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Mechanical Properties of Transition Metal Alloys from First-PrinciplesTheory
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]

The aim of the thesis is to investigate the alloying and temperature effects on the mechanical properties of body-centered cubic (bcc) random alloys. We employ the all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation. The second-order elastic constants reflect the mechanical properties of materials in the small deformation region, where the stress-strain relations are linear. Beyond the small elastic region, the mechanical properties of defect-free solids are described by the so called ideal strength. These two sets of physical quantities are the major topic of my investigations.

In part one (papers I and II), the elastic constants and the ideal tensile strengths (ITS) are investigated as a function of Cr and Ti for the bccV-based random solid solution. We find that alloys along the equi-composition region exhibit the largest shear modulus and Young’s modulus, which is a resultof the opposite alloying effects obtained for the two cubic shear elastic constants C′ and C44. The classical Labusch-Nabarro solid-solution hardening (SSH) model extended to ternary alloys predicts a larger hardening effect in V-Ti than in V-Cr alloy. By considering a phenomenological expression for the ductile-brittle transition temperature (DBTT) in terms of Peierls stress and SSH, we show that the present theoretical results can account for the observed variations of DBTT with composition. Under uniaxial [001] tensile loading, the ITS of V is 12.4 GPa and the lattice fails by shear. Assuming isotropic Poisson contraction, the ITSs are 36.4 and 52.0 GPa for V in the [111] and [110] directions, respectively. For the V-based alloys, Cr increases and Ti decreases the ITS in all principal directions. Adding the same concentration of Cr and Ti to V leads to ternary alloys with similar ITS values as that of pure V. We show that the ITS correlates with the fcc-bcc structural energy difference and explain the alloying effects on the ITS based on electronic band structure theory.

In part two (paper III), the alloying effect on the ITS of four bcc refractory HEAs based on Zr, V, Ti, Nb, and Hf is studied. Starting from ZrNbHf, we find that the ITS decreases with equimolar Ti addition. On the other hand, if both Ti and V are added to ZrNbHf, the ITS is enhanced by about 42%. An even more captivating effect is the ITS increase by about 170%, if Ti and V are substituted for Hf. We explain the alloying effect on the ITS based on the d-band filling. We explore an intrinsic brittle-to-ductile transition, which arises due to an alloying-driven change of the failure mode under uniaxial tension. Our results indicate that intrinsically ductile HEAs with high intrinsic strength can be achieved by controlling the proportion of group four elements to group five elements.

In part three (papers IV and V), the ITS of bcc ferromagnetic Fe-based random alloys is calculated as a function of compositions. The ITS of Fe is calculated to be 12.6 GPa under [001] direction tension, which is in good agreement with the available theoretical data. For the Fe-based alloys, we predict that V, Cr, and Co increase the ITS, while Al and Ni decrease it. Manganese yields a weak non-monotonic alloying behavior. We show that the previously established ideal tensile strengths model based on structural energy differences for the nonmagnetic V-based alloys is of limited use in the case of Fe-bases alloys, which is attributed to the effect of magnetism. We find that upon tension all investigated solutes strongly alter the magnetic response of the Fe host from the unsaturated towards a stronger ferromagnetic behavior.

In part four (paper VI), the temperature effect on the ITS of bcc Fe and Fe0.9Co0.1alloy is studied. We find that the ITS of Fe is only slightly temperature dependent below∼500K but exhibits large thermal gradients at higher temperatures. Thermal expansion and electronic excitations have an overall moderate effect, but magnetic disorder reduces the ITS with a pronounced 90% loss in strength in the temperature interval∼500 - 920K. Such a dramatic temperature effect far below the magnetic transition temperature has not been observed for other micro-mechanical properties of Fe. We demonstrate that the strongly reduced Curie temperature of the distorted Fe lattices compared to that of bcc Fe is primarily responsible for the onset of the drop of the intrinsic strength. Alloying additions, which have the capability to partially restore the magnetic order in the strained Fe lattice, push the critical temperature for the strength-softening scenario towards the magnetic transition temperature of the undeformed lattice. This can result in a surprisingly large alloying-driven strengthening effect at high temperature as illustrated in our work in the case of Fe-Co alloy

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , xi, 93 p.
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-169493ISBN: 978-91-7595-635-0 (print)OAI: oai:DiVA.org:kth-169493DiVA: diva2:821700
Public defence
2015-08-28, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20150616

