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  • 1. Al-Zoubi, Noura
    et al.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Schonecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Uppsala, Sweden .
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Uppsala, Sweden; Wigner Research Center for Physics, Budapest, Hungary .
    Influence of manganese on the bulk properties of Fe-Cr-Mn alloys: a first-principles study2014In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 89, no 12, p. 125702-Article in journal (Refereed)
    Abstract [en]

    We investigate the effect of manganese on lattice stability and magnetic moments of paramagnetic Fe-Cr-Mn steel alloys along the Bain path connecting the body-centered cubic (bcc) and face-centered cubic (fcc) structures. The calculations are carried out using the ab initio exact muffin-tin orbital method, in combination with the coherent potential approximation, and the paramagnetic phase is modeled by the disordered local magnetic moment scheme. For all Fe-Cr-Mn alloys considered here, the local magnetic moments on Fe atoms have the minimum values for the fcc structure and the maximum values for the bcc structure, whereas the local magnetic moments on Mn have almost the same value along the constant-volume Bain path. Our results show that Mn addition to paramagnetic Fe-Cr solid solution stabilizes the bcc structure. However, when considering the paramagnetic fcc phase relative to the ferromagnetic bcc ground state, then Mn turns out to be a clear fcc stabilizer, in line with observations.

  • 2.
    Al-Zoubi, Noura
    et al.
    Tafila Tech Univ, Dept Appl Phys, Tafila, Jordan..
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Elastic properties of 4d transition metal alloys: Values and trends2019In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 159, p. 273-280Article in journal (Refereed)
    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.

  • 3.
    Huang, He
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China..
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Dong, Zhihua
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Huang, Shuo
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Meng, Daqiao
    Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China..
    Lai, Xinchun
    Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China..
    Liu, Tianwei
    Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China..
    Zhu, Shengfa
    Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China..
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Dept Phys & Astron, Div Mat Theory, SE-75120 Uppsala, Sweden.;Wigner Res Ctr Phys, Inst Solid State Phys & Opt, H-1525 Budapest, Hungary..
    Critical stress for twinning nucleation in CrCoNi-based medium and high entropy alloys2018In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 149, p. 388-396Article in journal (Refereed)
    Abstract [en]

    The CrCoNi-based medium and high entropy alloys (MHEAs) have drawn much attention due to their exceptional mechanical properties at cryogenic temperatures. The twinning critical resolved shear stress (CRSS) is a fundamental parameter for evaluating the strength-ductility properties of MHEAs. Here we construct and apply an extended twinning nucleation Peierls-Nabarro (P-N) model to predict the twinning CRSSes of face-centered cubic (FCC) CrCoNi-based MHEAs. The order of the twinning CRSSes of the selected alloys is CrCoNi > CrCoNiMn > CrCoNiFe > CrCoNiFeMn and the values are 291, 277, 274 and 236 MPa, respectively. These theoretical predictions agree very well with the experimental twinning CRSSes of CrCoNi and CrCoNiFeMn accounting for 260 +/- 30 and 235 +/- 10 MPa, respectively and are perfectly consistent with the strength-ductility properties including yield stress, ultimate tensile stress and uniform elongation for fracture of the FCC CrCoNi-based MHEAs obtained at cryogenic temperatures. The present method offers a first-principle quantum-mechanical tool for optimizing and designing new MHEAs with exceptional mechanical properties. 

  • 4.
    Huang, Shuo
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Huang, He
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Science and Technology on Surface Physics and Chemistry Laboratory, Mianyang, 621900, PR China.
    Li, Wei
    Department of Physics and Astronomy, Division of Materials Theory, Uppsala University, SE-75120, Uppsala, Sweden.
    Kim, Dongyoo
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Department of Physics, Pukyung National University, Busan, 608-737, Republic of Korea.
    Lu, Song
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Holmström, E.
    Kwon, S. K.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Twinning in metastable high-entropy alloys2018In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, no 1, article id 2381Article in journal (Refereed)
    Abstract [en]

    Twinning is a fundamental mechanism behind the simultaneous increase of strength and ductility in medium- and high-entropy alloys, but its operation is not yet well understood, which limits their exploitation. Since many high-entropy alloys showing outstanding mechanical properties are actually thermodynamically unstable at ambient and cryogenic conditions, the observed twinning challenges the existing phenomenological and theoretical plasticity models. Here, we adopt a transparent approach based on effective energy barriers in combination with first-principle calculations to shed light on the origin of twinning in high-entropy alloys. We demonstrate that twinning can be the primary deformation mode in metastable face-centered cubic alloys with a fraction that surpasses the previously established upper limit. The present advance in plasticity of metals opens opportunities for tailoring the mechanical response in engineering materials by optimizing metastable twinning in high-entropy alloys. 

