C15 Crystal Structure Research Articles

Hybrid DFT study of structural, electronic, magnetic and elastic properties of laves phase binary intermetallics RFe2 (R = La, Ce, Pr and Nd)

Hybrid density functional theory was used to investigate the structural, electronic, magnetic and elastic properties of the Laves phase binary intermetallics RFe2 (R = La, Ce, Pr and Nd) in C15 crystal structure. The calculated lattice constants of these materials are found in good agreement with the experiments. The band structures and density of states distribution confirm the metallic nature of all these intermetallics. The optimized energies in different magnetic phases and magnetic susceptibilities by post-DFT treatments confirm that all the understudy compounds are ferromagnetic in nature. Elastic parameters were calculated from the cubic elastic coefficients C11, C12 and C44. The elastic properties reveal that these intermetallics are incompressible, ductile, elastically anisotropic and mechanically stable. Based on the metallic nature and ferromagnetic properties, it is expected that these intermetallics are suitable materials for spintronic technology.

Experimental investigation of isothermal sections at 1373 and 1473 K in the Co–Nb–Ti system

The isothermal sections at 1373 and 1473 K in the Co–Nb–Ti system were constructed using annealed diffusion couples and equilibrated alloys by using scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), electron probe microanalysis (EPMA), and X-ray diffraction (XRD) techniques. Three three-phase regions, Co2(Nb,Ti) + Co7Nb6 + CoTi, Co7Nb6 + CoTi + bcc(Nb,Ti), and CoTi + liquid + bcc(Nb,Ti) were determined. Co3Nb and α-Co2Ti with a C36 crystal structure, and Co2Nb and β-Co2Ti with a C15 crystal structure at 1373 and 1473 K formed continuous solid solutions Co3(Nb,Ti) and Co2(Nb,Ti), respectively. The solubilities of Ti in Co7Nb6 and Nb in CoTi are determined to be ~11.4 at% and ~22.3 at% at 1373 K, and ~12.5 at% and ~24.8 at% at 1473 K, respectively. No ternary compounds were observed.

Predictions of thermomagnetic properties of Laves phase compounds: TbAl2, GdAl2 and SmAl2 performed with ATOMIC MATTERS MFA computation system

Recent calculations of properties of TbAl2 GdAl2 and SmAl2 single crystals, performed with our new computation system called ATOMIC MATTERS MFA are presented. We applied localized electron approach to describe the thermal evolution of Fine Electronic Structure of Tb3+, Gd3+ and Sm3+ ions over a wide temperature range and estimate Magnetocaloric Effect (MCE). Thermomagnetic properties of TbAl2, GdAl2 and SmAl2 were calculated based on the fine electronic structure of the 4f8, 4f7 and 4f5 electronic configuration of the Tb3+ and Gd3+ and Sm3+ ions, respectively. Our calculations yielded: magnetic moment value and direction; single-crystalline magnetization curves in zero field and in external magnetic field applied in various directions m(T,Bext); the 4f-electronic components of specific heat c4f(T,Bext); and temperature dependence of the magnetic entropy and isothermal entropy change with external magnetic field – ΔS(T,Bext). The cubic universal CEF parameters values used for all CEF calculations was taken from literature and recalculated for universal cubic parameters set for the RAl2 series: A4 = +7.164 Ka04 and A6 = −1.038 Ka06. Magnetic properties were found to be anisotropic due to cubic Laves phase C15 crystal structure symmetry. These studies reveal the importance of multipolar charge interactions when describing thermomagnetic properties of real 4f electronic systems and the effectiveness of an applied self-consistent molecular field in calculations for magnetic phase transition simulation.

