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1.
  • Hu, Qing-Miao, et al. (författare)
  • Hardness and elastic properties of covalent/ionic solid solutions from first-principles theory
  • 2008
  • Ingår i: Journal of Applied Physics. - 0021-8979. ; 103:8, s. 083505
  • Tidskriftsartikel (refereegranskat)abstract
    • Most of the engineering materials are alloys (solid solutions) and inevitably contain some impurities or defects such as vacancies. However, theoretical predictions of the hardness of this kind of materials have rarely been addressed in literature. In this paper, a hardness formula for multicomponent covalent solid solution is proposed based on the work of Simunek and Vackar [Phys. Rev. Lett. 96, 085501 (2006)]. With this formula, the composition dependence of the hardness is investigated for titanium nitrogencarbide (TiN1-xCx), off-stoichiometric transition-metal nitrides (TiN1-x and VN1-x), and B-doped semiconductors. The predicted hardness is in good agreement with experiments. To investigate the most frequently quoted correlation between hardness and elastic modulus, the elastic moduli of the systems involved in this paper have also been calculated. The results show that the elastic moduli cannot be used for rigorous predictions of the hardness of the solid solutions.
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2.
  • Hu, Qing-Miao, et al. (författare)
  • Magnetoelastic effects in Ni2Mn1+xGa1-x alloys from first-principles calculations
  • 2010
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 81:6, s. 064108-1-064108-5
  • Tidskriftsartikel (refereegranskat)abstract
    • The magnetic coupling between Mn atoms on Ga sublattice (Mn-Ga) and Mn atoms on Mn sublattice (Mn-Mn) in Ni2Mn1+xGa1-x alloy and its effect on the elastic modulus of the alloy are investigated by the use of first-principles methods. It is shown that, for x = 0.25, the state with antiparallel Mn-Ga-Mn-Mn magnetic coupling is slightly more stable than that with parallel coupling, whereas for x = 0.10, both magnetic states are almost degenerated. For both antiparallel and parallel Mn-Ga-Mn-Mn magnetic couplings, the bulk modulus (B) of Ni2Mn1+xGa1-x deviates from the general e/a-B relationship with e/a being the number of valence electrons per atom. The shear modulus C' versus the martensitic transformation temperature T-M for Ni2Mn1+xGa1-x with antiparallel Mn-Ga-Mn-Mn magnetic coupling is in line with the general C'-T-M relationship for Ni2MnGa-based alloys, in contrast to the case of parallel Mn-Ga-Mn-Mn magnetic coupling.
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3.
  • Hu, Qing Miao, et al. (författare)
  • Predicting hardness of covalent/ionic solid solution from first-principles theory
  • 2007
  • Ingår i: Applied Physics Letters. - 0003-6951. ; 91:12, s. 121918
  • Tidskriftsartikel (refereegranskat)abstract
    • We introduce a hardness formula for the multicomponent covalent and ionic solid solutions. This expression is tested on nitride spinel materials A3N4 (A=C,Si,Ge) and applied to titanium nitrogen carbide (TiN1-xCx with 0<=x<=1), off-stoichiometric transition-metal nitride (TiN1-x and VN1-x with x<=0.25), and B-doped semiconductors (C1-xBx, Si1-xBx, and Ge1-xBx with x<=0.1). In all cases, the theoretical hardness is in good agreement with experiments.
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4.
  • Hu, Qing-Miao, et al. (författare)
  • Site occupancy, magnetic moments, and elastic constants of off-stoichiometric Ni2MnGa from first-principles calculations
  • 2009
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 79:14
  • Tidskriftsartikel (refereegranskat)abstract
    • The site occupancy and elastic modulus of off-stoichiometric Ni2MnGa alloys are investigated by the use of the first-principles exact muffin-tin orbital method in combination with coherent-potential approximation. The stable site occupancy at 300 K is determined by comparing the free energies of the alloys with different site-occupation configurations. It is shown that, for most of the off-stoichiometric Ni2MnGa, the "normal" site occupation is favorable, i.e., the excess atoms of the rich component occupy the sublattice(s) of the deficient one(s). Nevertheless, for the Ga-rich alloys, the excess Ga atoms have strong tendency to take the Mn sublattice no matter if Mn is deficient or not. Based on the determined site occupancy, the elastic moduli of the off-stoichiometric Ni2MnGa are calculated. We find that, in general, the bulk modulus increases with increasing e/a ratio (i.e., the number of valence electrons per atom). The shear moduli C-' and C-44 change oppositely with e/a ratio: C-' decreases but C-44 increases with increasing e/a. However, the Mn-rich Ga-deficient alloys deviate significantly from this general trend. The correlation of calculated elastic moduli and available experimental martensitic transformation temperatures (T-M) demonstrates that the alloy with larger C-' than that of the perfect Ni2MnGa generally possesses lower T-M except for Ni2Mn1+xGa1-x.
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5.
  • Li, Chun-Mei, et al. (författare)
  • First-principles investigation of the composition dependent properties of Ni2+xMn1-xGa shape-memory alloys
  • 2010
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 82:2, s. 024201-1-024201-9
  • Tidskriftsartikel (refereegranskat)abstract
    • The composition dependent lattice parameter, phase stability, elastic moduli, and magnetic transition temperature of the Ni2+xMn1-xGa shape-memory alloys are studied by using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation. The lattice parameter and tetragonal shear modulus of the cubic L-21 austenite phase decreases linearly with increasing concentration x of excess Ni atoms. The heats of formation of both cubic L-21 and tetragonal beta''' phases and their difference increase with x, indicating decreasing stability of the cubic and tetragonal phases and increasing driving force for the L-21 to beta''' martensitic transition. Investigating the electronic density of states, we find that the Ni-induced decreasing phase stability can mainly be ascribed to the weakening of the covalent bonding between minority spin states of Ni and Ga. Using the computed parameters, the composition dependence of the martensitic transition temperature is discussed. The theoretical Curie temperature, estimated from the Heisenberg model in combination with the mean-field approximation, is larger for the beta''' phase than for the L-21 phase. For both phases, the Curie temperature decreases nearly linearly with increasing x.
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6.
  • Li, Chun-Mei, et al. (författare)
  • First-principles study of the elastic properties of In-Tl random alloys
  • 2010
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 82:9, s. 094201
  • Tidskriftsartikel (refereegranskat)abstract
    • The composition-dependent lattice parameters and elastic constants of In1-xTlx(0<x <= 0.4) alloy in face-centered-cubic (fcc) and face-centered-tetragonal (fct) crystallographic phases are calculated by using the first-principles exact muffin-tin orbitals method in combination with coherent-potential approximation. The calculated lattice parameters and elastic constants agree well with the available theoretical and experimental data. For pure In, the fcc phase is mechanically unstable as shown by its negative tetragonal shear modulus C'. With Tl addition, C' of the fcc phase increases whereas that of the fct phase decreases, indicating that the fcc phase becomes mechanically more stable and the fct phase becomes less stable. In addition, the structural energy difference between the fcc and fct phases decreases with x. Both of these effects account for the observed lowering of the fcc-fct martensitic transition temperature upon Tl addition to In. The density of states indicates that the stability of the fct phase relative to the fcc one at low temperatures is due to the particular electronic structure of In and In-Tl alloys.
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7.
  • Li, Chun-Mei, et al. (författare)
  • Interplay between temperature and composition effects on the martensitic transformation in Ni(2+x)Mn(1-x)Ga alloys
  • 2011
  • Ingår i: Applied Physics Letters. - 0003-6951. ; 98:26, s. 261903
  • Tidskriftsartikel (refereegranskat)abstract
    • Martensitic transformation in Ni(2+x)Mn(1-x)Ga alloys is known to be controlled by the soft tetragonal elastic constant C' of the high-temperature austenitic phase. The temperature (T) and composition (x) dependence of C'(T, x) are calculated using the first-principles exact muffin-tin orbitals method. We show that the temperature factor of C' is dominated by the phonon-smearing term. 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) is found to lead to nearly constant C'(T(M)(x), x) at the critical temperature TM(x). We demonstrate that a proper account of the temperature and composition dependence of C'(T, x) is indispensable for reasonable theoretical TM(x) values.
