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Atomistic simulation and experimental studies of transition metal systems involving carbon and nitrogen
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The present work was initiated to investigate the stability, structural and thermodynamic properties of transition metal carbides, nitrides and carbo-nitrides by atomistic simulations and experimentations.

The interatomic pair potentials of Cr-Cr, Mn-Mn, Fe-Fe, C-C, Cr-C, Mn-C, Fe-C, Cr-Fe, Cr-N and Mn-N were inverted by the lattice inversion method and ab initio cohesive energies, and then employed to investigate the properties of Cr-, Mn- and Fe-carbides by atomistic simulations in this work.

For the binary M7C3 carbide, the structural properties of M7C3 (M = Cr, Mn, Fe) were investigated by atomistic simulations. The results show that the stable structure for these compounds is hexagonal structure with P63mc space group. The cohesive energy of M7C3 calculated in this work indicates that the stability of carbides decreases with the increasing in metal atomic number. Further, the vibrational entropy of Cr7C3 was calculated at different temperatures and compared with the entropy obtained by experimentations. The comparison demonstrates that the main contribution to the entropy is made by the vibrational entropy.

For the binary τ-carbides, the structural properties of Cr23C6 and Mn23C6, as well as the vibrational entropy of Cr23C6 were computed. Further, the site preference of ternary element Fe among 4a, 8c, 32f and 48h symmetry sites in Cr23-xFexC6 was studied. It has been seen that Fe atoms would firstly occupy 4a sites and then 8c sites. The lattice constant and stability of Cr23-xFexC6 were also computed with different Fe content.

In order to understand the relative stability of the transition metal carbides and nitrides, the standard formation Gibbs energies of carbides and nitrides for Cr, Mn and Fe were compared. The order of carbon and nitrogen affinities for Cr, Mn and Fe was further clarified by the comparison of the interatomic pair potentials among Cr-C, Mn-C, Fe-C, Cr-N and Mn-N. It was found that Cr-N interaction was very strong in comparison with other binary interactions above and consequently, nitrogen addition would lead to a strong decrease in the thermodynamic activity of chromium in Cr-containing alloys. This was confirmed by the investigations of thermodynamic activities of Cr in the Fe-Cr-N and Fe-Cr-C-N alloys.

The activities were measured in the temperature range 973-1173 K by solid-state galvanic cell method involving CaF2 solid electrolyte under the purified N2 gas. In addition, the analysis of nitrogen content and phase relationships in the Fe-Cr-N and Fe-Cr-C-N alloys equilibrated at 1173 K were carried out by inert-gas fusion thermal conductivity method, X-ray diffraction and scanning electron microscopy technique. The experimental results show that the solubility of nitrogen in the alloys decreases with the decreasing chromium content, as well as the increasing temperature. The addition of nitrogen to the alloys was found to have a strong negative impact on the Cr activity in Fe-Cr-N and Fe-Cr-C-N systems.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , vii, 40 p.
Keyword [en]
Transition metal carbides, Transition metal nitrides, Transition metal carbo-nitrides, Lattice inversion method, Interatomic potential, Atomistic simulation
National Category
Metallurgy and Metallic Materials
Identifiers
URN: urn:nbn:se:kth:diva-4228ISBN: 91-7178-487-X (print)OAI: oai:DiVA.org:kth-4228DiVA: diva2:11345
Public defence
2006-12-15, Salongen, KTHB, Osquars backe 31, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100929Available from: 2006-12-12 Created: 2006-12-12 Last updated: 2010-09-29Bibliographically approved
List of papers
1. Atomistic study on the structure and thermodynamic properties of Cr7C3, Mn7C3, Fe7C3
Open this publication in new window or tab >>Atomistic study on the structure and thermodynamic properties of Cr7C3, Mn7C3, Fe7C3
Show others...
2005 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 53, no 9, 2727-2732 p.Article in journal (Refereed) Published
Abstract [en]

The crystal structures and stabilities of Cr7C3, Mn7C3, Fe7C3 have been investigated using the interatomic potentials obtained by the lattice inversion method. The calculated structures of Cr7C3, Mn7C3 and Fe7C3 are proposed to be hexagonal with P6(3)mc space group and the calculated lattice constants are in basic agreement with the experimental data. The calculated cohesive energies indicate that the increase in the atomic number of the metal is accompanied by the decrease in the stability of its carbides. The phonon density of states and vibrational entropy related to dynamic phenomena are also evaluated. This work provides a new method for studying the properties of carbides with complex structure.

