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Thermodynamic investigation of systems related to TWIP steels
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Computational Thermodynamics.ORCID iD: 0000-0001-9010-525X
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The world is facing serious challenges regarding environmental issues. Carbon dioxide levels increase every day. In an attempt to decrease carbon emissions the automotive industry searches for lighter, stronger materials. TWinning Induced Plasticity (TWIP) steels show an impressive combination of strength and ductility. The possibility of adding high amounts of aluminum that decreases the density while maintaining the mechanical properties makes this type of steel very interesting for use in automotive applications. The only thing keeping the TWIP steels from being used in the automotive industry is that the Yield Strength (YS) is too low. The TWIP steels usually have a YS around 400 MPa. For them to be useful in automotive applications YS around 600-700 MPa is necessary. One of the most promising ways of improving the YS is by precipitation hardening. This work has been performed within a European Research Fund for Coal and Steel (RFCS) project called Precipitation in High Manganese steels (PrecHiMn). As the name of the project suggests, the goal of this project has been to study the precipitation in TWIP steels. The precipitation hardening is achieved through the addition of carbide and carbonitride formers such as Nb, Ti and V. In order to build advanced models to simulate precipitation it is important to have a good understanding of the thermodynamics of systems related to TWIP steels.

The goal of this work has been to study the thermodynamic properties of systems related to TWIP steels, more specifically the system forming the matrix phases of TWIP steels. Therefore the Al-C-Fe-Mn system has been studied as well as the Al-Ti-V system. Complete thermodynamic descriptions that reproduce the experimental data well have been produced including descriptions of order-disorder transformations.

Abstract [sv]

Varlden star infor stora utmaningar nar det galler miljofragor. Koldioxidnivaerna okar varje dag. I ett forsok att minska koldioxidutslappen soker bilindustrinefter lattare och starkare material. TWinning Induced Plasticity (TWIP)stal uppvisar en imponerande kombination av hallfasthet och duktilitet. Mojlighetenatt tillsatta hoga halter av aluminium som minskar densiteten med bibehallnamekaniska egenskaper gor denna typ av stal mycket intressanta for anvandning ifordonstillampningar. Det enda som hindrar TWIP stal fran att anvandas inomfordonsindustrin ar att strackgransen ar for lag. TWIP stal har vanligtvis enstrackgrans runt 400 MPa. For att de ska vara anvandbara i biltillampningarkravs en strackrans omkring 600-700 MPa. Ett av de mest lovande satten attforbattra strackgransen ar genom utskiljningshardning. Detta arbete har utfortsinom en europeisk forskningsfond for kol och stal (RFCS) projekt kallat Precipitationin High Manganese steels (PrecHiMn). Som namnet pa projektetantyder, har malet med projektet varit att studera utskiljning i TWIP stal.Utskiljningshardning uppnas genom tillsats av karbid- och karbonitridbildaresasom Nb, Ti och V. For att bygga avancerade modeller for simulering av utskiljningar det viktigt att ha en god forstaelse for termodynamiken i systemrelaterade till TWIP stal.Malet med detta arbete har varit att studera de termodynamiska egenskapernahos system med anknytning till TWIP stal. Mer specikt har systemetsom bildar matrisfaserna i TWIP stal. Drfr har Al-C-Fe-Mn-systemet studeratsliksom Al-Ti-V-systemet. Fullstandiga termodynamiska beskrivningar somaterger experimentella data val har producerats inklusive beskrivningar av ordningsomvandlingar.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , v, 48 p.
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-166224ISBN: 978-91-7595-516-2 (print)OAI: oai:DiVA.org:kth-166224DiVA: diva2:809940
Public defence
2015-05-22, B2, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
RFSR-CT-2010-00018
Note

QC 20150506

Available from: 2015-05-06 Created: 2015-05-05 Last updated: 2015-05-06Bibliographically approved
List of papers
1. The Al-Fe-Mn system revisited-An updated thermodynamic description using the most recent binaries
Open this publication in new window or tab >>The Al-Fe-Mn system revisited-An updated thermodynamic description using the most recent binaries
2013 (English)In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 43, 86-93 p.Article in journal (Refereed) Published
Abstract [en]

The so-called TWinning Induced Plasticity (TWIP) steels have gained a lot of attention in the last couple of years due to their excellent mechanical properties; they show very high strength and are at the same time very ductile. The TWIP steels are austenitic and form mechanical twins under deformation. All TWIP steels have very high manganese content and a certain sub-group of these steels, lightweight steels with induced plasticity (so-called L-IP), also have high aluminum content. These steels are the lightweight versions of the TWIP steels and are therefore of high interest to the automotive industry. However, the grades existing today have too low yield strength. The yield strength can be improved by alloying and/or by precipitation hardening. Both these techniques require detailed insight on the thermodynamic properties of the alloy system in question. In this work, a thermodynamic reassessment of the entire Al-Fe-Mn system has been performed as a first step to describe the Fe-Al-C-Mn system, the core system for L-IP steels. All available experimental information has been taken into consideration and a set of data has been selected to be used in the optimization. The new thermodynamic description is based on the most recent thermodynamic descriptions of the constituent binaries and reproduces the experimental information in a satisfactory manner. This description, as opposed to previous descriptions has been optimized to fit experimental results in both the aluminum-rich part and the iron-manganese rich part of the system.