Microstructure of MgAl5Ca3Sr alloy after creep deformation and high‐temperature heat treatment

The paper presents results of microstructural investigations of MgAl5Ca3Sr magnesium alloys in the as‐cast condition, after creep tests at 180 °C, and after heat treatment at 450 °C for 4.5 hours. The microstructure of MgAl5Ca3Sr alloy is composed of α‐Mg solid solution, irregular shaped (Mg,Al)2Ca phase with C36 crystal structure, bulky (Mg,Al)17(Sr,Ca)2 phase, fine lamellar Mg2Ca phase with C14 structure, needle‐shaped Al2Ca precipitates with the C15 crystal structure. The precipitation of the needle‐shaped Al2Ca phase in the α‐Mg grains and spheroidization of the C14 phase were found after heat treatment at 450 °C in argon atmosphere. The (Mg,Al)2Ca (C36) and (Mg,Al)17(Sr,Ca)2 phases seems to be stable at 450 °C, however, the increasing of aluminum content in C36 compound was observed suggesting the initial stage of C36 → C15 transformation. After creep deformation at 180 °C precipitates of the Al2Ca phase were found in α‐Mg phase. The intermetallic compounds are stable at 180 °C. The MgAl5Ca3Sr alloy exhibits good creep resistance up to 75 MPa. Tensile properties are comparable to those of Mg‐RE‐Zn–Zr alloys.

A single law for the activation energies of self‐diffusion of various cubic metals, intermetallic compounds, ionic crystals and oxides

AbstractIn previous publications the author put forward a relation which, for face‐centred cubic metals and intermetallic compounds with B2 and L12 structures, connected the activation energy of self‐diffusion with lattice constant and shear modulus. It is one aim of the present study to show that this formalism can be extended to intermetallic compounds with C1, D03 and C15 crystal structures. Since the covalently bonded cubic semiconductors and ceramics obey a different law, the question concerning the influence of the chemical bond was additionally investigated. Therefore ionic crystals and oxides with B1, B2 and C1 structures were analysed. It is demonstrated that these materials obey the same law as the metals and intermetallic compounds, for the B1 structure, however, the prefactor of the common rule is different. To be able to evaluate such differences, the proposed relation had to be more quantitatively derived than it was done before. Some cubic transition metals do not fit in the general picture. The deviations are traced back to the binding properties of the electronic d‐bands. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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The present paper describes processing, microstructures and phase relationships in directionally solidified Cr2NbNb and Cr2NbCr eutectics. Both of these eutectics, and the stoichiometric Laves phase Cr2Nb, were directionally solidified using cold crucible Czochralski crystal growth with growth rates from 1 to 15 mm/min. Cr2NbNb had a rod/ribbon-type structure and Cr2NbCr had a lamellar structure. The crystallographic oreintation relationships between both Cr and Nb and the Cr2Nb Laves phase in the individual eutectics were characterized using transmission electron microscopy. Inter-phase orientation relationships in these eutectics were complicated by the transformation of Cr2Nb from the C14 to the C15 crystal structure on post-solidification cooling. Twins were observed in the C15 Cr2Nb phase in both the single phase and eutectic samples. The relationship between the twins, the C14–C15 transformation, and the eutectic morphology is also discussed.

Mössbauer and X-ray Diffraction Studies of Zr1-xTixFe2Laves Phase Compounds

Mossbauer spectroscopy of 5 7 Fe and X-ray diffraction measurements in the Zr 1 - x Ti x Fe 2 phase have been made. Our X-ray diffraction spectra revealed that the samples have the cubic C15 crystal structure for low Ti concentration and the hexagonal C14 structure for high Ti concentration. In the range of 0.2 0.80 and magnetic for x ≤ 0.8. In the magnetic samples we found four different values of hyperfine magnetic fields. The presence of two hyperfine magnetic fields in the C14 phase suggests magnetic moments on Fe atoms at 2a sites. The obtained e Q vs. the x dependence indicates the spin canted antiferromagnetic structure in the hexagonal phase in the range of 0.2 < x < 0.8.

