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Stacking fault energy and deformation behaviour of austenitic stainless steels: a joint theoretical-experimental study
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [sv]

Austenitiska rostfria stål är främst kända för sin exceptionella korrosionsbeständighet. De har en ytcentrerad kubisk (FCC) struktur som stabiliseras genom att nickel, mangan eller kväve tillsätts till Fe-Cr legeringen. Fe-Cr-Ni-systemet kan utökas ytterligare genom tillsats av andra element såsom Mo, Cu, Ti, 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 är det plastiska deformationsbeteendet hos austenitiska rostfria stål undersökt med teoretiskt och experimentellt. I FCC material spelar staplingsfelsenergin (SFE) en viktig roll vid förutsägelsen och beskrivning av deformationsmekanism. Baserat på storleken av SFE kan olika deformationsmekanismer observeras, såsom martensitbildning, tvillingbildning, eller dislokationsglidning. Alla dessa funktioner påverkar beteendet, därför är det önskvärt att förutsäga och kontrollera 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 affinitet 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.

Korrelation mellan staplingsfelenergi och deformationsbeteendet för fyra olika austenitiska rostavstavning stål diskuteras i detta arbete. SFE för de valda legeringarna erhålls genom ab initio beräkningar och baserat på olika modeller, deras tendens till tvillingbildning och den kritiska tvillingsspänningen kan förutsägas. Deras mekaniska beteende och affinitet till tvilling och martensitisk transformation kartläggs över ett brett temperaturområde (−70°C to +500°C) för de fyra legeringarna. De teoretiska förutsägelserna jämförs med resultat från dragprov och bakåtspridd elektrondiffraktion (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 på låga spänningsnivåer. Betydelsen av temperatur och mellanliggande legering på mekaniskt beteende undersökt också i detta arbete.

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, manganese or nitrogen to the Fe-Cr alloy. The Fe-Cr-Ni system can be further extended by adding other elements such as Mo, Cu, Ti, C, etc. 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, the plastic deformation behaviour of austenitic stainless steels is investigated by theoretical and experimental approaches. In FCC materials the stacking fault energy (SFE) plays an important role in the description and prediction of the deformation modes. Based on the magnitude of the SFE different deformation modes can be observed such as martensite formation, deformation twinning, or dislocation glide. All these deformation modes influence the material behaviour, therefore it is desired to predict and control their occurrence. Alloying elements and temperature have a 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 describe the alloy's affinity to twinning and the critical twinning stress representing the minimum resolved shear stress required to initiate the deformation twinning 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.

The correlation between the stacking fault energy and the deformation behaviour for four different austenitic stainless steels is discussed in this work. The SFE of the selected alloys is obtained by ab initio calculations and based on different models, their tendency for twinning and their critical twinning stress is predicted. The mechanical behaviour and the affinity for twinning and martensitic transformation is mapped across a broad range of temperature (-70°C to +500°C) for the four alloys. 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. EBSD measurements on interrupted and fractured specimens, and during in situ tensile tests were carried out to closely follow the development of the microstructure. In the present thesis, a technique is proposed that can provide accurate unstable stacking fault energy values for any austenitic alloy exhibiting twinning at low stress values. The importance of temperature and interstitial alloying on mechanical behaviour is also investigated.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2019. , p. 80
Series
TRITA-ITM-AVL ; 2019:33
Keywords [en]
austenitic stainless steel, deformation properties, stacking fault energy, electron backscatter diffraction, ab initio
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-263169ISBN: 978-91-7873-316-3 (print)OAI: oai:DiVA.org:kth-263169DiVA, id: diva2:1366950
Public defence
2019-11-28, F3, Lindstedsvägen 26, Stockholm, 09:00 (English)
Opponent
Supervisors
Available from: 2019-11-05 Created: 2019-10-31 Last updated: 2019-11-05Bibliographically approved
List of papers
1. Effect of temperature on the stacking fault energy and deformation behaviour in 316L austenitic stainless steel
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.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Deformation twinning, microstructure, first principles, stacking fault energy, stainless steel
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-252728 (URN)10.1016/j.msea.2019.05.079 (DOI)000472813900052 ()
Funder
Vinnova, 2014-03374
Note

QC 20190731

Available from: 2019-06-04 Created: 2019-06-04 Last updated: 2019-10-31Bibliographically approved
2. Experimental study of the γ-surface of austenitic stainless steels
Open this publication in new window or tab >>Experimental study of the γ-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
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-252729 (URN)10.1016/j.actamat.2019.04.057 (DOI)2-s2.0-85065259834 (Scopus ID)
Funder
Vinnova, 2014-03374
Note

QC 20190617

Available from: 2019-06-04 Created: 2019-06-04 Last updated: 2019-10-31Bibliographically approved
3. Deformation properties of austenitic stainless steels with different stacking fault energies
Open this publication in new window or tab >>Deformation properties of austenitic stainless steels with different stacking fault energies
2018 (English)In: Mater. Sci. Forum, Trans Tech Publications Ltd , 2018, p. 190-197Conference paper, Published paper (Refereed)
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
Trans Tech Publications Ltd, 2018
Keywords
Ab initio, Deformation, EBSD, Modelling, SEM, Stainless Steel, Tensile Testing, Thermec’2018
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-251777 (URN)10.4028/www.scientific.net/MSF.941.190 (DOI)000468152500032 ()2-s2.0-85064085774 (Scopus ID)9783035712087 (ISBN)
Conference
9 July 2018 through 13 July 2018
Funder
Swedish Research CouncilVinnova, 2014-03374Swedish Foundation for Strategic Research
Note

QC 20190528

Available from: 2019-05-28 Created: 2019-05-28 Last updated: 2019-10-31Bibliographically approved

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