Showing papers on "Laves phase published in 1989"

INCONEL 718: A solidification diagram

Abstract: As part of a program studying weldability of Ni-base superalloys, results of an integrated analytical approach are used to generate a constitution diagram for INCONEL 718* in the temperature range associated with solidification. Differential thermal analysis of wrought material and optical and scanning electron microscopy, electron probe microanalysis, and analytical electron microscopy of gas tungsten arc welds are used in conjunction with solidification theory to generate data points for this diagram. The important features of the diagram are an austenite (γ)/Laves phase eutectic which occurs at ≈19.1 wt pct Nb between austenite containing ≈9.3 wt pct Nb and a Laves phase which contains ≈22.4 wt pct Nb. The distribution coefficient for Nb was found to be ≈0.5. The solidification sequence of INCONEL 718 was found to be (1) proeutectic γ, followed by (2) a γ/NbC eutectic at ≈1250°C, followed by (3) continued γ solidification, followed by (4) a γ/Laves phase eutectic at ≈1200°C. An estimate of the volume fraction eutectic is made using the Scheil solidification model, and the fraction of each phase in the eutectic is calculatedvia the lever rule. These are compared with experimentally determined values and found to be in good agreement.

The solidification metallurgy of Alloy 718 and other Nb-containing superalloys

Abstract: The solidification behavior of Alloy 718 and other Nb-bearing austenitic superalloys has been examined using an integrated analytical approach All alloys of this type begin solidification with the formation of Nb-lean austenitic dendrites Interdendritic eutectic-type solidification constituents involving MC-type carbides and a Nb-rich Laves phase occur in these alloys The gamma/Laves eutectic constituent terminates solidification in these alloys Nb is the dominant element in the evolution of solidification microstructure with C and Si affecting the amounts of gamma/MC and gamma/Laves constituent observed Simple solidification models predict reasonably well the amount of eutectic constituent observed 11 refs, 9 figs, 2 tabs

Neutron diffraction studies of the crystalline and magnetic properties of UFe2

Abstract: Studies using both polarised and unpolarised neutrons have been performed on single crystals of the Laves phase intermetallic compound UFe2. The authors describe studies of the twinning characteristics (two crystals with a common (1, 1, 1) axis) of the sample, a feature that is probably common in these cubic compounds. They then describe studies with polarised neutrons to determine the magnitude of the spin and orbital moments on the uranium site. In agreement with a recent theoretical prediction these two moments are found to be almost equal, but oppositely directed in real space. The net moment at the U site is only 0.01 mu B. The polarised neutron experiments also allow them to put an upper limit on any possible anisotropy in the magnetisation. High-field magnetisation experiments up to (20 T) on the same crystal as used in the neutron experiments allow them to determine the conduction-electron polarisation.

Competition of magnetism in iron Laves phase compounds

Abstract: To investigate the varied magnetism of Fe Laves phase compounds, the electronic structures were calculated by the LMTO method within the framework of the local-spin-density approximation. Calculating the total energy as a function of the lattice constant and the magnetic moment, the authors found that two types of ferromagnetism, antiferromagnetism and paramagnetism complete with each other in the (Sc1-xTix)Fe2 and (Zr1-xNbx)Fe2 systems. The lattice constant or the concentration x is an important factor in the appearance of these different types of magnetism. The difference between the two types of ferromagnetism is clearly seen in the density of states. One of them is weakly itinerant ferromagnetism.

Hydrogen absorption and phase transitions in rapidly quenched LaNi alloys

Abstract: Several LaNi binary alloys were rapidly quenched by melt spinning. A metastable phase and an uniform Laves phase were obtained. Differences in behaviour were observed between hydrogenation of quenched materials and of as-cast materials whose compositions were identical. The quenched materials, in contrast to the as-cast phases, had a tendency to form amorphous LaNiH alloys. The amorphous alloy was stable, in particular when the ratio La:Ni was 1:2.

Superstructure in the intermetallic compound TmNi2

Abstract: The results of neutron and X-ray diffraction experiments at 300 and 4.2 K on the intermetallic compound TmNi 2 indicate that the crystal structure of the two modifications of TmNi 2 observed by us are closely related to the cubic Laves phase structure (C15). From the fit between the experimental and calculated data a consistent set of parameters describing the atomic positions in the F 43 m space group is obtained for one of these modifications, the corresponding lengths of the unit cell being doubled in all three dimensions with respect to the C15 structure. The other observed modification of TmNi 2 has a simple cubic structure, the unit cell being the same size as found for the C15 type.