Magnetic and structural properties of DyMn2Hx (0≤x≤4.2)

The magnetic properties of Dy(57Fe0.01Mn0.99)2Hx compounds were investigated by means of 57Fe Mössbauer spectroscopy and temperature dependent magnetic susceptibility (DC, AC) and magnetisation measurements. Their structural properties in the temperature range 70 K≤T≤400 K were studied by means of powder X-ray diffraction. It was found that, at room temperature, the lattice parameters increase continuously upon hydrogen absorption in the 0.8≤x≤3.5 range and the cubic C15 crystal structure is preserved. At 70 K the system differentiates into several distinct hydride phases and two-phase ranges. The lattice expansion together with the chemical effect of hydrogen results in dramatic changes in the properties of the hydrides as compared to the host compound. By means of 57Fe Mössbauer spectroscopy, values of the average isomer shifts at 4.2 and 300 K and the average hyperfine fields at 4.2 K were determined. From isomer shift changes at room temperature, the relative importance of the volume expansion and band effects due to hydrogen absorption were deduced. A nonlinear increase of the magnetic ordering temperatures TM versus x and an abrupt increase of the average hyperfine fields for x≥2.5 were also found. Based on TM values for Y(57Fe0.005Mn0.995)2Hx as reference the magnitude of the 4f–3d indirect interaction was inferred. In addition, a strong decrease of this interaction with increasing x (resulting in its cancellation for x>2.8) and a weak dependence on the Dy–Mn distance are clearly shown.

Factors affecting the room-temperature mechanical properties of TiCr 2-base Laves phase alloys

Various effects on the room-temperature mechanical properties of TiCr 2-base alloys have been assessed in efforts to improve the toughness of Laves phase intermetallics (AB 2). Systematic studies were performed on: (i) single-phase stoichiometric and nonstoichiometric TiCr 2, (ii) TiCr 2-base ternary Laves phases, and (iii) two-phase binary alloys containing TiCr 2. In order to make quantitative comparisons among the different alloys, Vickers indentation was used to obtain hardness and fracture toughness values. Within the single-phase field, constitutional defects accounted for the compositional dependencies of properties, and may aid the synchroshear deformation process. Ternary Laves phases comprised of Fe, Nb, V or Mo additions to TiCr 2 indicated that alloying elements which partition to both A and B sublattices can improve the toughness. Stabilization of the cubic C15 crystal structure also resulted in higher toughness values. Small amounts of the bcc β-phase effectively reduced crack lengths in the Laves phase, and two-phase alloys of (Ti,Cr)+TiCr 2 exhibited significant improvements in toughness.

Laves intermetallics in stainless steel–zirconium alloys

Laves intermetallics have a significant effect on properties of metal waste forms being developed at Argonne National Laboratory. These waste forms are stainless steel-zirconium alloys that will contain radioactive metal isotopes isolated from spent nuclear fuel by electrometallurgical treatment. The baseline waste form composition for stainless steel-clad fuels is stainless steel-15 wt.% zirconium (SS-15Zr). This article presents results of neutron diffraction measurements, heat-treatment studies and mechanical testing on SS-15Zr alloys. The Laves intermetallics in these alloys, labeled Zr(Fe,Cr,Ni){sub 2+x}, have both C36 and C15 crystal structures. A fraction of these intermetallics transform into (Fe,Cr,Ni){sub 23}Zr{sub 6} during high-temperature annealing; the authors have proposed a mechanism for this transformation. The SS-15Zr alloys show virtually no elongation in uniaxial tension, but exhibit good strength and ductility in compression tests. This article also presents neutron diffraction and microstructural data for a stainless steel-42 wt.% zirconium (SS-42Zr) alloy.

