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  • 1.
    Ehteshami, Hossein
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. National University of Science and Technology, Russia.
    Thermophysical properties of paramagnetic Fe from first principles2017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, no 22, article id 224406Article in journal (Refereed)
    Abstract [en]

    A computationally efficient, yet general, free-energy modeling scheme is developed based on first-principles calculations. Finite-temperature disorder associated with the fast (electronic and magnetic) degrees of freedom is directly included in the electronic structure calculations, whereas the vibrational free energy is evaluated by a proposed model that uses elastic constants to calculate average sound velocity of the quasiharmonic Debye model. The proposed scheme is tested by calculating the lattice parameter, heat capacity, and single-crystal elastic constants of alpha-, gamma-, and delta-iron as functions of temperature in the range 1000-1800 K. The calculations accurately reproduce the well-established experimental data on thermal expansion and heat capacity of gamma- and delta-iron. Electronic and magnetic excitations are shown to account for about 20% of the heat capacity for the two phases. Nonphonon contributions to thermal expansion are 12% and 10% for alpha- and delta-Fe and about 30% for gamma-Fe. The elastic properties predicted by the model are in good agreement with those obtained in previous theoretical treatments of paramagnetic phases of iron, as well as with the bulk moduli derived from isothermal compressibility measurements [N. Tsujino et al., Earth Planet. Sci. Lett. 375, 244 (2013)]. Less agreement is found between theoretically calculated and experimentally derived single-crystal elastic constants of gamma- and delta-iron.

  • 2.
    Ehteshami, Hossein
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ruban, Andrei V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Mat Ctr Leoben Forsch GmbH.
    High-temperature thermophysical properties of gamma- and delta-Mn from first principles2018In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 2, no 3, article id 034405Article in journal (Refereed)
    Abstract [en]

    Thermophysical properties of gamma-and delta-Mn phases have been investigated using first-principles calculations in their thermodynamically stable temperature range. An adiabatic approximation is used for partitioning of the Helmholtz free energy into electronic, magnetic, and vibrational contributions from the corresponding temperature induced excitations, where the fastest degree of freedom has been included in the slower ones. Namely, electronic excitations (on a one-electron level) have been included directly in the first-principles calculations at the corresponding temperatures. Magnetic excitations in the paramagnetic state then have been taken into consideration in the two opposite limits: localized, considering only transverse spin fluctuations (TSF), and itinerant, allowing for the full coupling of transverse and longitudinal spin fluctuations (LSF). Magnetic contribution to the free energy has been included in the calculations of the vibrational one, which has been obtained within the Debye-Gruneisen model. The calculated thermophysical properties such as lattice constance, thermal lattice expansion, and heat capacity are in good agreement with available experimental data, especially in the case when the itinerant magnetic model is chosen. We also present our results for elastic properties at high temperatures.

  • 3.
    Ehteshami, Hossein
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Ruban, Andrei V.
    KTH, Superseded Departments (pre-2005), Materials Science and Engineering.
    High-temperature thermophysical properties of γ- and δ-Mn from first principlesIn: Physical Review Materials, ISSN 2475-9953Article in journal (Refereed)
  • 4.
    Ehteshami, Hossein
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ruban, Andrei V.
    KTH, Superseded Departments (pre-2005), Materials Science and Engineering.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Role of defects in Ti(O,C)Manuscript (preprint) (Other academic)
  • 5.
    Yan, Jia-Yi
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ehteshami, Hossein
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Borgenstam, Annika
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sigma 3(111) grain boundary of body-centered cubic Ti-Mo and Ti-V alloys: First-principles and model calculations2017In: PHYSICAL REVIEW MATERIALS, ISSN 2475-9953, Vol. 1, no 2, article id 023602Article in journal (Refereed)
    Abstract [en]

    The energetics and atomic structures of Sigma 3[1 (1) over bar0](111) grain boundary (GB) of body-centered cubic (bcc) Ti-Mo and Ti-V alloys are investigated using density-functional-theory calculations and virtual crystal approximation. The electron density in bcc structure and the atomic displacements and excess energy of the GB are correlated to bcc-omega phase stability. Model calculations based on pairwise interplanar interactions successfully reproduce thechemical part of GB energy. The chemical GB energy can be expressed as a sum of excess pairwise interactions between bcc (111) layers, which are obtained from Gaussian elimination of the total energies of a number of periodic structures. The energy associated with the relaxation near the GB is solved by numerical minimization using the derivatives of the excess interactions. Anharmonic interlayer interactions are necessary for obtaining accurate relaxation energy and excess GB volume from model calculations. The effect of GB on vibrational spectrum is also investigated. Segregation energies of B and Y to a substitutional site on the GB plane are calculated. Preliminary results suggest that Y tends to segregate, while B tends to antisegregate.

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