Showing papers on "Laves phase published in 1991"

3d-5d band magnetism in rare earth-transition metal intermetallics: total and partial magnetic moments of the RFe2 (R=Gd-Yb) Laves phase compounds

Abstract: The magnetic moments of the RFe2 (R=Gd-Yb) Laves phase compounds have been calculated. Agreement with published measured values for single crystals is excellent. The R 4f magnetic moments were obtained from the standard Russell-Saunders scheme but the radial 4f spin density was otherwise part of the self-consistent density functional calculation. The influence of the localized 4f magnetism upon the conduction band magnetism is examined in detail and exchange interactions extracted. The site- and angular-momentum-resolved contributions to the total conduction bands magnetic moments are calculated and shown to obey an approximate sum rule which conserves the total conduction band moment, despite the change of 4f moment, across the series.

Microstructure and mechanical properties of Laves-phase alloys based on Cr2Nb

Abstract: Mechanical properties of Laves-phase alloys based on Cr2Nb at temperatures up to 1000°C were examined and correlated with microstructures and phase relationships. Single-phase Cr2Nb alloys are very hard and brittle at ambient temperatures, indicating the difficulty in generation and glide of dislocations due to the complicated crystal structure (C-15). Examination of the CrCr2Nb two-phase region revealed the following: (a) the hardness decreases with increasing amounts of the soft chromium-rich phase; (b) the eutectic composition has a niobium concentration of 17 at.%, instead of 12 at.% as reported in the currently existing phase diagram; (c) heat treatments produce uniform dispersion of fine Laves-phase precipitates in primary chromium-rich patches for the hypoeutectic alloys, and these particles are very stable even at temperatures above 1000°C; (d) the soft particles are very effective in preventing crack propagation originating in the brittle Laves-phase matrix, which results in a high yield strength with moderate ductility up to 1000°C. These results demonstrate that the introduction of a soft chromium phase has promising effects in improving the mechanical properties of brittle Cr2Nb Laves-phase alloys.

The Effect of Laves Phase on the Mechanical Properties of Wrought and Cast + HIP Inconel 718

Abstract: The effect of varying amounts of Laves phase on the mechanical properties of wrought and cast + HIP Inconel718 is discussed. When present as a continuous or semicontinuous grain boundary network in wrought Inconel718, Laves phase dramatically reduces room temperature tensile ductility and ultimate tensile strength, with room temperature impact and fracture toughness properties and elevated temperature ductility also reduced. Laves may also act as a preferred crack initiation and propagation site, resulting in reduced low cycle fatigue (LCF) p blty d ca a i i an accelerated fatigue crack growth rates. Laves present as large globular aggregates in cast+HIP Inconel 718 significantly reduces room temperature tensile and elevated temperature stress rupture properties. In addition, the phase acts as a preferred crack initiation and propagation site, resulting in significant reductions in smooth and notch LCF capability and an accelerated fatigue crack growth rate. Methods for controlling Laves phase in wrought and cast +HIP Inconel 718 are discussed. Superalloys 718,625 and Various Derivatives Edited by Edward A. JJxia The Minerals, Metals & Materials Society, 1991

Anisotropy compensation and magnetostriction in TbxDy1−x(Fe1−yTy)1.9 (T=Co,Mn)

Abstract: From magnetization (M) and magnetostriction (λ) measurements as a function of magnetic field and stress, the temperatures of anisotropy compensation, Tm, for technologically important TbxDy1−x(Fe1−yTy)1.9 [T=Co,Mn (0.3≤x≤0.5) (0≤y≤0.3)] were determined. Measurements of M and λ encompassing Tm were made under compressive stresses from 8.8 to 36 MPa and for temperatures from −196 to +130 °C. In agreement with earlier measurements, Tm decreases with increasing Tb. Substitution of Mn for Fe for fixed x also decreases Tm. In contrast with these observations is the increase of the anisotropy compensation temperature with the replacement of Fe by small amounts of Co. In the cases of both (1) increasing Tb content and (2) increasing Co content, the Curie temperature TC increases, yielding, in general, a higher magnetic moment and saturation magnetostriction of these alloys. Thus, compensation at a given temperature may be obtained in an improved class of Laves phase compounds, R(1)xR(2)1−x(Fe1−yCoy)2, where rare ...

