Showing papers on "Laves phase published in 1994"

Isotropic and anisotropic magnetoelastic interactions in heavy and light RCo2 Laves phase compounds

Abstract: The thermal expansion and anisotropic magnetostriction of the RCo2 Laves phases were studied in the temperature range 4-500 K using the X-ray powder diffraction method. In the heavy RCo2 the magnetic moment of the itinerant d electron subsystem derived from the magnetovolume effect was found to fit well with the magnetization curve of YCo2. A pronounced paraprocess above the metamagnetic transition has been observed when increasing the f-d exchange field. The type and temperature variation of the distortion of the cubic unit cell of the three compounds PrCo2, NdCo2 and SmCo2 have been studied in detail. The corresponding magnetostriction constants lambda 111 or lambda 100 were calculated. At 4 K the following values have been obtained: PrCo2, tetragonal distortion, easy axes (100), lambda 100=-3.4*10-3: SmCo2, rhombohedral distortion, easy axes (111), lambda 111=-4.6*10-3; NdCo2, tetragonal distortion and easy axes (100) for 42 K

Thermodynamic reassessment and calculation of FeTi phase diagram

Abstract: A Thermodynamic description of the FeTi system is obtained by a least-square optimisation of the thermochemical and phase diagram data from literature. SGTE recommended expressions for the Gibbs energies of pure elements are used. The Laves phase is modelled using a three-sublattice model that takes care of its homogeneity range. Calculated phase diagram and the thermochemlcal properties show good agreement with the experimental data.

Microstructural and crystallographic relationships in directionally solidified NbCr2Nb and CrCr2Nb eutectics

Abstract: The present paper describes processing, microstructures and phase relationships in directionally solidified Cr[sub 2]Nb-Nb and Cr[sub 2]Nb-C eutectics. Both of these eutectics, and the stoichiometric Laves phase Cr[sub 2]Nb, were directionally solidified using cold crucible Czochralski crystal growth with growth rates from 1 to 15 mm/min. Cr[sub 2]Nb-Nb had a rod/ribbon-type structure and Cr[sub 2]Nb-Cr had a lamellar structure. The crystallographic orientation relationships between both Cr and Nb and the Cr[sub 2]Nb 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[sub 2]Nb from the C14 to the C15 crystal structure on post-solidification cooling. Twins were observed in the C15 Cr[sub 2]Nb 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.

A pulsed-field-gradient NMR study of hydrogen diffusion in the Laves-phase compounds ZrCr2Hx

Abstract: Hydrogen diffusivities D in C14-ZrCr2H0.4 and C15-ZrCr2Hx (0.2

Studies on creep/stress rupture behaviour of superalloy 718 weldments used in gas turbine applications

Abstract: Superalloy 718 in the solution-treated condition was welded autogenously by electron beam welding and gas tungsten are welding processes. The weldrnents after suitable heat treatment were subjected. to creep/stress rupture testing at 650°C and 690 MPa. The results showed that the Laves phase resulting in the weld metalis deterimental to the creep rupture life of weld metals when present With the continuous morphology found in gas tungsten are welds. The lower amounts and discontinuous morphology of the Laves phase in electron beam weldments in combination With favourable gramonentauon resulted in relatively better rupture properties for these weldments.

Total energy and electronic structure calculations of C15 Laves-phase compounds MV2 (M=Zr, Hf or Ta): Elastic properties

Abstract: The total energy and electronic structure of early-transition-metal cubic Laves-phase compounds MV2 (M─Zr, Hf or Ta) have been calculated using the linear muffin-tin orbital method within the local-density and atomic sphere approximations. The elastic properties of the compounds have been examined from these results. The bulk moduli of the cubic MV2 alloys (M─Z, Hf or Ta) obtained from the total-energy calculations are 162, 172 and 218 GPa respectively. It is found from the electronic structures that there is a double degeneracy of electron energy levels with a linear dispersion relationship in the neighbourhood of the X point of the Brillouin zone and that the Fermi surfaces of the alloys pass near X-point of the Brillouin zone with an energy gap Δ∊. The contribution c(e) 44 from these electrons to the shear modulus c44 of the alloys, is found to be anomalous at high temperatures (T>400K), such that c(e) 44 increases with increasing temperature.

