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Multi-length scale modeling of martensitic transformations in stainless steels
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Physical Metallurgy.ORCID iD: 0000-0002-7656-9733
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.ORCID iD: 0000-0002-9920-5393
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2012 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 60, no 19, 6508-6517 p.Article in journal (Refereed) Published
Abstract [en]

In the present work a multi-length scale model is developed to study both the athermal and stress-assisted martensitic transformations in a single crystal of 301 type stainless steel. The microstructure evolution is simulated using elastoplastic phase-field simulations in three dimensions. The input data for the simulations is acquired from a combination of computational techniques and experimental works. The driving force for the transformation is calculated by using the CALPHAD technique and the elastic constants of the body-centered cubic phase are calculated by using ab initio method. The other input data is acquired from experimental works. The simulated microstructures resemble a lath-type martensitic microstructure, which is in good agreement with the experimental results obtained for a stainless steel of similar composition. The martensite habit plane predicted by the model is in accordance with experimental results. The Magee effect, i.e. formation of favorable martensite variants depending on the loading conditions, is observed in the simulations. The results also indicate that anisotropic loading conditions give rise to a significant anisotropy in the martensitic microstructure.

Place, publisher, year, edition, pages
2012. Vol. 60, no 19, 6508-6517 p.
Keyword [en]
phase-field simulations, ab initio, multi-length scale model, martensitic transformation, microstructure
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-95306DOI: 10.1016/j.actamat.2012.08.012ISI: 000311188400007Scopus ID: 2-s2.0-84867582531OAI: oai:DiVA.org:kth-95306DiVA: diva2:527635
Projects
hero-m
Funder
Vinnova
Note

QC 20121119. Updated from accepted to published.

Available from: 2012-05-21 Created: 2012-05-21 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Martensitic Transformations in Steels: A 3D Phase-field Study
Open this publication in new window or tab >>Martensitic Transformations in Steels: A 3D Phase-field Study
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Martensite is considered to be the backbone of the high strength of many commercial steels. Martensite is formed by a rapid diffusionless phase transformation, which has been the subject of extensive research studies for more than a century. Despite such extensive studies, martensitic transformation is still considered to be intriguing due to its complex nature. Phase-field method, a computational technique used to simulate phase transformations, could be an aid in understanding the transformation. Moreover, due to the growing interest in the field of “Integrated computational materials engineering (ICME)”, the possibilities to couple the phase-field method with other computational techniques need to be explored. In the present work a three dimensional elastoplastic phase-field model, based on the works of Khachaturyan et al. and Yamanaka et al., is developed to study the athermal and the stress-assisted martensitic transformations occurring in single crystal and polycrystalline steels. The material parameters corresponding to the carbon steels and stainless steels are considered as input data for the simulations. The input data for the simulations is acquired from computational as well as from experimental works. Thus an attempt is made to create a multi-length scale model by coupling the ab-initio method, phase-field method, CALPHAD method, as well as experimental works. The model is used to simulate the microstructure evolution as well as to study various physical concepts associated with the martensitic transformation. The simulation results depict several experimentally observed aspects associated with the martensitic transformation, such as twinned microstructure and autocatalysis. The results indicate that plastic deformation and autocatalysis play a significant role in the martensitic microstructure evolution. The results indicate that the phase-field simulations can be used as tools to study some of the physical concepts associated with martensitic transformation, e.g. embryo potency, driving forces, plastic deformation as well as some aspects of crystallography. The results obtained are in agreement with the experimental results. The effect of stress-states on the stress-assisted martensitic microstructure evolution is studied by performing different simulations under different loading conditions. The results indicate that the microstructure is significantly affected by the loading conditions. The simulations are also used to study several important aspects, such as TRIP effect and Magee effect. The model is also used to predict some of the practically important parameters such as Ms temperature as well as the volume fraction of martensite formed. The results also indicate that it is feasible to build physically based multi-length scale model to study the martensitic transformation. Finally, it is concluded that the phase-field method can be used as a qualitative aid in understanding the complex, yet intriguing, martensitic transformations.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. xii, 56 p.
Keyword
Phase-field method, Martensitic transformations, Plastic de formation, Multi-length scale modeling, Microstructure, Stress states, Steels
National Category
Materials Engineering Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-95316 (URN)978-91-7501-388-6 (ISBN)
Public defence
2012-06-15, B2, Materialvetenskap, Brinellvägen 23, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
Hero-m
Note
QC 20120525Available from: 2012-05-25 Created: 2012-05-22 Last updated: 2012-08-07Bibliographically approved
2. Thermodynamic and kinetic properties of Fe-Cr and TiC-ZrC alloys from Density Functional Theory
Open this publication in new window or tab >>Thermodynamic and kinetic properties of Fe-Cr and TiC-ZrC alloys from Density Functional Theory
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The complete and accurate thermodynamic and kinetic description of any systemis crucialfor understanding and predicting its properties. A particular interest is in systemsthat are used for some practical applications and have to be constantly improved usingmodification of their composition and structure. This task can be quite accuratelysolved at a fundamental level by density functional theory methods. Thesemethods areapplied to two practically important systems Fe-Cr and TiC-ZrC.The elastic properties of pure iron and substitutionally disordered Fe-Cr alloy are investigatedas a function of temperature and concentration using first-principles electronicstructurecalculations by the exact muffin-tin orbitals method. The temperature effectson the elastic properties are included via the electronic, magnetic, and lattice expansioncontributions. It is shown that the degree of magnetic order in both pure iron andFe90Cr10 alloy mainly determines the dramatic change of the elastic anisotropy of thesematerials at elevated temperatures. A peculiarity in the concentration dependence ofthe elastic constants in Fe-rich alloys is demonstrated and related to a change in theFermi surface topology.A thermodynamic model for the magnetic alloys is developed from first principles andapplied to the calculation of bcc Fe-Cr phase diagram. Various contributions to the freeenergy (magnetic, electronic, and phonon) are estimated and included in the model. Inparticular, it is found that magnetic short range order effects are important just abovethe Curie temperature. The model is applied for calculating phase equilibria in disorderedbcc Fe-Cr alloys. Model calculations reproduce a feature known as a Nishizawahorn for the Fe-rich high-temperature part of the phase diagram.The investigation of the TiC-ZrC system includes a detailed study of the defect formationenergies and migration barriers of point defects and defect complexes involvedin the diffusion process. It is found, using ab initio atomistic simulations of vacancymediateddiffusion processes in TiC and ZrC, that a special self-diffusion mechanism isoperative for metal atom diffusion in sub-stoichiometric carbides. It involves a noveltype of a stable point defect, a metal vacancy ”dressed” in a shell of carbon vacancies.It is shown that this vacancy cluster is strongly bound and can propagate through thelattice without dissociating.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. viii, 62 p.
Keyword
ab initio, first principles, point defects, vacancy clusters, alloys, steels, iron, carbides, diffusion, phase diagram, density functional theory, elastic constants, elastic properties, thermodynamic modelling
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-96285 (URN)978-91-7501-340-4 (ISBN)
Public defence
2012-06-12, F2, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
HERO-M
Note

QC 20120604

Available from: 2012-06-04 Created: 2012-06-01 Last updated: 2013-04-02Bibliographically approved

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Borgenstam, AnnikaKorzhavyi, Pavel. A.

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