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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.
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: urn:nbn:se:kth:diva-28611ISBN: 978-91-7515-755-0 OAI: oai:DiVA.org:kth-28611DiVA: diva2:388290
Opponent
Supervisors
Note
QC 20110117Available from: 2011-01-17 Created: 2011-01-17 Last updated: 2011-01-17Bibliographically approved
List of papers
1. 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
2. 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
3. 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
4. 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

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