Showing papers on "Laves phase published in 1979"

Hydrogen sorption properties of some ab2 laves phase compounds

Abstract: The hydrogen sorption properties of the hydrides of the Laves phase compounds GdB2 (B=Mn, Fe, Co, Ni, Rh, Ru) and LaRh2 have been studied at various temperatures in the pressure range up to 20 atm. Hydrogen absorption in LaB2 (B=Ru, Pt, Ir, Al) and GdB2 (B=Ir, Al) was measured in the pressure range up to 70 atm. A model that calculates the prefered interstitial sites occupied by the hydrogen is presented for the AB2 compounds. A gradual decrease of the hydrogen absorption capacity is observed for the GdNi2 and GdCo2 compounds after several absorption-desorption cycles. The hydrogen-cycled GdRh2 exhibited a change in its thermodynamic properties.

Magnetic ordering and anomalous ground state in CeAl2

Abstract: The intermetallic Laves phase compound CeAl2 presents some unusual properties, which makes the exact nature of its ground state a matter of great interest. A first step towards an explanation would be the knowledge of any magnetic structure below 3.8 K (phase transition temperature). We have carried out very accurate neutron diffraction experiments on a polycrystalline sample, completed by experiments on a single crystal. Several magnetic reflections have been found which are characteristic of the propagation vector k↘=1/2+τ, 1/2−τ, 1/2. The corresponding magnetic structure consists of antiferromagnetic (110) planes in which the magnetic moments, lying along [111], are modulated according to a sine wave propagation along [110]. No variation of k↘ nor any tendency towards antiphase structure appear down to 0.4 K. This result is not coherent with the Kramers’ degeneracy of the Ce3+ ion because it implies a non magnetic level as a ground state. On the other hand previous polarized neutron experiments have shown an antiparallel coupling between the spins of the cerium and the conduction electrons. Both results, as well as other physical properties, agree with the existence of a singlet ground state associated with a strong d‐f coupling.

Effect of hydrogen on sublattice magnetization of Laves‐phase rare earth iron compounds

Abstract: We have used elastic neutron scattering to examine the effect of hydriding on the sublattice magnetization of the Laves‐phase compounds RFe2 for R=Ho and Er. On addition of hydrogen or deuterium a stable phase of composition about RFe2D3.5 is formed. The pure compounds RFe2 are ferrimagnetic, with the R moment equal to the free ion value and Fe moment of ∼1.6 μB. The bulk Curie temperatures Tc of the hydrides and deuterides are considerably lower than the corresponding RFe2 values (e.g., 440 K for ErFe2D3.5 versus 574 K for ErFe2). The 10 K R moment is also significantly reduced in the deuterides (4.4 and 5.2 μB versus 9 and 10 μB for Er and Ho in RFe2) and drops steadily with increased temperature, reaching zero below Tc in the case of Er. The Fe moment remains nearly constant with temperature until just below Tc where it drops rapidly to zero; in ErFe2D3.5 and HoFe2D3.5 its magnitude is 1.6 μB, and 1.9 μB, respectively. These results indicate that the exchange interactions involving the R atoms are markedly weakened in the hydrides. The distortion of the local anisotropy field due to random site occupancy of H and the weak exchange coupling of R produces a ’’fanning’’ of the R moment which reduces the spatially averaged moment measured in the neutron experiment.

Some improvements in laves phase high-field superconductors

Abstract: Improvements in superconducting properties and workability have been achieved both in V 2 (Hf,Zr) and in V 2 (Hf,Nb), through the effects of additional elements. The V 2 (Hf,Zr) can be fabricated into a conductor form by a composite diffusion process in which a composite of vanadium sheath and Zr-Hf alloy core is cold-worked and then heat treated. The addition of hafnium to the vanadium sheath considerably improves the critical current density J c , and moreover it makes the fabrication of the composite easier by reducing the difference in workability between the sheath and the core. The multifilamentary V-(1-2)at%Hf/Zr-25at%Hf wire may be practically useful for generating magnetic fields up to 15T. The V 2 (Hf,Nb) ternary alloys containing hafnium less than 17at% can be fabricated by direct plastic deformation at room temperature. The titanium addition to the V 2 (Hf,Nb) significantly improves both the superconducting properties and the workability. In the cold-rolled V-19at%Hf-6at%Ti-8.3at%Nb, transition temperature T c of 9.9K, upper critical field H c2 (4.2K) of about 22T and overall J c (4.2K) of 1 × 104A/cm2at 13T are obtained.