A phase diagram approach to study liquation cracking in alloy 718

Abstract: The constitutional liquation of NbC wrought alloy 718 is discussed in terms of a diffusion couple between NbC and they matrix in a pseudo-ternary of γ-NbC-Laves. Theoretical arguments are presented to show that the diffusion path cannot be a straight line connecting the terminal compositions. Experimental results indicate the existence of the NbC-γ diffusion couple and also support the hypothesis that the diffusion path is not a straight line connecting NbC and γ. Consequently, the temperature at which carbide liquation can occur is below the equilibrium NbC solidus, and hence, carbide liquation is constitutional in nature. The lowest temperature at which carbide liquation initiates depends upon the extent of deviation of the diffusion path from the straight line path. The dependence of the relative volume fractions of carbide and Laves phase in cast alloys on Nb/C ratio is explained using the γ-NbC-Laves pseudo-ternary. The Laves phase present in cast alloys is shown to liquate at the terminal eutectic temperature of the alloy, which could be lower than the carbide liquation temperature.

The relationship between carbon content, microstructure, and intergranular liquation cracking in cast nickel alloy 718

Abstract: The ability of NbC and Laves phases to promote intergranular liquation cracking in the weld heat-affected zone (HAZ) of cast alloy 718 is examined in this paper. The term liquation cracking as used in this paper is synonymous with the terms microfissuring and hot cracking as used by the authors in previous papers. Many alloys are susceptible to intergranular liquation cracking due to the formation of incipient intergranular liquid films which can open into intergranular cracks. Previous studies on alloy 718 showed that both NbC and Laves phases produce intergranular liquid films when subjected to the rapid thermal cycles of the HAZ. When seeking a correlation between microstructure and liquation cracking, it is thus necessary to account for changes in the volume fractions and distributions of both NbC and Laves phases. The volume fractions and distributions of NbC and Laves phases were varied by heat treatment and also by using two bulk carbon concentrations. Increasing the bulk carbon from 0.02 to 0.06 wt pct caused a 70 pct increase in the volume fraction of NbC but no significant change in the amount of Laves phase. This increase in NbC was accompanied by an 18 pct increase in liquation cracking in the as-cast metal as measured using the spot varestraint test. When the volume fraction of NbC and Laves phases were varied using heat treatment, a very different behavior was observed. One hour heat treatments at 1200°F (649°C) and 1700°F (927 °C) reduced the amount of NbC by 40 pct, and the 1200°F (649°C) treatment also reduced the volume of Laves phase. Surprisingly, although the 1200°F (649°C) treatment reduced the amounts of both Laves phase and NbC, it produced a 23 pct increase in liquation cracking, while the 1700°F (927°C) treatment produced an opposite 22 pct decrease in liquation cracking. The effect of carbon content on the as-cast liquation cracking susceptibility is discussed in terms of its effect on the volume fraction of liquid produced during the welding thermal cycle and the subsequent resolidification kinetics of the liquid during cooling. The effects of different heat treatments are discussed in terms of their ability to redistribute solute, impurities, and precipitates within the matrix and to grain boundaries in deleterious or beneficial ways.

A redetermination of the LaNi phase diagram from LaNi to LaNi5 (50–83.3 at.% Ni)

Abstract: The La-Ni phase diagram was re-investigated over the composition range (50–83.3 at.% Ni). The major changes from the earlier published phase diagrams are: 1. (1) the non-existence of the LaNi 2 Laves phase as an equilibrium phase; 2. (2) the existence of La 0.875 Ni 2 (LaNi 2.286 ), a pseudo-Laves-like phase; 3. (3) the liquidus region between 58 and 70 at.% Ni; the α-β transformation in La 2 Ni 7 at 976 °C (previously no transformation was reported); and 4. (4) the peritectic melting temperature for LaNi 3 at 811 °C (previously reported at 975 °C). Minor differences were found for several invariant temperature horizontals (eutectic and peritectic temperatures) and the compositions of the LaNi-“La 2 Ni 3 ” eutectic and the so-called “La 2 Ni 3 ” phase.

