First-principle calculations of structural,elastic and thermodynamic properties of Fe–B compounds

The structural, elastic and thermodynamic properties of FeB, Fe₂B, orthorhombic and tetrahedral Fe₃B, FeB₂ and FeB₄ iron borides are investigated by first-principle calculations. The elastic constants and polycrystalline elastic moduli of Fe–B compounds are usually large especially for FeB₂ and FeB₄, whose maximum elastic constant exceeds 700 GPa. All of the six compounds are mechanically stable. The Vickers hardness of FeB₂ is estimated to be 31.4 GPa. Fe₂B and FeB₂ are almost isotropic, while the other four compounds have certain degree of anisotropy. Thermodynamic properties of Fe–B compounds can be accurately predicted through quasi-harmonic approximation by taking the vibrational and electronic contributions into account. Orthorhombic Fe₃B is more stable than tetrahedral one and the phase transition pressure is estimated to be 8.3 GPa.     

Structural,electronic and magnetic properties of the Mn54−xAl46Tix (x = 2; 4) alloys

The structural, electronic and magnetic behavior of the as-cast and annealed Mn₅₂Al₄₆Ti₂ and Mn₅₀Al₄₆Ti₄ alloys have been studied through electronic band structure calculations, X-ray diffraction and magnetic measurements in the temperature range 4–850 K and magnetic field up to 7 T. Band structure calculations show a preference for Ti atoms to occupy the Mn sites in the plane of Al atoms with their magnetic moments (−0.68 μ_B/Ti) coupled antiparallel relative to the Mn magnetic moments in the plane of Mn atoms (2.33 μ_B/Mn). The as-cast and annealed samples were phase mixtures with different values of the hard ferromagnetic τ phase content. Except the as-cast and annealed at 1050 °C Mn₅₂Al₄₆Ti₂ alloys, all the analyzed samples include, along with the τ and γ₂ phases, a soft κ phase (CsCl - structure type) with T_C around 530 K. The best magnetic characteristics were obtained for Mn₅₂Al₄₆Ti₂ alloy annealed at 470 °C for 6 h: M_S = 116 A m²/Kg at 4 K and T_C = 668 K, in good agreement with the values reported in the literature for the τ phase of the MnAl system. The effects of the composition and of the preparation route on the electronic and magnetic properties are discussed in comparison with the properties of Mn₅₄Al₄₆ parent alloy.     

Structural,magnetic and thermal properties of intermetallics Nd3Fe27−xCoxV2 (x = 0, 2 and 4) and their hydrides

Intermetallics of the Nd₃Fe_27?xCo_xV₂ (x = 0, 2 and 4) system with monoclinic type structure and their hydrides were prepared by arc melting and exposed to hydrogenation. The effects of Co substitution of Fe on the magnetic hyperfine fields of intermetallic compound Nd₃Fe₂₇V₂, as well as the effects of interstitial hydrogen additions on the symmetry and local magnetic properties in these systems were studied. The investigations were performed by powder X-ray diffraction, Mössbauer spectroscopy and simultaneous TGA-DSC. Both site populations and magnetic hyperfine field strength were analyzed in order to discriminate between the Co and H contributions to the local atomic environments in these systems. The values of magnetic hyperfine fields corresponding to inequivalent lattice sites were found to be enhanced after the increase in Co content and hydrogenation. Slight changes in the relative site occupancies were observed. Simultaneous TGA-DSC studies showed that the amount of hydrogen insertion decreased slightly with the increase of Co contents, while the enthalpy associated with hydrogen desorption decreased sharply under heat treatment. The Nd₃Fe_27?xCo_xV₂ (x = 0, 2 and 4) and their hydrides are not stable under treatment and two exothermic peaks are observed in the DSC curves, corresponding to the decomposition of the monoclinic phase with the precipitation of α-Fe phase.     

