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Defects in Austenitic Steels and Hard Metals - A DFT-based Study
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. (Applied Materials Physics)ORCID iD: 0000-0002-7421-9203
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 [en]
defects, austenitic steels, cemented carbides, ab-initio
National Category
Condensed Matter Physics
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-273258ISBN: 978-91-7873-497-9 (print)OAI: oai:DiVA.org:kth-273258DiVA, id: diva2:1429722
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)VinnovaAvailable from: 2020-05-12 Created: 2020-05-12 Last updated: 2020-05-27Bibliographically approved
List of papers
1. Quantum mechanics basis of quality control in hard metals
Open this publication in new window or tab >>Quantum mechanics basis of quality control in hard metals
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2019 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 169, p. 1-8Article in journal (Refereed) Published
Abstract [en]

Non-destructive and reliable quality control methods are a key aspect to designing, developing and manufacturing new materials for industrial applications and new technologies. The measurement of the magnetic saturation is one of such methods and it is conventionally employed in the cemented carbides industry. We present a general quantum mechanics based relation between the magnetic saturation and the components of the binder phase of cemented carbides, which can be directly employed as a quality control. To illustrate our results, we calculate the magnetic saturation of a binder phase, 85Ni15Fe binary alloy, using ab-initio methods and compare the theoretical predictions to the magnetic saturation measurements. We also analyse interface and segregation effects on the magnetic saturation by studying the electronic structure of the binder phase. The excellent agreement between calculations and measurements demonstrates the applicability of our method to any binder phase. Since the magnetic saturation is employed to ensure the quality of cemented carbides, the present method allows us to explore new materials for alternative binder phases efficiently.

Place, publisher, year, edition, pages
Acta Materialia Inc, 2019
Keywords
Ab-initio calculations, Binder phase, Hard metal, Magnetic saturation
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-246425 (URN)10.1016/j.actamat.2019.02.036 (DOI)000465365300001 ()2-s2.0-85062451846 (Scopus ID)
Note

QC 20190401

Available from: 2019-04-01 Created: 2019-04-01 Last updated: 2020-05-12Bibliographically approved
2. Generalized stacking fault energy of carbon-alloyed paramagnetic gamma-Fe
Open this publication in new window or tab >>Generalized stacking fault energy of carbon-alloyed paramagnetic gamma-Fe
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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
Keywords
C-alloyed gamma-Fe, GSFE, paramagnetism, ab initio
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-241183 (URN)10.1088/1361-648X/aaf2fa (DOI)000454553700001 ()30524044 (PubMedID)2-s2.0-85059403568 (Scopus ID)
Note

QC 20190121

Available from: 2019-01-21 Created: 2019-01-21 Last updated: 2020-05-12Bibliographically approved
3. 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
4. A comparative study of microstructure and magnetic properties of a Ni–Fe cemented carbide: Influence of carbon content
Open this publication in new window or tab >>A comparative study of microstructure and magnetic properties of a Ni–Fe cemented carbide: Influence of carbon content
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2019 (English)In: International Journal of Refractory Metals and Hard Materials, ISSN 0263-4368, Vol. 80, p. 181-187Article in journal (Refereed) Published
Abstract [en]

Due to the renewed interest in alternative binders for cemented carbides it is important to understand how the binder composition influences not only mechanical properties but also the microstructure and related measurements for quality control. Microstructure and chemical composition of WC-Co is often evaluated by magnetic measurements. However, when the binder composition deviates significantly from conventional Co-based binders it should not be assumed that the standard measurements can be used to directly evaluate the same parameters. In this paper we investigate the influence of relative C-content on the microstructure and magnetic properties of an alternative binder cemented carbide. It is shown that the saturation magnetization is related to the relative C-content and the magnetic coercivity is related to the microstructure, more specifically to the binder phase distribution, but could not be directly linked to the carbide grain size in the same manner as for standard WC-Co. Furthermore, a direct correlation between Curie temperature and saturation magnetization is observed for this system which means that the Curie temperature potentially could be used for calibration of empirical relations or as a method to accurately determine the binder volume fraction.

Place, publisher, year, edition, pages
Elsevier Ltd, 2019
Keywords
Alternative binder, Cemented carbide, Cermet, Cobalt substitution, Magnetic properties, Metal-matrix composite, Microstructure
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-246465 (URN)10.1016/j.ijrmhm.2019.01.014 (DOI)000460992100018 ()2-s2.0-85060087544 (Scopus ID)
Note

QC 20190326

Available from: 2019-03-26 Created: 2019-03-26 Last updated: 2020-05-12Bibliographically approved
5. Understanding Quality Control of Hard Metals in Industry - A Quantum Mechanics Approach
Open this publication in new window or tab >>Understanding Quality Control of Hard Metals in Industry - A Quantum Mechanics Approach
Show others...
2019 (English)In: Advanced Theory and Simulations, ISSN 2513-0390, Vol. 2, no 6, article id 1900035Article in journal (Refereed) Published
Abstract [en]

For many decades, the magnetic saturation of, for example, hard metals (HM) such as WC-Co-based cemented carbides, has been used as process and quality control in industry to ensure consistency of product properties. In an urge to replace cobalt as a binder phase, a demand on understanding the magnetic response as a function of composition on the atomic scale is growing. In this paper, a theoretical description of the measured weight-specific magnetic saturation of hard metals as a function of the tungsten weight fraction present in the cobalt binder phase, based on first-principle calculations, is established for standard WC-Co. The predicted magnetic saturation agrees well with the experimental one. Furthermore, it is proposed that the theoretical description can be extended to alternative and more complex binder phases which allows to transfer the production control to those hard metals.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2019
Keywords
ab initio calculations, alternative binder, Co substitution, hard metal, magnetic saturation
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-254083 (URN)10.1002/adts.201900035 (DOI)000470158200009 ()
Note

QC 20190625

Available from: 2019-06-25 Created: 2019-06-25 Last updated: 2020-05-12Bibliographically approved

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The full text will be freely available from 2020-12-31 12:23
Available from 2020-12-31 12:23

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