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Molnar, D., Sun, X., Lu, S., Li, W., Engberg, G. & Vitos, L. (2019). Effect of temperature on the stacking fault energy and deformation behaviour in 316L austenitic stainless steel. Materials Science & Engineering: A, 759, 490-497
Open this publication in new window or tab >>Effect of temperature on the stacking fault energy and deformation behaviour in 316L austenitic stainless steel
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2019 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 759, p. 490-497Article in journal (Refereed) Published
Abstract [en]

The stacking fault energy (SFE) is often used as a key parameter to predict and describe the mechanical behaviour of face centered cubic material. The SFE determines the width of the partial dislocation ribbon, and shows strong correlation with the leading plastic deformation modes. Based on the SFE, one can estimate the critical twinning stress of the system as well. The SFE mainly depends on the composition of the system, but temperature can also play an important role. In this work, using first principles calculations, electron backscatter diffraction and tensile tests, we show a correlation between the temperature dependent critical twinning stress and the developing microstructure in a typical austenitic stainless steel (316L) during plastic deformation. We also show that the deformation twins contribute to the strain hardening rate and gradually disappear with increasing temperature. We conclude that, for a given grain size there is a critical temperature above which the critical twinning stress cannot be reached by normal tensile deformation, and the disappearance of the deformation twinning leads to lower strain hardening rate and decreased ductility.

Keywords
Deformation twinning, microstructure, first principles, stacking fault energy, stainless steel
National Category
Metallurgy and Metallic Materials
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-30092 (URN)10.1016/j.msea.2019.05.079 (DOI)000472813900052 ()2-s2.0-85066090205 (Scopus ID)
Funder
Vinnova, 2014-03374
Available from: 2019-05-23 Created: 2019-05-23 Last updated: 2019-07-22Bibliographically approved
Molnar, D., Engberg, G., Li, W., Lu, S., Hedström, P., Kwon, S. K. & Vitos, L. (2019). Experimental study of the gamma-surface of austenitic stainless steels. Acta Materialia, 173, 34-43
Open this publication in new window or tab >>Experimental study of the gamma-surface of austenitic stainless steels
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2019 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 173, p. 34-43Article in journal (Refereed) Published
Abstract [en]

We introduce a theory-guided experimental approach to study the γ-surface of austenitic stainless steels. The γ-surface includes a series of intrinsic energy barriers (IEBs), which are connected to the unstable stacking fault (USF), the intrinsic stacking fault (ISF), the unstable twinning fault (UTF) and the extrinsic stacking fault (ESF) energies. The approach uses the relationship between the Schmid factors and the effective energy barriers for twinning and slip. The deformation modes are identified as a function of grain orientation using in situ electron backscatter diffraction measurements. The observed critical grain orientation separating the twinning and slip regimes yields the USF energy, which combined with the universal scaling law provides access to all IEBs. The measured IEBs and the critical twinning stress are verified by direct first-principles calculations. The present advance opens new opportunities for modelling the plastic deformation mechanisms in multi-component alloys.

Keywords
stacking fault energy, twinning, electron backscatter diffraction, plasticity, first-principles
National Category
Metallurgy and Metallic Materials
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-29982 (URN)10.1016/j.actamat.2019.04.057 (DOI)000472812300005 ()2-s2.0-85065259834 (Scopus ID)
Available from: 2019-05-07 Created: 2019-05-07 Last updated: 2019-07-23Bibliographically approved
Molnar, D., Vida, Á., Huang, S. & Chinh, N. Q. (2019). The effect of cooling rate on the microstructure and mechanical properties of NiCoFeCrGa high-entropy alloy. Journal of Materials Science, 54(6), 5074-5082
Open this publication in new window or tab >>The effect of cooling rate on the microstructure and mechanical properties of NiCoFeCrGa high-entropy alloy
2019 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 54, no 6, p. 5074-5082Article in journal (Refereed) Published
Abstract [en]

The effect of cooling rate on the microstructure and mechanical properties of equimolar NiCoFeCrGa high-entropy alloy (HEA) was studied by scanning electron microscopy, energy-dispersive X-ray spectroscopy and electron backscatter diffraction (EBSD), as well as by microhardness tests. Experimental results show that the cooling rate has a crucial impact on the developing microstructure which has a mixture of two—FCC and BCC—phases, leading to a self-similarity of the solidified structure formed in the sample. Furthermore, the cooling rate influences both the composition of the two phase-components and the ratio of their volume fractions, determining the mechanical properties of the sample. The present results confirm the grouping of Co, Fe and Cr in the FCC phase and that of Ni and Ga in BCC phase in the NiCoFeCrGa high-entropy alloy system. An empirical rule is suggested to predict how the phase-components can be expected in this complex high-entropy alloy.