Microstructure,phase evolution and magnetic properties of melt-spun SmCo6.8−xSnxZr0.2 (x = 0, 0.1 and 0.3) ribbons

SmCo_6.8?xSn_xZr_0.2 (x = 0, 0.1 and 0.3) melt-spun ribbons have been produced from bulk as-cast samples by varying wheel speeds from 8 to 32 m/s. The microstructure, phase evolution and magnetic properties of as-spun ribbon samples were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM), respectively. The XRD results show that the addition of Sn to SmCo_6.8Zr_0.2 alloy has significant role to shift the TbCu₇-structure into Sm₂Co₁₇(H) and Sm₂Co₁₇(R) -phases for the Sn content of 0.1 and 0.3, respectively. The SEM analysis reveals that the grain size of the as-spun ribbon samples is gradually refined from several microns to 600?800 nm with the increase of wheel speed from 8 to 32 m/s. A wheel speed of 24 m/s has been found to be optimal in achieving reasonably good combination of coercivity (H_c) and magnetization (M_s) values in the as-spun SmCo_6.8-xSn_xZr_0.2 (x = 0, 0.1 and 0.3) ribbons. Maximum values of H_c of 5.1 kOe and M_s of 76.9 emu/g were achieved for the SmCo_6.5Sn_0.3Zr_0.2 ribbons spun at 24 m/s; upon annealing at 550 °C, the H_c values of these ribbons were significantly improved (10.4 kOe) with not much destruction in the M_s value.     

Predictions of high-pressure structural, electronic and thermodynamic properties of α-Si3N4

The plane-wave pseudo-potential method within the framework of first-principles technique is used to investigate the fundamental structural properties of Si3N4 . The calculated ground-state parameters agree quite well with the experimental data. Our calculation reveals that α-Si3N4 can retain its stability to at least 45 GPa when compressed below 300 K. No phase transition can be seen in the pressure range of 0-45 GPa and the temperature range of 0-300 K. Actually, the α→β transition occurs at 1600 K and 7.98 GPa. Many thermodynamic properties, such as bulk modulus, heat capacity, thermal expansion, Gru¨neisen parameter and Debye temperature of α-Si3N4 were determined at various temperatures and pressures. Significant differences in these properties were observed at high temperature and high pressure. The calculated results are in good agreement with the available experimental data and previous theoretical values. Therefore, our results may provide useful information for theoretical and experimental investigations of the N-based hard materials like α-Si3N4.     

Crystal structure of τ5–TiNi2−xAl5 (x = 0.48) and isotypic {Zr,Hf}Ni2−xAl5−y

The crystal structure of the compound in the Al-rich region of the Ti–Ni–Al system, τ₅–TiNi_2?xAl₅, x = 0.48, has been derived from X-ray powder and single crystal, neutron powder and electron diffraction (space group I4/mmm, a = 0.3984(2) nm, c = 1.4073(3) nm, R_F2 = 0.0133). Titanium atoms were unambiguously located from neutron powder data. τ₅ is isotypic with the crystal structure of ZrNi₂Al₅. Detailed transmission electron microscopy (TEM) in several crystallographic directions confirmed the lattice parameters and crystal symmetry. Although occupancy of Ni in the 4e site revealed a defect (occ. = 0.76), no significant homogeneity region was observed for this phase at 1020°C. Rietveld analyses of X-ray powder diffraction data for the Zr- and Hf-homologues confirmed for both compounds isotypism and revealed defects in the Ni sites and to a lesser extent also in the Al sites: ZrNi_2?xAl_5?y, x = 0.4, y = 0.4 and HfNi_2?xAl_5?y, x = 0.5, y = 0.2. The crystallographic relations among the structure types of Cu, TiAl₃, ZrNi₂Al₅ and Zr(Ni,Ga)₇ have been defined in terms of a Bärnighausen scheme.     

Stability,elastic and electronic properties of the Rh–Zr compounds from first-principles calculations

A systematic investigation on structural, elastic and electronic properties of Rh–Zr intermetallic compounds is conducted using first-principles electronic structure total energy calculations. The equilibrium lattice parameters, enthalpies of formation (E_for), cohesive energies (E_coh) and elastic constants are presented. Of the eleven considered candidate structures, Rh₄Zr₃ is most stable with the lowest E_for. The two orthogonal-type, relative to the CsCl-type, are the competing ground-state structures of RhZr. The result is in agreement with the experimental reports in the literature. The analysis of E_for and mechanical stability excludes the presence of Rh₂Zr and RhZr₄ at low temperature mentioned by .Curtarolo et al. [Calphad 29, 163 (2005)]. It is found that the bulk modulus B increases monotonously with Rh concentration, whereas all other quantities (shear modulus G, Young's modulus E, Poisson's ratio σ and ductility measured by B/G) show nonmonotonic variation. RhZr₂ exhibits the smallest shear/Young's modulus, the largest Poisson's ratio and ductility. Our results also indicate that all the Rh–Zr compounds considered are ductile. Furthermore, the detailed electronic structure analysis is implemented to understand the essence of stability.     

