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Elastic properties and phase stability of shape memory alloys from first-principles theory
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
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: urn:nbn:se:kth:diva-38456ISBN: 978-91-7501-063-2 (print)OAI: oai:DiVA.org:kth-38456DiVA: diva2:436911
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
List of papers
1. First-principles study of the elastic properties of In-Tl random alloys
Open this publication in new window or tab >>First-principles study of the elastic properties of In-Tl random alloys
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2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 9, 094201- p.Article in journal (Refereed) Published
Abstract [en]

The composition-dependent lattice parameters and elastic constants of In1-xTlx(0<x <= 0.4) alloy in face-centered-cubic (fcc) and face-centered-tetragonal (fct) crystallographic phases are calculated by using the first-principles exact muffin-tin orbitals method in combination with coherent-potential approximation. The calculated lattice parameters and elastic constants agree well with the available theoretical and experimental data. For pure In, the fcc phase is mechanically unstable as shown by its negative tetragonal shear modulus C'. With Tl addition, C' of the fcc phase increases whereas that of the fct phase decreases, indicating that the fcc phase becomes mechanically more stable and the fct phase becomes less stable. In addition, the structural energy difference between the fcc and fct phases decreases with x. Both of these effects account for the observed lowering of the fcc-fct martensitic transition temperature upon Tl addition to In. The density of states indicates that the stability of the fct phase relative to the fcc one at low temperatures is due to the particular electronic structure of In and In-Tl alloys.

Keyword
INDIUM-THALLIUM ALLOYS, SHAPE-MEMORY ALLOY, MARTENSITIC-TRANSFORMATION, POTENTIAL MODEL, HIGH-PRESSURE, APPROXIMATION, TEMPERATURE, CONSTANTS, TRANSITION, BEHAVIOR
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-26674 (URN)10.1103/PhysRevB.82.094201 (DOI)000282004500004 ()2-s2.0-77957606575 (Scopus ID)
Note
QC 20101130Available from: 2010-11-30 Created: 2010-11-26 Last updated: 2017-12-12Bibliographically approved
2. Site occupancy, magnetic moments, and elastic constants of off-stoichiometric Ni2MnGa from first-principles calculations
Open this publication in new window or tab >>Site occupancy, magnetic moments, and elastic constants of off-stoichiometric Ni2MnGa from first-principles calculations
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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.

Keyword
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:nbn:se:kth:diva-18410 (URN)10.1103/PhysRevB.79.144112 (DOI)000265943200036 ()2-s2.0-65649134421 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
3. Magnetoelastic effects in Ni2Mn1+xGa1-x alloys from first-principles calculations
Open this publication in new window or tab >>Magnetoelastic effects in Ni2Mn1+xGa1-x alloys from first-principles calculations
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2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 6, 064108-1-064108-5 p.Article in journal (Refereed) Published
Abstract [en]

The magnetic coupling between Mn atoms on Ga sublattice (Mn-Ga) and Mn atoms on Mn sublattice (Mn-Mn) in Ni2Mn1+xGa1-x alloy and its effect on the elastic modulus of the alloy are investigated by the use of first-principles methods. It is shown that, for x = 0.25, the state with antiparallel Mn-Ga-Mn-Mn magnetic coupling is slightly more stable than that with parallel coupling, whereas for x = 0.10, both magnetic states are almost degenerated. For both antiparallel and parallel Mn-Ga-Mn-Mn magnetic couplings, the bulk modulus (B) of Ni2Mn1+xGa1-x deviates from the general e/a-B relationship with e/a being the number of valence electrons per atom. The shear modulus C' versus the martensitic transformation temperature T-M for Ni2Mn1+xGa1-x with antiparallel Mn-Ga-Mn-Mn magnetic coupling is in line with the general C'-T-M relationship for Ni2MnGa-based alloys, in contrast to the case of parallel Mn-Ga-Mn-Mn magnetic coupling.

Keyword
shape-memory alloys, electronic-structure, martensitic phase, potential, model, heusler alloys, ni2mnga, transformation, approximation, strain
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-19262 (URN)10.1103/PhysRevB.81.064108 (DOI)000274998100037 ()2-s2.0-77954823239 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
4. First-principles investigation of the composition dependent properties of Ni2+xMn1-xGa shape-memory alloys
Open this publication in new window or tab >>First-principles investigation of the composition dependent properties of Ni2+xMn1-xGa shape-memory alloys
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2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 2, 024201-1-024201-9 p.Article in journal (Refereed) Published
Abstract [en]

The composition dependent lattice parameter, phase stability, elastic moduli, and magnetic transition temperature of the Ni2+xMn1-xGa shape-memory alloys are studied by using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation. The lattice parameter and tetragonal shear modulus of the cubic L-21 austenite phase decreases linearly with increasing concentration x of excess Ni atoms. The heats of formation of both cubic L-21 and tetragonal beta''' phases and their difference increase with x, indicating decreasing stability of the cubic and tetragonal phases and increasing driving force for the L-21 to beta''' martensitic transition. Investigating the electronic density of states, we find that the Ni-induced decreasing phase stability can mainly be ascribed to the weakening of the covalent bonding between minority spin states of Ni and Ga. Using the computed parameters, the composition dependence of the martensitic transition temperature is discussed. The theoretical Curie temperature, estimated from the Heisenberg model in combination with the mean-field approximation, is larger for the beta''' phase than for the L-21 phase. For both phases, the Curie temperature decreases nearly linearly with increasing x.

