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Levämäki, H., Tian, Y., Vitos, L. & Ropo, M. (2019). An automated algorithm for reliable equation of state fitting of magnetic systems. Computational materials science, 156, 121-128
Open this publication in new window or tab >>An automated algorithm for reliable equation of state fitting of magnetic systems
2019 (English)In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 156, p. 121-128Article in journal (Refereed) Published
Abstract [en]

In computational physics and materials science ground-state properties are often extracted from an equation of state fit to energy-volume data. Magnetic systems often have multiple magnetic phases present in the energy-volume data, which poses a challenge for the fitting approach because the results are sensitive to the selection of included fitting points. This is because practically all popular equation of state fitting functions, such as Murnaghan and Birch-Murnaghan, assume just one phase and therefore cannot correctly fit magnetic energy-volume data that contains multiple phases. When fitting magnetic energy-volume data it is therefore important to select the range of fitting points in such a way that only points from the one relevant phase are included. We present a simple algorithm that makes the point selection automatically. Selecting fitting points automatically removes human bias and should also be useful for large-scale projects where selecting all fitting points by hand is not feasible.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
EOS, Equation of state fitting, Ground state, Magnetism, Automated algorithms, Computational physics, Equation of state, Fitting functions, Ground state properties, Large-scale projects, Magnetic energies, SIMPLE algorithm, Equations of state
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-236337 (URN)10.1016/j.commatsci.2018.09.026 (DOI)000449375500015 ()2-s2.0-85053772857 (Scopus ID)
Note

QC 20181109

Available from: 2018-11-09 Created: 2018-11-09 Last updated: 2018-11-28Bibliographically approved
Tian, L., Levämäki, H., Kuisma, M., Kokko, K., Nagy, A. & Vitos, L. (2019). Density functional theory description of random Cu-Au alloys. Physical Review B, 99(6), Article ID 064202.
Open this publication in new window or tab >>Density functional theory description of random Cu-Au alloys
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2019 (English)In: Physical Review B, Vol. 99, no 6, article id 064202Article in journal (Refereed) Published
Abstract [en]

Density functional alloy theory is used to accurately describe the three core effects controlling the thermodynamics of random Cu-Au alloys. These three core effects are exchange correlation (XC), local lattice relaxations (LLRs), and short-range order (SRO). Within the real-space grid-based projector augmented-wave (GPAW) method based on density functional theory (DFT), we adopt the quasinonuniform XC approximation (QNA), and take into account the LLR and the SRO effects. Our approach allows us to study the importance of all three core effects in a unified way within one DFT code. The results demonstrate the importance of the LLR term and show that going from the classical gradient level approximations to QNA leads to accurate formation energies at various degrees of ordering. The order-disorder transition temperatures for the 25%, 50%, and 75% alloys reach quantitative agreement with the experimental values only when also the SRO effects are considered.

Place, publisher, year, edition, pages
American Physical Society, 2019
Keywords
Binary alloys, Copper alloys, Gold alloys, Lunar surface analysis, Thermodynamics, Density functionals, Exchange correlations, Experimental values, Formation energies, Local lattice, Projector augmented waves, Quantitative agreement, Short-range order, Density functional theory
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-252070 (URN)10.1103/PhysRevB.99.064202 (DOI)000459222700003 ()2-s2.0-85061991346 (Scopus ID)
Note

QC 20190731

Available from: 2019-07-31 Created: 2019-07-31 Last updated: 2019-07-31Bibliographically approved
Tian, L.-Y., Levämäki, H., Eriksson, O., Kokko, K., Nagy, A., Delczeg-Czirjak, E. K. & Vitos, L. (2019). Density Functional Theory description of the order-disorder transformation in Fe-Ni. Scientific Reports, 9, Article ID 8172.
Open this publication in new window or tab >>Density Functional Theory description of the order-disorder transformation in Fe-Ni
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2019 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 9, article id 8172Article in journal (Refereed) Published
Abstract [en]