On the magnetic properties of Gd(Fe1-xAlx)2 compounds

Abstract: The magnetic properties of the Laves phase series Gd(Fe1-xAlx)2 have been studied by means of magnetisation measurements in fields up to 160 kOe for 0 or=0.8. The mean magnetic moment increases almost linearly from GdFe2 to GdAl2. This implies a constant iron moment for all iron atoms throughout the whole concentration range. The Gd atoms appear to be the magnetising element of the diluted Al-Fe matrix.

Pressure-composition phase diagram for hydrides of rare earth-Fe2 laves compounds

Abstract: The pressure-composition-temperature phase diagram for the absorption of hydrogen in the cubic Laves phase compound ErFe2 has been investigated. At least three stable hydrides ErFe2Hn are found with n ≈ 2, 3.4 and 4. At 20 °C, the equilibrium pressures for these phases are 0.04, 10 and 4000 Torr, respectively. The implications of these results on the preparation of samples for the measurement of physical properties is discussed.

Mössbauer effect investigations of the electronic and magnetic properties of rare earth metal and intermetallic hydrides

Abstract: Mossbauer spectroscopy has been used in a number of investigations of elemental rare earth (RE) metal and RE–transition metal intermetallic hydrides. Analysis of hyperfine parameters has established the crystalline electric field (CEF) ground state for Dy3+ in DyH2 and for Er3+ in ErH2 and YH2. In all cases the results are compatible with the presence of negatively charged hydrogen atoms. Long range magnetic ordering is observed in the RE‐H2 compounds. In DyH2, short‐range magnetic ordering is seen at temperatures up to twice the long‐range transition temperature. This arises from the changes in CEF symmetry due to the presence of a few H atoms which are not in regular tetrahedral sites. These effects are much more pronounced in slightly non‐stoichiometric compounds and can result in dramatic changes in the magnetic ordering temperature. As an example of an intermetallic hydride, we have studied the cubic Laves phase RE‐iron compounds. These form several stable hydride phases RE‐Fe2Hx, with the maximum x ...

A thermodynamically calculated phase diagram for the Co-Cr-Zr system

Abstract: The phase diagram for the Co-Cr-Zr system has been calculated from thermodynamic data for the binary systems involved. Ten isothermal sections for temperatures between 1900 and 1300 K are given. Portions of the diagram, indicating eutectic behaviour between fcc Co-based solutions and the (Co,Cr)2Zr Laves phase, are in excellent agreement with experimental studies made subsequent to the calculations. A set of thermodynamic data for the Co-Zr system which is consistent with the experimental phase diagram has been derived.

Temperature dependence of the magnetostriction and magnetization in single crystal HoFe2

Abstract: The magnetostriction (λ100) and the magnetic moment of single crystal HoFe2 were measured as a function of temperature down to T=4.2 K. HoFe2 forms a cubic (C15) Laves phase compound with the easy axis of magnetization along the [100] crystallographic direction. At room temperature the magnetostriction is rather small with λ100=−67×10−6 and λ111=200×10−6. λ100 increases substantially with decreasing temperature reaching a value of −745×10−6 at T=4.2 K. This behavior is contrary to that observed in DyFe2 (also [100] easy) where λ111≫λ100. The decrease in λ100 with increasing temperature was found to be much more rapid than could be explained by lowest order single‐ion magnetoelastic theory. On the other hand, magnetostriction measurements of λ111 over the temperature range 275–325 K result in a temperature dependence which can satisfactorily be explained by the single‐ion model. The spontaneous magnetic moment at T=0 K is 134.5 emu/g. Taking the the gJ value of 10 μb for the Ho3+ sublattice moment results ...