Keyword
NI-MN-GA, EXCHANGE INTERACTIONS, FERROMAGNETIC METALS, PHASE-TRANSFORMATION, ELECTRONIC-STRUCTURE, MARTENSITIC PHASE, POTENTIAL MODEL, NI2MNGA, APPROXIMATION, TEMPERATURES
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-28609 (URN)10.1103/PhysRevB.82.024201 (DOI)000279600400001 ()2-s2.0-77956504751 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20110117Available from: 2011-01-17 Created: 2011-01-17 Last updated: 2017-12-11Bibliographically approved
5. Interplay between temperature and composition effects on the martensitic transformation in Ni(2+x)Mn(1-x)Ga alloys
Open this publication in new window or tab >>Interplay between temperature and composition effects on the martensitic transformation in Ni(2+x)Mn(1-x)Ga alloys
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2011 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 98, no 26, 261903- p.Article in journal (Refereed) Published
Abstract [en]

Martensitic transformation in Ni(2+x)Mn(1-x)Ga alloys is known to be controlled by the soft tetragonal elastic constant C' of the high-temperature austenitic phase. The temperature (T) and composition (x) dependence of C'(T, x) are calculated using the first-principles exact muffin-tin orbitals method. We show that the temperature factor of C' is dominated by the phonon-smearing term. The competition between the negative alloying effect (partial derivative C'/partial derivative x < 0) and the positive temperature effect (partial derivative C'/partial derivative T > 0) is found to lead to nearly constant C'(T(M)(x), x) at the critical temperature TM(x). We demonstrate that a proper account of the temperature and composition dependence of C'(T, x) is indispensable for reasonable theoretical TM(x) values.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-37167 (URN)10.1063/1.3603935 (DOI)000292335700013 ()2-s2.0-79960090172 (Scopus ID)
Available from: 2011-08-02 Created: 2011-08-02 Last updated: 2017-12-08Bibliographically approved
6. Temperature dependence of elastic properties of Ni(2+x)Mn(1-x)Ga and Ni(2)Mn(Ga(1-x)Al(x)) from first principles
Open this publication in new window or tab >>Temperature dependence of elastic properties of Ni(2+x)Mn(1-x)Ga and Ni(2)Mn(Ga(1-x)Al(x)) from first principles
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2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 17, 174117- p.Article in journal (Refereed) Published
Abstract [en]

The temperature dependence of the elastic properties of Ni(2+x)Mn(1-x)Ga and Ni(2)Mn(Ga(1-x)Al(x)) (x = 0.0, 0.1, and 0.2) random alloys are investigated by using the first-principles exact muffin-tin orbitals method. At 0 K, the calculated equilibrium parameters in both cubic L2(1) and nonmodulated tetragonal beta'''-Ni(2)MnGa are in good agreement with the available experimental data and other theoretical results. Separating the thermal effects into single electron excitation, volume expansion, phonon smearing, and magnetic terms, we find that phonon smearing gives the dominating positive tetragonal elastic constant (C') versus temperature (T) slope for the cubic phase. For Ni(2+x)Mn(1-x)Ga, the competition between the negative alloying effect (partial derivative C'/partial derivative x < 0) and the positive temperature effect (partial derivative C'/partial derivative T > 0) leads to nearly constant C'[x, T(M)(x)] at the martensitic transition temperature T(M)(x). For Ni(2)Mn(Ga(1-x)Al(x)), where both partial derivative C'/partial derivative x and partial derivative C'/partial derivative T are positive, however, due to the significantly decrease of T(M)(x), the critical C'[x, T(M)(x)] slightly decreases with Al doping. Furthermore, it is demonstrated that both the composition and the temperature dependence of C' are indispensable to get a reasonable theoretical T(M)(x).

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-38591 (URN)10.1103/PhysRevB.84.174117 (DOI)000297294500004 ()2-s2.0-82455217332 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20111228Available from: 2011-08-29 Created: 2011-08-29 Last updated: 2017-12-08Bibliographically approved
7. Site preference and elastic properties of Fe-, Co-, and Cu- doped Ni(2)MnGa shape memory alloys from first principles
Open this publication in new window or tab >>Site preference and elastic properties of Fe-, Co-, and Cu- doped Ni(2)MnGa shape memory alloys from first principles
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2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 2, 024206- p.Article in journal (Refereed) Published
Abstract [en]

The site preference and elastic properties of Fe-, Co-, and Cu-doped Ni(2)MnGa alloys are investigated by using the first-principles exact muffin-tin orbital method in combination with coherent-potential approximation. It is shown that 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 alloys with stable site occupations, both the electron density n and the shear modulus C' can be considered as predictors of the composition dependence of the martensitic transition temperature T(M) of the alloys. The physics underlying the composition-dependent C' are discussed based on the calculated density of states.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-37155 (URN)10.1103/PhysRevB.84.024206 (DOI)000292599600002 ()2-s2.0-79961179970 (Scopus ID)
Available from: 2011-08-03 Created: 2011-08-02 Last updated: 2017-12-08Bibliographically approved

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