Keyword
carbides, lattice inversion, crystal structure, dynamic phenomena, thermodynamics
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6588 (URN)10.1016/j.actamat.2005.02.039 (DOI)000229329300020 ()2-s2.0-17644423456 (Scopus ID)
Note
QC 20100928Available from: 2006-12-12 Created: 2006-12-12 Last updated: 2017-12-14Bibliographically approved
2. Atomistic simulation on the structural properties and phase stability for Cr23C6 and Mn23C6
Open this publication in new window or tab >>Atomistic simulation on the structural properties and phase stability for Cr23C6 and Mn23C6
2006 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 420, no 1-2, 269-272 p.Article in journal (Refereed) Published
Abstract [en]

Based on the successful applications of lattice inversion method in many fields, the crystal structure and phase stability of Cr23C6 and Mn23C6 are investigated using the interatomic potentials obtained by lattice inversion method in this research. The calculated atomic coordinates and crystal parameters are in good agreement with the experimental results and the phase stability of Cr23C6 and Mn23C6 are tested by random atom shifts and global deformations. The calculated energy also indicates that the Cr23C6 is more stable than Mn23C6.

Keyword
tau-carbides, lattice inversion, structural properties, phase stability
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6589 (URN)10.1016/j.jallcom.2005.10.027 (DOI)000239299300047 ()2-s2.0-33745610461 (Scopus ID)
Note
QC 20100928Available from: 2006-12-12 Created: 2006-12-12 Last updated: 2017-12-14Bibliographically approved
3. Atomistic study on the site preference and thermodynamic properties for Cr23-xFexC6
Open this publication in new window or tab >>Atomistic study on the site preference and thermodynamic properties for Cr23-xFexC6
2005 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 53, no 20, 5305-5312 p.Article in journal (Refereed) Published
Abstract [en]

The site preference of Fe in Cr23-xFexC6 is investigated based on the interatomic potentials obtained by the lattice inversion method. The calculated results show that Fe atoms preferentially Substitute For Cr at 4a sites first and then 8c sites. The structural parameters of Cr23-xFexC6 With content x are calculated and the results are consistent with experimental results. The calculated cohesive energies indicate that the increase in x value is accompanied by the decrease in the stability of Cr23-xFexC6. The thermodynamic properties of Cr23C6, such as the phonon density of states and vibrational entropy, as well as the bulk modulus of Cr23-xFexC6 are evaluated. The calculated results are in good agreement with experimental results. This work provides a simple and promising method for Studying the properties of carbides with complex structures

Keyword
carbides, lattice inversion, crystal structure, site preference, thermodynamic properties
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6590 (URN)10.1016/j.actamat.2005.07.039 (DOI)000235919800005 ()2-s2.0-27144533138 (Scopus ID)
Note
QC 20100929Available from: 2006-12-12 Created: 2006-12-12 Last updated: 2017-12-14Bibliographically approved
4. Thermodynamic investigations of the Cr-Fe-N system with solid-state galvanic cell method
Open this publication in new window or tab >>Thermodynamic investigations of the Cr-Fe-N system with solid-state galvanic cell method
(English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Metallurgical and materials transaction AArticle in journal (Refereed) In press
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6591 (URN)
Note

QC 20100928

Available from: 2006-12-12 Created: 2006-12-12 Last updated: 2017-12-14Bibliographically approved
5. Thermodynamic studies of the Fe-Cr-C-N system by EMF measurements
Open this publication in new window or tab >>Thermodynamic studies of the Fe-Cr-C-N system by EMF measurements
2007 (English)In: Metallurgical and materials transactions. B, process metallurgy and materials processing science, ISSN 1073-5615, E-ISSN 1543-1916, Metallurgical and materials transaction B, Vol. 38, no 1, 93-100 p.Article in journal (Refereed) Published
Abstract [en]

In this work, the Fe-Cr-C-N alloys were synthesized by nitriding the Fe, Cr, and C powder mixtures at 1573 K in the N-2 gas (101 325 Pa). The nitrogen content and phase relationships at 1173 K in the alloys were investigated by the use of an equilibration technique. The thermodynamic activities of chromium in the alloys were studied using the solid-state galvanic cell method with CaF2 as the solid electrolyte in the temperature range 973 to 1173 K in an atmosphere of N-2 gas (101 325 Pa). The activities of chromium in the Fe-Cr-C-N alloys were calculated and compared with those of the corresponding Fe-Cr-C ternary alloys with pure bcc-Cr as standard state. X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods were used to identify the equilibrium phases and microstructures of the investigated alloys. The experimental results show that a Cr2N-based nitride was formed during the nitriding procedure in the alloys. The nitrogen content in the alloys decreases with the decreasing chromium content, as well as the increasing temperature. The addition of nitrogen to the ternary Fe-Cr-C alloy was found to have a strong negative impact on the Cr activity in the Fe-Cr-C-N system.

National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-6592 (URN)10.1007/s11663-007-9019-4 (DOI)000246113900010 ()2-s2.0-34247554322 (Scopus ID)
Note
QC 20100929Available from: 2006-12-12 Created: 2006-12-12 Last updated: 2017-12-14Bibliographically approved

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