Keywords
high entropy alloy, microstructure, EBSD
National Category
Metallurgy and Metallic Materials
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-29174 (URN)10.1007/s10853-018-03196-8 (DOI)000453927400051 ()2-s2.0-85058001002 (Scopus ID)
Available from: 2018-12-20 Created: 2018-12-20 Last updated: 2019-01-07Bibliographically approved
Fazakas, E., Heczel, A., Molnar, D., Varga, B., Zadorozhnyy, V. & Vida, A. (2018). Comparative microstructural and corrosion development of VCrNiCoFeCu equiatomic multicomponent alloy produced by induction melting and spark plasma sintering. In: IOP Conference Series: Materials Science and Engineering. Paper presented at Symposium on Solutions for Critical Raw Materials Under Extreme Conditions 2017; Warsaw University of Technology Warsaw; Poland. , 329(1), Article ID 012016.
Open this publication in new window or tab >>Comparative microstructural and corrosion development of VCrNiCoFeCu equiatomic multicomponent alloy produced by induction melting and spark plasma sintering
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2018 (English)In: IOP Conference Series: Materials Science and Engineering, 2018, Vol. 329, no 1, article id 012016Conference paper, Published paper (Refereed)
Abstract [en]

The present study focuses on the corrosion behavior of a single-phase FCC high entropy alloy (VCrNiCoFeCu) casted by two different methods: induction melting and spark plasma sintering. The corrosion resistance has been evaluated using immersion tests in 3.5% NaCl solution, the potentiodynamic polarization measurements and the results are compared how is dependent the corrosion rate as a function of the production methods. Our results show that induction melted sample is stable in salty environment. On the other hand, based on the changes of polarization curves, there must be an evolution of oxide films on the SPSed sample until reaching the stable oxide layer. 

Series
IOP Conference Series: Materials Science and Engineering, ISSN 1757-8981
National Category
Materials Engineering
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-27515 (URN)10.1088/1757-899X/329/1/012016 (DOI)000432425000016 ()2-s2.0-85045134705 (Scopus ID)
Conference
Symposium on Solutions for Critical Raw Materials Under Extreme Conditions 2017; Warsaw University of Technology Warsaw; Poland
Available from: 2018-04-23 Created: 2018-04-23 Last updated: 2018-06-05Bibliographically approved
Molnar, D., Engberg, G., Li, W. & Vitos, L. (2018). Deformation properties of austenitic stainless steels with different stacking fault energies. Paper presented at THERMEC 2018. Materials Science Forum, 941, 190-197
Open this publication in new window or tab >>Deformation properties of austenitic stainless steels with different stacking fault energies
2018 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 941, p. 190-197Article in journal (Refereed) Published
Abstract [en]

In FCC metals a single parameter – stacking fault energy (SFE) – can help to predict the expectable way of deformation such as martensitic deformation, deformation twinning or pure dislocation glide. At low SFE one can expect the perfect dislocations to dissociate into partial dislocations, but at high SFE this separation is more restricted. The role of the magnitude of the stacking fault energy on the deformation microstructures and tensile behaviour of different austenitic steels have been investigated using uniaxial tensile testing and electron backscatter diffraction (EBSD). The SFE was determined by using quantum mechanical first-principles approach. By using plasticity models we make an attempt to explain and interpret the different strain hardening behaviour of stainless steels with different stacking fault energies.

