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Träfflista för sökning "swepub ;pers:(Johansson Börje);conttype:(refereed);pers:(Hu Qing Miao)"

Sökning: swepub > Johansson Börje > Refereegranskat > Hu Qing Miao

  • Resultat 21-30 av 31
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21.
  • Luo, Hu-Bin, et al. (författare)
  • Magnetic properties and temperature-dependent half-metallicity of Co2Mn(Ga(1-x)Z(x)) (Z = Si, Ge, Sn) from first-principles calculation
  • 2013
  • Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 25:15, s. 156003-
  • Tidskriftsartikel (refereegranskat)abstract
    • Using the first-principles exact muffin-tin orbitals method in combination with the coherent potential approximation, we investigated the magnetic properties, exchange interactions, and temperature-dependent half-metallicity of the Co2Mn(Ga(1-x)Z(x)) (Z = Si, Ge, Sn) alloys. The total magnetic moment follows perfectly a previously proposed Slater-Pauling relation, i.e., mu(0) = N-t - 24, with N-t being the number of valence electrons. The Co-Mn and Co1-Co2 (inter-sublattice) interactions are dominated by direct exchange, whereas the Co1-Co1 (intra-sublattice) interaction is characterized by superexchange. The Mn-Mn exchange interaction in Co2MnGa is of long-ranged RKKY-type. However, the Mn-Mn exchange interactions in Co(2)MnZ are relatively localized and can be attributed to superexchange. The Co-Mn, Co1-Co2 and Co1-Co1 total exchange interactions increase with x, whereas the Mn-Mn total exchange interactions show convex behavior. The calculated Curie temperature (T-C) increases with x. The ability of Z to enhance T-C follows the sequence of Si > Ge > Sn, in agreement with the experimental findings. The temperature dependence of the spin polarization at the Fermi level [P(T)] is investigated based on the disordered local moment model. P(T) drops abruptly at temperatures much lower than T-C. At temperatures higher than 200 K, the composition with higher TC generally corresponds to larger P(T).
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22.
  • Luo, Hu-Bin, et al. (författare)
  • Phase stability of Ni-2(Mn1-xFex)Ga : A first-principles study
  • 2012
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:2, s. 024427-
  • Tidskriftsartikel (refereegranskat)abstract
    • Ni-2(Mn1-xFex)Ga ferromagnetic shape memory alloy shows unusual composition-dependent martensitic transformation temperature (T-M), namely, T-M decreases with increasing e/a ratio. In hope of understanding this unusual behavior, we investigated the composition dependence of the heat of formation (H-f) and shear elastic modulus (C') of the cubic austenite as well as the stability of the five-layer modulated (5M) tetragonal martensite relative to the austenite, using first-principles exact muffin-tin orbital method in combination with coherent potential approximation. Our calculations demonstrated that H-f of the austenite increases with the Fe content x. C' increases slightly with x up to 0.05 but decreases thereafter. The composition dependence of both H-f and C' cannot fully account for the trend of T-M against x although such correlations have been proposed in literature for other Ni-Mn-Ga based alloys. The structure of 5M martensite phase of Ni-2(Mn1-xFex)Ga with 0 < x < 0.2 is determined by optimizing both the shear (changing c/a) and wavelike shuffle of atoms in (110) planes along [1 (1) over bar0] direction adopting the experimentally determined modulation function. The energy difference Delta E-AM between the austenite and 5M phases decreases with increasing x up to 0.05, following the lower Delta E-AM corresponding to lower T-M rule. However, with x larger than 0.05, Delta E-AM increases, against the experimental T-M similar to x behavior. We propose that, if taking the temperature effect and the spin-orbital coupling into account, the Delta E-AM similar to x curve might be altered and may explain the unusual composition dependence of Ni-2(Mn1-xFex)Ga.
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23.