Precipitation in a CrCr 2Nb alloy

The microstructure of a CrCr 2Nb two-phase alloy containing 12 at. % Nb was examined in the as-cast and annealed conditions using optical and electron microscopy techniques. In the as-cast condition, a Cr solid-solution matrix and a eutectic mixture of Cr solid solution and C15 Cr 2Nb coexist. In the Cr matrix, adjacent to the CrCr 2Nb interface, dislocations are observed and are likely a consequence of the thermal mismatch between the two phases. The matrix solid solution, away from the eutectic Cr 2Nb, also exhibits a high dislocation density and intense deformation bands; no solid-state precipitation of Cr 2Nb in the matrix had occurred. Upon annealing for as little as 3 h at 1200°C, however, precipitation occurs; these precipitates, with the C15 crystal structure, have a rod morphology with a hexagonal cross-section and faceted end-caps. These precipitates are twinned, the twinning system being the same as in the eutectic Cr 2Nb. Mismatch dislocations are observed in the precipitate/matrix interface and dislocations arising from thermal contraction mismatch are also present in the Cr matrix adjacent to the precipitate/matrix interface. The precipitate-matrix orientation relationship was (101) m//(1 1 ) p and [ 1 01] m// 1 01] p. Exposure at 1200°C for extended times results in some precipitate coarsening. A second type of precipitate with a cube-on-cube orientation with the matrix and a lattice parameter of a ≅ 11.5 A ̊ was occasionally observed, persisted upon extended annealing, and was identified as an M 6C-type carbide (M = Cr, Nb) that may be stabilized by other interstitial contaminants such as oxygen.

Magnetic and electric properties of Gd(Al1−xSbx)2 and Gd(Al1−xSbx)2 compounds

Abstract The magnetic susceptibility χ and the electrical resistivity ρ of the intermetallic compounds Gd(Al1−xSbx)2 with M= Sb, Bi was measured in the range of x having the C15 crystal structure. The paramagnetic Curie temperature τp, the Curie temperature TC and the resistivity contribution ρs due to disordered magnetic moments of Gd above TC were observed to depend sensitively on the composition x. In general it is found that the variation of τp(x) and ρs(x) is similar, for both M = Sb and Bi. However, in the case of Sb this x-dependence is much more drastic than in the case of Bi. The composition dependence of the various quantities is discussed on the basis of electronic structure calculations. It is interpreted as a combined effect of changes in the total density of states N(EF) at the Fermi level EF on alloying of changes in the normalized partial density of states components nl for different angular momental l at EF, and of substitutional disorder effects in the Al/M sublattice.

Microstructural and crystallographic relationships in directionally solidified NbCr 2Nb and CrCr 2Nb eutectics

The present paper describes processing, microstructures and phase relationships in directionally solidified Cr 2NbNb and Cr 2NbCr eutectics. Both of these eutectics, and the stoichiometric Laves phase Cr 2Nb, were directionally solidified using cold crucible Czochralski crystal growth with growth rates from 1 to 15 mm/min. Cr 2NbNb had a rod/ribbon-type structure and Cr 2NbCr had a lamellar structure. The crystallographic oreintation relationships between both Cr and Nb and the Cr 2Nb Laves phase in the individual eutectics were characterized using transmission electron microscopy. Inter-phase orientation relationships in these eutectics were complicated by the transformation of Cr 2Nb from the C14 to the C15 crystal structure on post-solidification cooling. Twins were observed in the C15 Cr 2Nb phase in both the single phase and eutectic samples. The relationship between the twins, the C14–C15 transformation, and the eutectic morphology is also discussed.

Electrical resistivity of the Gd(Al 1− xM x) 2 compounds for M  In, Ga, Pd, Sn, Si and Ge

The electrical resistivity σ of the intermetallic compounds Gd(Al 1− x M x ) 2 with M  In, Ga, Pd, Sn, Si and Ge was measured, in the range of x giving the C15 crystal structure, between 4.2 and 300 K. The Curie temperatures T c and the resistivity contributions ρ s due to disordered magnetic moments of Gd above T c were observed to depend sensitively on x. This dependence was not so strong in the case of M  In and Ga, which are isoelectronic with Al, as in the case of the other M. In general it was found that the variation of T c( x) and ρ s( x) for a given M has a similar character. While the dependence on x of T c can be explained already by changes in the total density of states at the Fermi level E F on alloying, the dependence on x of ρ s points to the importance of the normalized partial density of states components n 1 for different angular momenta l at E F, in combination with the corresponding l-f exchange matrix elements. In accordance with the existence of such drastic band structure effects on alloying, it was found that the dependence on x of the residual resistivity ρ, does not follow the Nordheim rule ρ r ∼ x(1− x).