Available from: 2015-06-16 Created: 2015-06-15 Last updated: 2015-11-06Bibliographically approved
List of papers
1. Elastic properties of vanadium-based alloys from first-principles theory
Open this publication in new window or tab >>Elastic properties of vanadium-based alloys from first-principles theory
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2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 1, 014105- p.Article in journal (Refereed) Published
Abstract [en]

The effect of Cr and Ti on the fundamental mechanical properties of V-Cr-Ti alloys has been investigated using the all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation. The static lattice constant and elastic parameters have been calculated for the body-centered-cubic V1-x-yCrxTiy (0 <= x,y <= 0.1) random solid solution as a function of composition. Our theoretical predictions are in good agreement with the available experimental data. Alloys along the equicomposition region are found to exhibit the largest shear and Young's modulus as a result of the opposite alloying effects obtained for the two cubic shear elastic constants. The classical solid-solution hardening (SSH) model predicts larger strengthening effect in V1-yTiy than in V1-xCrx. By considering a phenomenological expression for the ductile-brittle transition temperature (DBTT) in terms of Peierls stress and SSH, it is shown that the present theoretical results can account for the variations of DBTT with composition.

Keyword
Cubic Transition-Metals, V-Cr Alloys, Mechanical-Properties, Fusion Applications, Temperature-Dependence, Structural Material, Internal-Friction, V-4cr-4ti, Ti, Constants
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-100159 (URN)10.1103/PhysRevB.86.014105 (DOI)000306308100002 ()2-s2.0-84863661838 (Scopus ID)
Funder
Swedish Research CouncilEU, European Research Council
Note

QC 20140312

Available from: 2012-08-06 Created: 2012-08-06 Last updated: 2017-12-07Bibliographically approved
2. Ideal strength of random alloys from first principles
Open this publication in new window or tab >>Ideal strength of random alloys from first principles
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2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 21, 214203- p.Article in journal (Refereed) Published
Abstract [en]

The all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation was employed to investigate the ideal tensile strengths of elemental V and Mo solids, and V-and Mo-based random solid solutions. Under uniaxial [001] tensile loading, the ideal tensile strength of V is 11.6 GPa and the lattice fails by shear. Assuming isotropic Poisson contraction, the ideal tensile strengths are 26.7 and 37.6 GPa for V in the [111] and [110] directions, respectively. The ideal strength of Mo is 26.7 GPa in the [001] direction and decreases when a few percent of Tc is introduced in Mo. For the V-based alloys, Cr increases and Ti decreases the ideal tensile strength in all principal directions. Adding the same concentration of Cr and Ti to V leads to ternary alloys with similar ideal strength values as that of pure V. The alloying effects on the ideal strength are explained using the electronic band structure.

Place, publisher, year, edition, pages
American Physical Society, 2013
Keyword
Vanadium-Base Alloys, Theoretical Tensile-Strength, Cubic Transition-Metals, Ab-Initio Calculations, Mechanical-Properties, Elastic-Constants, Temperature-Dependence, Fusion Applications, Bcc Metals, Stability
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-124722 (URN)10.1103/PhysRevB.87.214203 (DOI)000321060500002 ()2-s2.0-84879725158 (Scopus ID)
Funder
Swedish Research CouncilEU, European Research Council
Note

QC 20130730

Available from: 2013-07-30 Created: 2013-07-29 Last updated: 2017-12-06Bibliographically approved
3. Anomalous ideal tensile strength of ferromagnetic Fe and Fe-rich alloys
Open this publication in new window or tab >>Anomalous ideal tensile strength of ferromagnetic Fe and Fe-rich alloys
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2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 2Article in journal (Refereed) Published
Abstract [en]

Within the same failure mode, iron has the lowest ideal tensile strength among the transition metals crystallizing in the body-centered cubic structure. Here, we demonstrate that this anomalously low strength of Fe originates partly from magnetism and is reflected in unexpected alloying effects in dilute Fe(M) (M = Al, V, Cr, Mn, Co, Ni) binaries. We employ the structural energy difference and the magnetic pressure to disentangle the magnetic effect on the ideal tensile strength from the chemical effect. We find that the investigated solutes strongly alter the magnetic response of the Fe host from the weak towards a stronger ferromagnetic behavior, which is explained based on single-particle band energies.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-142686 (URN)10.1103/PhysRevB.90.024201 (DOI)000345446400001 ()2-s2.0-84903839928 (Scopus ID)
Note

Updated from manuscript to article in journal. Previous title: Anomalous ideal tensile strength of ferromagnetic Fe.

QC 20150126

Available from: 2014-03-12 Created: 2014-03-12 Last updated: 2017-12-05Bibliographically approved

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