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Ab initio prediction of the mechanical properties of alloys
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
##### Abstract [en]

At the time of the 50th anniversary of the Kohn-Sham method, ab initio calculations based on density functional theory have formed an accurate, efficient, and reliable method to work on the properties of engineering materials. In this thesis, we use the exact muffin-tin orbitals method combined with the coherent-potential approximation to study the mechanical properties of high-technology materials. The thesis includes two parts: a study of long-range chemical order effects and a study of alloying effects on the mechanical properties of alloys. In the first part, we concentrate on the impact of chemical ordering on the mechanical properties. The long range order effect on the elastic constants behaves in a very different way for non-magnetic materials and ferromagnetic materials. For a non-magnetic Cu3Au, the long-range order effect on the elastic constants is very small. The Debye temperature does not show a strong chemical order dependence either. For a ferromagnetic material, on the other hand, the long-range chemical order produces considerable influence on C' in the ferromagnetic state, but negligible effect on C' in the paramagnetic state. The lattice parameter, bulk modulus $B$, and shear elastic constant C44 change slightly with the degree of long-rang order for both magnetic states. The Young's modulus E and the shear modulus G increase significantly with the degree of order in the ferromagnetic state, but the effect becomes weak as the system approaches the random regime.In the second part, the alloying effect of Mn/Ni on the lattice parameter, elastic constants, surface energy, and unstable stacking fault energy of bcc Fe is examined. The calculated results show that the lattice parameter of ferrite Fe is slightly altered upon Ni/Mn alloying the trend of which can be explained by the magnetism-induced pressure. Nickel addition decreases C' but has a negligible effect on C44, whereas manganese addition increases C44 and has a weak influence on C'. In both systems, the bulk modulus B shows a smooth second order variation. On the other hand, the surface energy and the unstable stacking fault (USF) energy decrease by adding Mn or Ni to Fe. For both planar fault energies, Ni shows a stronger effect than Mn. Segregation seems to have a minor effect on the surface and USF energies for dilute Fe-Ni and Fe-Mn alloys. The ductility can be estimated using available physical parameters via traditional phenomenological criteria like the Pugh ratio B/G, the Poisson ratio ν, the Cauchy pressure C12-C44, and the Rice ratio γsu .According to dislocation theory, a dislocation can not cross a grain boundary. Therefore, the study of dislocations is assumed to be limited to single-crystals. Several theoretical studies indicate that the cleavage plane is {001} in bcc crystals. Based on these information, we suggest that the resolved single-crystal tensile strength E[001] and the resolved single crystal shear strength G{110}<111> should be used to describe brittle cleavage and dislocation movement rather than polycrystalline parameters such as B and G. We demonstrate that all shear moduli G{lmn}<111> associated with the <111> Burgers vector take the same value 3C44C'/(C'+2C44), which could in fact explain the observed multiple slip in bcc systems. Due to the discrepancy between the resolved single-crystal elastic constants and the averaged polycrystalline elastic constants, the Pugh ratio B/G and the traditional criteria based on polycrystalline elastic constants lead to large differences for magnetic systems. Finally, we propose a new measure of the ductile-bittle behavior based on the ratio σclevage/Gresolved which gives the right experimental trend for Fe-Mn and Fe-Ni system.

##### Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , xii, 67 p.
##### National Category
Condensed Matter Physics
##### Research subject
Industrial Engineering and Management
##### Identifiers
ISBN: 978-91-7595-636-7OAI: oai:DiVA.org:kth-169511DiVA: diva2:821751
##### Public defence
2015-08-26, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
##### Note

QC 20150616

Available from: 2015-06-16 Created: 2015-06-15 Last updated: 2015-06-16Bibliographically approved
##### List of papers
1. The effect of long-range order on the elastic properties of Cu3Au
Open this publication in new window or tab >>The effect of long-range order on the elastic properties of Cu3Au
2013 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 8, 085401- p.Article in journal (Refereed) Published
##### Abstract [en]

Ab initio calculations, based on the exact muffin-tin orbitals method are used to determine the elastic properties of Cu-Au alloys with Au/Cu ratio 1/3. The compositional disorder is treated within the coherent potential approximation. The lattice parameters and single-crystal elastic constants are calculated for different partially ordered structures ranging from the fully ordered L1(2) to the random face centered cubic lattice. It is shown that the theoretical elastic constants follow a clear trend with the degree of chemical order: namely, C-11 and C-12 decrease, whereas C-44 remains nearly constant with increasing disorder. The present results are in line with the experimental findings that the impact of the chemical ordering on the fundamental elastic parameters is close to the resolution of the available experimental and theoretical tools.

