Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Site occupancy, magnetic moments, and elastic constants of off-stoichiometric Ni2MnGa from first-principles calculations
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.
Institute of Strength Physics and Materials Science, Siberian Branch of Russian Academy of Science, Russia.
Show others and affiliations
2009 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 14Article in journal (Refereed) Published
Abstract [en]

The site occupancy and elastic modulus of off-stoichiometric Ni2MnGa alloys are investigated by the use of the first-principles exact muffin-tin orbital method in combination with coherent-potential approximation. The stable site occupancy at 300 K is determined by comparing the free energies of the alloys with different site-occupation configurations. It is shown that, for most of the off-stoichiometric Ni2MnGa, the "normal" site occupation is favorable, i.e., the excess atoms of the rich component occupy the sublattice(s) of the deficient one(s). Nevertheless, for the Ga-rich alloys, the excess Ga atoms have strong tendency to take the Mn sublattice no matter if Mn is deficient or not. Based on the determined site occupancy, the elastic moduli of the off-stoichiometric Ni2MnGa are calculated. We find that, in general, the bulk modulus increases with increasing e/a ratio (i.e., the number of valence electrons per atom). The shear moduli C-' and C-44 change oppositely with e/a ratio: C-' decreases but C-44 increases with increasing e/a. However, the Mn-rich Ga-deficient alloys deviate significantly from this general trend. The correlation of calculated elastic moduli and available experimental martensitic transformation temperatures (T-M) demonstrates that the alloy with larger C-' than that of the perfect Ni2MnGa generally possesses lower T-M except for Ni2Mn1+xGa1-x.

Place, publisher, year, edition, pages
2009. Vol. 79, no 14
Keyword [en]
density functional theory, elastic constants, elastic moduli, ferromagnetic materials, gallium alloys, linear muffin-tin orbital, method, magnetic moments, magnetomechanical effects, manganese alloys, martensitic transformations, nickel alloys, shape memory effects, shear, modulus, stoichiometry, shape-memory alloys, ni-mn-ga, martensitic-transformation, phase-transformations, potential model, heusler alloys, landau theory, transition, metals, approximation
Identifiers
URN: urn:nbn:se:kth:diva-18410DOI: 10.1103/PhysRevB.79.144112ISI: 000265943200036Scopus ID: 2-s2.0-65649134421OAI: oai:DiVA.org:kth-18410DiVA: diva2:336457
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Elastic properties and phase stability of shape memory alloys from first-principles theory
Open this publication in new window or tab >>Elastic properties and phase stability of shape memory alloys from first-principles theory
2010 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

 

Ni-Mn-Ga and In-Tl are two examples of shape memory alloys. Their shape memory effect is controlled by the martensitic transformation from high temperature face-centered-cubic (fcc) phase to the low temperature face-centered-tetragonal (fct) phase. Experimentally, it was found that the martensitic transformation is related to the elastic properties. In order to better understand the phase transition and facilitate the design of new materials with improved shape memory properties, the atomic scale description of the thermophysical properties of these alloys is needed. In the present thesis, the elastic properties and phase stability of Ni-Mn-Ga and In-Tl shape memory alloys are investigated by the use of first-principles exact muffin-tin orbital method in combination with coherent-potential approximation.

It is shown that the theoretical lattice parameters and elastic constants of stoichiometric Ni2MnGa and pure In agree well with the available theoretical and experimental data, indicating that the employed theoretical approach is suitable to study the elastic properties of both cubic and tetragonal crystals. For most of the off-stoichiometric Ni2MnGa, the excess atoms of the rich component prefer to occupy the sublattice of the deficient one, except for the Ga-rich alloys, where the excess Ga atoms have strong tendency to take the Mn sublattice irrespective of the Mn occupation. With increasing e/a ratio (the number of valence electrons per atom), it is found that the theoretical bulk modulus B and the shear constant C44 increase but the tetragonal elastic constant C′ decreases. Except for Mn-rich Ga-deficient alloys, C′ is generally inversely proportional and the energy difference between parent and martensitic phases is directly proportional to the martensitic tansformation temperature TM. For In1-xTlx alloys, the tetragonal lattice parameter c/a and the shear modulus C′ in the fct phase and the total energy difference between the fcc and fct phases decrease with Tl addition, whereas the negative C′ of the fcc phase increases with x turning positive around x=0.35. All of these composition dependent thermophysical properties can be understood by investigating the electronic structure of In and In-Tl alloys and they are in line with the experimentally observed lowering of TM with addition of Tl.