The thermodynamic ordering transformation of tetragonal FeNi system is investigated by the Exact Muffin-Tin Orbitals (EMTO) method. The tetragonal distortion of the unit cell is taken into account and the free energy is calculated as a function of long-range order and includes the configurational, vibrational, electronic and magnetic contributions. We find that both configurational and vibrational effects are important and that the vibrational effect lowers the predicted transformation temperature by about 480 K compared to the value obtained merely from the configurational free energy. The predicted temperature is in excellent agreement with the experimental value when all contributions are taken into account. We also perform spin dynamics calculations for the magnetic transition temperature and find it to be in agreement with the experiments. The present research opens new opportunities for quantum-mechanical engineering of the chemical and magnetic ordering in tetrataenite.

Place, publisher, year, edition, pages
Nature Publishing Group, 2019
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-254082 (URN)10.1038/s41598-019-44506-7 (DOI)000469912700013 ()31160612 (PubMedID)2-s2.0-85066778812 (Scopus ID)
Note

QC 20190624

Available from: 2019-06-24 Created: 2019-06-24 Last updated: 2019-06-24Bibliographically approved
Molnár, D. S., Sun, X., Lu, S., Li, W., Engberg, G. & Vitos, L. (2019). Effect of temperature on the stacking fault energy and deformation behaviour in 316L austenitic stainless steel. Materials Science & Engineering: A, 759, 490-497
Open this publication in new window or tab >>Effect of temperature on the stacking fault energy and deformation behaviour in 316L austenitic stainless steel
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2019 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 759, p. 490-497Article in journal (Refereed) Published
Abstract [en]

The stacking fault energy (SFE) is often used as a key parameter to predict and describe the mechanical behaviour of face centered cubic material. The SFE determines the width of the partial dislocation ribbon, and shows strong correlation with the leading plastic deformation modes. Based on the SFE, one can estimate the critical twinning stress of the system as well. The SFE mainly depends on the composition of the system, but temperature can also play an important role. In this work, using first principles calculations, electron backscatter diffraction and tensile tests, we show a correlation between the temperature dependent critical twinning stress and the developing microstructure in a typical austenitic stainless steel (316L) during plastic deformation. We also show that the deformation twins contribute to the strain hardening rate and gradually disappear with increasing temperature. We conclude that, for a given grain size there is a critical temperature above which the critical twinning stress cannot be reached by normal tensile deformation, and the disappearance of the deformation twinning leads to lower strain hardening rate and decreased ductility.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Deformation twinning, microstructure, first principles, stacking fault energy, stainless steel
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-252728 (URN)10.1016/j.msea.2019.05.079 (DOI)000472813900052 ()
Funder
Vinnova, 2014-03374
Note

QC 20190731

Available from: 2019-06-04 Created: 2019-06-04 Last updated: 2019-07-31Bibliographically approved
Al-Zoubi, N., Schönecker, S., Li, X., Li, W., Johansson, B. & Vitos, L. (2019). Elastic properties of 4d transition metal alloys: Values and trends. Computational materials science, 159, 273-280
Open this publication in new window or tab >>Elastic properties of 4d transition metal alloys: Values and trends
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2019 (English)In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 159, p. 273-280Article in journal (Refereed) Published
Abstract [en]