Structure and magnetic properties of intermetallic compounds Gd5Si2-xGe2-xM2x

1 Introduction Nowadays, there is much interest in magnetic refrigeration materials, as they offer the prospect of an energy-efficient and environment friendly alternative for the traditional vapour cycle refrigeration technique. Magnetic refrigeration is…     

Correlation of the atomic and electronic structures and the optical properties of the Σ5(2 1 0)/[ 0 01] symmetric tilt grain boundary in yttrium aluminum garnet

A series of atomic models of the Σ5(2 1 0)/[0 0 1] symmetric tilt grain boundary in yttrium aluminum garnet (YAG) are constructed according to the coincident site lattice theory. Calculations performed using empirical potentials show that the O-termination configurations, namely G(1) and G(2), are the most energetically favorable boundary structures. First-principles density functional theory calculations have been further performed to understand the atomic and electronic structures, effective charge, potential distributions and optical properties of G(1) and G(2) grain boundaries. The simulated high-resolution transmission electron microscopy images of G(1) and G(2) are generally in good agreement with the experimental micrographs of the Σ5(2 1 0)/[0 0 1] grain boundaries. Results show that the effective charges of the atoms in the grain boundary region are related to their coordination numbers and bonding lengths. Moreover, the overall total density of states of G(1) and G(2) has been found to have similar features with the bulk YAG, with the exception of some defect states being introduced at the top of the valence band, resulting in the reduction of the band gap. The calculated optical properties show that the refractive indices of both grain boundary models are slightly larger than those of the bulk YAG. This is in agreement with the experimental observation that the refractive index of the polycrystalline YAG is higher than that of its single crystalline counterpart.     

Structure and magnetic properties of SmxCo72−xFe16Zr7B4Cu1 (x = 8, 12, 15) alloys

Sm_xCo_72−xFe₁₆Zr₇B₄Cu₁ (x = 8, 12, 15) alloys with low level of Sm and mixed addition of several elements (Fe, Zr, B, Cu) were prepared via arc melting and rapid quenching technique. The influences of Sm content and cooling rate on the microstructure and magnetic properties of alloys were investigated. The ribbons melt-spun at 40 m/s present a novel composite microstructure with nanocrystalline phases embedded in amorphous matrix. Such a unique structure endows the x = 12 ribbons a coercivity as high as ∼20,000 Oe. Ferromagnetism of the amorphous matrix, the pinning effect of amorphous phase during the movement of domain-walls and exchange interactions among adjacent grains may contribute to the high coercivity performance.     

Structural,magnetic and magnetoelastic properties of Laves-phase Tb0.3Dy0.6Nd0.1(Fe1-xCox)1.93 compounds (0 ≤ x ≤ 0.40)

Structure, magnetization and magnetostriction of Tb_0.3Dy_0.6Nd_0.1(Fe_1-xCo_x)_1.93 (0 ≤ x ≤ 0.40) compounds have been investigated. X-ray diffraction (XRD) analysis shows the presence of single Laves phase with a cubic MgCu₂-type structure. The easy magnetization direction (EMD) is observed to be <111> direction when x ≤ 0.10, and deviates away with further increasing Co contents. The Co contributes an opposite role in the resultant magnetocrystalline-anisotropy compared to Tb. A small amount of Co substitution for Fe can enhance the Curie temperature for x ≤ 0.25. The magnetocrystalline-anisotropy compensation can be achieved in this system. A minimum in anisotropy is obtained for the Tb_0.3Dy_0.6Nd_0.1(Fe_0.8Co_0.2)_1.93 compound, which has good magneto-elastic properties, e.g., the large saturation magnetostriction λ_S (∼930 ppm) and the high low-field magnetostriction λ_a (∼670 ppm/3 kOe), and may make it technological interest in the field of magnetostrictive materials.     

