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By using first principles in combination with atomistic spin dynamics calculational methods, we determine the temperature-dependent free energies of the L21-and L10-Ni2(Mn1-xFex) Ga (0 ≤ x ≤ 1), including phonon vibrational and magnetic energies. The x-dependent martensitic phase transformation (MPT) temperature (TM) and the Curie temperature (TC) are well represented. It is found that, the abnormal nonmonotonic behavior of TM similar to x mainly originates from the phonon vibrational free energy. The magnetic energy, which does not change the trend of TM ~ x but decreases the driving force of the MPT, is indispensable as well to get reasonable TM values. This insight provides a good understanding of the physical mechanisms driving the MPT of Ni2(Mn1-xFex)Ga, and promotes the improvement of their magnetic shape memory properties. [ABSTRACT FROM AUTHOR]

The incorporation of cubic β/β0 phase improves significantly the ductility and workability of TiAl-based alloys. However, the ductility deteriorates with the precipitation of the ordered hexagonal ω0 phase in the β/β0 phase. To avoid the formation of the ω0 phase in alloy design, fundamentals about the β/β0 to ω0 phase transition and its alloying element dependence have to be known. In the present work, we investigated the phase transition and phase stability in Ti4Al3Nb and Ti4Al3Mo alloys using a first-principles method based on density functional theory. Our calculations indicate that for the β0 to ω0 transition, the "collapse-diffusion" pathway is easier than the "diffusion-collapse" one; however, both pathways are quite difficult due to the high-energy barriers. The pa to co" transition occurs through the "diffusion-collapse" pathway without an energy barrier (except for the activation energy for the short-range diffusion needed for atomic rearrangement). Comparing the total energies of different phases, we find that the ω0 phase of Ti4Al3Nb is more stable than β0 and vice versa for Ti4Al3Mo whereas the relative stability of ω" to β0 for Ti4Al3Nb is stronger than that for Ti4Al3Mo, indicating that the addition of Mo inhibits the transition from the β0- to the ω-related phases in Ti4Al3Nb. Our calculations demonstrate that ω" is more stable than ω0 for both Ti4Al3Nb and Ti4Al3Mo, in contrast to the previously proposed picture based on some experimental observations, the possible origins of which are discussed in detail. [ABSTRACT FROM AUTHOR]

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 L12 phase. The ferromagnetic L12 system possesses ~5.4% larger elastic Debye temperature than the paramagnetic L12 phase, which in turn has a similar ΘTD 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. [ABSTRACT FROM AUTHOR]

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 (Mn1) and Mn on Sn sublattice (Mn2) is favorable over the antiferromagnetic (AFM) coupling for X with the number of valence electrons [Nv(X)] of 8 and 9, and vice versa for X with Nv(X)=10 and 11, originated from the competition of the exchange interactions between X-Mn2 and Mn1-Mn2. In comparison with the FM Mn1-Mn2 coupling, the AFM coupling decreases significantly the shear elastic constant C′ but increases slightly C44, which results in increasing elastic anisotropy (A=C44/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. [ABSTRACT FROM AUTHOR]

Ab initio calculations are employed to determine the lower and upper bounds of the interfacial energy and work of separation of a fcc-Ag/bcc-Fe interface. The strain-free interfacial energy of the coherent interface is taken as the lower bound and the interfacial energy of the commensurate incoherent interface as the upper bound of the interfacial energy of a realistic semicoherent interface. The latter is estimated by applying an averaging scheme based on the interfacial energies obtained for the coherent interfaces. Similar calculations are performed for determining the bounds of the work of separation. We justify the use of the averaging scheme by carrying out large supercell calculations for a semicoherent interface. For a Fe(110)/Ag(111) semicoherent interface, we show that taking either Fe or Ag as the underlying lattice, our averaging scheme can yield a reasonable estimation of the work of separation of the semicoherent interface. However, when taking Ag as the underlying lattice, the averaged interfacial energy of the semicoherent interface is significantly underestimated due to the magnetism. The structure and magnetism at the coherent and semicoherent interfaces are discussed. [ABSTRACT FROM AUTHOR]

Ni2(Mn1-xFex)Ga ferromagnetic shape memory alloy shows unusual composition-dependent martensitic transformation temperature (TM), namely, TM decreases with increasing e/a ratio. In hope of understanding this unusual behavior, we investigated the composition dependence of the heat of formation (Hf) 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 Hf 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 Hf and C' cannot fully account for the trend of TM against x although such correlations have been proposed in literature for other Ni-Mn-Ga based alloys. The structure of 5M martensite phase of Ni2(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 [110] direction adopting the experimentally determined modulation function. The energy difference &Dgr;EAM between the austenite and 5M phases decreases with increasing x up to 0.05, following the lower &Dgr;EAM corresponding to lower TM rule. However, with x larger than 0.05, &Dgr;EAM increases, against the experimental TM ∼ x behavior. We propose that, if taking the temperature effect and the spin-orbital coupling into account, the &Dgr;EAM ∼ x curve might be altered and may explain the unusual composition dependence of Ni2(Mn1-xFex)Ga. [ABSTRACT FROM AUTHOR]

The structural phase transitions of yttrium under high pressure up to 300 GPa have been studied by using the first-principles, pseudopotential plane-wave method based on density functional theory. The experimental rare-earth structure sequence with increasing pressure is successfully reproduced. The yet unknown distorted lace-centered cubic (dfcc) phase is determined to be of the R3̄m space group. In addition, the P6222 phase is predicted to be energetically more stable beyond the pressure range for dfcc. Accurate phonon-dispersion calculations demonstrate that the P6222 phase is dynamically stable beyond pressure of about 206 GPa. The corresponding projected band structures, density of states, and charge density differences are given in order to clarify the electronic origins of the stability of the P6222 phase under pressure. [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]

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 L21 to tetragonal L10 accompanies the MnMn-Mnin 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 L21 phase are in good agreement with those determined by experiments, whereas for the L10 phase they are slightly larger than the experimental data due to the possible Mn-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. [ABSTRACT FROM AUTHOR]

The temperature dependence of the elastic properties of Ni2+xMn1-xGa and Ni2Mn(Ga1-xAlx) (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 L21 and nonmodulated tetragonal β"'-Ni2MnGa 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 Ni2+xMn1-xGa, the competition between the negative alloying effect (∂C'/∂x < 0) and the positive temperature effect (∂C'/∂T > 0) leads to nearly constant C'[x, TM(x)] at the martensitic transition temperature TM(x). For Ni2Mn(Ga1-xAlx), where both ∂C'/∂x and ∂C'/∂T are positive, however, due to the significantly decrease of TM(x), the critical C'[x, TM(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 TM(x). [ABSTRACT FROM AUTHOR]

The site preference and elastic properties of Fe-, Co-, and Cu-doped Ni2MnGa 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 TM of the alloys. The physics underlying the composition-dependent C' are discussed based on the calculated density of states. [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]