Using the Exact Muffin-Tin Orbitals method within the Perdew-Burke-Ernzerhof exchange-correlation approximation for solids and solid surfaces (PBEso1), we study the single crystal elastic constants of 4d transition metals (atomic number Z between 39 and 47) and their binary alloys in the body centered cubic (bcc) and face centered cubic (fcc) structures. Alloys between the first neighbors Z(Z + 1) and between the second neighbors Z(Z + 2) are considered. The lattice constants, bulk moduli and elastic constants are found in good agreement with the available experimental and theoretical data. It is shown that the correlation between the relative tetragonal shear elastic constant C-fcc'-2C(bcc)' and the structural energy difference between the fcc and bcc lattices Delta E is superior to the previously considered models. For a given crystal structure, the equiatomic Z(Z + 2) alloys turn out to have similar structural and elastic properties as the pure elements with atomic number (Z + 1). Furthermore, alloys with composition Z(1-x)(Z + 2)(x) possess similar properties as Z(1-2x)(Z + 1)(2x). The present theoretical data on the structural and the elastic properties of 4d transition metal alloys provides consistent input for coarse scale modeling of material properties.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2019
Keywords
Exact muffin-tin orbital method, 4d transition metals, Binary alloys, Elastic constants, First-principles, Structural properties, EGAN RA, 1968, JOURNAL OF PHYSICS PART C SOLID STATE PHYSICS, V1, P763 glas D., 1975, Physical Metallurgy of Zirconium, tos L, 2001, PHYSICAL REVIEW B, V64, dersen O. K., 1998, Tight-Binding Approach to Computational Materials ScienceMRS Symposia Proceedings No. 491, Y GK, 1982, JOURNAL DE PHYSIQUE, V43, P327
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-244490 (URN)10.1016/j.commatsci.2018.12.027 (DOI)000457856900027 ()2-s2.0-85058676887 (Scopus ID)
Note

QC 20190327

Available from: 2019-03-27 Created: 2019-03-27 Last updated: 2019-04-04Bibliographically approved
Molnár, D. S., Engberg, G., Li, W., Lu, S., Hedström, P., Kwon, S. K. & Vitos, L. (2019). Experimental study of the γ-surface of austenitic stainless steels. Acta Materialia, 173, 34-43
Open this publication in new window or tab >>Experimental study of the γ-surface of austenitic stainless steels
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2019 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 173, p. 34-43Article in journal (Refereed) Published
Abstract [en]

We introduce a theory-guided experimental approach to study the γ-surface of austenitic stainless steels. The γ-surface includes a series of intrinsic energy barriers (IEBs), which are connected to the unstable stacking fault (USF), the intrinsic stacking fault (ISF), the unstable twinning fault (UTF) and the extrinsic stacking fault (ESF) energies. The approach uses the relationship between the Schmid factors and the effective energy barriers for twinning and slip. The deformation modes are identified as a function of grain orientation using in situ electron backscatter diffraction measurements. The observed critical grain orientation separating the twinning and slip regimes yields the USF energy, which combined with the universal scaling law provides access to all IEBs. The measured IEBs and the critical twinning stress are verified by direct first-principles calculations. The present advance opens new opportunities for modelling the plastic deformation mechanisms in multi-component alloys.

Keywords
stacking fault energy, twinning, electron backscatter diffraction, plasticity, first-principles
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-252729 (URN)10.1016/j.actamat.2019.04.057 (DOI)2-s2.0-85065259834 (Scopus ID)
Funder
Vinnova, 2014-03374
Note

QC 20190617

Available from: 2019-06-04 Created: 2019-06-04 Last updated: 2019-07-29Bibliographically approved
Li, X., Schönecker, S., Li, X., Hao, S., Zhao, J., Johansson, B. & Vitos, L. (2019). First-principles study of crystal-face specificity in surface properties of Fe-rich Fe-Cr alloys. PHYSICAL REVIEW MATERIALS, 3(3), Article ID 034401.
Open this publication in new window or tab >>First-principles study of crystal-face specificity in surface properties of Fe-rich Fe-Cr alloys
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2019 (English)In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 3, no 3, article id 034401Article in journal (Refereed) Published
Abstract [en]