Control of hydrogen equilibrium pressure for C14-type laves phase alloys

Abstract: It is very important to control continuously the hydrogen equilibrium pressure P -temperature T characteristics of hydrogen storage alloys to have them applicable in a wide range of temperatures. The relation between the alloy phases and the P-C-T characteristics was determined for C14 (MgZn 2 ) type Laves phase alloys based on Ti(Zr)Mn 2 by changing the alloy composition. The P-C-T characteristics, hysteresis and plateau pressure were changedsubstantially by suchtituting zirconium for titanium in the A site and other elements such as chromium, copper or nickel in the B site. In this case, the crystal lattice constants and hydrogen equilibrium pressure were changed more effectively by substituting zirconium for titanium in the A site. The range of equilibrium pressures covered was from 10 to 10 8 Pa at 20°C (this corresponds to the range from −50°C to +250°C at 1.013×10 5 Pa). The equilibrium pressure changes particularly markedly for lattice constants a =(4.84–5.05)×10 −10 m and c =(7.94–8.28)×10 −10 m. These results show that the C14-type Laves phase alloys with excellent flatnessof the plateau pressure and small hysteresis, are applicable in these ranges of temperatures and pressures for hydrogen storage. We have confirmed these results by fabricating a prototype heat pump with ahydrogen storage alloy that utilizes high temperature waste heat.

Relation between equilibrium hydrogen pressure and lattice parameters in pseudobinary ZrMn alloy systems

Abstract: The influence of transition elements (M ≡ V, Fe, Co, Ni) in ZrMn2 − xMx alloy systems on the equilibrium hydrogen pressure and lattice parameters was studied in order to develop metal hydride materials for use in heat storage and heat transportation systems operating at temperatures between 100 and 250 °C, for which there is great demand in industrial heat processes. The equilibrium characteristics and lattice parameters of as-cast pseudobinary alloys ZrMn2 − xMx (M ≡ V, Fe, Co, Ni; 0 ⩽ x ⩽ 0.6) were evaluated by measuring pressure-composition isotherms at 200 °C and by analysing X-ray diffraction patterns using Rietveld's method. The crystal structures of all the pseudobinary alloys were the C14-type Laves phase. The decrease of the unit cell volume increased the equilibrium hydrogen pressure in the alloys modified by vanadium, iron or cobalt, but the alloys modified by nickel showed an opposite tendency. These results were interpreted in terms of Miedema's rule. Consequently, cobalt and vanadium were found to be the most effective elements for control of the equilibrium hydrogen pressure and therefore for extension of the available temperature range of pseudobinary ZrMn alloy systems of the C14-type Laves phase.

General features of hydrogen-induced amorphization in RM2 (R = rare earth, M = transition element) Laves phases

Abstract: From a literature survey, the hydrogen-induced amorphization behaviour in RM2 (R=rare earth, M = Ni, Fe, Co) Laves phases is classified into two groups according to whether a crystalline hydride phase appears during the transition to an amorphous state or not. The major factor that determines the presence or non-presence of the crystalline metal hydrides is related to the stability of the compound, which can be estimated by the heat of formation (δHf) or the decomposition temperature of the compound itself. The crystalline metal hydrides are found only in the compounds with high stability. It is proposed that the mechanism for the two types of amorphization is related to the degree of lattice expansion with hydrogen absorption, which is a function of the elastic modulus of the compounds. To explain the dependence of the elastic modulus of the compound on the two types of amorphization behaviour, the concept of a nucleation barrier for amorphization is adopted. Compounds with a large lattice expan...

Grain boundary segregation in Nimonic PE16

Abstract: The changes in composition of the grain boundary regions in Nimonic PE16, which occur during heat treatment and on neutron irradiation, are described. The results show that the composition of these regions is sensitive to heat treatment variables, bulk composition, and irradiation. In particular, molybdenum has a strong tendency to segregate to grain boundaries both on cooling from solution treatment and on aging. The non-equilibrium segregation which occurs on cooling increases with increasing boron content of the alloy. Chromium also segregates to grain boundaries under these conditions, though there is no dependence on boron content. Aging at 700°C causes molybdenum and chromium to segregate to grain boundaries until the onset of precipitation of M23C6, M6C, and Laves phase causes a decrease in the concentrations of these elements. The effects of neutron irradiation are dependent on the boron content of the alloy.MST/1336

Thermodynamic properties of the Zr0.8Ti0.2(MnxCr1−x)FeH2 system

Abstract: The effects of the substitution of chromium by manganese in Zr0.8Ti0.2(MnxCr1−x)Fe Laves phase alloys on the pressure-composition-isotherm (PCT) relations, especially on the plateau slopes and hysteresis, were systematically investigated by PCT measurements. There is no dependence of the hydrogen storage capacity on composition as manganese is substituted for chromium. Increasing manganese substitution increases the extent of hysteresis but decreases the degree of the plateau slopes as well as the hydride stability. This is caused by the more sensitive change in the equilibrium pressures for hydride formation than for hydride decomposition in the two-phase region as the alloy composition x varies. It is suggested from the above results that the compositional dependence of the PCT relations of the Zr0.8Ti0.2(MnxCr1−x)FeH2 system originates from the difference in nature of the hysteresis behavior between ZrMn2-type and ZrCr2-type Laves phase alloy systems, since the hysteresis energies show a drastic change around x = 0.5. The difference in hysteresis nature is probably related to the difference in mechanical properties of the alloy itself, i.e. between ZrCr2-type and ZrMn2-type Laves phases.