  • 5.
    Huang, Shuo
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Bergqvist, Lars
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Holmström, E.
    Varga, L. K.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Wigner Research Centre for Physics, Hungary; Uppsala University, Sweden.
    Mechanism of magnetic transition in FeCrCoNi-based high entropy alloys2016In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 103, p. 71-74Article in journal (Refereed)
    Abstract [en]

    First-principles alloy theory and Monte-Carlo simulations are performed to investigate the magnetic properties of FeCrCoNiAlx high entropy alloys. Results show that face-centered-cubic (fcc) and body-centered-cubic (bcc) structures possess significantly different magnetic behaviors uncovering that the alloy's Curie temperature is controlled by the stability of the Al-induced single phase or fcc-bcc dual-phase. We show that the appearance of the bcc phase with increasing Al content brings about the observed transition from the paramagnetic state for FeCrCoNi to the ferromagnetic state for FeCrCoNiAl at room-temperature. Similar mechanism is predicted to give rise to room-temperature ferromagnetism in FeCrCoNiGa high entropy alloy.

  • 6.
    Huang, Shuo
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Uppsala Univ, Div Mat Theory, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Huang, He
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Sci & Technol Surface Phys & Chem Lab, Mianyang 621900, Peoples R China..
    Holmström, Erik
    Sandvik Coromant R&D, S-12680 Stockholm, Sweden..
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala Univ, Div Mat Theory, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden.;Wigner Res Ctr Phys, Inst Solid State Phys & Opt, POB 49, H-1525 Budapest, Hungary..
    Mechanical performance of FeCrCoMnAlx high-entropy alloys from first-principle2018In: Materials Chemistry and Physics, ISSN 0254-0584, E-ISSN 1879-3312, Vol. 210, p. 37-42Article in journal (Refereed)
    Abstract [en]

    The elastic parameters and ideal tensile strength in the 10011 direction for the body-centered cubic solid solution phase of FeCrCoMnAlx (0.6 <= x <= 1.5) high-entropy alloys are determined using first-principle alloy theory. Based on the estimated theoretical Curie temperatures, all alloys considered here are predicted to order ferromagnetically at room temperature. The mechanical behaviors are analyzed through the single-crystal and polycrystalline elastic moduli, Pugh ratio, and Debye temperature by making use of a series of phenomenological models. High ideal tensile strength is found for the equiatomic FeCrCoMnAl system, and the intrinsic strength increases with decreasing Al content.

  • 7.
    Li, Guijiang
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Delczeg-Czirjak, Erna K.
    Kvashnin, Yaroslav O.
    Eriksson, Olle
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden.
    Kinetic arrest induced antiferromagnetic order in hexagonal FeMnP0.75Si0.25 alloy2014In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 105, no 26, p. 262405-Article in journal (Refereed)
    Abstract [en]

    The magnetic state of the FeMnP0.75Si0.25 alloy was investigated by first principles calculations. The coexistence of ferromagnetic and antiferromagnetic phases in FeMnP0.75Si0.25 with the same hexagonal crystal structure was revealed. It was found that kinetic arrest during the transition from the high temperature disordered paramagnetic phase to the low temperature ordered ferromagnetic phase results in the intermediate metastable and partially disordered antiferromagnetic phase. We propose that the ratio of the ferromagnetic and antiferromagnetic phases in the FeMnP0.75Si0.25 sample can be tuned by adjusting the kinetic process of atomic diffusion. The investigations suggest that careful control of the kinetic diffusion process provides another tuning parameter to design candidate magnetocaloric materials.

  • 8. Li, R.
    et al.
    Zhang, P.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Zhang, C.
    Zhao, J.
    First-principles study of the behavior of O, N and C impurities in vanadium solids2013In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 435, no 1-3, p. 71-76Article in journal (Refereed)
    Abstract [en]

    Vanadium alloys are promising candidate for the structural materials of first-wall in future fusion reactor. In realistic vanadium alloys, there always exist some impurities (e.g. oxygen, nitrogen and carbon). To understand the microscopic behavior of these impurities, we investigated energetic and diffusion of O, N and C impurities as well as O-O/N-N/C-C interactions in pure vanadium using first-principles calculations. The O, N and C atoms prefer to occupy an octahedral interstitial site, and exhibit high diffusion barrier with 1.23 eV, 1.48 eV and 1.14 eV via diffusing between two neighboring octahedral interstitial sites, respectively. Such high barriers indicate that these impurities are hard to diffuse inside bulk vanadium. The corresponding diffusion coefficients as function of temperature were estimated using the Arrhenius diffusion equation. Our theoretical results provide the fundamental parameters for understanding the impurity effects in early stage of irradiation damage.

