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First-principles based thermodynamic model of phase equilibria in bcc Fe-Cr alloys
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 17, p. 174111-Article in journal (Refereed) Published
Abstract [en]

A first-principles based thermodynamic model for magnetic alloys is applied to the calculation of the Fe-Cr phase diagram restricted by the bcc structure. The model includes magnetic, electronic, phonon, and local atomic relaxations contributions to the free-energy derived from ab initio calculations. Atomic short-range-order effects are found to be relatively small and they have been neglected in the calculations, assuming that alloys are in the completely random state. In contrast, we have taken into consideration magnetic short-range-order effects, which are found to be very important in particular above the Curie temperature. The calculated phase diagram is in reasonable agreement with the latest CALPHAD assessment. Our calculations reproduce a feature known as a Nishizawa horn for the Fe-rich high-temperature part of the phase diagram.

Place, publisher, year, edition, pages
2012. Vol. 86, no 17, p. 174111-
Keywords [en]
Generalized Gradient Approximation, High-Temperature Calorimeter, Initio Molecular-Dynamics, Total-Energy Calculations, Augmented-Wave Method, Iron-Chromium Alloys, Miscibility Gap, Electronic-Structure, Magnetic-Properties, Finite-Temperature
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-96420DOI: 10.1103/PhysRevB.86.174111ISI: 000311138600001Scopus ID: 2-s2.0-84870014447OAI: oai:DiVA.org:kth-96420DiVA, id: diva2:530705
Funder
VinnovaSwedish Research CouncilEU, European Research Council, 228074
Note

QC 20121217. Updated from submitted to published.

Available from: 2012-06-04 Created: 2012-06-04 Last updated: 2017-12-07Bibliographically approved
In thesis
1. 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. p. viii, 62
Keywords
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
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HERO-M
Note

QC 20120604

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

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