Itinerant electron metamagnetic transition in exchange enhanced paramagnetic compounds Lu(Co1-xSnx)2

Abstract: Magnetization measurements of C-15 type Laves phase compounds Lu(Co1-xSnx)2 have been carried out up to 100 T in order to investigate the itinerant electron metamagnetic transition. By partial replacement of Co by Sn, the lattice constant increases and the critical magnetic field decreases linearly with increasing x. The susceptibility increases until x=0.04 and then decreases above x=0.06. A clear itinerant electron metamagnetic transition has been observed in the whole concentration range. It has been demonstrated that this transition occurs even in a pure compound LuCo2 and its critical magnetic-transition field is determined to be 74 T, not so different from the value predicted by the band calculation. On the other hand, the observed magnetization jump of LuCo2 is about 0.5 mu B/Co, which is much larger than the theoretical value.

Characteristics of electrodes using amorphousAB2 hydrogen storage alloys

Abstract: The hydrogen sorption behaviour of RENi 2 (RE=La, Ce, Pr, Mm (misch metal))Laves phase alloys was investigated by both the coulometric titration technique and the gas volumetric method. These alloys became amorphous by charging and kept the amorphous structure during charge-discharge cycles. The shapes of the pressure-composition isotherms indicated that the amorphous hydrides were compounds of the solid solution type. The discharge capacities of the amorphous hydride electrodes in the electrochemical cell were less than one-fourth of the capacities evaluated from the gas volumetric measurement, though they increased with increasing temperature. The discharge capacity did not decay during the first 500 cycles.

Solubility of hydrogen in Zr1 − xHoxCo2 (0 ⩽ x ⩽ 1) alloys

Abstract: Hydrogen absorption studies were carried out on the pseudobinary system Zr 1 − x Ho x Co 2 ( x = 0.2, 0.4, 0.6 and 0.8) belonging to the C15-type Laves phases in the temperature range RT ⩽ T (° C ) ⩽ 300 and the pressure range 0.001 ⩽ P (bar) ⩽ 1. Hydrogen absorption was found to increase with increasing holmium concentration. For the compositions x = 0.6 and 0.8, the pressure-composition isotherms indicated the occurrence of a phase transformation. The relative partial molar enthalpy and entropy of hydrogen solution calculated for all the concentrations were found to be approximately −(7–18) kJ (mol H) −1 and −(10–48) J K −1 (mol H) −1 respectively.

Undercooling experiments on quasicrystal-forming melts

Abstract: We show that the undercooling of AlMgZn emulsions is limited by the easy nucleation of the MgZn 2 Laves phase, and that spontaneous nucleation from the melt of the AlCuFe icosahedral phase can be forced in small droplets that solidify in a drop tube.

Hydrogen storage alloy electrode and process for producing the electrode

Abstract: The present invention provides a hydrogen storage alloy electrode made of a pentanary or higher multi-component hydrogen storage alloy or a hydride thereof where the alloy comprises at least Zr, Mn, Cr, Ni, and M where M is one or more elements selected from V and Mo, and a major component of the alloy phase is C₁₅ (MgCu₂ ) type Laves phase. This hydrogen storage alloy electrode may be enhanced in its performance by subjecting the alloy after the production thereof to a homogenizing heat-treatment at a temperature of 900 to 1300 °C in vacuum or in an inert gaseous atmosphere.