Elastic moduli of the Laves-phase pseudobinary compounds Zr( Al x Fe 1 − x ) 2 as determined by ultrasonic measurements

Abstract: Ultrasonic methods have been used to measure the polycrystalline elastic moduli of the Laves-phase pseudobinary compounds Zr(${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Fe}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${)}_{2}$ for 0.04\ensuremath{\le}x\ensuremath{\le}1.0. The bulk modulus decreases with increasing x up to the change from cubic (C15) to hexagonal (C14) structure at x=0.25. The shear modulus is essentially constant in the cubic phase. At the phase boundary the bulk modulus is about 6% higher in the hexagonal phase than in the cubic phase, while the shear modulus is the same in the two phases. As x increases in the hexagonal phase the bulk modulus decreases while the shear modulus increases. The softening of the bulk modulus in the cubic phase is correlated with the softening of the Fe bonding strength previously observed by nuclear methods, and with a sharp increase in the hydrogen capacity of these compounds. The stiffening of the shear modulus in the hexagonal phase may be correlated with the decrease of the hydrogen capacity in the corresponding compounds.

Stoichiometry and Alloying Effects on the Phase Stability and Mechanical Properties of TiCr2-Base Laves Phase Alloys

Abstract: Ti-Cr alloys near the TiCr2 composition have been studied to determine the single-phase Laves field and the associated defects accompanying off-stoichiometry. A combination of metallography, x-ray diffraction, lattice parameter measurements, density measurements and electron microprobe analysis have been used to establish a narrow single-phase region extending towards Ti-rich compositions. All three Laves crystal structures (C14, C36 and C15) were found to exist at different temperatures. Hardness and fracture toughness values determined by Vickers microindentation were studied as a function of alloy composition. Effects of adding Fe, Nb, Mo, and V to TiCr2 on lattice parameter, crystal structure, hardness and fracture toughness are reported.

Anomalous elastic properties in a C15 Laves-phase compound

Abstract: Low-temperature (70–300 K) elastic constants of a C15 Laves-phase compound Hf25V60Nb15 have been studied using resonant ultrasound spectroscopy. The results show anomalous elastic properties in this intermetallic alloy. The shear modulus and Young's modulus increase with increasing temperature. The bulk modulus is virtually constant in the tested temperature region. The room-temperature shear elastic constants of the alloy are smaller than those of the constituent elements and the average values derived from the rule of mixtures. The room-temperature Poisson's ratio of the alloy is higher than those of the constituent elements. The elastic properties of the C15 compound can be qualitatively understood using the electronic structure obtained from ab-initio calculations.

The new laves phase Na2Ba

Abstract: The crystal structure of Na 2 Ba was determined from single crystal X-ray diffraction data. It is isostructural with the hexagonal Laves phase Zn 2 Mg; P6 3 /mmc, a=739.3(4) pm, c=1199.9(9) pm. The compound Na 4 Ba reported in the phase diagram literature is actually Na 2 Ba. Comparisons are made with other Laves phases using a strain parameter diagram

Existence of Laves Phase in Nb-Hardened Superalloys

Abstract: The possibility of forming the topologically close-packed Laves phase has been investigated in various Ni-base superalloy systems that contain a significant amount of Nb. Simplified alloy compositions consisting of various combinations of major alloying elements, including Cr, Fe, and Co, were prepared; a high level of Nb was added in each alloys to simulate the dendritic segregation in the real casting process. In addition to the fee dendrites, the as-cast microstructure through a slow solidification rate developed the regions of eutectic decomposition at the end of solidification. The eutectic regions consisted of only two phases in every alloy studied. The intermetallic phases that formed the eutectic with the fee Ni matrix were identified by SEM-EDAX, X-ray diffraction, and DTA analysis. The results suggested that Laves phase was not expected to exist in Ni-Cr-Co base alloys, and that other Nb-hardened superalloys, especially those Ni-Cr-Fe base alloys, would likely develop Laves phase in the Nbsegregated regions during casting. Alloy chemistry theory was proposed to discuss the alloying effect on the existence of Laves phase in Ni base alloy systems. The combination of Cr and Fe alloying additions would be the essential ‘criteria to allow Laves phase appeared in a slow solidification process. The DTA analysis indicated that the alloy with the Laves phase had a low eutectic melting point. These important results can provide a comprehensive understanding of what observed in those Nb-strengthened superalloys with complex chemical compositions.

The lattice constant and itinerant electron metamagnetic transition in Laves-phase pseudo-binary Lu(Co1-xSix)2 compounds

Abstract: The lattice constant, low-temperature specific heat and high-field magnetization of the Laves-phase itinerant electron metamagnets Lu(Co1-xSix)2 have been investigated. By partial substitution of Co by Si, the critical field Hc of the metamagnetic transition decreases and the temperature Tmax of the magnetization maximum shifts to lower temperatures with increasing x regardless of the decrease in the lattice constant. Both the electronic specific heat coefficient gamma and the magnetic susceptibility chi increase with increasing x up to x=0.09 and decrease above this concentration. The present results suggest that the decrease in the critical field is not due to the increase in the lattice constant but is due to the hybridization between 3d electrons of Co and 3p electrons of Si.