A density-functional theory investigation of the (100) and (110) surfaces of the body-centered cubic (bcc) Fe1-xbCrxb binary alloys, x(b) <= 15 at.%, is reported. The energies and segregation energies of these surfaces were calculated for chemically homogeneous concentration profiles and for Cr surface contents deviating from the nominal one of the bulk. The implications of these results for the surface alloy phase diagram are discussed. The surface chemistry of Fe-Cr(100) is characterized by a transition from Cr depletion to Cr enrichment in a critical bulk Cr composition window of 6 < x(b) < 9 at.%. In contrast, such threshold behavior of the surface Cr content is absent for Fe-Cr(110) and a nearly homogeneous Cr concentration profile is energetically favorable. The strongly suppressed surface-layer relaxation at both surfaces is shown to be of magnetic origin. The compressive, magnetic contribution to the surface relaxation stress is found to correlate well with the surface magnetic moment squared at both surface terminations. The stability of the Cr surface magnetic moments against bulk Cr content is clarified based on the surface electronic structure.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2019
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-247823 (URN)10.1103/PhysRevMaterials.3.034401 (DOI)000460683400001 ()2-s2.0-85062963048 (Scopus ID)
Note

QC 20190327

Available from: 2019-03-27 Created: 2019-03-27 Last updated: 2019-05-16Bibliographically approved
Ji, Z.-W., Lu, S., Hui, Y., Qingmiao, H., Vitos, L. & Rui, Y. (2019). First-Principles Study on the Impact of Antisite Defects on the Mechanical Properties of TiAl-Based Alloys. Acta Metallurgica Sinica, 55(5), 673-682
Open this publication in new window or tab >>First-Principles Study on the Impact of Antisite Defects on the Mechanical Properties of TiAl-Based Alloys
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2019 (English)In: Acta Metallurgica Sinica, ISSN 0412-1961, Vol. 55, no 5, p. 673-682Article in journal (Refereed) Published
Abstract [en]

Microalloying is an effective approach to improve the mechanical properties of TiAl-based alloys which have been applied as high-temperature structure materials. The antisite defects may be regarded as special alloying elements. However, the detailed information about the effect of antisite defects on mechanical behavior (full slip and twinning), which may be described theoretically by generalized stacking fault energy (GSFE), of TiAl-based alloys are scarce. In this work, the composition dependent GSFEs of off-stoichiometric gamma- TiAl were calculated by using the first-principles exact muffin-tin orbitals method in combination with coherent potential approximation. With the calculated GSFE, the energy barriers for various deformation modes including twin (TW), ordinary dislocation (OD), and superlattice dislocation (SDI and SDII) were determined. The selection of the deformation mode under external shear stress with various directions was analyzed. The effects of the Ti-Al and Al(Ti )antisite defects on the mechanical properties of gamma-TiAl were then discussed. The results showed that the Ti-Al antisite defect decreases the energy barrier for the TW deformation leading by the superlattice intrinsic stacking fault (SISF) partial dislocation and increases the angle window of the applied shear stress within which TW deformation may be activated. Therefore, Ti-Al antisite defect is expected to improve the plasticity of gamma-TiAl. The effect of Al-Ti antisite defect is opposite. The Al-Ti antisite defect decreases the energy barriers for the OD and SDII deformations leading by complex stacking fault (CSF) partial dislocation and increases their operating angle window, indicating that Al-Ti facilitates the slip of OD and SDII. Considering that the energy barrier for CSF is much higher than that for SISF, the plasticity induced by OD and SDII should be lower than that induced by TW. Calculations in this work explain the experimental finding that Ti-Al antisite defect improves the plasticity of gamma-TiAl more significantly than Al-Ti antisite defect.