Keyword
Al-Fe-Mn, TWIP-steel, L-IP steel, CALPHAD, Computational thermodynamics, Modeling
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-139286 (URN)10.1016/j.calphad.2013.05.001 (DOI)000328236100011 ()2-s2.0-84888858476 (Scopus ID)
Note

QC 20140108

Available from: 2014-01-08 Created: 2014-01-08 Last updated: 2017-12-06Bibliographically approved
2. Ordering in ternary BCCalloys applied to the Al-Fe-Mn system
Open this publication in new window or tab >>Ordering in ternary BCCalloys applied to the Al-Fe-Mn system
(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-166245 (URN)
Note

QS 2015

Available from: 2015-05-06 Created: 2015-05-06 Last updated: 2015-05-06Bibliographically approved
3. A thermodynamic re-assessment of Al-V toward an assessment of the ternary Al-Ti-V system
Open this publication in new window or tab >>A thermodynamic re-assessment of Al-V toward an assessment of the ternary Al-Ti-V system
2015 (English)In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 51, 75-88 p.Article in journal (Refereed) Published
Abstract [en]

Titanium alloys are highly sought after due to their excellent mechanical properties. One of the most commonly used Ti alloys is Ti-6Al-4V, which contains 6% Al and 4% V by weight. Despite the popularity of this alloy, no thermodynamic description of the ternary Al-Ti-V system has been published in the open literature. In this work an assessment procedure of the ternary Al-Ti-V system was initiated based on the binary descriptions by Witusiewitcz et al. (J. Alloys Compds. 465 (2008) 64-77 [1]) for (Al-Ti), Gong et al. (Int. J. Mater. Res. 95 (2004) 978-986 [2]) for (Al-V) and Saunders (COST 507, 2 (1998) 297-298 [3]) for (Ti-V). When combining the three binary systems and looking at the extrapolated ternary isothermal sections, it was found that there was a very large miscibility gap in the bcc phase. The origin of this miscibility gap was mainly the Al-V system and therefore it was decided to reassess this system. The Al-V system was reassessed according to available experimental data along with the enthalpies of formation of all compounds as well as the enthalpies of mixing for all terminal phases obtained by first-principles calculations based on the density functional theory. For the Al8V5 phase there are two different sets of data for the enthalpies of formation. These two sets are investigated in this work and it is found that the set not used by Gong et al. in their assessment of the Al-V binary system gives better extrapolations. The final description produced improved extrapolated ternary isothermal sections.

Place, publisher, year, edition, pages
Elsevier, 2015
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-166246 (URN)10.1016/j.calphad.2015.07.002 (DOI)000366790500009 ()2-s2.0-84940901655 (Scopus ID)
Note

Updated from "Manuscript" to "Article". QC 20151208. QC 20160121

Available from: 2015-05-06 Created: 2015-05-06 Last updated: 2017-12-04Bibliographically approved
4. A thermodynamic investigationof the Al-C-Fe-Mn system
Open this publication in new window or tab >>A thermodynamic investigationof the Al-C-Fe-Mn system
(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-166248 (URN)
Note

QS 2015

Available from: 2015-05-06 Created: 2015-05-06 Last updated: 2015-05-06Bibliographically approved
5. Low-Density Steels: Complex Metallurgy for Automotive Applications
Open this publication in new window or tab >>Low-Density Steels: Complex Metallurgy for Automotive Applications
Show others...
2014 (English)In: JOM: The Member Journal of TMS, ISSN 1047-4838, E-ISSN 1543-1851, Vol. 66, no 9, 1747-1758 p.Article in journal (Refereed) Published
Abstract [en]

The current aim in the development of third-generation steels for lightweighting automotive applications is to increase strength keeping at least the same formability as current steel concepts. In this philosophy, an optimal concept would be one that brings, in addition, a lower density. For this purpose, low-density steels have been designed with important aluminum additions obtaining density reductions of 8-10% or higher in comparison with low-carbon steels. At the levels required for lightweighting, aluminum introduces complex phenomena in steels. Here, some of the effects of aluminum in phase stability, CALPHAD-type modeling, and microstructure development are described, the latter in relation with mechanical properties. Finally, the potential of two families of lightweight steels for automotive applications is assessed by comparison with a steel currently present in automotive structures.

Keyword
Al-C System, Mechanical-Properties, Phase-Equilibria, Spinodal Decomposition, Thermodynamic Calculation, Ternary Alloys, Fe System, Aluminum, Carbide, Ferrite
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-154385 (URN)10.1007/s11837-014-1084-y (DOI)000341916100032 ()2-s2.0-84920253175 (Scopus ID)
Note

QC 20141020

Available from: 2014-10-20 Created: 2014-10-20 Last updated: 2017-12-05Bibliographically approved

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