  • Tian, Li-Yun, et al. (författare)
  • Elastic constants of random solid solutions by SQS and CPA approaches : the case of fcc Ti-Al
  • 2015
  • Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 27:31
  • Tidskriftsartikel (refereegranskat)abstract
    • Special quasi-random structure (SQS) and coherent potential approximation (CPA) are techniques widely employed in the first-principles calculations of random alloys. Here we scrutinize these approaches by focusing on the local lattice distortion (LLD) and the crystal symmetry effects. We compare the elastic parameters obtained from SQS and CPA calculations, taking the random face-centered cubic (fcc) Ti1-xAlx (0 <= x <= 1) alloy as an example of systems with components showing different electronic structures and bonding characteristics. For the CPA and SQS calculations, we employ the Exact Muffin-Tin Orbitals (EMTO) method and the pseudopotential method as implemented in the Vienna Ab initio Simulation Package (VASP), respectively. We show that the predicted trends of the VASP-SQS and EMTO-CPA parameters against composition are in good agreement with each other. The energy associated with the LLD increases with x up to x = 0.625 similar to 0.750 and drops drastically thereafter. The influence of the LLD on the lattice constants and C12 elastic constant is negligible. C-11 and C-44 decrease after atomic relaxation for alloys with large LLD, however, the trends of C-11 and C-44 are not significantly affected. In general, the uncertainties in the elastic parameters associated with the symmetry lowering turn out to be superior to the differences between the two techniques including the effect of LLD.
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24.
  • Wang, Guisheng, et al. (författare)
  • Ab initio investigation of the elastic properties of Ni3Fe
  • 2013
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:17, s. 174205-
  • Tidskriftsartikel (refereegranskat)abstract
    • Ab initio alloy theory, formulated within the exact muffin-tin orbitals method in combination with the coherent-potential approximation, is used to determine the elastic properties of Ni-Fe alloys with Fe:Ni ratio 1:3. The interplay between magnetic and chemical effects is investigated by computing the lattice parameters and the single-and polycrystal elastic moduli for different partially ordered structures in the ferro-and paramagnetic states. It is found that the influence of long-range chemical order on the bulk properties strongly depends on the magnetic state. The largest magnetic-order-induced changes are obtained for the chemically ordered L1(2) phase. The ferromagnetic L1(2) system possesses similar to 5.4% larger elastic Debye temperature than the paramagnetic L1(2) phase, which in turn has a similar Theta(D) as the chemically disordered face-centered cubic phase in either the ferro-or paramagnetic state. It is concluded that magnetic ordering has a substantially larger impact on the bulk parameters of Ni3Fe than chemical ordering. The calculated trends are explained based on the electronic structure of nonmagnetic, ferromagnetic, and paramagnetic ordered and disordered phases.
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25.
  • Wang, Guisheng, 1985-, et al. (författare)
  • Ab initio prediction of the mechanical properties of alloys : The case of Ni/Mn-doped ferromagnetic Fe
  • 2015
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:22, s. 224203-
  • Tidskriftsartikel (refereegranskat)abstract
    • First-principles alloy theory, formulated within the exact muffin-tin orbitals method in combination with the coherent-potential approximation, is used to study the mechanical properties of ferromagnetic body-centered cubic (bcc) Fe1−xMx alloys (M=Mn or Ni, 0≤x≤0.1). We consider several physical parameters accessible from \emph{ab initio} calculations and their combinations in various phenomenological models to compare the effect of Mn and Ni on the properties of Fe. Alloying is found to slightly alter the lattice parameters and produce noticeable influence on elastic moduli. Both Mn and Ni decrease the surface energy and the unstable stacking fault energy associated with the {110} surface facet and the {110}⟨111⟩ slip system, respectively. Nickel is found to produce larger effect on the planar fault energies than Mn. The semi-empirical ductility criteria by Rice and Pugh consistently predict that Ni enhances the ductility of Fe but give contradictory results in the case of Mn doping. The origin of the discrepancy between the two criteria is discussed and an alternative measure of the ductile-brittle behavior based on the theoretical cleavage strength and single-crystal shear modulus G{110}⟨111⟩ is proposed.
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26.
  • Wang, Gui-Sheng, et al. (författare)
  • The effect of long-range order on the elastic properties of Cu3Au
  • 2013
  • Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 25:8, s. 085401-
  • Tidskriftsartikel (refereegranskat)abstract
    • Ab initio calculations, based on the exact muffin-tin orbitals method are used to determine the elastic properties of Cu-Au alloys with Au/Cu ratio 1/3. The compositional disorder is treated within the coherent potential approximation. The lattice parameters and single-crystal elastic constants are calculated for different partially ordered structures ranging from the fully ordered L1(2) to the random face centered cubic lattice. It is shown that the theoretical elastic constants follow a clear trend with the degree of chemical order: namely, C-11 and C-12 decrease, whereas C-44 remains nearly constant with increasing disorder. The present results are in line with the experimental findings that the impact of the chemical ordering on the fundamental elastic parameters is close to the resolution of the available experimental and theoretical tools.