Electronic structure and anomalous properties of UAl2, NpAl2, and PuAl2.

The electronic ground states of UAl/sub 2/, NpAl/sub 2/, and PuAl/sub 2/ in the C15 crystal structure were determined by performing self-consistent, semirelativistic, warped muffin-tin linear augmented-plane-wave electronic band-structure calculations. Then the densities of states and band states were determined by obtaining solutions including spin-orbit interactions with the self-consistent semirelativistic potential. The partial density of states functions obtained for these systems indicate strong d-f hybridization. A consistent procedure for determining the spin fluctuation parameters from the experimental data was obtained. This procedure was used to obtain the spin-fluctuation parameters from UAl/sub 2/. It is found that this procedure yields parameters that are different from previous estimates and indicates that the calculated parameters such as the bare density of states at the Fermi energy N(0), and the occupied f-band width are consistent with the band-structure values. It is found that although the actinide-actinide separation is about the same, the electronic structure near E/sub F/ is different. These calculations indicate that both d-f hybridization and spin-orbit coupling are important in the electronic structure of these systems.

J-B-T-ε interaction and strain limits in A15, B1, and C15 crystal structure superconductors

Experimental evidence is presented which indicates, that the elastic strain effect on the critical current of high-field compound superconductors correlates strongly with the type of superconductor crystal structure. Large strain effects are observed in all practical A15 superconductors examined to date, including Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn, Nb-Hf/Sn-Ga, Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Ge, Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Al, and V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Ga. Strain is observed to have no measurable effect, however, on either the critical current or the critical field of superconductors having the B1 crystal structure [NbN] or the C15 crystal structure [V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> (Hf, Zr)]. Strain limits placed on the mechanical design of superconducting devices are evaluated as a function of magnetic field for several A15 superconductors (Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn, Nb-Hf/Cu-Sn-Ga, and V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Ga) and compared with the strain limits imposed by B1 and C15 superconductors. It is shown that the latter materials have mechanical design advantage in applications where the superconductor is subjected to strain in excess of 0.2% (either compressive or tensile).

Crystallographic and magnetic investigations of the intermetallic system ErCo 2-ErAl 2

The crystalline structure and magnetic properties of polycrystalline ErCo 2-ErAl 2 Laves phase compounds with the C14 and C15 crystal structures were investigated. It was found that for the C15 cubic structure it is possible to obtain compounds with a disordered structure. X-ray examination showed that for the ErCo 2 compound the exchange of cobalt atom between 8a and 16d sites is almost 1 atom per unit cell. After thermal treatment the crystal structure of the ErCo 2 compound shows a tendency towards ordering. For compounds with the hexagonal C14 structure the possibility of a layer distribution of atoms, alternately magnetic and nonmagnetic, was tested. This assumption is well satisfied by the ErCoAl compound. For this compound a certain anomaly was also observed, i.e. a small lattice constant c. Magnetic investigations indicated that the magnetic properties of the compounds investigated, particularly the Curie temperature, are markedly dependent on the degree of crystal structure ordering. For the remaining compounds with the C15 structure it was not possible to obtain fully ordered compounds but after thermal treatment they exhibit a tendency towards ordering.

Electrical Resistivity of Gd(Al1-xCux)2

Electrical resistivity ρ of the intermetallic compounds Gd(Al 1- x Cu x ) 2 in the range of x having the C15 crystal structure was measured from room temperature to liquid He temperature. The resis...