Supersaturation of the AI2y laves phase by rapid solidification

Abstract: 1987, vol. 22, p. 244. 20. V. Laurent, D. Chatain, C. Chattillon, and N. Eustathopoulos: Acta Metall. Mater., 1988, vol. 36 (7), p. t797. 21. N. Eustathopoulos, D. Chatain, and L. Coudurier: Mater. Sci. Eng., 1991, vol. A135, p. 83. 22. P. Koitsalis, J.G. Li, L. Coudurier, and N. Eustathopoulos: J. Mater. Sci. Lett., 1990, vol. 9, p. 1332. 23. C.S. Kantetkar, A.S. Kacar, and D.M. Stefanescu: Metall. Trans. A, 1988, vol. 19A, pp. 1833-39. 24. T.M. Valentine: Mater. Sci. Eng., 1977, vol. 30, pp. 205-10. 25. T.M. Valentine: Mater. Sci. Eng., 1977, vol. 30, pp. 211-18. 26. C.A. Ward and A.W. Neumann: J. Colloid Interface Sci., 1974, vol. 49 (2), p. 286. 27. A.W. Neumann, R.J. Good, C.J. Hope, and M. Sejpal: J. Colloid Interface Sci., 1974, vol. 49 (2), p. 291.

Microstructure and magnetic properties of Sm-Fe-Ta based alloys

Abstract: The microstructure and magnetic properties of Sm/sub 2/Fe/sub 17-x/Ta/sub x/(0/spl les/x/spl les/2) cast alloys were investigated by means of microstructural and thermomagnetic analysis. A process for obtaining the Sm/sub 2/Fe/sub 17/ phase nearly free of iron in the as-cast state is described. The method consists of adding 4 to 5 at% of Ta to the melt. By this addition of Ta to the basic alloy a two phase structure of the as-cast ingots consisting of Sm/sub 2/Fe/sub 17/ phase and Pauli paramagnetic TaFe/sub 2/ hexagonal Laves phase can be obtained. Approximately 0.5 wt.% of free iron in the cast ingot with 5 at% of Ta addition was found by isothermal magnetic analysis. It was also shown that there is some solid solubility of Ta in the Sm/sub 2/Fe/sub 17/ phase, which results in enhancement of the Curie temperature. >

Lattice relaxation and three‐dimensional reflection high‐energy electron‐diffraction analysis of strained epitaxial Co/Mn superlattices

Abstract: In situ observations of reflection high‐energy electron diffraction (RHEED) are used to study the structure and growth of epitaxial Co/Mn superlattices on hcp (0001) Ru buffer layers. Mn deposited on fcc (111) or hcp (0001) Co presents a singular growth behavior which can be interpreted as an incoherent growth with compressive stresses perpendicular to the surface. Moreover, the Mn structure is found to switch from a sixfold in‐plane symmetric 1×1 structure, which probably corresponds to strained fcc γ‐Mn, to a sixfold in‐plane symmetric √3×√3−30° structure with in‐plane lattice parameter a=4.69 A. This structural change occurs at a critical thickness of 5–6 monolayers, after the partial relaxation of the Mn in‐plane lattice parameter. An analysis of the three‐dimensional contribution to the RHEED patterns shows that the √3×√3−30° structure is probably identical to the Cu2Mg Laves phase which in turn closely resembles the α‐Mn phase.

Metamagnetic transition in LuCo2 and its pseudo-binary compounds

Abstract: High field magnetizations up to 100 T, lattice constants and electronic specific heat coefficients have been measured for the Laves phase Lu(Co 1− x M x ) 2 (M = Ga, Sn and Si) compounds which exhibit itinerant electron metamagnetism for small x . The lattice constant increases with increasing x for M = Ga, or Sn, while it decreases linearly in the case of M = Si. The metamagnetic characteristics strongly depend on the substitutional element M. The critical field for the metamagnetic transition decreases with increasing x for M = Ga and Si, whereas it remains constant around 40 T above x = 0.06 for M = Sn. The magnetization jump due to the transition increases for M = Sn and rapidly decreases for M = Si. The different characteristics may result from the difference in the shape of the density of states modified by the substitutional element.