Microstructural evolution of intermetallic compounds in Sn–3.5Ag–X (X = 0, 0.75Ni, 1.0Zn and 1.5In)/Cu solder joints during liquid aging

In this paper, the microstructural evolution of IMCs in Sn–3.5Ag–X (X = 0, 0.75Ni, 1.0Zn, 1.5In)/Cu solder joints and their growth mechanisms during liquid aging were investigated by microstructural observations and phase analysis. The results show that two-phase (Ni₃Sn₄ and Cu₆Sn) IMC layers formed in Sn–3.5Ag–0.75Ni/Cu solder joints during their initial liquid aging stage (in the first 8 min). While after a long period of liquid aging, due to the phase transformation of the IMC layer (from Ni₃Sn₄ and Cu₆Sn phases to a (Cu, Ni)₆Sn₅ phase), the rate of growth of the IMC layer in Sn–3.5Ag–0.75Ni/Cu solder joints decreased. The two Cu₆Sn₅ and Cu₅Zn₈ phases formed in Sn–3.5Ag–1.0Zn/Cu solder joints during the initial liquid aging stage and the rate of growth of the IMC layers is close to that of the IMC layer in Sn–3.5Ag/Cu solder joints. However, the phase transformation of the two phases into a Cu–Zn–Sn phase speeded up the growth of the IMC layer. The addition of In to Sn–3.5Ag solder alloy resulted in Cu₆(Sn_x,In_1?x)₅ phase which speeded up the growth of the IMC layer in Sn–3.5Ag–1.5In/Cu solder joint.     

Magnetocaloric properties of a novel ferromagnet Gd3Co4+xAl12−x (x = 0.50)

A novel gadolinium and cobalt based aluminide, namely Gd₃Co_4+xAl_12−x (x = 0.50(3)) is shown to crystallize in the hexagonal Gd₃Ru₄Al₁₂ structure-type (space-group P6₃/mmc, a = 8.6830(1) Å, c = 9.3164(2) Å at room temperature). The investigation of its magnetic properties revealed a reversible ferromagnetic ordering at T_C ≈ 40 K but expanding in a wide temperature range, potentially due to the geometrical magnetic frustrations expected in this structure-type. The magnetocaloric properties of this intermetallic, determined from both magnetization and specific heat data, are dominated by a maximum entropy variation of −3.28 J kg⁻¹ K⁻¹ at 39(1) K and an adiabatic temperature change of about 1.5 K extending from 20 to 40 K, highlighting some interest for low temperature applications.     

Role of crystalline precipitates on the mechanical properties of (Cu0.50Zr0.50)100−xAlx (x = 4, 5, 7) bulk metallic glasses

The mechanical behaviour upon compression of (Cu_0.50Zr_0.50)_100?xAl_x (x = 4, 5, 7) rods with 2 and 3 mm diameter was systematically studied and compared with the literature data. Fully amorphous bulk metallic glasses (x = 5, 7) show little permanent deformation and reproducible yield stress values. The remarkable fracture strain, observed for some apparently X-ray diffraction amorphous samples (x = 4), was found to be due to significant amounts (at least 20%) of the B2-CuZr crystalline phase. The effect of possible flaws on the external surface of the rods was evaluated by mechanical testing of either as cast and machined samples.     

Microstructure and shape-memory characteristics of Ni56Mn25−xCoxGa19 (x = 4, 8) high-temperature shape-memory alloys

NiMnCoGa alloys were studied as candidate of high-temperature shape-memory alloys with Mn substituted by Co. The results show that the hot workability and room-temperature ductility of NiMnGa alloys can be greatly improved by forming ductile γ phase through Co addition. When the amount of γ phase is about 19%, the tensile stress and strain are 491 MPa and 8.2%, respectively. When the amount of γ phase increased to about 43%, the tensile stress and strain are 729.3 MPa and 14.1%, respectively. The reversible strain due to shape-memory effect is 2.1% under a prestrain of 5.3% for x = 4, and the shape-memory ability almost disappears for x = 8 due to large amount of γ phase, which seriously hampers the reorientation of martensitic variants and/or detwinning.     

