Iron intermetallic phases in the Al corner of the Al-Si-Fe system

The iron intermetallics observed in six dilute Al-Si-Fe alloys were studied using thermal analysis, optical microscopy, and image, scanning electron microscopy/energy dispersive X-ray, and electron probe microanalysis/wavelength dispersive spectroscopy (EPMA/WDS) analyses. The alloys were solidified in two different molds, a preheated graphite mold (600 °C) and a cylindrical metallic mold (at room temperature), to obtain slow (∼0.2 °C/s) and rapid (∼15 °C/s) cooling rates. The results show that the volume fraction of iron intermetallics obtained increases with the increase in the amount of Fe and Si added, as well as with the decrease in cooling rate. The low cooling rate produces larger-sized intermetallics, whereas the high cooling rate results in a higher density of intermetallics. Iron addition alone is more effective than either Si or Fe+Si additions in producing intermetallics. The alloy composition and cooling rate control the stability of the intermetallic phases: binary Al-Fe phases predominate at low cooling rates and a high Fe:Si ratio; the β-Al₅FeSi phase is dominant at a high Si content and low cooling rate; the α-iron intermetallics (e.g., α-Al₈Fe₂Si) exist between these two; while Si-rich ternary phases such as the δ-iron Al₄FeSi₂ intermetallic are stabilized at high cooling rates and Si contents of 0.9 wt pct and higher. Calculations of the solidification paths representing segregations of Fe and Si to the liquid using the Scheil equation did not conform to the actual solidification paths, due to the fact that solid diffusion is not taken into account in the equation. The theoretical models of Brody and Flemings^[44] and Clyne and Kurz^[45] also fail to explain the observed departure from the Scheil behavior, because these models give less weight to the effect of solid back-diffusion. An adjusted 500 °C metastable isothermal section of the Al-Si-Fe phase diagram has been proposed (in place of the equilibrium one), which correctly predicts the intermetallic phases that occur in this part of the system at low cooling rates (∼0.2 °C/s).     

Nucleation of Fe-intermetallic phases in the Al-Si-Fe alloys

Nucleation of Fe-intermetallic phases (i.e. binary Al-Fe, α-AlFeSi, β-AlFeSi, δ-AlFeSi, and q₁-AlFeSi phases) on the surface of different inclusions in six experimental Al-Si-Fe alloys was studied through a quantitative evaluation of the number of inclusion particles that have a direct physical contact with the nucleated phase as seen through the optical microscope. It was found that nucleation of each of the Fe-intermetallic phases was promoted on the surface of several inclusions under the same conditions of alloy composition and cooling rates. Some inclusions exhibited high potency for the nucleation of particular Fe-intermetallic phases under certain conditions and poor potency under other conditions. The potent nucleants for the primary α-Al phase such as γ-Al₂O₃ exhibited poor potency for the nucleation of the Fe-intermetallic particles that lie within the primary phase (intragranular particles). Reactive inclusions such as CaO and SiC are very potent nucleants for the intragranular Fe-intermetallic phase particles. The nucleation of the Fe-intermetallic phases in Al-Si-Fe alloys obeys the general features of nucleation, in particular, the effect of cooling rate and solute concentration on the potency of the nucleant particles: (1) it was observed that increasing the cooling rate enhances the heterogeneous nucleation of the Fe-intermetallic phases on the surface of different inclusions, and (2) the nucleation potency of inclusion particles in both α-Al and interdendritic regions improves with increasing solute concentration up to a certain level. Above this level, the solute concentration poisons the nucleation sites. Nucleation of the Fe-intermetallics in the alloys studied does not seem to be largely affected by the type of the nucleating surface.     