  • 9.
    Li, Xiaojie
    et al.
    Dalian Univ Technol, Minist Educ, Key Lab Mat Modificat Laser Electron & Ion Beams, Dalian 116024, Peoples R China.;KTH Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden..
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Ruihuan
    Changzhou Vocat Inst Mechatron Technol, Inst Mold Technol, Changzhou 213164, Peoples R China..
    Zhao, Jijun
    Dalian Univ Technol, Minist Educ, Key Lab Mat Modificat Laser Electron & Ion Beams, Dalian 116024, Peoples R China..
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala Univ, Div Mat Theory, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden.;Res Inst Solid State Phys & Opt, POB 49, H-1525 Budapest, Hungary..
    Understanding the mechanical properties of reduced activation steels2018In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 146, p. 260-272Article in journal (Refereed)
    Abstract [en]

    Reduced activation ferritic/martensitic (RAFM) steels are structural materials with potential application in Generation-IV fission and fusion reactors. We use density-functional theory to scrutinize the micro-mechanical properties of the main alloy phases of three RAFM steels based on the body-centered cubic FeCrWVMn solid solution. We assess the lattice parameters and elastic properties of ferromagnetic alpha-Fe and Fe91Cr9, which are the main building blocks of the RAFM steels, and present a detailed analysis of the calculated alloying effects of V, Cr, Mn, and W on the mechanical properties of Fe91Cr9. The composition dependence of the elastic parameters is decomposed into electronic and volumetric contributions and studied for alloying levels that cover the typical intervals in RAFM steels. A linear superposition of the individual solute effects on the properties of Fe91Cr9 is shown to provide an excellent approximation for the ab initio values obtained for the RAFM steels. The intrinsic ductility is evaluated through Rice's phenomenological theory using the surface and unstable stacking fault energies, and the predictions are contrasted with those obtained by empirical criteria. Alloying with V or W is found to enhance the ductility, whereas additional Cr or Mn turns the RAFM base alloys more brittle.

  • 10.
    Li, Xiaojie
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Dalian University of Technology, China.
    Schonecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Ruihuan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Wang, Yuanyuan
    Zhao, Jijun
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Department of Physics and Astronomy, Division of Materials Theory, Sweden.
    Vitos, Levente
    Department of Physics and Astronomy, Division of Materials Theory, Sweden; Wigner Research Center for Physics, Hungary.
    Ab initio calculations of mechanical properties of bcc W-Re-Os random alloys: effects of transmutation of W2016In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 28, no 29, article id 295501Article in journal (Refereed)
    Abstract [en]

    To examine the effect of neutron transmutation on tungsten as the first wall material of fusion reactors, the elastic properties of W1-x-yRexOsy (0 <= x, y <= 6%) random alloys in body centered cubic (bcc) structure are investigated systematically using the all-electron exact muffin-tin orbitals (EMTO) method in combination with the coherent-potential approximation (CPA). The calculated lattice constant and elastic properties of pure W are consistent with available experiments. Both Os and Re additions reduce the lattice constant and increase the bulk modulus of W, with Os having the stronger effect. The polycrystalline shear modulus, Young's modulus and the Debye temperature increase (decrease) with the addition of Re (Os). Except for C-11, the other elastic parameters including C-12, C-44, Cauchy pressure, Poisson ratio, B/G, increase as a function of Re and Os concentration. The variations of the latter three parameters and the trend in the ratio of cleavage energy to shear modulus for the most dominant slip system indicate that the ductility of the alloy enhances with increasing Re and Os content. The calculated elastic anisotropy of bcc W slightly increases with the concentration of both alloying elements. The estimated melting temperatures of the W-Re-Os alloy suggest that Re or Os addition will reduce the melting temperature of pure W solid. The classical Labusch-Nabarro model for solid-solution hardening predicts larger strengthening effects in W1-yOsy than in W1-xRex. A strong correlation between C' and the fcc-bcc structural energy difference for W1-x-yRexOsy is revealed demonstrating that canonical band structure dictates the alloying effect on C'. The structural energy difference is exploited to estimate the alloying effect on the ideal tensile strength in the [0 0 1] direction.