Glass-like thermal conductivities in (La1-x1Yx1)2(Zr1-x2Yx2)2O7-x2 (x = x1 + x2, 0 ⩽ x ⩽ 1.0) solid solutions

The evolution of structure and thermal conductivity (k) has been studied for a range of Y–La₂Zr₂O₇ solid solutions. Within the pyrochlore range (x < 0.40) Y³⁺ solely substitutes for La³⁺ below a critical composition factor (x = 0.15), above which it substitutes for both La³⁺ and the Zr⁴⁺. A glass-like k, approaching the amorphous limit, is observed within a certain composition range (0.20 ? x < 0.40). The glass-like k behaviour is attributed to a phonon localization effect that arises from small and weakly bound Y³⁺ cations (rattlers) oscillating locally and independently in oversized anionic cages [(La/Y)O₈]. The ultralow and glassy k makes Y³⁺-doped La₂Zr₂O₇ pyrochlores promising candidate materials for high temperature thermal barrier coating topcoats.     

Manufacturing of the bulk amorphous Fe61Co10Zr2+xHf3−xW2Y2B20 alloys (where x = 1, 2, 3) their microstructure,magnetic and mechanical properties

In this paper the microstructure, magnetic properties and mechanical studies results for Fe₆₁Co₁₀Zr_2+xHf_3?xW₂Y₂B₂₀ (for x = 1, 2 or 3) alloys are presented. The samples used in the investigations were obtained by a suction-casting method. The samples were produced in the forms of rods with diameter of 1 mm and length of about 20 mm and plates with thickness of 0.5 mm and surface area of about 100 mm². The results show that the best soft magnetic properties were achieved by Fe₆₁Co₁₀Zr₃Hf₂W₂Y₂B₂₀ amorphous alloy in the form of a plate. This sample has the highest value of saturation magnetization (1.09 T) and the smallest values of coercivity (H_C = 1.5 A/m) and core losses. All investigated samples of amorphous alloys were characterized by substantially greater values of microhardness and, unfortunately, slightly lower values of wear resistance compared with their crystalline counterparts.     

Why UFexAl12−x phase does not crystallize with ThMn12-structure type,when x = 2?

The crystallographic structure of recently reported orthorhombic UFe₂Al₁₀ phase, which belongs to the YbFe₂Al₁₀-structure type is different from the well known tetragonal UFe_xAl_12?x (3 < x < 8) having ThMn₁₂-structure type. Comparative Density Functional Theory (DFT) study of the relative stability of the orthorhombic UFe₂Al₁₀ phase with respect to different models of tetragonal ThMn₁₂-type structures with the same composition is carried out to explain this unusual phenomenon. It is shown that the fine interplay between the overlapping of 3d-states of Fe and 5f-states of U in the region ?2.5 ÷ ?0.3 eV below the Fermi energy dictates the lower total energy of orthorhombic UFe₂Al₁₀ phase in comparison with tetragonal UFe₂Al₁₀ phase crystallized in ThMn₁₂ structure. On the example of UFe₂Al₁₀ it is also directly demonstrated that DFT calculations in the framework of the Full Potential + Linear Augmented Plane Wave method within the local density approximation may be used for refining the structure of complicate compounds. The calculated equilibrium volume, lattice parameters and atomic Wyckoff positions of UFe₂Al₁₀ intermetallic compound are in very good agreement with experimental data.     

Brilliant effect of Ca substitution in the appearance of magnetic memory in Dy0.5(Sr1−xCax)0.5MnO3 (x = 0.3) manganites

The present work undertakes an investigation of magnetic memory effect in Dy_0.5(Sr_1−xCa_x)_0.5MnO₃ (x = 0 and x = 0.3) nanoparticles heated at 700 °C and 1350 °C. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements displayed the existence of magnetic memory effect in only samples with a Ca concentration x = 0.3. However, there is no sign of magnetic memory in samples with x = 0 of Ca amount. This confirms that the effect of substituting the Sr by Ca on the appearance of magnetic memory is of great importance, admitting that magnetic memory is mainly due to the powerful inter-particle interactions.