Phase equilibria and intermetallic phases in the Ni-Si-Mg ternary system

The Ni-Si-Mg ternary phase diagram has been established after homogenization and slow cooling to room temperature. The chemical compositions of the alloys and their phases were obtained using fully quantitative energy dispersive X-ray spectroscopy (EDS) with standard spectrum files created from intermetallic compounds Mg₂Ni and Ni₂Si. The following intermetallic phases have been observed: (a) four new ternary intermetallic phases, designated as ν, ω, μ, and τ, (b) a ternary intermediate phase Mg(Ni,Si)₂ based on the binary MgNi₂ phase containing Si; (c) three ternary intermetallic phases, η, κ, and ζ, previously reported by the present authors;[10] and (d) Mg₂SiNi₃ (Fe₂Tb type),^[9] previously reported by Noreus et al. ^[8] The MgNi₆Si₆ phase, which was also previously reported,^[7] was not observed at the corresponding composition in the present work. However, the MgNi₆Si₆ phase reported as being of hexagonal symmetry (Cu₇Tb type),^[9] with the lattice parameters a=0.4948 nm and c=0.3738 nm, possibly corresponds to the μ phase (Mg(Si_0.48Ni_0.52)₇) discovered in the present work. The lattice structure of the newly discovered ω phase was determined with the help of the X-ray indexing program TREOR (developed by Werner et al. ^[13]) to be a hexagonal structure of the Ag₇Te₄ type ((Mg_0.52Ni_0.48)₇Si₄) with the lattice parameters a=1.3511 nm and c=0.8267 nm.     

Iron-rich intermetallic phases and their role in casting defect formation in hypoeutectic Al−Si alloys

Iron is the most common and detrimental impurity in aluminum casting alloys and has long been associated with an increase in casting defects. While the negative effects of iron are clear, the mechanism involved is not fully understood. It is generally believed to be associated with the formation of Fe-rich intermetallic phases. Many factors, including alloy composition, melt superheating, Sr modification, cooling rate, and oxide bifilms, could play a role. In the present investigation, the interactions between iron and each individual element commonly present in aluminum casting alloys, were investigated using a combination of thermal analysis and interrupted quenching tests. The Fe-rich intermetallic phases were characterized using optical microscope, scanning electron microscope, and electron probe microanalysis (EPMA), and the results were compared with the predictions by Thermocalc. It was found that increasing the iron content changes the precipitation sequence of the β phase, leading to the precipitation of coarse binary β platelets at a higher temperature. In contrast, manganese, silicon, and strontium appear to suppress the coarse binary β platelets, and Mn further promotes the formation of a more compact and less harmful α phase. They are therefore expected to reduce the negative effects of the β phase. While reported in the literature, no effect of P on the amount of β platelets was observed. Finally, attempts are made to correlate the Fe-rich intermetallic phases to the formation of casting defects. The role of the β phase as a nucleation site for eutectic Si and the role of the oxide bifilms and AlP as a heterogeneous substrate of Fe intermetallics are also discussed.     

Some aspects of the precipitation of metastable intermetallic phases in INCONEL 718

Some aspects of the precipitation of the metastable intermetallic phases —γ″ and γ″—in the commercial nickel base superalloy, INCONEL 718, have been investigated over a wide range of aging temperatures. It has been confirmed that the spherical γ″ particles and the ellipsoidal γ″ particles evolve predominantly through homogeneous nucleation. Precipitation of the former does not appear to precede that of the latter in this alloy. The tetragonal distortion associated with the γ″ particles has been found to increase with increasing precipitate size. It has been observed that at certain temperatures, physical association between precipitates of the two types occurs frequently, leading to the development of different composite precipitate morphologies. During coarsening, the precipitate size has been found to depend linearly on the cube root of the aging time for γ′ as well as γ″ particles.     