  • 11.
    Li, Xiaojie
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Dalian Univ Technol, Minist Educ, Key Lab Mat Modificat Laser Electron & Ion Beams, Dalian 116024, Peoples R China..
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Hao, Shengzhi
    Dalian Univ Technol, Minist Educ, Key Lab Mat Modificat Laser Electron & Ion Beams, Dalian 116024, Peoples R China..
    Zhao, Jijun
    Dalian Univ Technol, Minist Educ, Key Lab Mat Modificat Laser Electron & Ion Beams, Dalian 116024, Peoples R China..
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Uppsala Univ, Div Mat Theory, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden..
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Uppsala Univ, Div Mat Theory, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden.;Wigner Res Ctr Phys, Res Inst Solid State Phys & Opt, POB 49, H-1525 Budapest, Hungary..
    First-principles study of crystal-face specificity in surface properties of Fe-rich Fe-Cr alloys2019In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 3, no 3, article id 034401Article in journal (Refereed)
    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.

  • 12.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    First-principles study of the third-order elastic constants and related anharmonic properties in refractory high-entropy alloys2018In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 142, p. 29-36Article in journal (Refereed)
    Abstract [en]

    The third-order elastic constants (TOECs) and elastic anharmonic behavior in four body-centered cubic refractory high-entropy alloys (HEAs) based on elements of the fourth, fifth, and sixth groups are investigated using density-functional simulations. We find that the values of the TOECs C-111 are the largest in magnitude among the studied six independent TOECs and strongly increase with increasing average valence electron concentration (VEC). Interestingly, the TOEC C-456 undergos a sign change as a function of the VEC. Using the obtained TOECs, we investigate the mode Griineisen constants gamma(i) as well as the low temperature limit (gamma) over bar, derive the long-wavelength acoustic nonlinearity parameters a, and reveal the pressure derivatives of effective elastic constants and polycrystalline moduli as a function of the VEC. Our results show that,6 displays a different directional order along the pure mode [100], [110], and [111] directions for the four considered refractory HEAs. Furthermore, we show that the directional order of,8 is not correlated to the crystal symmetry. With the help of the obtained pressure derivatives of polycrystalline moduli, we predict the low temperature volume expansion coefficient.

  • 13.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Mechanical Properties of Transition Metal Alloys from First-Principles Theory2014Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The aim of the thesis is to investigate the alloying effect on the mechanical properties of random alloys using the all-electron exact muffin-tin orbitals methodin combination with the coherent-potential approximation. The second-order elastic constants describe the mechanical properties of materials in the small deformation region, where the stress-strain relations arelinear. Beyond the small elastic region, the mechanical properties of dislocation-free solids are described by the ideal strength.

    The elastic constants and ideal tensile strengths have been investigated as a function of Cr and Ti for the body centered cubic V-based random solidsolution. Alloys along the equi-composition region are found to exhibit the largest shear and Young’s modulus as a result of the opposite alloying effectsobtained for the two cubic shear elastic constants C' and C44.Classical solid-solution hardening (SSH) model predicts larger hardening effect in V-Ti thanin V-Cr alloy. By considering a phenomenological expression for the ductile-brittle transition temperature (DBTT) in terms of Peierls stress and SSH, itis shown that the present theoretical results can account for the variations of DBTT with composition. Under uniaxial [001] tensile loading, the ideal tensilestrength of V is 12.4 GPa and the lattice fails by shear. Assuming isotropic Poisson contraction, the ideal tensile strength are 36.4 and 52.0 GPa for V inthe [111] and [110] directions, respectively. For the V-based alloys, Cr increases and Ti decreases the ideal tensile strength in all principal directions. Addingthe same concentration of Cr and Ti to V leads to ternary alloys with similar ideal tensile strength values as that of pure V. The alloying effects on the idealtensile strength are explained using the electronic band structure.

    The ideal tensile strengths of bcc ferromagnetic Fe-based random alloys have been calculated as a function of compositions. The ideal tensile strength of Fe in the [001] direction is calculated to be 12. 6GPa,in agreement with the available data. For the Fe-based alloys, we predict that V, Cr, and Co increase the ideal tensile strength, while Al and Ni decrease it. Manganese yields a weak non-monotonous alloying behavior. We show that the limited use of the previouslyestablished ideal tensile strengths model based on structural energy differences in the case of Fe-bases alloys is attributed to the effect of magnetism. We find that upon tension all the investigated solutes strongly alter the magneticresponse of the Fe host from the unsaturated towards a stronger ferromagnetic behavior.