##### Keyword
Coherent-Potential Approximation, Au-Cu Alloys, Mg Alloys, Constants, Energy, Metals, Gold, Transition, Pressure, Model
##### National Category
Other Physics Topics
##### Identifiers
urn:nbn:se:kth:diva-119452 (URN)10.1088/0953-8984/25/8/085401 (DOI)000314780400009 ()23360774 (PubMedID)2-s2.0-84873301825 (ScopusID)
##### Funder
Swedish Research CouncilEU, European Research Council, 228074
##### Note

QC 20130315

Available from: 2013-03-15 Created: 2013-03-14 Last updated: 2015-06-16Bibliographically approved
2. Ab initio investigation of the elastic properties of Ni3Fe
Open this publication in new window or tab >>Ab initio investigation of the elastic properties of Ni3Fe
2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 17, 174205- p.Article in journal (Refereed) Published
##### Abstract [en]

Ab initio alloy theory, formulated within the exact muffin-tin orbitals method in combination with the coherent-potential approximation, is used to determine the elastic properties of Ni-Fe alloys with Fe:Ni ratio 1:3. The interplay between magnetic and chemical effects is investigated by computing the lattice parameters and the single-and polycrystal elastic moduli for different partially ordered structures in the ferro-and paramagnetic states. It is found that the influence of long-range chemical order on the bulk properties strongly depends on the magnetic state. The largest magnetic-order-induced changes are obtained for the chemically ordered L1(2) phase. The ferromagnetic L1(2) system possesses similar to 5.4% larger elastic Debye temperature than the paramagnetic L1(2) phase, which in turn has a similar Theta(D) as the chemically disordered face-centered cubic phase in either the ferro-or paramagnetic state. It is concluded that magnetic ordering has a substantially larger impact on the bulk parameters of Ni3Fe than chemical ordering. The calculated trends are explained based on the electronic structure of nonmagnetic, ferromagnetic, and paramagnetic ordered and disordered phases.

##### Keyword
Single-Crystal Alloys, Coherent-Potential Approximation, Order-Disorder, Stiffness Coefficients, Configurational Order, Room-Temperature, Atomic Volume, Phase, Stability, Constants
##### National Category
Other Physics Topics
##### Identifiers
urn:nbn:se:kth:diva-139203 (URN)10.1103/PhysRevB.88.174205 (DOI)000327933500003 ()2-s2.0-84890386334 (ScopusID)
##### Funder
Swedish Research CouncilEU, European Research Council
##### Note

QC 20140113

Available from: 2014-01-13 Created: 2014-01-08 Last updated: 2015-06-16Bibliographically approved
3. Large magneto-chemical-elastic coupling in highly magnetostrictive Fe-Ga alloys
Open this publication in new window or tab >>Large magneto-chemical-elastic coupling in highly magnetostrictive Fe-Ga alloys
2013 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, no 23, 231903- p.Article in journal (Refereed) Published
##### Abstract [en]

The strong softening of the tetragonal shear elastic constant (C') is the main reason for the second magnetostriction peak observed in Fe100-xGax alloys. Here, we study the coupling between chemical order and magnetism with the aim to understand the origin of the elastic softening. We demonstrate that C' strongly depends on the degree of order of Ga atoms in alpha-Fe. The B-2 type ordering proves to have an important role on the elastic softening for x < 19%, whereas the extreme shear lattice softening and the anomalous temperature dependence of C' are found to be due to the strong magnetochemical coupling in the DO3 phase.

##### Keyword
Chemical order, Degree of order, Elastic softening, Fe-Ga alloy, Shear elastic constants, Temperature dependence
##### National Category
Other Physics Topics
##### Identifiers
urn:nbn:se:kth:diva-140150 (URN)10.1063/1.4838657 (DOI)000328634900024 ()2-s2.0-84889823587 (ScopusID)
##### Funder
Swedish Research CouncilEU, European Research Council, 228074
##### Note

QC 20140120

Available from: 2014-01-20 Created: 2014-01-17 Last updated: 2015-06-16Bibliographically approved
4. Ab initio prediction of the mechanical properties of alloys: The case of Ni/Mn-doped ferromagnetic Fe
Open this publication in new window or tab >>Ab initio prediction of the mechanical properties of alloys: The case of Ni/Mn-doped ferromagnetic Fe
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 22, 224203- p.Article in journal (Refereed) Published
##### Abstract [en]

First-principles alloy theory, formulated within the exact muffin-tin orbitals method in combination with the coherent-potential approximation, is used to study the mechanical properties of ferromagnetic body-centered cubic (bcc) Fe1−xMx alloys (M=Mn or Ni, 0≤x≤0.1). We consider several physical parameters accessible from \emph{ab initio} calculations and their combinations in various phenomenological models to compare the effect of Mn and Ni on the properties of Fe. Alloying is found to slightly alter the lattice parameters and produce noticeable influence on elastic moduli. Both Mn and Ni decrease the surface energy and the unstable stacking fault energy associated with the {110} surface facet and the {110}⟨111⟩ slip system, respectively. Nickel is found to produce larger effect on the planar fault energies than Mn. The semi-empirical ductility criteria by Rice and Pugh consistently predict that Ni enhances the ductility of Fe but give contradictory results in the case of Mn doping. The origin of the discrepancy between the two criteria is discussed and an alternative measure of the ductile-brittle behavior based on the theoretical cleavage strength and single-crystal shear modulus G{110}⟨111⟩ is proposed.

##### National Category
Condensed Matter Physics
##### Identifiers
urn:nbn:se:kth:diva-169494 (URN)10.1103/PhysRevB.91.224203 (DOI)000356404800003 ()
##### Note

QP 201506

QC 20160617

Available from: 2015-06-15 Created: 2015-06-15 Last updated: 2016-06-17Bibliographically approved
5. Theory of unstable stacking fault energy of transition metals
Open this publication in new window or tab >>Theory of unstable stacking fault energy of transition metals
##### National Category
Condensed Matter Physics
##### Identifiers
urn:nbn:se:kth:diva-169498 (URN)
##### Note

QS 2015

Available from: 2015-06-15 Created: 2015-06-15 Last updated: 2015-06-16Bibliographically approved

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