Showing papers on "Laves phase published in 1996"

High-Pressure Compounds in Methane-Hydrogen Mixtures

Abstract: The effect of pressure on chemical interactions in molecular mixtures is important for problems spanning fundamental chemistry, planetary science, and materials science. Diamond-anvil cell studies reveal pressure-induced chemistry in the CH4-H2 system. The system, which has no known compounds at ambient conditions, formed four molecular compounds, CH4(H2)2, (CH4)2H2, CH4(H2)4, and CH4H2, at pressures up to 10 gigapascals. These have been characterized by synchrotron single-crystal x-ray diffraction, polycrystalline x-ray diffraction, Raman spectroscopy, and visual observation. Although CH4(H2)2 crystallizes in the MgZn2-type, hexagonal Laves phase structure, (CH4)H2 has a body-centered tetragonal structure that is similar to that of Al2Cu. The 1:1 and 1:2 compounds are stable to at least 30 gigapascals.

Solidification of an alloy 625 weld overlay

Abstract: The solidification behavior (microsegregation, secondary phase formation, and solidification temperature range) of an Alloy 625 weld overlay deposited on 2.25Cr-1Mo steel by gas metal arc welding was investigated by light and electron optical microscopy, electron microprobe, and differential thermal analysis techniques. The overlay deposit was found to terminate solidification at {approx}1,216 C by a {gamma}/Laves eutectic-type reaction. The Laves phase was highly enriched in Nb, Mo, and Si. The solidification reaction and microsegregation potential of major alloying elements in the overlay deposit are compared to other Nb-bearing Ni base alloys and found to be very similar to those for Alloy 718. Solidification cracks observed in the overlay were attributed to the wide solidification temperature range ({approx}170 C) and formation of interdendritic ({gamma} + Laves) constituent. Reasonable agreement is obtained between the calculated and measured volume percent ({gamma} + Laves) constituent with the Scheil equation by treating the overlay system as a simple {gamma}-Nb binary and using an experimentally determined k{sub Nb} value from electron microprobe data.

Metallurgy and creep properties of new 9-12%Cr steels

Abstract: Current understanding of the creep strengthening mechanisms in newly developed 9-12%Cr creep resistant steels is discussed. It is demonstrated how new computer programs for calculation of equilibrium phases in the steels can contribute to increased understanding of the microstructure of these steels. The calculations are useful for, e.g. estimation of the risk of δ-ferrite formation, and of balancing of the addition of V, Nb and N so M2X formation can be avoided. Combination of the equilibrium calculations with kinetic models and experiment has led to a description of the precipitation of Laves phase in W-alloyed 9%Cr steels. Combination of these results with creep tests indicated that Laves phases have a creep strengthening effect through precipitation hardening, and that any solid solution strengthening effect of W is small.

Total-energy study of electronic structure and mechanical behavior of C15 Laves phase compounds: NbCr2 and HfV2.

Abstract: First-principles electronic structure calculations based on a full-potential linear muffin-tin orbital method have been used to study the electronic and mechanical properties of the early transition metal C15 Laves phase compounds ${\mathrm{NbCr}}_{2}$ and ${\mathrm{HfV}}_{2}$. For both compounds, total energies are computed and compared for the two Laves phase crystal structures C15 and C14. The lower total-energy structure for ${\mathrm{NbCr}}_{2}$ is found to be the C15 structure as opposed to the C14 structure for ${\mathrm{HfV}}_{2}$. We have calculated the equilibrium unit cell volumes, bulk moduli, cohesive energies, and heats of formation. We have obtained the density of states and charge density contour plots. Based on these results, we discuss the elastic properties, the stability of the C15 phase, and the bonding and deformation mechanisms in Laves phases. \textcopyright{} 1996 The American Physical Society.