Place, publisher, year, edition, pages
Switzerland: , 2018
Keywords
Ab Initio, Deformation, EBSD, Modelling, SEM, Stainless Steel, Tensile Testing, Thermec’2018
National Category
Metallurgy and Metallic Materials
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-29197 (URN)10.4028/www.scientific.net/MSF.941.190 (DOI)000468152500032 ()2-s2.0-85064085774 (Scopus ID)
Conference
THERMEC 2018
Available from: 2018-12-26 Created: 2018-12-26 Last updated: 2019-06-10Bibliographically approved
Vida, Á., Maksa, Z., Molnar, D., Huang, S., Kovac, J., Varga, L. K., . . . Chinh, N. Q. (2018). Evolution of the phase structure after different heat treatments in NiCoFeCrGa high entropy alloy. Journal of Alloys and Compounds, 743, 234-239
Open this publication in new window or tab >>Evolution of the phase structure after different heat treatments in NiCoFeCrGa high entropy alloy
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2018 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 743, p. 234-239Article in journal (Refereed) Published
National Category
Materials Engineering
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-27277 (URN)10.1016/j.jallcom.2018.01.407 (DOI)000427511200027 ()2-s2.0-85041387848 (Scopus ID)
Available from: 2018-02-22 Created: 2018-02-22 Last updated: 2018-04-05Bibliographically approved
Molnar, D. (2018). Generalised stacking fault energy and plastic deformation of austenitic stainless steels. (Licentiate dissertation). Stockholm: KTH Royal Institute of Technology
Open this publication in new window or tab >>Generalised stacking fault energy and plastic deformation of austenitic stainless steels
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Austenitic stainless steels are primarily known for their exceptional corrosion resistance. They have the face centred cubic (FCC) structure which is stabilised by adding nickel to the Fe-Cr alloy. The Fe-Cr-Ni system can be further extended by adding other elements such as Mn, Mo, N, C, etc. in order to improve the properties. Since austenitic stainless steels are often used as structural materials, it is important to be able to predict their mechanical behaviour based on their composition, microstructure, magnetic state, etc.

In this work, we investigate the plastic deformation behaviour of austenitic stainless steels by theoretical and experimental approaches. In FCC materials the stacking fault energy (SFE) plays an important role in the prediction of the deformation modes. Based on the magnitude of the SFE different deformation modes can be observed such as martensite formation, deformation twinning, dissociated or undissociated dislocation glide. All these features influence the behaviour differently, therefore it is desired to be able to predict their occurrence. Alloying and temperature have strong effect on the SFE and thus on the mechanical properties of the alloys. Several models based on the SFE and more recently on the so called generalised stacking fault energy (GSFE or γ-surface) are available to predict the alloy's affinity to twinning and the critical twinning stress representing the minimum resolved shear stress required to initiate the twinning deformation mechanism. One can employ well established experimental techniques to measure the SFE. On the other hand, one needs to resort to ab initio calculations based on density functional theory (DFT) to compute the GSFE of austenitic steels and derive parameters like the twinnability and the critical twinning stress. 

We discuss the effect of the stacking fault energy on the deformation behaviour for two different austenitic stainless steels. We calculate the GSFE of the selected alloys and based on different models, we predict their tendency for twinning and the critical twinning stress. The theoretical predictions are contrasted with tensile tests and electron backscatter diffraction (EBSD) measurements. Several conventional and in situ tensile test are performed to verify the theoretical results. We carry out EBSD measurements on interrupted and fractured specimens and during tensile tests to closely follow the development of the microstructure. We take into account the role of the intrinsic energy barriers in our predictions and introduce a new and so far unique way to experimentally obtain the GSFE of austenitic stainless steels. Previously, only the SFE could be measured precisely by well-designed experiments. In the present thesis we go further and propose a technique that can provide accurate unstable stacking fault energy values for any austenitic alloy exhibiting twinning. 

Abstract [sv]

Austenitiska rostfria stål är främst kända för sin exceptionella korrosionsbeständighet. De har en ytcentrerad kubisk (FCC) struktur som stabiliseras genom att nickel tillsätts till Fe-Cr legeringen. Fe-Cr-Ni-systemet kan utökas ytterligare genom tillsats av andra element såsom Mn, Mo, N, C, etc. för att förbättra egenskaperna. Eftersom austenitiska rostfria stål ofta används som konstruktionsmaterial är det viktigt att kunna förutsäga deras mekaniska egenskaper baserat på deras sammansättning, mikrostruktur, magnetiska tillstånd, etc.

