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The bain path of paramagnetic Fe-Cr based 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.
Uddeholms AB, Hagfors, Sweden.
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.ORCID iD: 0000-0003-2832-3293
2011 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 110, no 1, 013708- p.Article in journal (Refereed) Published
Abstract [en]

Employing the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, we calculated the total energy and local magnetic moments of paramagnetic Fe-Cr-M (M = Cr, Mn, Fe, Co, Ni) alloys along the tetragonal distortion (Bain) path connecting the body centered cubic (bcc) and the face centered cubic (fcc) structures. The paramagnetic phase is modeled by the disordered local magnetic moment scheme. For all alloys, the local magnetic moments on Fe atoms decrease from the maximum value corresponding to the bcc phase toward the minimum value realized for the fcc phase. Cobalt atoms have non-vanishing local magnetic moments only for tetragonal lattices with c/a < 1.30, whereas the local magnetic moments of Mn show weak crystal structure dependence. We find that Cr stabilizes the bcc lattice and increases the energy barrier as going from the bcc toward the fcc phase. Both Co and Ni favor the fcc lattice and decrease the energy barrier relative to the bcc phase. On the other hand, the tetragonal distortion around the fcc phase is facilitated by Cr and to a somewhat lesser extent also by Ni, but strongly impeded by Co. Manganese has negligible effect on the structural energy difference as well as on the energy barrier along the Bain path. Our findings on the alloying induced softening or hardening of Fe-Cr based alloys against tetragonal distortions are important for understanding the interstitial driven martensitic transformations in alloy steels.

Place, publisher, year, edition, pages
2011. Vol. 110, no 1, 013708- p.
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-31079DOI: 10.1063/1.3603024ISI: 000292776500055Scopus ID: 2-s2.0-79960548131OAI: oai:DiVA.org:kth-31079DiVA: diva2:402404
Note
Updated from submitted to published.Available from: 2011-03-08 Created: 2011-03-08 Last updated: 2017-12-11Bibliographically approved
In thesis
1. Describing interstitials in close-packed lattices: first-principles study
Open this publication in new window or tab >>Describing interstitials in close-packed lattices: first-principles study
2010 (English)Licentiate thesis, comprehensive summary (Other academic)
Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2010. 32 p.
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-31124 (URN)978-91-7415-880-9 (ISBN)
Presentation
2011-03-25, Konferensrummet, Materialvetenskap, KTH, Brinellvägen 23, Stockholm, 10:00
Note
QC 20110309Available from: 2011-03-09 Created: 2011-03-09 Last updated: 2011-03-09Bibliographically approved
2. Describing Interstitials in Close-packed Lattices: First-principles Study
Open this publication in new window or tab >>Describing Interstitials in Close-packed Lattices: First-principles Study
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Describing interstitial atoms in intermetallics or simple mono-atomic close-packed metals is a straightforward procedure in common full-potential calculations. One establishes a sufficiently large supercell, introduces the interstitial impurity and performs the electronic structure and total energy calculation. Real systems, however, are rarely mono-atomic or ordered metals. In most of the cases, the matrix is a random or quasirandom mixture of several chemically and/or magnetically distinct components. Because of that a proper computational tool should incorporate advanced alloy theory and at the same time have sufficiently high accuracy to describe interstitial positions in close-packed solids. The purpose of the present thesis is to make a step towards solving this fundamental problem in computational materials science. To this end, in the first part of the thesis a prestudy on some selected metals and compounds was presented, and in the second part tools were applied to investigate the effect of interstitial carbon on the structural properties of steels.

For the prestudy, the equation of state for the selected Al, Cu and Rh was investigated in two equivalent phases: in conventional face-centered-cubic lattice (fcc, str-I) and in a face-centered-cubic lattice with one atomic and three interstitial empty potentialwells per primitive cell (str-II). A proper basis set of the exact muffin-tin orbitals as well as a proper potential sphere radius were established by calculating the equilibrium Wigner-Seitz radius and bulk modulus of the above elements in str-I and str-II using the exact muffin-tin orbitals (EMTO) first-principle density functional method. It was found that for Al spd orbitals are sufficient to describe the equilibrium bulk properties in both structures, while for str-II Rh and Cu at least five orbitals (spdfg) are needed to get accurate equilibrium volume and bulk modulus. Furthermore, it was shown that in general, for the str-II type of structure (close-packed structure with interstitials) the optimized overlapping muffin-tin potential in combination with spdfg orbitals ensures well converged bulk properties.