  • 14.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Third-order elastic constants and anharmonic properties of three fcc high-entropy alloys from first-principles2018In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 764, p. 906-912Article in journal (Refereed)
    Abstract [en]

    Third-order elastic constants (TOECs) are very important for understanding the nonlinear mechanical response of materials and evaluating the anharmonicity of crystal lattices. Here, we are concerned with investigating the six independent TOECs and related anharmonic properties of three face-centered cubic (fcc) high-entropy alloys (HEAs), namely CrFeCoNi, CrMnFeCoNi, and Cr10Mn40Fe40C10, using density-functional simulations. To benchmark computational accuracy, three ab initio codes are used to obtain the complete set of TOECs for fcc Ni. For the HEAs, we observe that the TOECs C-123 and C-456 are positive, and C-123 is particularly large. The Cauchy relations for the TOECs are partially satisfied for the three studied HEAs. With the help of the derived TOECs, the average TOECs for an isotropic polycrystal are estimated. Using the obtained TOECs, we reveal the pressure derivatives of the effective second-order elastic constants and polycrystalline moduli as well as derive the nonlinearity constant delta. The obtained pressure derivative of bulk modulus agrees very well with the available experimental data for CrMnFeCoNi. For the three considered HEAs, delta along high-symmetry directions orders as delta([011]) > delta([111]) > delta ([100]).

  • 15.
    Li, Xiaoqing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Irving, Douglas L.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. KTH, School of Engineering Sciences (SCI), Applied Physics.
    First-principles investigation of the micromechanical properties of fcc-hcp polymorphic high-entropy alloys2018In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, article id 11196Article in journal (Refereed)
    Abstract [en]

    High-entropy alloys offer a promising alternative in several high-technology applications concerning functional, safety and health aspects. Many of these new alloys compete with traditional structural materials in terms of mechanical characteristics. Understanding and controlling their properties are of the outmost importance in order to find the best single-or multiphase solutions for specific uses. Here, we employ first-principles alloy theory to address the micro-mechanical properties of five polymorphic high-entropy alloys in their face-centered cubic (fcc) and hexagonal close-packed (hcp) phases. Using the calculated elastic parameters, we analyze the mechanical stability, elastic anisotropy, and reveal a strong correlation between the polycrystalline moduli and the average valence electron concentration. We investigate the ideal shear strength of two selected alloys under shear loading and show that the hcp phase possesses more than two times larger intrinsic strength than that of the fcc phase. The derived half-width of the dislocation core predicts a smaller Peierls barrier in the fcc phase confirming its increased ductility compared to the hcp one. The present theoretical findings explain a series of important observations made on dual-phase alloys and provide an atomic-level knowledge for an intelligent design of further high-entropy materials.

  • 16.
    Li, Xiaoqing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    First-principles prediction of the stacking fault energy of gold at finite temperature2017In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 135, p. 88-95Article in journal (Refereed)
    Abstract [en]

    The intrinsic stacking fault energy (ISFE) γ is a material parameter fundamental to the discussion of plastic deformation mechanisms in metals. Here, we scrutinize the temperature dependence of the ISFE of Au through accurate first-principles derived Helmholtz free energies employing both the super cell approach and the axial Ising model (AIM). A significant decrease of the ISFE with temperature, −(36–39) % from 0 to 890 K depending on the treatment of thermal expansion, is revealed, which matches the estimate based on the experimental temperature coefficient dγ/dT closely. We make evident that this decrease predominantly originates from the excess vibrational entropy at the stacking fault layer, although the contribution arising from the static lattice expansion compensates it by approximately 60%. Electronic excitations are found to be of minor importance for the ISFE change with temperature. We show that the Debye model in combination with the AIM captures the correct sign but significantly underestimates the magnitude of the vibrational contribution to γ(T). The hexagonal close-packed (hcp) and double hcp structures are established as metastable phases of Au. Our results demonstrate that quantitative agreement with experiments can be obtained if all relevant temperature-induced excitations are considered in first-principles modeling and that the temperature dependence of the ISFE is substantial enough to be taken into account in crystal plasticity modeling.

  • 17.
    Li, Xiaoqing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Wei
    Varga, Lajos K.
    Irving, Douglas L.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Tensile and shear loading of four fcc high-entropy alloys: A first-principles study2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 9, article id 094102Article in journal (Refereed)
    Abstract [en]

    Ab initio density-functional calculations are used to investigate the response of four face-centered-cubic (fcc) high-entropy alloys (HEAs) to tensile and shear loading. The ideal tensile and shear strengths (ITS and ISS) of the HEAs are studied by employing first-principles alloy theory formulated within the exact muffin-tin orbital method in combination with the coherent-potential approximation. We benchmark the computational accuracy against literature data by studying the ITS under uniaxial [110] tensile loading and the ISS for the [11 (2) over tilde](111) shear deformation of pure fcc Ni and Al. For the HEAs, we uncover the alloying effect on the ITS and ISS. Under shear loading, relaxation reduces the ISS by similar to 50% for all considered HEAs. We demonstrate that the dimensionless tensile and shear strengths are significantly overestimated by adopting two widely used empirical models in comparison with our ab initio calculations. In addition, our predicted relationship between the dimensionless shear strength and shear instability are in line with the modified Frenkel model. Using the computed ISS, we derive the half-width of the dislocation core for the present HEAs. Employing the ratio of ITS to ISS, we discuss the intrinsic ductility of HEAs and compare it with a common empirical criterion. We observe a strong linear correlation between the shear instability and the ratio of ITS to ISS, whereas a weak positive correlation is found in the case of the empirical criterion.