Polytypic structures in close-packed Zr(FeCr)2 Laves phases

Abstract: The polytypic structures formed in bulk Zr(Fe1 − xCrx)2 alloys (x = 0 to 1) in the as-cast and heattreated conditions (30 min at 850 or 900 °C) were determined by electron diffraction techniques. ZrCr2 was the only bulk alloy to exhibit a range of polytypic structures. The hexagonal polytypes 2H, 4H, 8H and 12H were present in as-cast ZrCr2. Upon heat treatment at 850 or 900 °C the hexagonal polytypes were replaced with the cubic polytype 3R and 3R-twinned. The cubic polytype 3R was the only structure formed in as-cast or heat-treated ZrFe2. The hexagonal polytype 2H was the only structure formed in Zr(Fe1 − xCrx)2 alloys (x = 0.25, 0.45 and 0.75) in all metallurgical conditions. This behaviour is compared with that of Zr(Fe1 − xCrx)2 (x ≠ 1) precipitates in zirconium-rich alloys where a variety of polytypes are observed. The presence of the polytype structures in the precipitates is attributed to the lattice misfit and associated distortional stresses present between the precipitates and the α-Zr matrix in the Zr-rich alloys. No such stresses would be present in the bulk alloys.

Low-temperature specific heat of Laves phase Ce(Fe1-xCox)2

Abstract: The authors have studied the low-temperature specific heat of the pseudo-binary Ce(Fe1-xCox)2 system, which shows a double transition from a ferromagnetic to antiferromagnetic state for 0.04

Phase separation in amorphous ErNi2Hx prepared by hydrogenation

Abstract: X‐ray diffraction and observations of phase separation are reported in a hydrogen‐induced amorphous ErNi2 alloy. Two separate diffuse maxima in x‐ray diffraction and two principal halos in electron diffraction, which are evidence of phase separation into two amorphous states, are observed at hydrogenation conditions of 50 atm H2 and elevated temperatures. The phenomenon of phase separation is manifest with increasing hydrogenation temperature. The compositions of both phases are estimated by comparing the nearest‐neighbor distances calculated by using the Ehrenfest relation with all possible atomic distances in amorphous ErNi2 hydride. These distances are calculated by assuming that the configurational atomic arrangement in the amorphous phase is composed of tetrahedral units. The compositions of the two phases seem to correspond to an Er‐rich phase (mixture of 4‐Er and 3Er+1Ni tetrahedra) and a Ni‐rich phase (mixture of 4‐Ni and 1Er+3Ni tetrahedra). This phase separation may be caused by the motion of Ni atoms over short distances.

Electronic structure and magnetization in Ce(FexAl1-x)2 and Ce(FexMn1-x)2

Abstract: We have studied the intermetallic compound CeFe 2 in Laves phase using a discrete variational method. The iron and cerium moments are antiparallel. The magnetization falls off rapidly when Al is substituted at the Fe site but only a small reduction in moments is noticed for Mn substitution. The calculated results are in agreement with recent experiments. The partial density of states is plotted to understand the nature of hybridization responsible for this anomalous magnetic behavior.

Laves-phase pseudobinaries a(fe1-xbx)2 (a=zr,hf;b=al,si): some structural and magnetic properties

Abstract: The lattice parameters and the saturation magnetization are measured for the series of intermetallic compounds A(Fe,B)2, A=Zr, Hf and B=Si and Al, with concentrations less than 15%. The magnetization measurements and the lattice parameter behavior are qualitatively discussed.

Structural and magnetic behaviour of LuFe2Hx

Abstract: The alloy LuFe2 (Laves phase C15 type) absorbs up to 3.3 hydrogen atoms per formula unit and exhibits a cubic to rhombohedral cell transformation, previously observed with the related compounds TbFe2 and ErFe2. The unit cell distortion was studied by neutron diffraction and nuclear magnetic resonance techniques. The magnetoelastic parameters which favour such a transformation are probably not dominant.

Crystallization process and Mössbauer study of the melt-quenched amorphous FeGd alloys

Abstract: Amorphous FeGd alloys, from 16 to 70 at.% Gd, were prepared with a melt-quenching method in a vacuum. An X-ray diffraction study on the crystallization process showed that these amorphous alloys crystallized into two phases, that is the h.c.p. Gd phase and the Laves phase (F 2 Gd). Mossbauer measurements of the amorphous phases showed that the internal hyperfine field was at its maximum of 276 kOe in the neighborhood of 42 at.% Gd. The isomer shifts were about + 0.1 mm s −1 .

Crystal field effects on the magnetic fluctuations in RAl2 and RNi2

Abstract: Zero-field μSR spectra have been recorded for some ferromagnetic Laves phase compounds. A comparison of the temperature dependence of the exponential damping rate, λ(T), for our samples shows that λ(T) is strongly influenced by crystal field effects.