I denna avhandling undersöker vi det plastiska deformationsbeteendet hos austenitiska rostfria stål både teoretiskt och experimentellt. I FCC material spelar staplingsfelsenergin (SFE) en viktig roll vid förutsägelsen av deformationsmekanism. Baserat på storleken av SFE kan olika deformationsmekanismer observeras, såsom martensitbildning, tvillingbildning, dissocierad eller odissocierad dislokationsglidning. Alla dessa funktioner påverkar beteendet på olika sätt, därför är det önskvärt att kunna förutsäga deras förekomst. Legering och temperatur har stark inverkan på SFE och därmed legeringarnas mekaniska egenskaper. Flera modeller, baserade på SFE och mer nyligen på den så kallade generaliserade staplingsfelenergin (GSFE eller γ-surface), är tillgängliga för att förutsäga legeringens benägenhet till tvillingbildning och den kritiska spänning som representerar den minsta upplösta skjuvspänningen som krävs för att initiera tvillingbildning. Man kan använda ab initio beräkningar baserade på täthetsfunktionalteori (DFT) för att beräkna GSFE för austenitiska stål och härleda parametrar som twinnability och kritisk tvillingsspänning.

Vi diskuterar effekten av staplingsfelenergi på deformationsbeteendet för två olika austenitiska rostfria stål. Vi beräknar GSFE för de valda legeringarna och baserat på olika modeller, förutsäger vi deras tendens till tvillingbildning och den kritiska tvillingsspänningen. De teoretiska förutsägelserna jämförs med resultat från dragprov och bakåtspridd elektron diffraktion (EBSD). Flera konventionella och in situ dragprov utfördes för att verifiera de teoretiska resultaten. Vi utförde EBSD-mätningar på dragprov som avbrutits vid olika töjningar och efter brott samt med in situ dragprov för att följa utvecklingen av mikrostrukturen noggrant. Vi tar hänsyn till de inre energibarriärernas roll i våra förutsägelser och presenterar ett nytt sätt att experimentellt få GSFE av austenitiska rostfria stål. Tidigare kunde endast SFE mätas tillförlitligt genom väl utformade experiment. I den aktuella avhandlingen går vi vidare och föreslår en teknik som kan ge noggranna värden för den instabila staplingsfelenergin för alla austenitiska legeringar som uppvisar tvillingbildning.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. p. 44
Keywords
stacking fault energy, stainless steel, plasticity, deformation, ab initio
National Category
Materials Engineering
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-28668 (URN)978-91-7729-850-2 (ISBN)
Presentation
2018-09-28, Konferensrum (Kuben), N111, Kungliga Tekniska Högskolan, 13:00 (English)
Opponent
Supervisors
Available from: 2018-10-08 Created: 2018-10-08 Last updated: 2018-10-08Bibliographically approved
Huang, S., Vida, A., Li, W., Molnar, D., Kwon, S. K., Holmstrom, E., . . . Vitos, L. (2017). Thermal expansion in FeCrCoNiGa high-entropy alloy from theory and experiment. Applied Physics Letters, 110(24), Article ID 241902.
Open this publication in new window or tab >>Thermal expansion in FeCrCoNiGa high-entropy alloy from theory and experiment
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2017 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 110, no 24, article id 241902Article in journal (Refereed) Published
Abstract [en]

First-principle alloy theory and key experimental techniques are applied to determine the thermal expansion of FeCrCoNiGa high-entropy alloy. The magnetic transition, observed at 649 K, is accompanied by a significant increase in the thermal expansion coefficient. The phase stability is analyzed as a function of temperature via the calculated free energies accounting for the structural, magnetic, electronic, vibrational and configurational contributions. The single-and polycrystal elastic modulus for the ferro-and paramagnetic states of the face-centered and body-centered cubic phases are presented. By combining the measured and theoretically predicted temperature-dependent lattice parameters, we reveal the structural and magnetic origin of the observed anomalous thermal expansion behavior. Published by AIP Publishing.

Place, publisher, year, edition, pages
AMER INST PHYSICS, 2017
National Category
Materials Engineering
Research subject
Steel Forming and Surface Engineering
Identifiers
urn:nbn:se:du-25504 (URN)10.1063/1.4985724 (DOI)000403678300010 ()
Available from: 2017-07-06 Created: 2017-07-06 Last updated: 2017-07-06Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-7355-1941

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