Place, publisher, year, edition, pages
SCIENCE PRESS, 2019
Keywords
antisite defect, TiAl-based alloy, generalized stacking fault energy, plastic deformation
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-251470 (URN)10.11900/0412.1961.2018.00349 (DOI)000464751100014 ()
Note

QC 20190522

Available from: 2019-05-22 Created: 2019-05-22 Last updated: 2019-05-22Bibliographically approved
Ren, G.-d., Dai, C.-r., Mei, W., Sun, J., Lu, S. & Vitos, L. (2019). Formation and temporal evolution of modulated structure in high Nb-containing lamellar gamma-TiAl alloy. Acta Materialia, 165, 215-227
Open this publication in new window or tab >>Formation and temporal evolution of modulated structure in high Nb-containing lamellar gamma-TiAl alloy
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2019 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 165, p. 215-227Article in journal (Refereed) Published
Abstract [en]

The formation and temporal evolution of the modulated structure in a lamellar gamma-based Ti-45Al-8.5Nb alloy have been investigated by transmission electron microscopy (TEM) in combination with first-principles theory in this work. The results show that the Nb-rich O phase as a constituent of the modulated structure is thermodynamically stable below 650 degrees C in the alpha(2) lamellae. The morphology of the O phase variants changes from thin plate-like shape with a low volume fraction at initial annealing to rectangle/square shape with a high volume fraction after a prolonged annealing, and the retransformed alpha(2), named as alpha(2-II) hereafter, emerges at intersections of the variants with two orthogonal habit planes due to their elastic interactions. The partitioning coefficient of Nb between the O phase and alpha(2) is about 2 at 600 degrees C. The diffusion coefficient of Nb derived from growth kinetics of the O phase is about (1.3 +/- 0.2) x 10(-22) m(2)s(-1) in the alpha(2) lamellae. Significant precipitation hardening effect of the O phase has been revealed for the alpha(2) lamellae and gamma/(alpha(2)+O) lamellar microstructure, which is supposed to be attributed to refining the alpha(2) lamellae associated with elastic strain energy from the alpha(2) -> O phase transformation and introducing the interface between the modulated lamella and adjacent gamma phase. All rights reserved.

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2019
Keywords
Titanium aluminides, Orthorhombic phase, Microstructural evolution, Diffusion, Nano-indentation
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-244506 (URN)10.1016/j.actamat.2018.11.041 (DOI)000457665100020 ()2-s2.0-85057440485 (Scopus ID)
Note

QC 20190401

Available from: 2019-04-01 Created: 2019-04-01 Last updated: 2019-04-04Bibliographically approved
Xie, R., Li, W., Lu, S., Song, Y. & Vitos, L. (2019). Generalized stacking fault energy of carbon-alloyed paramagnetic gamma-Fe. Journal of Physics: Condensed Matter, 31(6), Article ID 065703.
Open this publication in new window or tab >>Generalized stacking fault energy of carbon-alloyed paramagnetic gamma-Fe
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2019 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 31, no 6, article id 065703Article in journal (Refereed) Published
Abstract [en]

Generalized stacking fault energy (GSFE) is an important parameter for understanding the underlying physics governing the deformation mechanisms in face-centred cubic (fcc) materials. In the present work, we study the long-standing question regarding the influence of C on the GSFE in austenitic steels at paramagnetic state. We calculate the GSFE in both gamma-Fe and Fe-C alloys using the exact muffin-tin orbitals method and the Vienna Ab initio Simulation Package. Our results show that the GSFE is increased by the presence of interstitial C, and the universal scaling law is used to verify the accuracy of the obtained stacking fault energies. The C-driven change of the GSFE is discussed considering the magnetic contributions. The effective energy barriers for stacking fault, twinning and slip formation are employed to disclose the C effect on the deformation modes, and we also demonstrate that the magnetic structures as a function of volume explain the effect of paramagnetism on the C-driven changes of the stacking fault energies as compared to the hypothetical non-magnetic case.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2019
Keywords
C-alloyed gamma-Fe, GSFE, paramagnetism, ab initio
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-241183 (URN)10.1088/1361-648X/aaf2fa (DOI)000454553700001 ()30524044 (PubMedID)2-s2.0-85059403568 (Scopus ID)
Note

QC 20190121

Available from: 2019-01-21 Created: 2019-01-21 Last updated: 2019-03-04Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0003-2832-3293

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