  • 18.
    Li, Xiaoqing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Zhao, J.
    Johansson, Börje
    Uppsala University, Sweden.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden; Wigner Research Center for Physics, Hungary.
    Alloying effect on the ideal tensile strength of ferromagnetic and paramagnetic bcc iron2016In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 676, p. 565-574Article in journal (Refereed)
    Abstract [en]

    Using ab initio alloy theory formulated within the exact muffin-tin orbitals theory in combination with the coherent potential approximation, we investigate the ideal tensile strength (ITS) in the [001] direction of bcc ferro-/ferrimagnetic (FFM) and paramagnetic (PM) Fe1-xMx (M = Al, V, Cr, Mn, Co, or Ni) random alloys. The ITS of ferromagnetic (FM) Fe is calculated to be 12.6 GPa, in agreement with available data, while the PM phase turns out to posses a significantly lower value of 0.7 GPa. Alloyed to the FM matrix, we predict that V, Cr, and Co increase the ITS of Fe, while Al and Ni decrease it. Manganese yields a weak non-monotonic alloying behavior. In comparison to FM Fe, the alloying effect of Al and Co to PM Fe is reversed and the relative magnitude of the ITS can be altered more strongly for any of the solutes. All considered binaries are intrinsically brittle and fail by cleavage of the (001) planes under uniaxial tensile loading in both magnetic phases. We show that the previously established ITS model based on structural energy differences proves successful in the PM Fe-alloys but is of limited use in the case of the FFM Fe-based alloys. The different performance is attributed to the specific interplay between magnetism and volume change in response to uniaxial tension. We establish a strong correlation between the compositional effect on the ITS and the one on the shear elastic constant C' for the PM Fe-alloys and briefly discuss the relation between hardenability and the ITS.

  • 19.
    Li, Xiaoqing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Zhao, Jijun
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden;.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden; Wigner Research Center for Physics, Hungary.
    Anomalous ideal tensile strength of ferromagnetic Fe and Fe-rich alloys2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 2Article in journal (Refereed)
    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.

  • 20.
    Li, Xiaoqing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Zhao, Jijun
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ideal strength of random alloys from first principles2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 21, p. 214203-Article in journal (Refereed)
    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.

  • 21.
    Li, Xiaoqing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Zhao, Jijun
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Ieal tensile strength of ferromagnetic Fe-based alloys from first-principles theoryManuscript (preprint) (Other academic)
  • 22.
    Li, Xiaoqing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Tian, Fuyang
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. University of Science and Technology Beijing, China.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Zhao, Jijun
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Uppsala University, Sweden; Wigner Research Center for Physics, Hungary.
    Ab initio-predicted micro-mechanical performance of refractory high-entropy alloys2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, article id 12334Article in journal (Refereed)
    Abstract [en]

    Recently developed high-entropy alloys (HEAs) consisting of multiple principal elements represent a new field of metallurgy and have demonstrated appealing properties for a wide range of applications. Using ab initio alloy theory, we reveal the alloying effect on the elastic properties and the ideal tensile strength (ITS) in the [001] direction of four body-centered cubic (bcc) refractory HEAs based on Zr, V, Ti, Nb, and Hf. We find that these HEAs show high elastic anisotropy and large positive Cauchy pressure, suggesting good extrinsic ductility. Starting from ZrNbHf, it is found 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. The alloying effect on the ITS is explained by the d-band filling. An intrinsic brittle-to-ductile transition is found in terms of the failure mode under uniaxial tension. These investigations suggest that intrinsically ductile HEAs with high ideal strength can be achieved by controlling the proportion of group four elements to group five elements.

  • 23.
    Li, Xiaoqing
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Zhang, Hualei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Lu, Song
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Zhao, Jijun
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Elastic properties of vanadium-based alloys from first-principles theory2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 1, p. 014105-Article in journal (Refereed)
    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.

