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Finite temperature properties of elements and alloy phases from first principles
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.ORCID iD: 0000-0001-5518-3308
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

First principles calculations are usually concerned with properties calculated at temperature 0 K. However, the industrially important materials are functioning at finite temperatures. To fill such a gap a first-principles based modeling of free energy has been developed in this thesis and finite temperature properties of different phases of Fe and Mn have been calculated and contrasted with available experimental data.

In particular, using partitioning of the Helmholtz free energy, thermophysical properties of paramagnetic Fe have been reported. The heat capacity, lattice constant, thermal expansion and elastic moduli of γ- and δ-Fe show a good agreement with available experimental data. In the case of α-Fe, we observe a good agreement for elastic moduli and thermal expansion with experiments but the heat capacity is not well-reproduced in the calculations because of the large contribution of magnetic short-range which our models are not capable of capturing.

α- and β-Mn theoretically pose a challenge for direct simulations of thermodynamic properties because of the complexity of magnetic and crystal structure. The partitioning of free energy has been used and thermodynamics of these phases have been derived. The obtained results show a good agreement with experimental data suggesting that, despite the complexities of these phases, a rather simple approach can well describe their finite temperature properties. High temperature phases of Mn, γ and δ, are also theoretically challenging problems. Employing a similar approach to Fe, thermophysical properties of these high symmetry phases of Mn have been reported which also show good agreement with available experimental data.

The point defect and metal-self diffusion in titanium carbide (TiC), a refractory material, have been investigated in the present work. The common picture of metal-vacancy exchange mechanism for metal self-diffusion was shown to be unable to explain the experimentally observed values of activation energy. Several new clusters of point defects such as vacancies and interstitials have been found and reported which are energetically lower that a single metal vacancy. In a subsequent study, we showed that some of these clusters can be considered as mediators of metal self-diffusion in TiC.

Evaluation of structural properties of Ti(O,C), a solid solution of TiC and β-TiO, from supercell approach is an extremely difficult task. For a dilute concentration of O, we show the complexity of describing an impurity of O in TiC using supercell approach. A single-site method such as the exact muffin-tin orbital method in the coherent potential approximation (EMTO-CPA) is a good alternative to supercell modeling of Ti(O,C). However, a study of Ti(O,C) using EMTO-CPA requires a further development of the technique regarding the partitioning of space. The shape module of EMTO has been modified for this purpose. With the help of the modified module, Ti(O,C) have been studied using EMTO-CPA. The results for the divacancy concentration and corresponding lattice parameter variations show good agreement with experimental data.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2018. , p. 78
National Category
Condensed Matter Physics
Research subject
Materials Science and Engineering; Physics
Identifiers
URN: urn:nbn:se:kth:diva-223668ISBN: 978-91-7729-687-4 (print)OAI: oai:DiVA.org:kth-223668DiVA, id: diva2:1186245
Public defence
2018-03-26, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
VINNOVA
Note

QC 20180228

Available from: 2018-02-28 Created: 2018-02-27 Last updated: 2018-03-08Bibliographically approved
List of papers
1. Thermophysical properties of paramagnetic Fe from first principles
Open this publication in new window or tab >>Thermophysical properties of paramagnetic Fe from first principles
2017 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 22, article id 224406Article in journal (Refereed) Published
Abstract [en]

A computationally efficient, yet general, free-energy modeling scheme is developed based on first-principles calculations. Finite-temperature disorder associated with the fast (electronic and magnetic) degrees of freedom is directly included in the electronic structure calculations, whereas the vibrational free energy is evaluated by a proposed model that uses elastic constants to calculate average sound velocity of the quasiharmonic Debye model. The proposed scheme is tested by calculating the lattice parameter, heat capacity, and single-crystal elastic constants of alpha-, gamma-, and delta-iron as functions of temperature in the range 1000-1800 K. The calculations accurately reproduce the well-established experimental data on thermal expansion and heat capacity of gamma- and delta-iron. Electronic and magnetic excitations are shown to account for about 20% of the heat capacity for the two phases. Nonphonon contributions to thermal expansion are 12% and 10% for alpha- and delta-Fe and about 30% for gamma-Fe. The elastic properties predicted by the model are in good agreement with those obtained in previous theoretical treatments of paramagnetic phases of iron, as well as with the bulk moduli derived from isothermal compressibility measurements [N. Tsujino et al., Earth Planet. Sci. Lett. 375, 244 (2013)]. Less agreement is found between theoretically calculated and experimentally derived single-crystal elastic constants of gamma- and delta-iron.