Effects of alloy additions on the microstructure and properties of CrCr2Nb alloys

Abstract: Alloying additions of Ni, Co, Fe, Al and Re at levels up to 16 at.% were added to Cr Cr2Nb alloys containing 5.6–17 at.% Nb for the study of their microstructure, oxidation behavior, and mechanical properties. These alloys contain patches of primary Cr-rich solid solution surrounded by the eutectic structure having Cr2Nb and Cr(∼6% Nb) phases. The supersaturated Cr-rich solid solution in the cast alloys precipitated out secondary Cr2Nb (Laves-phase) particles upon annealing at and above 900 °C. The studies by TEM and electron microprobe analysis indicated that the transition elements of Fe, Co, and Ni partition strongly in the Cr2Nb-type Laves phase whereas rhenium and aluminum were only moderately enriched in the Laves phase. The Cr Cr2Nb alloys with ≤ 12% Nb exhibited considerable plastic deformation under compression tests. The yield strength of the alloys depended strongly on the volume fraction of the hard Laves phase. The oxidation resistance also increased with this volume fraction. Among the alloying additions, rhenium was the only element that substantially hardened the Cr Cr2Nb alloys at room temperature and 1000 °C. The hardening behavior is discussed in terms of partitioning and sublattice occupation of the alloying elements as well as altering the volume fracture of the hard Laves phase. Multilayer oxide products formed on these alloys upon exposure to air of an elevated temperature. Little beneficial effect of any of the alloying additions on oxidation behavior at 950 °C was found.

Giant magnetostriction in amorphous (Tb1−xDyx)(Fe0.45Co0.55)y films

Abstract: The magnetization and magnetostriction have been studied in amorphous (Tb1−xDyx)(Fe0.45Co0.55)y films. A well‐defined easy axis is created by magnetic field annealing and the sperimagnetic cone structure, characteristic of these amorphous alloys, is reduced. The anisotropy has a minimum at x=0.73 as in the R Fe2 Laves phase. This points to the similarity between the local environments in the amorphous and crystalline states. A giant magnetoelastic coupling coefficient b of 60 MPa is developed at 300 K in low applied field for x=0, optimally annealed films. Assuming a Young’s modulus and a Poisson ratio of 80 GPa and 0.31, respectively, the magnetostriction is evaluated at 1020×10−6. This is much larger than previously reported values in other amorphous films.

Temperature- and pressure-induced structural transitions in rare-earth-deficient (R = Y, Sm, Gd, Tb) Laves phases

Abstract: Electrical resistivity, thermal expansion, and temperature-dependent x-ray diffraction measurements on compounds give mutually consistent evidence for structural phase transitions at 740 K, 550 K, 600 K, and 450 K respectively for R = Y, Sm, Gd, Tb; 0 < x < 005 Arguments are given as to why most of the rare-earth - nickel compounds with the 1:2 ratio do not crystallize in the simple cubic Laves phase (C15 type) but show a superstructure of the cubic Laves phase at room temperature and at ambient pressure This superstructure with the space group and a doubled cell parameter is characterized by ordered vacancies on the R sites It is shown that the observed structural instabilities result in transitions to the cubic Laves phase (space group ), however with disordered vacancies at high temperatures High-pressure x-ray powder diffraction experiments show that the phase transition in shifts down to room temperature for a pressure of 27 GPa

Macrotransport-solidification kinetics modeling of equiaxed dendritic growth: Part II. Computation problems and validation on INCONEL 718 superalloy castings

Abstract: In Part I of the article, a new analytical model that describes solidification of equiaxed dendrites was presented. In this part of the article, the model is used to simulate the solidification of INCONEL 718 superalloy castings. The model was incorporated into a commercial finite-element code, PROCAST. A special procedure called microlatent heat method (MLHM) was used for coupling between macroscopic heat flow and microscopic growth kinetics. A criterion for time-stepping selection in microscopic modeling has been derived in conjunction with MLHM. Reductions in computational (CPU) time up to 90 pct over the classic latent heat method were found by adopting this coupling. Validation of the model was performed against experimental data for an INCONEL 718 superalloy casting. In the present calculations, the model for globulitic dendrite was used. The evolution of fraction of solid calculated with the present model was compared with Scheil’s model and experiments. An important feature in solidification of INCONEL 718 is the detrimental Laves phase. Laves phase content is directly related to the intensity of microsegregation of niobium, which is very sensitive to the evolution of the fraction of solid. It was found that there is a critical cooling rate at which the amount of Laves phase is maximum. The critical cooling rate is not a function of material parameters (diffusivity, partition coefficient,etc.). It depends only on the grain size and solidification time. The predictions generated with the present model are shown to agree very well with experiments.

Evaluation of structural stability and creep resistance of 9-12% Cr steels

Abstract: During creep exposure of modified chromium steels lowering of solid solution strengthening due to precipitation of Laves phase as well as coarsening of all precipitates causes degradation of creep resistance. Two distinct domains of the stress dependence of creep rate and time to rupture have been observed in precipitation strengthened modified chromium steels. The stress characterizing the transition between these domains was found to be closely related to the Orowan stress. This stress consists in these steels of the contribution from large particles on subgrain boundaries (mainly M 23 C 6 and during the limited time also Laves phase) and from small precipitates (Nb(C,N) and VN) inside subgrains. This has to be considered when measuring the interparticle spacing and calculating Orowan stress. Larson-Miller parametric equation is used to elucidate the necessity of long-term creep testing. By means of two heats of CrMoVNbN steel it is shown that reliable extrapolation of creep properties is possible only in a stress and temperature domain in which only one creep creep rupture mechanism operate. In the high stress domain Larson-Miller constant C LM is well above 30 while in the low stress domain this constant does not exceed 25. When the extrapolation is based mainly on short-term creep tests, the C LM constant is close to that valid in the high stress domain and therefore it overestimates long-term creep strength.

Icosahedral-phase formation and stability in Ti Zr Co alloys

Abstract: We present the first report of icosahedral phase i-phase formation in rapidly quenched alloys of Ti Zr Co . Electron diffraction patterns of i TiZrCo 53 27 20 contain features, such as anisotropic spot shapes and arcs of diffuse scattering, that are characteristic of the disordered icosahedral phases found in Ti 3d transition metal Si O and Ti Zr Fe alloys. The features are less prominent than in those alloys, however, suggesting that this i-phase may have structural order intermediate between those strongly disordered i-phases and more ordered Ti Zr Ni i-phases, showing none of these features. The quasilattice constant for a i TiZrCo, 0 51 nm, is close to that of i TiZrNi . The i-phase in rapidly q quenched Ti Zr Co alloys is deeply metastable and transforms exothermically o to the hexagonal Laves phase about 500 C. The Laves phase transforms to the o T bcc solid solution phase b-Ti and the Ti Ni-type fcc structure for 630 C. A 2 reversible transformation between the b-Ti and the hexagonal solid solution...

Hydrogen storage alloy and electrode therefrom

Abstract: Addition of Mo to a Zr--Mn--V--Cr--Co--Ni, a Zr--Mn--Cr--Co--Ni hydrogen storage alloy, or those including Ti as substitution for Zr improves high-rate discharge characteristics of the hydrogen storage alloy at low temperatures The hydrogen storage alloy is of the general formula ZrMn a V b Mo c Cr d Co e Ni f , wherein 04≦a≦08, 0≦b<03, 0

Stacking-fault energy in the C15 Laves phase Cr2Nb

Abstract: Transmission electron microscopy observations of extended nodes in Cr2Nb compressed at 1400 and 1500°C have been used to obtain a value of γ = 25 mJ m-2 for the stacking-fault energy above the ductile-to-brittle transition temperature of 1200°C. Such a modest value for γ may help to explain the significance of deformation twinning and the formation of polytypic structures in this compound.

Phase Equilibria in the Ni–Al–Ta System

Abstract: Phase equilibria in the Ni-Al-Ta system were determined at 1000 and 1250°C. From metallography, X-ray diffraction and electron probe microanalysis (EPMA), two isothermal sections for Ta contents < 50 at. % were set up. The stability ranges of the Laves phase NiAlTa and the Heusler phase Ni 2 AlTa were fixed and that of Ni 6 AlTa is confirmed. Several other ternary intermetallic phases, which had been reported in the literature, are not confirmed. Lattice constants are given for all phases studied. For those phases with a large solid solubility range, i.e. NiTa and NiAlTa, they are given as a function of composition.

First-principles calculations of the magnetic properties of and its hydrides

Abstract: Self-consistent, general potential, electronic structure calculations have been performed for the Laves phase compound and its hydrides (x = 0.5 and 1.0). The parent material, , is found to be an itinerant antiferromagnet with a magnetic moment of per Mn atom whereas for the hydrides an additional ferrimagnetic component appears. This is in good agreement with experiment. The dependence of the calculated atom-projected magnetic moments and hyperfine fields on the Mn - H interaction is analysed. We have also calculated the total energy for three different H positions and established that the lowest energy is found for the experimentally observed position.

The Effect of Phosphorus, Sulphur and Silicon on Segregation, Solidification and Mechanical Properties in Cast Alloy 718

Abstract: Laves forming behavior of Nb [2]. R. G. Thompson et al reported that P segregated uniformly to grain boundary Effects of trace elements P, S and Si on the surfaces when it was present as an intentional additive microstructures and mechanical properties in cast alloy [3]. In the solidification of alloy IN738, it has been 718 have been studied. The results show that P, S and Si shown that P increased the segregation of Al and Ti to promote the segregation of Nb and formation of Laves the interdendritic region and ‘the formation of y/y’ phase. It was found that P and Si are significant Laves eutectic which reduced stability of alloys[4]. Much attenformers, with P enrichment in the Laves phase of about tion has focused on sulphur in high-titanium-content l.Owt%, and Si enrichment of 2.06wt%. DTA results alloys. The formation of T&SC phase could reduce the demonstrate that, as P, S contents in the alloys effect of sulphur to some extent, but in alloy 718, increased, the solidification temperature range increased titanium content is lower (about l.Owt-%), and more obviously and the complete solidification was further de- work is necessary on the problem . Silicon may be used layed. On the contrary, Si raised the y / Laves eutectic as a refining addition during melting, but its presence in reaction temperature. Tensile testing indicated that P, S the final alloys is considered detrimental and therefore and Si had harmful effects on the tensile strength and upper limits are usually fixed at a low level in most alloy ductility at RT and 650°C. When P, S and Si content ex- specifications. This research was aimed at the behavior ceeded O.O13wt%, O.O14wt%, 0.34wt% in the alloy, of trace elements P, S and Si and their effects on the segrespectively, smooth stress-rupture life and elongations regation of Nb during solidification , and the mechanical were reduced markedly. properties under different P, S and Si contents.

Magnetic order in

Abstract: Neutron powder diffraction has been used to study the evolution of magnetic order with increasing Zr substitution in the C14 Laves phase compounds . For compounds with x = 0.3 and x = 0.4 we find a simple antiferromagnetic structure, similar to that reported for isostructural . In this structure the Fe moments at the 6h sites are aligned along the c-axis, ferromagnetically coupled within the a - b plane, with adjacent planes antiferromagnetically coupled. This spin structure results in a cancellation of the molecular field at the interplanar 2a sites, and the Fe atoms at these sites carry no ordered moment. The neutron diffraction measurements on compounds in the composition range provide evidence of a low-temperature spin-canted structure in which the antiferromagnetic structure described above is modified by the appearance of a basal-plane ferromagnetic component which in turn leads to a small ordered Fe moment at the 2a site. The temperature dependence of the staggered magnetization in the antiferromagnetic state of the x = 0.4 compounds is found to closely follow the form , as predicted for weak itinerant antiferromagnets by SCR spin-fluctuation theory.

The effect of Hf and Ti additions on microstructure and properties of Cr2Nb–Nb in situ composites

Abstract: The present paper describes the effect of Hf and Ti additions on the microstructures and mechanical properties of two-phase composites based on the Cr2Nb–Nb eutectic The microstructures of directionally solidified in situ composites containing 50–70% by volume of the Laves phase Cr2Nb which was modified with Hf (75–92%) and Ti (165–26%) are described Partitioning of Hf and Ti between the two phases is discussed using microprobe and EDS results The tensile properties at 1100 and 1200 °C are described and compared with those of an analogous niobium silicide-based composite The Cr2(Nb)–(Nb) composite tensile yield strengths at 1200 °C were increased over that of monolithic Cr2Nb to ∼130 MPa However, at 1200 °C the yield strengths of the silicide-based composites were approximately twice those of the Cr2(Nb)–(Nb) composites