Showing papers on "Laves phase published in 1984"

Lattice structure and phase transitions of hydrogen in intermetallic compounds

Abstract: Structural aspects of the behaviour of hydrogen in intermetallic compounds are discussed. A similar pattern of hydrogen behaviour is shown to occur in Laves phase intermetallic compounds and metals of groups IV and V: a change in the hydrogen coordination, the occurrence of short-range order in the arrangement of interstitial atoms, the formation of superstructures etc. Some special features which produce a complex structure of the metallic matrix are discussed.

Neutron diffraction in Ti1.2Mn1.8 deuteride: Structural and magnetic aspects

Abstract: The hydrogen storage compound Ti1.2Mn1.8 is an off-stoichiometric hexagonal Laves phase (C14) where the excess titanium atoms are inserted in the manganese sublattice and are located exclusively on the 2a sites. This leads to the formation of Ti3Mn tetrahedra which do not exist in stoichiometric TiMn2. Our neutron diffraction study revealed that the three deuterium atoms per formula unit do not enter TiMn3 or Mn4 sites but are distributed among the Ti2Mn2 and the Ti3Mn sites, where the latter have an increased occupancy because of their higher hydrogen affinity. An exclusive volume exists around each deuterium atom; owing to the high deuterium concentration this results in liquid-like short-range order with the first and second coordination spheres at about 2.1 A and 2.8 A respectively. The manganese atoms on the 6h sites in the magnetically ordered state of Ti1.2Mn1.8D3 form ferromagnetic layers, whereas the other manganese atoms and the titanium atoms remain non-magnetic.

NMR Study of Coexistence of Ferro- and Antiferromagnetism in the Itinerant Electron System (Zr 1− x Nb x )Fe 2 . II

Abstract: Pulsed NMR investigations of 93 Nb have been performed in the hexagonal Laves phase intermetallic compound (Zr 1- x Nb x )Fe 2 with 0.8 ≤ x ≤1.0. From the measurements of the Knight shift and the nuclear spin-lattice relaxation time T 1 , it is demonstrated that NbFe 2 is a nearly ferromagnetic compound and (Zr 1- x Nb x )Fe 2 with x =0.95 and 0.85 are weakly ferromagnetic compounds having a Curie temperature of 30 K and 40 K, respectively, in contrast with the itinerant electron antiferromagnetism for 0.5 ≤ x ≤0.7. In the composition of x =0.8 where the antiferromagnetic order occurs below 85 K and ferromagnetic moment appears below 30 K, it was found out from T 1 measurement that the system is in a coexistent state of ferro- and antiferromagnetism.

Theory of the bound state between phonons and a CEF excitation in CeAl2

Abstract: The coupling of lattice vibrations to the crystalline electric field (CEF) states of 4f electrons plays an important role in rare-earth compounds. It is shown that under favourable conditions this magnetoelastic interaction can lead to a 'bound state' between CEF excitations and lattice vibrations. The existence of this bound state in the intermetallic Laves phase CeAl2 explains the results of inelastic neutron scattering investigations which have not been understood before. In this compound, a Gamma 7- Gamma 8CEF excitation and low-lying vibrational modes of the Ce sublattice are involved in bound-state formation. The properties of this new type of excitation and its consequences for thermodynamic quantities are investigated. In addition the phonon softening at low temperatures found in CeAl2 is discussed.

Exchange Enhanced Pauli Paramagnetism of ScCo 2

Abstract: The temperature dependence of magnetic susceptibility, χ, of a cubic Laves phase compound ScCo 2 was measured in the temperature range 4.2–1018 K and found to have a broad maximum at about 600 K. The temperature dependent Knight shift, K , of Sc was obtained from NMR experiment below room temperature. The K vs χ plot gives a straight line resulting in -41 kOe/ µ B as a transferred hyperfine field from Co. It was concluded that ScCo 2 is an exchange enhanced Pauli paramagnet of the same kind of YCo 2 and LuCo 2 .

Recent experimental results on the electronic structure of binary and ternary hydrides

Abstract: Recent experimental results on the electronic structure of hydrides of palladium, the rare earths, niobium and magnesium, of the intermetallic compounds LaNi 5 , FeTi and Mg 2 Ni, of the zirconium-based Laves phase compounds and of a few glassy metals are reviewed and compared with results of band structure calculations. The review focuses on results obtained by photoelectron and X-ray spectroscopy methods. Emphasis is placed on missing data to stimulate further investigations.