Place, publisher, year, edition, pages
American Physical Society, 2017
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-220460 (URN)10.1103/PhysRevB.96.224406 (DOI)000417075100001 ()2-s2.0-85039421203 (Scopus ID)
Funder
VINNOVASwedish National Infrastructure for Computing (SNIC), 2015/16-50
Note

QC 20180103

Available from: 2018-01-03 Created: 2018-01-03 Last updated: 2018-02-27Bibliographically approved
2. Thermodynamic properties of paramagnetic α- and β−Mn from first principles: The effect of transverse spin fluctuations
Open this publication in new window or tab >>Thermodynamic properties of paramagnetic α- and β−Mn from first principles: The effect of transverse spin fluctuations
2017 (English)In: Physical Review Materials, Vol. 1, no 073803Article in journal (Refereed) Published
Abstract [en]

First-principles-based thermodynamic modeling of cubic alpha and beta phases of Mn represent a challenge due to their structural complexity and the necessity of simultaneous treatment of several types of disorder (electronic, magnetic, and vibrational) that have very different characteristic time scales. Here we employ mean-field theoretical models to describe the different types of disorder and then we connect each layer of theory to the others using the adiabatic principle of separating faster and slower degrees of freedom. The slowest (vibrational) degrees of freedom are treated using the Moruzzi, Janak, and Schwarz formalism [Phys. Rev. B 37, 790 (1988)] of the Debye-Gruneisen model parametrized based on the first-principles calculated equation of state which includes the free-energy contributions due to the fast (electronic and magnetic) degrees of freedom via the Fermi-Dirac distribution function and a mean-field theory of transverse spin fluctuations. The magnetic contribution due to transverse spin fluctuations has been computed self-consistently within the disordered local moment picture of the paramagnetic state. The obtained results for thermodynamic properties such as lattice parameter, linear thermal expansion coefficient, and heat capacity of both phases show a good agreement with available experimental data. We also tested the assumption about the nature (localized versus delocalized) of magnetic moment on site IV in alpha-Mn and site I in beta-Mn on the thermodynamic properties of these two phases. Similar to the findings of experimental studies, we conclude that magnetic moment on site IV in alpha-Mn is not of a localized character. However, a similar analysis suggests that the magnetic moment of site I in beta-Mn should be treated as localized.

National Category
Condensed Matter Physics
Research subject
Physics; Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-223663 (URN)10.1103/PhysRevMaterials.1.073803 (DOI)000419032600001 ()2-s2.0-85053854667 (Scopus ID)
Funder
Vinnova, 2012-02892
Note

QC 20180228

Available from: 2018-02-27 Created: 2018-02-27 Last updated: 2019-03-19Bibliographically approved
3. High-temperature thermophysical properties of γ- and δ-Mn from first principles
Open this publication in new window or tab >>High-temperature thermophysical properties of γ- and δ-Mn from first principles
(English)In: Physical Review Materials, ISSN 2475-9953Article in journal (Refereed) Submitted
National Category
Condensed Matter Physics
Research subject
Materials Science and Engineering; Physics
Identifiers
urn:nbn:se:kth:diva-223664 (URN)
Funder
VINNOVA, 2012-02892
Note

QC 20180228

Available from: 2018-02-27 Created: 2018-02-27 Last updated: 2018-02-28Bibliographically approved
4. Role of defects in Ti(O,C)
Open this publication in new window or tab >>Role of defects in Ti(O,C)
(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Research subject
Materials Science and Engineering; Physics
Identifiers
urn:nbn:se:kth:diva-223666 (URN)
Funder
VINNOVA, 2012-02892
Note

QC 20180228

Available from: 2018-02-27 Created: 2018-02-27 Last updated: 2018-02-28Bibliographically approved
5. Structure and energy of point defects in TiC: A system ab intitio study
Open this publication in new window or tab >>Structure and energy of point defects in TiC: A system ab intitio study
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 13, article id 134111Article in journal (Refereed) Published
Abstract [en]

We employ first-principles calculations to study the atomic and electronic structure of various point defects such as vacancies, interstitials, and antisites in the stoichiometric as well as slightly off-stoichiometric Ti-1-C-c(c) (including both C-poor and C-rich compositions, 0.49 <= c <= 0.51). The atomic structure analysis has revealed that both interstitial and antisite defects can exist in split conformations involving dumbbells. To characterize the electronic structure changes caused by a defect, we introduce differential density of states (dDOS) defined as a local perturbation of the density of states (DOS) on the defect site and its surrounding relative to the perfect TiC. This definition allows us to identify the DOS peaks characteristic of the studied defects in several conformations. So far, characteristic defect states have been discussed only in connection with carbon vacancies. Here, in particular, we have identified dDOS peaks of carbon interstitials and dumbbells, which can be used for experimental detection of such defects in TiC. The formation energies of point defects in TiC are derived in the framework of a grand-canonical formalism. Among the considered defects, carbon vacancies and interstitials are shown to have, respectively, the lowest and the second-lowest formation energies. Their formation energetics are consistent with the thermodynamic data on the phase stability of nonstoichiometric TiC. A cluster type of point defect is found to be next in energy, a titanium [100] dumbbell terminated by two carbon vacancies.

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-158031 (URN)10.1103/PhysRevB.91.134111 (DOI)000353448900001 ()2-s2.0-84928781075 (Scopus ID)
Funder
VINNOVA
Note

QC 20150807. Updated from manuscript to article in journal.

Available from: 2014-12-19 Created: 2014-12-19 Last updated: 2018-02-27Bibliographically approved
6. Self-diffusion of Ti interstitial based point defects and complexes in TiC
Open this publication in new window or tab >>Self-diffusion of Ti interstitial based point defects and complexes in TiC
(English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453Article in journal (Refereed) Submitted
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-223667 (URN)10.1016/j.actamat.2018.11.056 (DOI)000457665100034 ()2-s2.0-85058046095 (Scopus ID)
Note

QC 20180228

Available from: 2018-02-27 Created: 2018-02-27 Last updated: 2019-04-24Bibliographically approved

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