scholar

doi
pubmed
urn-nbn

Altmetric score

doi
pubmed
urn-nbn
Total: 31 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf