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First principle calculations using supercell approach and coherent potential approximation (CPA) are performed to investigate the electronic and magnetic structures of Fe3-xMnxSi and Fe3-yMnySiy alloys, where x, y=0, 0.25, 0.50, 0.75, 1.00, 1.25, 1.5, 1.75, and 2.25. Using supercell calculations we obtained a metallic behavior for x=0, 0.25, and 0.5 in Fe3-xMnxSi alloys with spin polarizations of 24%, 39%, and 93%, respectively. The behavior starts to be half-metallic at x=0.75 with a small direct band gap that increases for higher concentrations of Mn. Among the half-metallic systems, only those of L21 structure at x=1 and 2 possess indirect band gaps along Γ-X symmetry line. The change of Si concentration in Fe3-yMnySiy structures retrieve the metallic behavior for all concentrations except y=1.25 that shows a half-metallic behavior with a direct band gap of 0.27 eV. We obtained a good agreement between supercell and CPA calculations for the values of the magnetic moment and the trends of the formation energies, which reveals the validity of the supercell approach in predicting the magnetic structure and the energetics of doped Heusler alloys. [ABSTRACT FROM AUTHOR]

Density functional theory calculations are performed to investigate the electronic and magnetic properties of FeCrSi alloys with Cr concentration in the range of 0.25 ≤ x ≤ 2.75. The L2 phase is found to be a more stable one in comparison with the A15 phase for x ≤ 1.50 beyond which the A15 phase becomes more stable. Alloys with the stable L2 phase are found to be metallic for x ≤ 0.75, however, a half metallic behavior is found at x = 1.00, 1.25, and 1.50 with band gaps of 0.60, 0.24, and 0.21 eV, respectively. In contrast, all A15 structures are found to be metallic. The total magnetic moments are found to decrease for L2 phase from 14.4 μ/cell at x = 0.25 to zero at x = 2.00 with non-integer values for the metallic structures and integer values for the half-metallic. However, a monotonic decrease is found for the case of A15 phase with values larger than those of L2 phase. [ABSTRACT FROM AUTHOR]

The influences of doping elements (Ti, Zr, Hf, V, Ta, Cr, Mo, W and Al) on the heats of formation of Nb, Nb5Si3 and Nb3Si have been studied using first-principles pseudopotential plane-wave method based on density functional theory. Site preferences of dopants in Nb5Si3 and Nb3Si intermetallics are first determined by comparing the heats of formation of the systems with different site occupations of the doping atoms. The partitionings of dopants between the equilibrium phases Nb and Nb5Si3 of Nb-Si in situ composites are then discussed and compared with experimental results. Phase partitioning behaviours of the alloying elements are found to depend strongly on the number of their valence electrons. By calculating the enthalpy of reaction regarding the eutectoid decomposition of Nb3Si phase, we conclude that Ta is a Nb3Si stabiliser while other dopants destabilise Nb3Si at the investigated concentration. [ABSTRACT FROM AUTHOR]

Martensitic transformation in Ni2+xMn1-xGa 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 (∂C′/∂x < 0) and the positive temperature effect (∂C′/∂T > 0) is found to lead to nearly constant C′(TM(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. [ABSTRACT FROM AUTHOR]

The composition-dependent crystal structure, elastic modulus, phase stability, and magnetic property of Ni2-xCoxMn1.60Sn0.40(0≤x≤0.50) are studied by using first-principles calculations in combination with atomistic spin dynamics method. It is shown that the present lattice parameters and Curie temperature (TC) are in agreement with the available experimental data. The martensitic phase transformation (MPT) occurs for x<0.43, where the austenite is in the ferromagnetic (FM) state whereas the martensite is in the antiferromagnetic (AFM) one at 0 K. The x dependence of the lattice parameter, elastic modulus, and energy difference between the FM austenite and the AFM martensite well accounts for the decrease of the MPT temperature (TM) with the Co addition. With increasing x, the increase of the magnetic excitation energy between the paramagnetic and FM austenite of these alloys is in line with the TC~x. The Ni 3d as well as the Co 3d electronic states near the Fermi level are confirmed mainly dominating the phase stability of the studied alloys. [ABSTRACT FROM AUTHOR]

First-principles alloy theory, formulated within the exact muffin-tin orbital 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 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 semiempirical 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. [ABSTRACT FROM AUTHOR]