Structure of Vacuum Brazed BNi-5 Joint of Inconel 718

Abstract: Structural investigations of the brazed joint of Inconel 718 with BNi-5 filler metal were carried out on specimens with geometries simulating a real joint. Three identification methods applied in parallel were used. Fused and solidified filler metal showed the presence of at least four microstructural components: the dominant γ-solid solution, two binary eutectics (G phase + γ) and (θ-phase + γ@#@), and prismatic objects identified as cr-phase. The influence of brazing time and temperature was studied with specimens heated in accordance with three different thermal regimes. The Ni-base γ-phase solidifies in dendritic form and contains Cr, Fe, Nb, and Mo, the concentrations of which are dependent on the thermal regime and on the distance from the former liquid-solid interface. The intermetallics (G, 0) and the matrix form binary and ternary eutectics. In overheated brazed joints the filler metal penetrates into the grain boundaries of the base metal, resulting in the formation of new phases. The dominant phase was identified as a hexagonal Laves phase (λ,). The diffusion zone in the base metal can be divided into two subregions. In subregion I the precipitating phase is a Nb-rich G-phase, while in subregion II, the depth of which can be directly related to the width of the gap, preferentially oriented carbides of Nb and Ti are formed. The phase formation in the BNi-5 brazing of Inconel 718 may be described by a quasi-quarternary diagram on the Ni-Cr-Nb-Si system.

Quadrupole splitting observed in some cubic Laves-phase compounds by means of 169Tm Mossbauer spectroscopy

Abstract: The authors performed 169Tm Mossbauer spectroscopy on the cubic Laves-phase compounds TmNi2, TmRh2 and TmIr2 in the temperature range 2-300K and found well resolved quadrupolar splitting at low temperature. In cubic compounds this feature is uncommon and they explain it as indicating the presence of a static or dynamic distortion. None of the three compounds investigated shows magnetic ordering at 2K or higher temperatures, nor can any of the three compounds be characterised as a van Vleck paramagnet at low temperatures.

Structure of the Laves Phase Observed in Polystyrene Latexes

Abstract: Etude au microscope optique de deux latex de particules spheriques (diametre 1,4 et 1,7). Structure de MgCu 2 et MgZn 2 dans ces latex

Electrochemical electrode employing metal hydride

Abstract: PURPOSE:To produce an electrochemical electrode having high performance and long service life with low cost by employing alloy for forming metal hydride of special structure containing Ni while having C14 type Laves phase. CONSTITUTION:In alloys for forming metal hydride to be shown by general formula ABa, C14(MgZn2) type Laves phase structure alloy where main alloy phase is containing Ni by 5-50atom% is employed for electrochemical electrode. In the formula, A means more than one kind of Ti, Zr, Hf while B means more than one kind of V, Nb, Ta, Cr, Mo, Mn, Fe, Co, Cu, La, Ce as well as 5- 50wt% of Ni and a=1.5-2.5. Such alloy includes, for example, Ti0.3Zr0.7Mn0.8 Cr0.8Ni0.4 (where, Ni=13.3atom%). When employing such alloy, the performance and the serive life of metal hydride negative pole for alkali secondary cell are improved.

The electronic structure of Zr(V1−xCrx)2 laves phase compounds and their hydrides studied by photoelectron spectroscopy

Abstract: We report X-ray photoelectron spectroscopy core level and valence band spectra of the cubic C15 Laves phase compounds Zr(V 1− x Cr x ) 2 ( x = 0, 0.5, 1) and their hydrides together with UV photoelectron spectra of ZrV 2 and ZrCr 2 . Comparison of the UV photoelectron spectroscopy data with recent band structure calculations for ZrV 2 suggests that the difference in the stability of the hydrides of ZrV 2 and ZrCr 2 is due to an upward shift of the Fermi level by 0.65 eV. The metal core level shifts are proportional to the hydrogen content of the sample. Measured core level shifts normalized by the hydrogen content are found to have a characteristic value for each of the three elements in the system investigated.

Satellite Structure in 59Co NMR Spectrum of Magnetically Ordered Dy1-xYxCo2 Intermetallic Compound

Abstract: The magnetic environment effect of cobalt in Dy 1- x Y x Co 2 has been studied by means of bulk magnetization and 59 Co spin-echo NMR measurements at 4.2 K. Clearly resolved satellite structures of the NMR spectra have been observed. The hyperfine field distributions of 59 Co are decomposed into contributions of Co atoms in various nearest neighbor configurations of rare earth atoms. In this analysis the dipole field due to nearest neighbor rate earth moments plays an important role. The result indicates that the magnetic moment of Co in the RCo 2 cubic Laves phase pseudobinary compounds is quite sensitive to the nearest neighbor rare earth environment.