Acoustic emission interpretation of ductile fracture processes

Abstract: Detailed analyses of acoustic emission from several material classes have established that the predominant source in tensile testing is particle decohesion and fracture. Acoustic emission is a smoothly varying parameter with plastic strain in 4340 steel where small carbides predominate; however, in an IN 718 superalloy, it is both bimodal and exhibits a burst phenomena where very large carbides and nitrides as well as a medium size laves (δ) phase contribute. From microscopic and acoustic emission observations, it is found that the fracture process is sulfide decohesion followed by void sheet instability associated with carbides in 4340 steel. In IN 718, large carbide or nitride fractures are followed by void sheets associated with laves phase. Identification of the major particle nucleation sites have allowed an initial interpretation of ductile hole growth models. Application of McClintock hole growth and a Hahn and Rosenfield void sheet instability criterion to the ductile fracture process has provided a good correlation to tensile ductilities, plane-strain crack tip ductilities, and plane-strain fracture toughness, K IC.

Preparation and magnetic properties of YbFe2, NdFe2, PrFe2

Abstract: We report the preparation under high pressure and temperature of the Laves phase compounds YbFe2, NdFe2 and PrFe2 and their Mossbauer study with Fe57 and Yb170.

Influence of magnetocrystalline anisotropy effects on the magnetic isotherms of hexagonal Laves phase materials in the system Zr(Fe, Al) 2

Abstract: The M(H,T) isotherms of the hexagonal Laves phase materials Zr(Fe x Al 1-x ) 2 ( 056 \leq x \leq 074 ) were obtained experimentally in fields up to 72 kG and at a temperature 42 K The resulting Arrott plots (M2versus H/M) show downturns from the linear behavior characteristic of weak itinerant ferromagnets in fields in the range H/M about 05-10 kG/emu/g These deviations arise, at least in part, from magnetocrystalline anisotropy effects The relevant anisotropy fields were measured directly using the singular point detection method and indirectly by using a theoretical analysis based on domain rotation effects From the differences of the observed and calculated Arrott plots and from these anisotropy fields conclusions regarding other causes of the anomaly may be drawn

Tempering of a beta quenched Zr-1.9 wt % Cr alloy

Abstract: The precipitation of ZrCr2, which is a Laves phase with the C15 structure, in a supersaturated, martensitic matrix has been studied in a Zr-1.9 wt % Cr alloy using transmission electron microscopy. The nucleation, morphology and distribution of the precipitates has been examined at different tempering temperatures. The crystallographic orientation relationship between the α-Zr(Cr) and the ZrCr2 phases has been established and the precipitate habit plane determined.

Influence of heat treatment on the microstructure and mechanical properties of Alloy 718 base metal and weldments

Abstract: Effect of heat treatment on the metallurgical structure and tensile properties of three heats of Alloy 718 base metal and an Alloy 718 GTA weldment were characterized. Heat treatments employed were the conventional (ASTM A637) precipitation treatment and a modified precipitation treatment designed to improve the toughness of the weldments. The GTA weldments were characterized in the as-welded condition. Light microscopy, thin foil, and surface replica electron microscopy revealed that the microstructure of this superalloy was sensitive to heat treatment and heat-to-heat variations. The modified aging treatment resulted in a larger grain size and a more homogeneous microstructure than the conventional treatments. The morphology of the primary strengthening ..gamma.. phase was found to be finer and more closely spaced in the conventionally treated condition. Room and elevated temperature tensile testing revealed that the strength of the conventionally treated alloy was generally superior to that of the modified material. The conventional aging treatment resulted in greater heat-to-heat variations in tensile properties. This behavior was correlated with variations in the microstructure resulting from the precipitation heat treatments. The precipitate morphology of the GTA weldments was sensitive to heat treatment. The Laves phase was present in the interdendritic regions of both heat-treated welds. more » The modified aging treatment reduced the amount of Laves phase present in the weld zone. Room and elevated temperature tensile properties of the precipitation hardened weldments were relatively insensitive to heat treatment, suggesting that reduction in Laves phase from the weld zone had essentially no effect on tensile properties. As-welded GTA weldments exhibited lower strength levels and higher ductility values than heat-treated welds. « less