Characterization of phases in the Fe-Nb system

Abstract: The Fe-Nb system was investigated by means of X-ray diffraction and Mossbauer spectroscopy (at 300 and 77 K), in the range from 1 to 66.7 at%. We have found that the limit of solubility of Fe in Nb at 1100°C is between 3 and 4 at% Fe, and observed the coexistence of the Nb solid solution (Nbss) phase and Fe21Nb19 in the range from 4 to 40 at% Fe. The Mossbauer parameters of all the single phases are reported. The lattice parameters of Nbss phase present no significant variation with the Nb content. The X-ray pattern for the Fe21Nb19 phase could not be solved. The Laves phase Fe2Nb presents Mossbauer and X-ray parameters that agree with the literature.

Concentration and temperature dependence of electrical resistivity in Zr1−xHoxCo2 (0≤x≤1) intermetallic compounds and their hydrides

Abstract: The electrical resistivity studies have been carried out on the C 15‐type Laves phase intermetallic compounds Zr1−xHoxCo2 (0≤x≤1) and their hydrides in the temperature range 16–300 K. The first‐order magnetic transition occurring for x=1 disappears for lower Ho concentrations. Anomalies in the form of resistivity minima observed for the intermediate x values are attributed to the spin fluctuations in the vicinity of the magnetic ordering temperature. On hydrogen absorption, the first‐order transition observed for x=1.0 as well as the anomalies observed for the other x values disappear at higher hydrogen concentrations. A concentration‐dependent metal‐semiconductor‐like transition has been observed in all cases. This may be a consequence of charge transfer taking place between hydrogen and the 3d band, which leads to a strong decrease in the conduction electron density. The activation energy is evaluated from the ln ρ vs 1/T plots.

Effect of iron additions on microstructure and mechanical properties of Ni–Cr–Mo–Si hardfacing alloy

Abstract: The effect of iron additions in the range 5–15 wt-%Fe inclusive on the microstructure and properties of a Ni–Cr–Mo–Si wear resistant alloy (Tribaloy T7 00) has been investigated Iron additions were found to stabilise the hexagonal type structure of the Laves phase and to decrease the volume fraction of the primary Laves phase These microstructural modifications reduced the hardness, slightly improved the toughness, and significantly enhanced the strength of the as cast alloys Unlike T700, the alloys containing iron were not stable when heat treated at 700°C, and an increase in hardness was observed The mechanical properties of T700 have been compared with those of the cobalt base Tribaloy alloys (T400 and T8 00)MST/1936

Laves phase formation in solids

Abstract: Structural changes of the Fe-Mo, Fe-Ti and Fe-Nb binary alloys have been investigated by means of electron microscopy and X-ray diffraction. A tweed structure representing modulation of the composition always precedes the Laves phase precipitation. The latter fact allows confirmation that the Laves phase precipitation proceeds by the spinodal mechanism. A tweed structure formation results in softening of the Fe-Ti and Fe-Nb alloys; the hardness of the Fe-Mo alloys is unchangeable.

Influence of hydrogen on the magnetic properties of Terfenol‐D

Abstract: The reactions with hydrogen of the C15‐type Laves phase Tb0.27Dy0.73Fe2 have been studied by differential thermal analysis and the influence of this hydrogenation upon the magnetic properties was observed with a Curie–Faraday balance and a vibrating sample magnetometer. It has been found that a slightly rare earth rich composition allows the hydrogenation to take place at room temperature. Two endothermic peaks observed on heating under 1.2 bar of hydrogen are attributed to hydrogen desorption, due possibly to the form of the pressure isotherms. At a pressure of 1.2 bar, the amorphization and disproportionation peaks are combined as a single peak, but a higher pressures these two phenomena separate into two peaks. After the first absorption/desorption cycle, the hydrogenation occurs at temperatures as low as −75 °C, and is highly exothermic and very rapid in nature. The magnetic susceptibility of the compound suffers a dramatic drop when it is hydrided, and the magnetization at 1100 kA m−1 falls from 77.5 J T−1 kg−1 to 9.2 J T−1 kg−1. When heated in hydrogen, a peak in susceptibility was observed at about the same temperatures as those of the amorphization/disproportionation reactions. The Curie point of the amorphous hydrided material was found to be around 68 °C, with a magnetization at room temperature and 1100 kA m−1 of 46 J T−1 kg−1. The fully disproportionated material had a magnetization of 82 J T− kg−1 at 1100 kA m−1, which was related to the proportion of α‐Fe formed during the disproportionation reaction.

Study of itinerant electron metamagnetism in Y(Co1−xMx)2, (M = Ni, Fe) in very high magnetic fields

Abstract: Effects of Ni and Fe substitutions on the metamagnetic property of the Laves phase intermetallic compound YCo 2 are studied in pulsed magnetic fields up to 110T. A linear increase in the critical field B c is observed for the Ni substitution, whereas the Fe doping is found to lower B c . The results are qualitatively in accordance with the band picture of metamagnetism.