Bound state of phonons and a crystal field excitation in CeAl2 (invited)

Abstract: In rare earth compounds the coupling of lattice vibrations to the crystalline electric field (CEF) states of 4f‐electrons plays an important role. It is shown that under suitable conditions this magnetoelastic interaction can lead to a ‘‘bound state’’ between CEF excitations and lattice vibrations. The existence of this bound state in the intermetallic Laves phase CeAl2 explains the results of inelastic neutron scattering experiments which have not been understood before. The properties of these bound states and their consequences for thermodynamic quantities are discussed. In addition the phonon softening at low temperatures found in CeAl2 is discussed.

Relationship of the structure of the pseudobinary Laves phase compounds Sm‐Er‐Fe to their intrinsic magnetostriction

Abstract: A series of pseudo‐binary Laves phase compounds Sm1−xErxFe2 (x=0.1, 0.2...0.85) have been prepared and investigated. Using the method of x‐ray diffractometry, the samples are step scanned with monochromatic Cu radiation at Bragg angle 2θ ranging from 50° to 110°. A detailed report is presented here on the broadening and splitting of the line profiles in some specimens at higher Bragg angles, like the cubic (533), (622), (642), and (731) reflections. The results are discussed in connection with the intrinsic magnetostriction of the material. The Curie temperatures and the room temperature saturation magnetization of the compounds have also been studied.A series of pseudo‐binary Laves phase compounds Sm1−xErxFe2 (x=0.1, 0.2...0.85) have been prepared and investigated. Using the method of x‐ray diffractometry, the samples are step scanned with monochromatic Cu radiation at Bragg angle 2θ ranging from 50° to 110°. A detailed report is presented here on the broadening and splitting of the line profiles in some specimens at higher Bragg angles, like the cubic (533), (622), (642), and (731) reflections. The results are discussed in connection with the intrinsic magnetostriction of the material. The Curie temperatures and the room temperature saturation magnetization of the compounds have also been studied.

Dynamics of Laves Phase Intersections in a Concentration Gradient

Abstract: In the approximate interval 850–1150C, the Laves phase TiCr2 exhibits several composition dependent structures, ranging from two layer hexagonal (2H), denoted C14, to face-centered cubic, denoted C15, for Ti-deficient and Ti-excess material, respectively. Intermediate stacking variants such a dihexagonal (4H), denoted C36, may also exist. The Laves phase band in a Ti-Cr microdiffusion couple is examined in cross-section TEM at ambient temperature, after a 1000 C anneal. The band consists of very faulty hexagonal material, at least in part reflecting rapid shear transformation on cooling; in addition the cubic phase is present, in some instances within the same grain as the hexagonal phase. The cubic/hexagonal interfaces are approximately parallel rather than perpendicular to the bulk diffusion direction. Due to a difference in Ti and Cr fluxes, there is a gradual transformation of cubic to hexagonal by a shear mechanism. A model for the mobility of the hexagonal/cubic interface in a concentration gradient is proposed.

A new ordered structure of the C15-type Laves phase, Mg28.4Cu57.9Si13.7

Abstract: Cristallisation dans P4 1 32 (ou P4 3 32) avec a=6,9598 A, Z=24 atomes par maille unitaire; affinement jusqu'a R=0,046. Une nouvelle structure ordonnee de la phase de Laves a ete observee a la composition hors stœchiometrie Mg 2 Cu 3 Si. Elle est caracterisee par des arrangements helicoidaux d'ordre 4 ordonnes d'atomes Cu et Si localises sur quatre tetraedres dans la maille unitaire, en laissant les atomes Mg sur les sites 8(c). Estimation du parametre d'ordre S=α 1 -α 2 (α 1 et α 2 etant les facteurs d'occupation de Cu sur les sites 12(d) et 4(a) de P4 1 32)

Ternary metal hydrides for energy storage

Abstract: Many metal hydride systems have been proposed for energy storage purposes, but the one which seems to offer the best practical advantages is that based on the intermetallic compound FeTi. It is important that any FeTi alloy consists solely of the FeTi phase with the B2 (CsCl) crystal structure. Excess iron causes the formation of the Fe/sub 2/Ti Laves phase, which does not readily form a hydride. Excess titanium results in the presence of some Ti solid solution, which forms a highly stable hydride, locking up hydrogen in a nonrecoverable form. The presence of either of these phases reduces the efficiency of the hydride.