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8.
  • Li, Chun-Mei, et al. (författare)
  • Magnetic ordering and physical stability of X2Mn1+xSn1-x (X = Ru, Os, Co, Rh, Ni, Pd, Cu, and Ag) Heusler alloys from a first-principles study
  • 2013
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 88:1, s. 014109
  • Tidskriftsartikel (refereegranskat)abstract
    • The magnetic ordering and its effect on the physical stability of X2Mn1+xSn1-x (0 <= x <= 0.5, and X = Ru, Os, Co, Rh, Ni, Pd, Cu, and Ag) Heusler alloys are investigated systematically by the use of first-principles method. It is found that the ferromagnetic (FM) coupling between Mn on Mn sublattice (Mn-1) and Mn on Sn sublattice (Mn-2) is favorable over the antiferromagnetic (AFM) coupling for X with the number of valence electrons [N-v(X)] of 8 and 9, and vice versa for X with N-v(X) = 10 and 11, originated from the competition of the exchange interactions between X-Mn-2 and Mn-1-Mn-2. In comparison with the FM Mn-1-Mn-2 coupling, the AFM coupling decreases significantly the shear elastic constant C' but increases slightly C-44, which results in increasing elastic anisotropy (A = C-44/C') and consequently may facilitate the tetragonal shear lattice deformation. The hybridization of the minority electronic states between X d and Sn p plays a dominant role on the orientation of the magnetic coupling. The smaller change of the density of states in the Fermi level, induced by the lattice distortion for C', corresponds to the softer C' as well as the larger A in the AFM state than the FM one.
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9.
  • Li, Chun-Mei, et al. (författare)
  • Role of magnetic and atomic ordering in the martensitic transformation of Ni-Mn-In from a first-principles study
  • 2012
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 86:21, s. 214205
  • Tidskriftsartikel (refereegranskat)abstract
    • The composition-dependent lattice parameters, crystal structure, elastic properties, magnetic moment, and electronic structure of Ni2Mn1+xIn1-x (0 <= x <= 0.6) are studied by using first-principles calculations. It is shown that the martensitic phase transition (MPT) from cubic L2(1) to tetragonal L1(0) accompanies theMn(Mn)-Mn-In ferromagnetic (FM) to antiferromagnetic (AFM) transition, at around the critical composition x = 0.32, in agreement with the experimental measurement. The Mn-In atomic disorder leads to decreasing stability of the martensite relative to the austenite, which depresses the MPT. The shear elastic constant C' of the parent phase first decreases slightly with increasing x and then remains almost unchanged above x = 0.32, indicating C' alone cannot account for the increase of the MPT temperature with x. The total magnetic moments for the L2(1) phase are in good agreement with those determined by experiments, whereas for the L1(0) phase they are slightly larger than the experimental data due to the possibleMn-In atomic disorder in the sample. The calculated density of states demonstrate that the covalent bonding between the minority spin states of Ni and In plays an important role in both the magnetic and structural stability.
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10.
  • Li, Chun-Mei, et al. (författare)
  • Site preference and elastic properties of Fe-, Co-, and Cu- doped Ni(2)MnGa shape memory alloys from first principles
  • 2011
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 84:2, s. 024206
  • Tidskriftsartikel (refereegranskat)abstract
    • The site preference and elastic properties of Fe-, Co-, and Cu-doped Ni(2)MnGa alloys are investigated by using the first-principles exact muffin-tin orbital method in combination with coherent-potential approximation. It is shown that Fe atom prefers to occupy the Mn and Ni sublattices even in Ga-deficient alloys; Co has strong tendency to occupy the Ni sublattice in all types of alloys; Cu atoms always occupy the sublattice of the host elements in deficiency. For most of the alloys with stable site occupations, both the electron density n and the shear modulus C' can be considered as predictors of the composition dependence of the martensitic transition temperature T(M) of the alloys. The physics underlying the composition-dependent C' are discussed based on the calculated density of states.
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