  • 24.
    Qin, Gang
    et al.
    Harbin Inst Technol, Natl Key Lab Precis Hot Proc Met, Harbin 150001, Heilongjiang, Peoples R China..
    Chen, Ruirun
    Harbin Inst Technol, Natl Key Lab Precis Hot Proc Met, Harbin 150001, Heilongjiang, Peoples R China.;Harbin Inst Technol, State Key Lab Adv Welding & Joining, Harbin 150001, Heilongjiang, Peoples R China..
    Liaw, Peter K.
    Univ Tennessee, Dept Mat Sci & Engn, Knoxville, TN 37996 USA..
    Ga, Yanfei
    Univ Tennessee, Dept Mat Sci & Engn, Knoxville, TN 37996 USA..
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Zheng, Huiting
    Harbin Inst Technol, Natl Key Lab Precis Hot Proc Met, Harbin 150001, Heilongjiang, Peoples R China..
    Wang, Liang
    Harbin Inst Technol, Natl Key Lab Precis Hot Proc Met, Harbin 150001, Heilongjiang, Peoples R China..
    Su, Yanqing
    Harbin Inst Technol, Natl Key Lab Precis Hot Proc Met, Harbin 150001, Heilongjiang, Peoples R China..
    Guo, Jingjie
    Harbin Inst Technol, Natl Key Lab Precis Hot Proc Met, Harbin 150001, Heilongjiang, Peoples R China..
    Fu, Hengzhi
    Harbin Inst Technol, Natl Key Lab Precis Hot Proc Met, Harbin 150001, Heilongjiang, Peoples R China..
    A novel face-centered-cubic high-entropy alloy strengthened by nanoscale precipitates2019In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 172, p. 51-55Article in journal (Refereed)
    Abstract [en]

    A new single-phase face-centered-cubic (FCC) Co9Cr7Cu36Mn25Ni23 [atomic percent, similar hereinafter] high-entropy alloy (HEA) was prepared by arc melting. A uniform distribution of nanometer-sized precipitates was achieved. The tensile yield strength, ultimate tensile strength, and elongation were 401 MPa, 700 MPa, and 36%, respectively. The energy-dispersive spectrometer results showed that the nano-precipitates were rich in Co and Cr elements. Moreover, the crystal-forming behavior and the nanoscale-precipitates-forming mechanism were revealed. Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

  • 25.
    Schönecker, Stephan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Kwon, Se Kyun
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Thermal surface free energy and stress of iron2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, article id 14860Article in journal (Refereed)
    Abstract [en]

    Absolute values of surface energy and surface stress of solids are hardly accessible by experiment. Here, we investigate the temperature dependence of both parameters for the (001) and (110) surface facets of body-centered cubic Fe from first-principles modeling taking into account vibrational, electronic, and magnetic degrees of freedom. The monotonic decrease of the surface energies of both facets with increasing temperature is mostly due to lattice vibrations and magnetic disorder. The surface stresses exhibit nonmonotonic behaviors resulting in a strongly temperature dependent excess surface stress and surface stress anisotropy.

  • 26.
    Schönecker, Stephan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. Uppsala University, Sweden; Wigner Research Center for Physics, Hungary.
    Atomic long-range order effects on Curie temperature and adiabatic spin-wave dynamics in strained Fe-Co alloy films2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 94, no 6, article id 064410Article in journal (Refereed)
    Abstract [en]

    The strained Fe-Co alloy in body-centered tetragonal (bct) structure has raised considerable interest due to its giant uniaxial magnetocrystalline anisotropy energy. On the basis of the classical Heisenberg Hamiltonian with ab initio interatomic exchange interactions, we perform a theoretical study of fundamental finite temperature magnetic properties of Fe1-xCox alloy films as a function of three variables: chemical composition 0.3 <= x <= 0.8, bct geometry [a, c(a)] arising from in-plane strain and associated out-of-plane relaxation, and atomic long-range order (ALRO). The Curie temperatures T-C(x, a) obtained from Monte Carlo simulations display a competition between a pronounced dependence on tetragonality, strong ferromagnetism in the Co-rich alloy, and the beginning instability of ferromagnetic order in the Fe-rich alloy when c/a -> root 2. Atomic ordering enhances T-C and arises mainly due to different distributions of atoms in neighboring coordination shells rather than altering exchange interactions significantly. We investigate the ordering effect on the shape of the adiabatic spin-wave spectrum for selected pairs (x, a). Our results indicate that long-wavelength acoustic spin-wave excitations show dependencies on x, a, and ALRO similar to those of T-C. The directional anisotropy of the spin-wave stiffness d(x, a) peaks in narrow ranges of composition and tetragonality. ALRO exhibits a strong effect on d for near equiconcentration Fe-Co. We also discuss our findings in the context of employing Fe-Co as perpendicular magnetic recording medium.