Structural and magnetic properties of Gd(CoxIr1-x)2

Abstract: The crystal structure and magnetic properties of the pseudobinary system Gd(CoxIr1-x)2 are reported. The compounds crystallise in the C15 Laves phase with a sudden lattice contraction at x=0.4. Magnetic measurements indicate that Co develops a magnetic moment for x>0.4. The onset of this moment is coincident with a sudden contraction of the lattice. Electron spin resonance measurements of g shift and linewidth also show sharp increases at x=0.4. The correlated changes in the various properties are interpreted as due to a movement of the Fermi level as Co is substituted for Ir.

METALLIC HYDRIDES.Magnetic properties of laves-phase rare earth hydrides

Abstract: — Neutron scattering results show that the introduction of hydrogen into RFe2 compounds (R = Tm, Ho, and Er) significantly lowers the overall Curie temperature and produces a reduced 0 K moment on the rare earth site. The rare earth spins disorder at a temperature lower than the bulk Tc in ErFe 2H 3 5 . The 0 K iron sublattice moment in ErFe2H3 5 is essentially the same as that found in the non-hydride compounds and remains nearly constant to approximately 0.8 Tc.

Precipitation-strengthened austenitic Fe-Mn-Ti alloys

Abstract: The precipitation of the intermetallic compounds in the Fe-20 Mn-2 Ti and Fe-28 Mn-2 Ti alloy systems has been investigated over the temperature range 700 to 900/sup 0/C using hardness measurements, optical and scanning electron microscopy, and x-ray diffraction. In both systems only the equilbrium Laves phase was found to form during the ausaging process; no transition phase was observed. The precipitate was identified as a C14 (MgZn/sub 2/) type hexagonal Laves phase with a chemical composition close to Fe/sub 2/(Ti,Mn). The precipitation in an as-solution-treated sample occurred predominantly in a heterogeneous manner, predominantly along grain boundaries.

Magnetostriction of some rare earth-aluminum Laves phase compounds

Abstract: Anisotropic magnetostriction measurements of some R‐Al2 (R=Gd, Dy, Ho and Er) compounds have been carried out from 4.2 K up to their Curie temperatures (Tc). Magnetic fields (H) up to 2.5 Tesla were applied. The extrapolated (to 1/H=0 for H≳1.4 Tesla) saturation magnetostrictions determined at 4.2 K are 17×10−6, −1420×10−6, 60×10−6 and −920×10−6 for GdAl2, DyAl2, HoAl2 and ErAl2, respectively. A dramatic reduction in the magnitude of the strain observed for HoAl2 below 15 K may be associated with a change of easy direction of magnetization. For GdAl2, the strains observed above the saturation are found to be strongly field‐dependent. The temperature dependence of magnetostriction for DyAl2 is interpreted in terms of the Callen and Callen’s single‐ion model. Large volume strains are obtained for these compounds (except for GdAl2). An attempt is made to estimate the anisotropic volume constant h3 at 4.2 K.

E.P.R. and susceptibility measurements in XAl2 (X = Sc, Y, La, Yb, Lu) compounds containing Gd, Er, Dy and Nd impurities

Abstract: and W [I]. E.P.R. and magnetization measurements of the rare earth in metals give information about the C.E.F. and about the exchange interaction between the impurities and the conduction electrons (C.E.). In this paper we report E.P.R. of Er, Dy and Nd ions in cubic Laves phase compounds XAl, (X