As an application of the above work in alloys, (i) the chemical reaction between hydrogen H2 molecule and ScAl1−xMgx (0≤x≤0.3) random alloys, (ii) the phase stability of the hydrogenated alloys in different structures and (iii) the hydrogen absorption/desorption temperatures were studied by calculating the Gibbs energy for the components of the reaction. Experimental and theoretical studies by Sahlberg et al . showed that the ScAl0.8Mg0.2 compound with CsCl structure absorbs hydrogen by decomposing into ScH2 with CaF2 structure and fcc Al0.8Mg0.2. This reaction was found to be very fast, even without adding catalyst, and fully reversible. The theoretical hydrogen absorption/desorption temperatures agree well with the experimental values. On the other hand, the stability field of the hydrogenated alloys was found to be strongly depends on Mg content and on the microstructure of the hydrogenated alloys. For a given microstructure, the critical temperature for hydrogen absorption/desorption increases with the Mg concentration.

The second part of the thesis focused on steel materials with special emphases on the effect of interstitial carbon. Steels are considered to be one of the most important engineering materials. They are mainly composed of iron and carbon. Other alloying elements in steel are introduced to get specific properties like microstructure, corrosion resistance, hardness, brittleness, etc. In order to describe the effect of carbon interstitial in iron alloys, it is important to know how the substitutional alloying elements affect the softness and some other properties of iron alloys. For that reason, the alloying effects on the energetic and magnetic structure of paramagnetic Fe0.85Cr0.1M0.05 (M = Cr, Mn, Fe, Co and Ni) alloys along the tetragonal distortion path connecting the body centered cubic (bcc) and the face centered cubic (fcc) phases were investigated. It was shown that Cr stabilizes bcc phase and increases the energy barrier (relative to bcc phase) between fcc and bcc phases. Cobalt and Ni stabilize fcc structure. Cobalt increases whereas Ni slightly decreases the energy barrier relative to fcc structure. Manganese and iron have negligible effect on the structural energy difference as well as on the energy barrier along the Bain path. The local magnetic moments on Fe atoms have maximum values at bcc phase and minimum values at fcc phase. Cobalt atoms possess local magnetic moments only for tetragonal lattices with c/a < 1.30, and the Mn magnetic moments have almost constant value along the Bain path.

The tetragonality of Fe-C martensite was discovered in 1928. Early experimental works showed that the tetragonality of Fe-C is linearly depends on C content. However, Later many observations indicated that the tetragonality of martensite is influenced also by alloying and interstitial carbon distributions. Very few ab initio studies focus on investigating the tetragonality of Fe-C based alloys. In this thesis the interstitial carbon in ferromagnetic Fe-based alloys and it is impact on the tetragonal lattice ratio of Fe matrix as well as the alloying effect on the tetragonality of Fe-C system were investigated. It was found that the ferromagnetic Fe-C system with C content ∼ 1.3 wt. % has a body-centered tetragonal (bct) structure with c/a ∼ 1.07. Alloying has an impact on the tetragonality; adding 5% Al, Co or Ni enhances while 5% Cr addition decreases the tetragonal lattice ratio.

The electronic structure and total energy calculations from this thesis are based on firstprinciples exact muffin-tin orbitals method. The chemical and magnetic disorder was treated using coherent-potential approximation and the paramagnetic phase was modeled by the disordered local magnetic moments approach. Some test calculations involved also full-potential tools as implemented in Vienna ab-initio simulation package (VASP).

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. viii, 46 p.
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-47733 (URN)978-91-7501-124-0 (ISBN)
Public defence
2011-12-09, Sal K2, Teknikringen 28, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20111118Available from: 2011-11-18 Created: 2011-11-11 Last updated: 2011-11-18Bibliographically approved

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