  • 27.
    Schönecker, Stephan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Richter, Manuel
    IFW Dresden, D-01069 Dresden, Germany.;Dresden Ctr Computat Mat Sci, D-01069 Dresden, Germany..
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics. KTH, School of Engineering Sciences (SCI), Applied Physics.
    Lattice dynamics and metastability of fcc metals in the hcp structure and the crucial role of spin-orbit coupling in platinum2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 22, article id 224305Article in journal (Refereed)
    Abstract [en]

    We investigate the lattice dynamical properties of Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au in the nonequilibrium hcp structure by means of density-functional simulations, wherein spin-orbit coupling (SOC) was considered for Ir, Pt, and Au. The determined dynamical properties reveal that all eight elements possess a metastable hcp phase at zero temperature and pressure. The hcp Ni, Cu, Rh, Pd, and Au previously observed in nanostructures support this finding. We make evident that the inclusion of SOC is mandatory for an accurate description of the phonon dispersion relations and dynamical stability of hcp Pt. The underlying sensitivity of the interatomic force constants is ascribed to a SOC-induced splitting of degenerate band states accompanied by a pronounced reduction of electronic density of states at the Fermi level. To give further insight into the importance of SOC in Pt, we (i) focus on phase stability and examine a lattice transformation related to optical phonons in the hcp phase and (ii) focus on the generalized stacking fault energy (GSFE) of the fcc phase pertinent to crystal plasticity. We show that the intrinsic stable and unstable fault energies of the GSFE scale as in other common fcc metals, provided that the spin-orbit interaction is taken into account.

  • 28. Wei, D.
    et al.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Jiang, J.
    Heng, W.
    Koizumi, Y.
    Choi, W. -M
    Lee, B. -J
    Kim, H. S.
    Kato, H.
    Chiba, A.
    Novel Co-rich high performance twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) high-entropy alloys2019In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 165, p. 39-43Article in journal (Refereed)
    Abstract [en]

    The equiatomic CoCrMnNiFe high-entropy alloy (HEA) has attracted much attention owing to its exceptional mechanical properties. Here, we designed novel face-centered cubic (fcc) phase Co-rich non-equiatomic CoCrMnNiFe HEAs with tensile properties superior to the counterparts, derived from lowering stacking fault energy (SFE) via modifying constituent concentrations. The decrease of Mn, Ni, Fe meanwhile increase of Co, Cr concentrations does reduce the SFE value, based on ab initio and thermodynamics calculations. Hereinto, Co 35 Cr 20 Mn 15 Ni 15 Fe 15 and Co 35 Cr 25 Mn 15 Ni 15 Fe 10 HEAs overcame the strength-ductility trade-off, contributing to twinning-induced plasticity (TWIP) or transformation-induced plasticity (TRIP) effects, respectively. The present study sheds light on developing high performance HEAs.

  • 29.
    Xiaoqing, Li
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Mechanical Properties of Transition Metal Alloys from First-PrinciplesTheory2015Doctoral 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

  • 30.
    Zhang, Hualei
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Jesperson, Henrik
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Anomalous elastic hardening in Fe-Co alloys at high temperature2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 18, p. 184107-Article in journal (Refereed)
    Abstract [en]

    The elastic moduli of Fe1-cCoc (c <= 0.2) alloys are found to decrease strongly with increasing temperature, but show very weak alloying effects for both low-temperature ferromagnetic and high-temperature paramagnetic states. For temperatures slightly below and around the Curie temperature of Fe, Co addition significantly increases the elastic moduli. The variation of the tetragonal shear elastic constant upon 20% Co addition increases from a small negative value to more than 135% as the temperature rises from 0 to 1200 K. The expected elastic softening in the case of Al doping is not confirmed. Both anomalous trends are ascribed to the interplay between intrinsic chemical effects, magnetism, and temperature.

  • 31. Zhao, W.
    et al.
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Gong, S.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sun, Z.
    Thermo-mechanical properties of Ni-Mo solid solutions: A first-principles study2019In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 158, p. 140-148Article in journal (Refereed)
    Abstract [en]

    The mechanical strength of Ni-based single-crystal superalloys under service condition is related to the thermo-mechanical properties of the disordered γ matrix. Here we use density functional theory and quasi-harmonic approximation to determine the temperature-dependent bulk moduli and generalized stacking fault energies (GSFEs) of Ni-Mo solid solutions. We show that the increasing temperatures between 1000 K and 1400 K cause evident reductions in the bulk moduli and planar fault energies of Ni-Mo alloys. Furthermore, their negative slopes versus temperature are gradually diminished with increasing Mo concentration except that of the unstable stacking fault energy. Adopting recent theoretical models for twinning based on GSFE, increasing temperature enhances the twinnability of low-Mo alloys but has limited influences in the case of high-Mo alloys. The composition-dependent thermal expansion, the thermal electronic excitation and the magnetic transition are shown to be the main factors rendering the complex variations in the elastic properties and twinning behavior of Ni-Mo solid solution with temperature.

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