Structure and hardness of Ni-Mo-Cr-Si wear and corrosion resistant alloys

Abstract: The microstructure and hardness of Tribaloy 700 (T 700) of nominal composition (wt-%) 50Ni–32Mo–3Si–15Cr and two alloys of similar Ni, Mo, and Cr contents, but having no or 6%Si have been determined. The microstructure of the three alloys consisted of a primary Laves phase in a eutectic matrix, but the volume fraction and crystal structure of the primary Laves phase and the morphology of the eutectic matrix were different. The presence of Si increased the proportion of primary phase, stabilised the hexagonal form of the Laves phase, and produced an ill defined eutectic. The hardnesses of the as cast alloys were similar, but T 700 was the most stable at elevated temperatures.MST/881

The precipitation of Laves' phase in 9Cr1Mo steels

Abstract: The precipitation of Laves'-type Fe2Mo can have important consequences for the mechanical properties of high chromium ferritic steels. In this Letter available experimental results relating to the precipitation of this phase in 9Cr1Mo steel are analysed to obtain approximate thermodynamic and kinetic data for the reaction from ferrite. These have been used to obtain descriptions of the solubility of Laves'-type Fe2Mo and the rate of molybdenum removal from the matrix.

The Welding Metallurgy of Custom Age 625 PLUS Alloy

Abstract: The authors discuss the welding metallurgy (solidification behavior, mechanical properties of weld metal, and susceptibility to fusion zone hot cracking) of Carpenter Custom Age 625 PLUS alloy. This alloy solidifies to a nickel austenitic matrix with the formation of a Nb-rich {gamma}/Laves terminal solidification constituent. The low Si and C concentrations in combination with a lower Nb content (3.4wt-%) in this alloy result in a relatively small volume fraction of eutectic constituent relative to that previously observed in Alloy 718. Because of the low volume fraction of eutectic constituent, this alloy has better weldability than Alloy 718. In addition to Laves phase, {gamma} was observed adjacent to Laves phase {gamma} was observed adjacent to Laves in the fusion zone microstructure of the new alloy. Weld metal strengths approximately equal to aged wrought product can be achieved by direct aging of welds.

Formation and crystallization of hydrogen-induced amorphous SmFe2H3.6 alloy

Abstract: On hydrogenation of the Laves phase SmFe2, an amorphous SmFe2H3.6 (a-SmFe2H3.6) alloy was formed between 400 K and 500 K. The amorphous nature of the alloy was confirmed by X-ray diffraction, transmission electron microscopy and thermal analysis. However, SmFe2 absorbed hydrogen in the crystal state below 350 K and decomposed into SmH2 and α-Fe above 550 K. The crystallization behaviour of a-SmFe2H3.6 was investigated by differential scanning calorimetry in combination with electron microscopy. Even after considerable hydrogen desorption (Image ) by an endothermic reaction on heating, the amorphous state was retained. Crystallization of a-SmFe2H3.6 took place in two stages. The first stage involved the precipitation of α-Fe in the amorphous matrix. The second stage involved the decomposition of the remaining amorphous phase into the equilibrium phases SmH2 and SmFe2.

Neutron Inelastic Scattering in the HfV2H3.3 Laves Phase Hydride

Abstract: Both the ZrV^ and HfV cubic Laves phases can absorb hydrogen to form non-stoichiometric hydrides up to a hydrogen to metal ratio approaching 2. Above 320K neutron diffraction measurements /1,2/ on deuterated samples reveal that, at low concentrations, the hydrogen occupies tetrahedrally co-ordinated interstitial positions surrounded by two Hf(Zr) and two V atoms (2-2 sites) in the C15 lattice of the intermetallic compound. At higher concentrations, corresponding to HfV_H , where x>2.5, other sites (1-3) surrounded by one Hf(Zr) and 2 X three V atoms are also occupied. There are 48 2-2 and 16 1-3 sites in each fee cube of Hf(Zr) atoms. Below 320K the structure becomes tetragonal and the 2-2 sites divide into three sub-sets, which

A Microstructural and Mechanical Properties Comparison of P/M 718 and P/M TA 718

Abstract: The effects of Ta on phases and mechanical properties in conventional 718 type compositions have not been fully explored. While Ta and Nb have similar atomic sizes, the solubility of Ta in nickel is much greater than that of Nb. This difference in solubility would affect the initial segregation on solidification and subsequent phase reactions. To study the role of Ta in phase reactions and on alloy stability in a homogeneous material, a comparative study of conventional P/M 718 and P/M Ta 7 18 was undertaken. The results of this study showed that the heat treatment used for conventional 7 18 did not produce an significant strengthening phases in Ta 718 and a modified heat treatment was to reci necessa~ p p itate ?/’ and 1/ strengthening phases. The ?/’ phase in Ta 718 is still present at 1750 F. The r to delta transition in Ta 718 is more sluggish and occurs at higher temperatures than in conventional 718 materials. Data from tensile tests at 1400’F indicates that Ta 718 has a higher temperature capability than conventional 718. Introduction Ta and Nb atoms are generally believed to have similar effects on structures in high temperature superalloys, largely because of their similar atomic size and location in the periodic table. The role of Ta in Ni-based superalloys has been the subject of numerous papers, but the effect of Ta in Alloy 718 has not been fully explored. In Alloy 718, Nb has been shown to be not only part of the strengthening phases, but also the main cause of segregation because of its limited solubility in y matrix. Homogenization efforts to eliminate the Laves phase and diffuse the Nb are time consuming since the rate of diffusion of Nb is slow, and large amounts of residual segregation are commonly found in so-called homogenized materials. Recently, Loewenkamp, Radavich, and Kelly studied the effects of a one to one atomic base substitution of Ta for Nb in cast Alloy 718. Their study showed that Ta reduced the amount of initial segregation which they attributed to the higher solubility of Ta in the y matrix. More importantly, Ta additions, similar to Nb, were found to produce strengthening v’ phase. This y’ phase was still present at temperatures up to 1800F. The transition of the y’ to 6 phase found in Alloy 7 18 was not obvious in the cast Ta-modified 7 18. In the study by Loewenkamp, Radavich, and Kelly, the pattern of y’ precipitation at 18OOF suggested that residual segregation was present and that it would influence the temperature of stability of both the 6 and y’ phases. To eliminate the segregation effects on Superalloy 718-Metallurgy and Applications Edited by E.A. Loria The Minerals, Metals & Materials Society, 1989 623 phase behavior, especially the strengthening 1/’ phase, a program was carried out to characterize the phases formed in alloy 7 18 to the phases in Ta 718 using homogeneous consolidated powder samples. The important goals of this program were to study the effect of Ta on the precipitation and stability of 1/’ and relate the 1/’ behavior to mechanical properties. Materials Preparation The Alloy 7 18 and Ta 718 powders were produced commercial rapid solidification processes. The compositions shown in Table 1. Table 1: Alloy Compositions using argon atomization and of the powders produced are wt% TA 718 4Y5 2% 179 Nb Ta MO Ti Al B C S 7.2 33 9 05 0001 003 001 Alloy 718 52:5 18:5 18:5 5:l 3:0 ::9 0:5 0:oos 0:04 Oil03 Both powder products were consolidated at atmospheric pressure (CAP) and were hot rolled at 2025F and heat treated at 1825F/l hr./WC + 1325F/8 hrs./FC to 115OF/S hrs./AC. Room temperature tensile, 1200F tensile, and stress rupture tests were carried out. For Ta 718, 1400F tensile tests were also conducted. Microstructural characterization was performed using scanning electron microscopy, and phase identification was carried out using EDAX analysis and X-ray diffraction. Diffraction samples were prepared from residue of phase extractions using a 10% HCl-Methanol solution with a Ta cathode at 5 volts for approximately 2 hours. The sample preparation for the SEM study was as follows: 1. Mechanical polish to 600 grit Sic paper and 6p, diamond polish. 2. Electropolish using a stainless steel beaker as the cathode and a solution of 20% HzS04 and 80% Methanol. Polishing time was 20 seconds at 5 volts. 3. Electrolytic etch in a CrO3 etchant ( 172cc H~POJ, 1Occ HzSOb, 16g 003) for 10 seconds at 5 volts. Mechanical Properties Analysis The results of the room temperature tensile and 1200F tensile tests for both conventional 7 18 and Ta 7 18 are shown in Figure 1 along with comparative literature values for the same heat treatment. The lower results for the Ta-modified product are due to the lack of adequate strengthening precipitates and porosity resulting from low densification of the Ta 718. The conventional Alloy 718 properties are also lower than comparative literature values as a result of the same low densification. Fractography micrographs of the room temperature tensile samples, Figures 2A and 2B, illustrate the presence of residual porosity due to the lack of complete consolidation of the powder. When the consolidated samples were fully 100% dense, the mechanical properties improved dramatically. The important effect of densification on mechanical properties is shown in Figure 3. Heat Treatment Study SEM studies of the heat treated Ta 718 material showed very little precipitation of the y” and y’ strengthening phases when given the standard 1325OF to 1150°F age. The comparison in strengthening phase precipitation between Alloy 718 and Ta 718 is shown in Figures 4A and 4B. The precipitation in the Ta 718 is very difficult to resolve even at 30K magnification indicating the standard heat treatment was too low for Ta 718. Both figures show precipitated particles that are finely dispersed. Although Alloy 718 shows a greater disnersion of strengthening particles, neither alloy shows sufficient precipitation for optimum mechanical properties as evidenced by the high magnification necessary to view the y” and r’ particles. 624

Electrochemistry of Multiphase Nickel‐Base Alloys in Aqueous Systems

Abstract: A study has been conducted to evaluate the effect of galvanic coupling between precipitated phases, matrix, and near grain boundary chemistries in nickel-base alloys. Corrosion potential, potentiodynamic polarization, behavior, galvanic effects, and passivation kinetics have been determined for grain boundary {gamma}{prime}, Laves phase, and Ni-Cr-Fe matrices for a wide range of chromium contents in aqueous systems over the temperature range 30{degrees} - 288{degrees}C. Electrochemical activity of {gamma}{prime}, Laves phase, and sensitized chemistries is significantly higher than the matrix and varies with composition and temperature. Galvanic couples, {gamma}{prime}/matrix, and Laves/matrix in a simulated crack tip environment exhibit higher current density than a sensitized chemistry (6.3 weight percent Cr)/matrix couple. The results of the study show that the electrochemical relationships between grain boundary phases and their couples with the matrix can provide for an extremely local (i.e., at the crack tip) source of anodic and cathodic processes. A correlation between grain boundary {gamma}{prime} (Ni{sub 3}Al)/matrix galvanic coupling and environmentally assisted intergranular cracking in alloy X-750 has been made. At low temperatures, around 100{degrees}C, hydrogen embrittlement is proposed as the dominant mechanism. At high temperature it is suggested that both hydrogen embrittlement and stress corrosion cracking are viable mechanisms for crack propagation. Amore » schematic model is presented which describes the relationship between embrittlement mechanisms and local electrochemistry at the crack tip.« less

Production of hydrogen-occlusive alloy

Abstract: PURPOSE:To obtain the title alloy free from developing cracks etc., capable of stably retaining its shape outstanding in strength, by covering the surface of the first lamellar or thin bandlike hydrogen-occlusive alloy with the specific second hydrogen-occlusive alloy. CONSTITUTION:The objective alloy can be obtained by covering (A) the surface of the first lamellar or thin bandlike hydrogen-occlusive alloy 1 to 50mu thick made up of LaNi5, CaNi5, etc., with (B) the second hydrogen-occlusive alloy made up of e.g., a Laves phase alloy which is to be converted into noncrystalline one by hydrogen-absorbing reaction (e.g., LaNi2, CeNi2, SmNi2) through the sputterling, vapor deposition or MBE process so that the thickness of the alloy B fall, e.g., between 0.01 and 1mu.

Intermetallic Precipitates in Zirconium-Niobium Alloys

Abstract: Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) have been used to study the composition and structure of intermetallic precipitates, principally those containing iron and chromium, in Zr-2.5wt%Nb pressure tubing and Zr-1wt%Nb fuel sheathing. The Zr-lwt%Nb fuel sheathing contained Zr(FeCr) 2 . Laves phase precipitates with either the C-14 hexagonal or C-15 cubic structure. Orthorhombic Zr 3 Fe precipitates were also tentatively identified. Tetragonal Zr 2 Fe precipitates were found in Zr-2.5wt%Nb pressure tubing containing 700-ppm iron. The volume fraction of Zr 2 Fe was very small (<0.1% on an area basis) and was much smaller than the volume fraction of Zr(FeCr) 2 precipitates in Zr-1wt%Nb. The difference in type and volume fraction of iron-containing precipitates in Zr-lwt%Nb and Zr-2.5wt%Nb despite similar iron and chromium contents is tentatively attributed to the different volume fractions of the β-phase. Iron is a β-stabilizer and concentrates in the β-phase. The Zr-2.5wt%Nb alloy, which contains the higher volume fraction of β-phase, takes in more iron in solution in the p-phase thus leaving less to precipitate out as an intermetallic.