Characterization of dispersed intermetallic phases in rapidly quenched Al-Ti-Ce alloys

The microstructures of Al-3Ti-lCe (wt pct) and Al-5Ti-5Ce alloys melt-spun under controlled He atmosphere have been characterized using analytical electron microscopy. The rapidly solidified microstructures comprise uniform, fine-scale dispersions of intermetallic phase in an aluminum matrix, and particular attention has been given to identification of the dispersed phases. In the Al-3Ti-lCe alloy, the dispersed particles are polycrystalline with a complex twinned substructure and a diamond cubic crystal structure(a _o =1.44 ±0.01 nm) and composition consistent with the ternary compound Al₂₀Ti₂Ce (Al₁₈Cr₂Mg₃ structure type, space group Fd3m). In the Al-5Ti-5Ce alloy, there is, in addition to the dispersed ternary phase, a separate uniform array of fine-scale particles of the binary compound Al₁₁Ce₃. The majority of such particles have the body-centered orthorhombic structure of the low-temperature polymorph, α-Al₁₁Ce₃, but there is evidence to suggest that at least some particles developvia initial formation of the high-temperature body-centered tetragonal phase, β-Al₁₁Ce₃. The accumulated evidence suggests that both binary and ternary particles formed as primary phases directly from the melt during rapid solidification, leaving only small concentrations of solute in aluminum matrix solid solution. Both phases are observed to be resistant to coarsening for up to 240 hours at 400 °C. Formerly Research Fellow, Department of Materials Engineering, Monash University.     

Thermal oxidation of the intermetallic Al3Y surface

The oxidation of the intermetallic Al₃Y surface in air at temperatures of 673 and 823 K is studied by ellipsometry. The dependences of the oxide film thickness on the oxidation time are obtained, and the optical constants of oxide films and the intermetallic compound are determined at a light wavelength of 0.6328 μm.     

High-temperature creep of the intermetallic alloy Ni3Al

Constant stress creep tests have been conducted on Ni₃Al (Hf, B) single crystals in an attempt to characterize the high-temperature creep behavior of this alloy. In contrast to intermediate temperature creep behavior, steady-state creep was observed at 1273 K. This extended steady-state region was formed in less than 1 pct creep strain and lasted for the duration of the creep tests. Primary creep was, however, observed to be limited in nature and consistent with inversetype creep behavior. These observations, preliminary transmission electron microscopy (TEM) observations, and the measured values for the stress exponent(n = 4.3 ± 0.1) and activation energy (Q _c = 398 ± 41 kJ/mole) all suggest that high-temperature creep involves both dislocation mobility and the recovery of dislocation substructure. Attempts to identify a single dislocation mechanism for high-temperature creep were unsuccessful, and it was concluded that a number of slip systems were active at the high temperatures used in these experiments. Formerly Graduate Student, Department of Materials Science and Engineering, Stanford University     

Ternary intermetallic compounds in the system Ni-Ti-Nb

Conclusions Four ternary intermetallic compounds -Ni₈₀Ti₅Nb₁₅, Ni₇₅Nb₅Ti₂₀, Ni₇₅Nb₁₃Ti₁₂ and Ni₅₆Ti₂₉Nb₁₅ -were dis-covered in the system Ni-Ti-Nb by the methods of x-ray diffraction analysis. Isothermal phase diagram sections at 1000 and 900C have been plotted.Translated from Poroshkovaya Metallurgiya, No. 8(44), pp. 61–69, August, 1966.     

Ni_3Al金属间化合物的研究进展

Ni3Al金属间化合物因具有熔点高、抗蠕变强度大、密度低、耐腐蚀、耐氧化以及其它优异性能,已被广泛应用于航空、冶金、机械、电化学、环保工业等领域,并有进一步发展的潜力和扩大应用的需求.该文对Ni3Al金属间化合物的制备及性能的研究进展进行综合评述,并着重论述粉末冶金法制备Ni3Al金属间化合物及其产品的抗氧化与催化性能等的研究现状.     

Selective isolation of intermetallic and carbide phases present in a Ni-Cr-Ti-Al-C alloy

Conclusions A study was made of the electrochemical behavior of the alloy in various electrolytes. On the basis of this study, electrolytes have been chosen for the selective isolation of the intermetallic and carbide phases. The origin of the isolated phases and their exact quantitative composition have been determined.Translated from Poroshkovaya Metallurgiya, No. 8(44), pp. 93–97, August, 1966.