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27.
  • Zhang, Hualei, et al. (författare)
  • Static equation of state of bcc iron
  • 2010
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:13, s. 132409-
  • Tidskriftsartikel (refereegranskat)abstract
    • Body-centered-cubic (bcc) iron is one of the most investigated solid-state systems. Using four different density-functional methods, we show that there is a magnetic transition close to the ground-state volume of bcc Fe, which originates from the particular magnetic band structure. The common equation of state functions, used to determine the basic ground-state physical quantities from the calculated total energies, cannot capture the physics of this magnetic transition leading to serious underestimation of the Fe bulk modulus. Ignorance of the magnetic transition found here is reflected by large scatter of the published theoretical bulk moduli of ferromagnetic bcc Fe. Due to the low performance of the exchange-correlation functionals, most of the erroneous results are accidentally in good agreement with the experimental values. The present finding is of fundamental importance, especially taking into account that bcc Fe is frequently used as a test system in assessing the performance of exchange-correlation approximations or total-energy methods.
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28.
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29.
  • Li, Chun-Mei, et al. (författare)
  • Temperature dependence of elastic properties of Ni(2+x)Mn(1-x)Ga and Ni(2)Mn(Ga(1-x)Al(x)) from first principles
  • 2011
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:17, s. 174117-
  • Tidskriftsartikel (refereegranskat)abstract
    • The temperature dependence of the elastic properties of Ni(2+x)Mn(1-x)Ga and Ni(2)Mn(Ga(1-x)Al(x)) (x = 0.0, 0.1, and 0.2) random alloys are investigated by using the first-principles exact muffin-tin orbitals method. At 0 K, the calculated equilibrium parameters in both cubic L2(1) and nonmodulated tetragonal beta'''-Ni(2)MnGa are in good agreement with the available experimental data and other theoretical results. Separating the thermal effects into single electron excitation, volume expansion, phonon smearing, and magnetic terms, we find that phonon smearing gives the dominating positive tetragonal elastic constant (C') versus temperature (T) slope for the cubic phase. For Ni(2+x)Mn(1-x)Ga, the competition between the negative alloying effect (partial derivative C'/partial derivative x < 0) and the positive temperature effect (partial derivative C'/partial derivative T > 0) leads to nearly constant C'[x, T(M)(x)] at the martensitic transition temperature T(M)(x). For Ni(2)Mn(Ga(1-x)Al(x)), where both partial derivative C'/partial derivative x and partial derivative C'/partial derivative T are positive, however, due to the significantly decrease of T(M)(x), the critical C'[x, T(M)(x)] slightly decreases with Al doping. Furthermore, it is demonstrated that both the composition and the temperature dependence of C' are indispensable to get a reasonable theoretical T(M)(x).
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30.
  • Lu, Song, et al. (författare)
  • First-principles determination of the alpha-alpha ' interfacial energy in Fe-Cr alloys
  • 2010
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:19, s. 195103-
  • Tidskriftsartikel (refereegranskat)abstract
    • The interfacial energies (gamma) between the Cr-rich alpha'-FexCr1-x and Fe-rich alpha-Fe1-yCry phases (0 < x, y < 0.35) are calculated to be between similar to 0.02 and similar to 0.33 J m(-2) for the ferromagnetic state and between similar to 0.02 and similar to 0.27 J m(-2) for the paramagnetic state. Although for both magnetic states, the interfacial energy follows a general decreasing trend with increasing x and y, the fine structures of the gamma(x, y) maps exhibit a marked magnetic state dependence. The subtleties are shown to be ascribed to the magnetic interaction between the Fe and Cr atoms near the interface. The theoretical results are applied to estimate the critical grain size for nucleation and growth in Fe-Cr stainless steel alloys.
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  • Resultat 21-30 av 31

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