Place, publisher, year, edition, pages
Stockholm: US-AB, 2010. 46 p.
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-28611 (URN)978-91-7515-755-0 (ISBN)
Opponent
Supervisors
Note
QC 20110117Available from: 2011-01-17 Created: 2011-01-17 Last updated: 2011-01-17Bibliographically approved
2. Elastic properties and phase stability of shape memory alloys from first-principles theory
Open this publication in new window or tab >>Elastic properties and phase stability of shape memory alloys from first-principles theory
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Ni-Mn-Ga and In-Tl are two examples of shape memory alloys. Their shape memory effect is controlled by the martensitic transformation from the high temperature cubic phase to the low temperature tetragonal phase. Experimentally, it was found that the martensitic transformation, related to the elastic properties, is highly composition-dependent.In order to better understand the phase transition and facilitate the design of new materials with improved shape memory properties, the atomic scale description of the thermophysical properties of these alloys is needed. Therefore, in the presen tthesis, the elastic properties and phase stability of Ni-Mn-Ga and In-Tl shape memory alloys are investigated by the use of first-principles exact muffin-tin orbitals method in combination with the coherent-potential approximation.

We present a theoretical description of the equilibrium properties of pure In and standard stoichiometric Ni2MnGa alloy with both cubic and tetragonal structures. In In-Tl alloys, all the calculated composition-dependent thermophysical properties: lattice parameter c/a, tetragonal shear modulus C" = (C11 - C12)/2, energy difference between the austenitic and martensitic phases, as well as electronic structures are shown to be in line with the experimentally observed lowering of the martensitic transition temperature TM with the addition of Tl. For most of the off-stoichiometric Ni2MnGa, the excess atoms of the rich component prefer to occupy the sublattice of the deficient one, except for the Ga-rich alloys, where the excess Ga atoms have strong tendency to take the Mn sublattice irrespective of the Mn occupation. In Ni-Mn-Ga-X (X=Fe, Co, and Cu) quarternary alloys, Fe atom prefers to occupy the Mn and Ni sublattices even in Ga-deficient alloys; Co has strong tendency to occupy the Ni-sublattice in all types of alloys; Cu atoms always occupy the sublattice of the host elements in deficiency. For most of the studied Ni-Mn-Ga and Ni-Mn-Ga-X alloys with stable site-occupations, the shear modulus C" can be considered as a predictor of the composition dependence of TM of the alloys: the alloy with larger C" than that of the perfect Ni2MnGa generally possesses lower TM except for Ni2Mn1+xGa1-x and Ni2Mn1-xGaFex. The failure of C" as a factor of TM in these two types of alloys may be ascribed that the compositiondependentmagnetic interactions and the temperature-dependent C0, which also playan important role on the martensitic transformation in these alloys. Furthermore, wedemonstrate that a proper account of the temperature and composition dependence ofC0 gives us reasonable theoretical TM(x) values in Ni2+xMn1-xGa alloys. Also in this type of Ni-rich and Mn-deficient alloys, by using the Heisenberg model in combination with the mean-field approximation, the abnormal trend of experimental magnetic transition temperature TC(x) with respect to the composition x is shown to be well captured by the theory.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. ix, 60 p.
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-38456 (URN)978-91-7501-063-2 (ISBN)
Public defence
2011-09-16, Sal B2, Brinellvägen 23, KTH, Stockholm, 14:00 (English)
Opponent
Supervisors
Note
QC 20110830Available from: 2011-08-30 Created: 2011-08-25 Last updated: 2012-03-21Bibliographically approved

Open Access in DiVA

No full text

Other links

Publisher's full textScopus

Authority records BETA

Vitos, Levente

Search in DiVA

By author/editor
Hu, Qing-MiaoLi, Chun-MeiYang, RuiJohansson, BörjeVitos, Levente
By organisation
Applied Material Physics
In the same journal
Physical Review B. Condensed Matter and Materials Physics

Search outside of DiVA

GoogleGoogle Scholar

doi
urn-nbn

Altmetric score

doi
urn-nbn
Total: 97 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf