Showing papers on "Laves phase published in 1997"

The formation and control of Laves phase in superalloy 718 welds

Abstract: Weld heat input/cooling rate (affected by welding process, parameters, technique, tooling, etc.) was found to influence the microstructural characteristics and segregational features in alloy 718 welds, with low heat inputs proving beneficial. Laves phase formed in the interdendritic regions of the weld metals as a result of segregation. The morphology and composition of Laves phase depended strongly on heat input/cooling rate and influenced its response to subsequent homogenization post-weld heat treatment. The various factors affecting the formation and control of Laves phase in alloy 718 welds are highlighted.

Binary alkali metal compounds with the Zintl anions [Ge9]4- and [Sn9]4-

Abstract: The binary germanides M12Ge17 and M4Ge9 (M Na, K, Rb, Cs) and the stannides M12Sn17 and M4Sn9 (M K, Rb, Cs) were identified by a combination of direct synthesis, thermogravimetric analysis, vibrational spectroscopy, X-ray powder data and single crystal structure analysis. The M12E17 phases contain the cluster anions [E9]4− and [E4]4− in the ratio 1:2, forming a hierarchical structure with the cluster anions at the atomic positions of the hexagonal Laves phase MgZn2. Like the M4E4 phases, the M4Ge9 compounds are hierarchical derivatives of the cubic Cr3Si structure but with [Ge9]4− anions. The thermogravimetric analyses give strong evidence for the existence of at least one more phase with [E9]4− and [E4]4− clusters and of the clathrate phases M6E136 in addition to the well-known M8E44□2 chlathrates.

Effect of electron concentration on the phase stability of NbCr2-based Laves phase alloys

Abstract: The phase stability in NbCr2-based transition-metal Laves phases was investigated, using data from binary X–Cr, Nb–X, and ternary Nb–Cr–X phase diagrams. It was shown that when the atomic size ratios are kept identical, the average electron concentration factor, e/a, is the dominating factor in controlling the phase stability of NbCr2-based transition-metal Laves phases. The e/a ratios for different Laves polytypes were determined as follows: with e/a 7.65, the C15 structure is stabilized again. A further increase in the electron concentration factor (e/a>8) leads to the disordering of the alloy.

Laves intermetallics in stainless steel–zirconium alloys

Abstract: Laves intermetallics have a significant effect on properties of metal waste forms being developed at Argonne National Laboratory. These waste forms are stainless steel-zirconium alloys that will contain radioactive metal isotopes isolated from spent nuclear fuel by electrometallurgical treatment. The baseline waste form composition for stainless steel-clad fuels is stainless steel-15 wt.% zirconium (SS-15Zr). This article presents results of neutron diffraction measurements, heat-treatment studies and mechanical testing on SS-15Zr alloys. The Laves intermetallics in these alloys, labeled Zr(Fe,Cr,Ni){sub 2+x}, have both C36 and C15 crystal structures. A fraction of these intermetallics transform into (Fe,Cr,Ni){sub 23}Zr{sub 6} during high-temperature annealing; the authors have proposed a mechanism for this transformation. The SS-15Zr alloys show virtually no elongation in uniaxial tension, but exhibit good strength and ductility in compression tests. This article also presents neutron diffraction and microstructural data for a stainless steel-42 wt.% zirconium (SS-42Zr) alloy.

Elastic and mechanical properties of Nb(Cr, V)2 C15 Laves phases

Abstract: The effect of vanadium alloying additions to NbCr2 has been studied, focusing on the phase stability, defect structures, elastic properties, and mechanical behavior of these C15 Laves phases. First, the ternary Nb–Cr–V phase diagram is presented. The C15 phase field of NbCr2 is significantly extended by V alloying along the constant niobium isoplethal section, suggesting that V atoms substitute primarily on Cr sites rather than Nb sites. Next, the elastic properties of the C15 Nb(Cr, V)2 Laves phases were examined using resonant ultrasound spectroscopy. The vanadium alloyed Laves phase alloys have slightly higher shear modulus values and lower Poisson's ratios than binary NbCr2. Finally, the mechanical properties of the C15 phases were investigated by indentation tests at different temperatures. Increasing the vanadium content modestly increases the strength but slightly decreases the fracture toughness and increases the brittle-to-ductile transition temperature in the alloys. The correlations between the phase stability, defect structures, elastic properties, and mechanical response of the C15 phases are discussed using a combination of atomic size arguments and electronic structure analyses. From these interpretations, an alloying strategy for improving the mechanical properties in C15 NbCr2 is proposed.

The Ce-Mg-Y system

Abstract: On the basis of a critical assessment, the Ce-Mg-Y system has been investigated, employing X-ray diffraction (XRD), optical microscopy, and microprobe analyses. Phase relations have been determined in the complete isothermal section at 500 °C, revealing the existence of two novel ternary compounds: Ce1−xYxMg5−y (0.39 ≤ x ≤ 0.84; 0 ≤ y ≤ 0.60) with the GdMg5, or the defect Sm11Cd45-type and the cubic Laves phase Ce1−xYxMg2 (x ≈ 0.67). The binary system Ce-Y has been reinvestigated, and a constitutional diagram, temperature vs concentration, has been proposed. In contrast to earlier versions, the so-called δ phase with the αSm-type has been shown to be metastable.

Microstructures of two-phase Ti–Cr alloys containing the TiCr2 Laves phase intermetallic

Abstract: Microstructures of two-phase Ti–Cr alloys (Ti-rich bcc + TiCr2 and Cr-rich bcc + TiCr2) are analyzed. A variety of TiCr2 precipitate morphologies is encountered with different nominal alloy compositions and annealing temperatures. Lattice constants and crystal structures are determined by x-ray diffraction (XRD) and transmission electron microscopy (TEM). Orientation relationships between the beta bcc solid solution and C15 TiCr2 Laves phase are understood in terms of geometrical packing, and are consistent with a Laves phase growth mechanism involving twinning.

Itinerant-electron metamagnetism and the onset of ferromagnetism in Laves phase compounds

Abstract: Detailed magnetization measurements have been carried out for Laves phase compounds which have a C15-type structure up to x = 0.220. A clear increase in the lattice constant occurs at room temperature in the vicinity of x = 0.095, corresponding to the critical concentration of the onset of ferromagnetism. A representative first-order metamagnetic transition was observed in the compounds with x = 0.090 and 0.093 in relatively low fields below 9 T. The critical temperature from the first- to second-order metamagnetic transition in both compounds and the susceptibility maximum temperature for several paramagnetic compounds were obtained. It should be noted that practically coincides with for x = 0.093, very close to the critical concentration of the onset of ferromagnetism. This result is well explained by the recent theory associated with spin fluctuations. The temperature shifts to lower temperatures with increasing x and disappears at x = 0.095 and above, correlated with the onset of ferromagnetism also. The spontaneous magnetization per Co atom exhibits a maximum value of at x = 0.110, whereas the Curie temperature does not show a maximum value at the same composition.

Microstructure and mechanical properties of a 10% chromium steel with improved creep resistance at 600°C

Abstract: A powder metallurgical 10%Cr steel containing molybdenum, tungsten, vanadium, niobium, and boron was studied. Results from tensile, impact, and creep rupture tests are reported. The alloy studied had a considerably higher creep resistance than the standard steel X 21 CrMoV12 1. After tempering, the microstructure consisted of tempered martensite with M23 C6 and M2N precipitates at grain and lath boundaries and inhomogeneously distributed disc shaped fine MX precipitates a few nanometers thick and 5–20 nm in diameter. Boron was found inside the M23 C6 precipitates after tempering and after creep testing at 600°C. Laves phase (Fe,Cr)2(Mo,W) precipitated at grain and lath boundaries during creep. Atom probe analyses of the matrix in the tempered and creep tested material indicated the presence of very small precipitates rich in chromium, vanadium, iron, and nitrogen. A latent creep resistance of the material, giving rise to a dynamic nucleation of small precipitates at dislocations, is suggested.

High temperature x-ray and calorimetric studies of phase transformations in quasicrystalline Ti-Zr-Ni alloys

Abstract: We present the first high temperature x-ray diffraction (HTXRD) studies of in situ quasicrystal-crystal and crystal-crystal transformations in Ti–Zr–Ni alloys. Together with differential scanning calorimetry studies, these x-ray measurements indicate three separate paths for the Ti–Zr–Ni quasicrystal-crystal transformation: single exothermic, single endothermic, or multiple endothermic. The mode of transformation depends on the alloy composition and the level of environmental oxygen. The crystalline products include the Ti2Ni, MgZn2 Laves, α−(Ti, Zr), and β−(Ti, Zr) phases. In the absence of oxygen, the endothermic transformation of the quasicrystal demonstrates that it is the lowest free energy (stable) phase at the Ti53Zr27Ni20 composition. Oxygen stabilizes the Ti2Ni phase, eliminating both the quasicrystal and the MgZn2 Laves phase, at partial pressures as low as a few hundred ppm.

Deformation behaviour of the C15 Laves phase Cr2Nb

Abstract: Compression tests have been used to show the temperature dependence of the deformation behaviour for stoichiometric samples of the C15 Laves phase Cr2Nb. The yield stress varied from 550 MPa at 1300°C where a significant yield drop occurred to 150 MPa at 1550°C, where a creep-type response was obtained, with an apparent change in the slope of the σy versus T plot close to 1400°C. The deformation microstructures observed using transmission electron microscopy were dominated by twins up to 1400°C, whereas at higher temperatures the deformation appeared to proceed by a mixture of twinning and slip. In all cases the twins appeared on a single composition plane within each grain and this could be due to an autocatalytic nucleation effect. This microstructure may be related to the onset of brittle behaviour at a temperature where residual components of the imposed strain cannot be compensated for by the activation of other deformation modes.

High-temperature deformation of the NbCr2-based Laves intermetallics in Nb-Cr-V and Nb-Cr-Mo alloy systems

Abstract: High-temperature deformation of the C15 NbCr2-based intermetallics in ternary Nb-Cr-V and Nb-Cr-Mo alloy systems is observed in terms of microstructural and compositional effect, by compression testing and transmission electron microscopy (TEM). The observed 0.2% yield stress and stress—strain curve were largely dependent on the equilibrating b.c.c. phase (or the C15 phase), their volume fraction and morphology. Among the observed alloys, the Nb-Cr-Mo alloys consisting of the C15 phase and ‘Nb-rich’ b.c.c. phase showed slightly enhanced 0.2% yield stress and largely decreased brittle—ductile transition temperature (BDTT) in comparison to the unalloyed single-phase C15 NbCr2 alloy, indicating a better high-temperature mechanical property with a favorable balance of strength and fracture toughness. TEM observation indicates that high density of dislocations are observed above BDTT in both constituent phases in both alloy systems. Also, the Burgers vector and morphological feature of dislocations are determined.

Computational Modeling of NbC/Laves Formation In INCONEL 718 Equiaxed Castings

Abstract: The goal of this work is to develop a solidification kinetics model for prediction of NbC and Laves phase evolution during casting solidification. Previous studies on alloy 718 showed that both NbC and Laves produce intergranular liquid films due to the intergranular distribution of Nb and C. Also, the ability of Laves to promote intergranular liquation cracking (microfissuring and hot cracking) during heat treatment is much higher than that of NbC. This is because the temperature of Laves phase formation is usually lower than that of NbC, i.e., liquation initiates at the eutectic-Laves temperature. Experimental evidence demonstrates that the amount of NbC and Laves in cast alloy 718 is different from that predicted by phase equilibrium. The reason for this difference is that while in equilibrium processes mass diffusion transport is very fast compared with solidification kinetics (V {much_lt} D/L), in casting processes, solidification kinetics is much closer to diffusivity (V {le} D/L). Thus, solidification kinetics cannot be ignored.

Phase changes in superaustenitic steels after long-term annealing

Abstract: A structural study was performed on the austenitic steels Avesta 254 SMO and Avesta 654 SMO after annealing at 700 C for 500, 3188 and 6170 h. Both Avesta steels initially show an unexpectedly large amount of the Laves phase, followed by a relative slow development of the Sigma phase with equilibrium apparently not yet reached after 3188 h. Thermodynamic calculations confirm that the driving forces for alternative precipitates are very similar thus making it easy to form metastable precipitates that only change very slowly to the equilibrium state. TTT calculations also comnfirm that the Laves phase precipitates earlier than the Sigma phase as the temperature is lowered.

The alloying effect on the high temperature deformation of Laves phase NbCr2 intermetallic compound

Abstract: The effect of ternary alloying elements (such as V, Mo and W) on high temperature deformation of the C15 NbCr 2 was investigated by compression test and transmission electron microscopy. The addition of Mo to C15 NbCr 2 resulted in a marked decrease in the 0.2% yield stress and also BDTT. The addition of V resulted in a slight increase in the 0.2% yield stress and almost the same BDTT as that in the unalloyed C15 NbCr 2 . The addition of W to the C15 NbCr 2 resulted in a marked increase in the 0.2% yield stress and also BDTT. The high temperature deformation of the ternary C15 NbCr 2 alloys was discussed on the basis of the easiness of the activation of the dislocations and the counteracting solid solution hardening by the ternary element.