Experimental Investigation on the Rheological Behavior of Hypereutectic Al-Si Alloys by a Precise Rotational Viscometer

Determining the viscosity of low solid volume fraction semisolid alloys is important for predicting the gradient of primary particles within functionally graded materials (FGMs), produced by in-situ casting processes. In this study, a new precise rotational viscometer was developed and used to measure the viscosity of Al-22 pct Si and Al-30 pct Si semisolid alloys up to solid volume fractions of 9 and 20 pct, respectively. Three kinds of typical curves, viscosity (η) vs solid volume fraction (f _s ), shear time (t), and shear rate ([(g)\dot] \dot{\gamma } ), were derived from the results of viscosity measurements for both Al-Si alloys. In the semisolid Al-Si alloys, thixotropic behavior was not detected at low solid volume fractions, but this behavior was obviously observed with increasing solid volume fraction and could be described by an analytical model. Finally, the results showed that the equilibrium viscosity of the semisolid alloys with thixotropic properties decreased by increasing shear rate according to the Ostwald–De Waele power law. A special test, developed in this research, was used to show the effect of agglomeration on the viscosity of the semisolid alloys.     

Specific permeability of partially solidified dendritic networks of Al-Si alloys

Porous dendritic networks of Al-4 pet Si and Al-4 pct Si-0.25 pet Ti alloys with volume fraction solid,g _sA > 0.628 were prepared by removing the segregated interdendritic liquid from partially solidified samples of the alloys. In the equiaxed samples, available channels for flow were predominately between the grains. Specific permeabilities of the porous dendritic networks were measured with a triaxial cell permeameter. Measured values of specific permeability were 1 × 10^-9 to 3.5 × 10^-11 cm² in the Al-4 pet Si alloy for volume fractions solid of 0.655 to 0.94, respectively. Specific permeabilities in Al-4 pct Si-0.25 pct Ti alloy were 4.82 × 10^-10 to 7.6 × 10^-11 cm² for volume fractions solid of 0.628 to 0.837, respectively. For equivalent volume fractions solid, the measured specific permeabilities were consistently lower for the grain refined samples. Flow through the porous dendritic networks obeys D’Arcy’s law and equations derived from the capillaric flow model for volume fraction liquid less than ∼0.35. Formerly a graduate student in the Department of Metallurgy and Materials Science, M.I.T.     

The low strain tensile behavior of U-7.5 wt pct Nb-2.5 wt pct Zr

The stress-strain response of polycrystalline, γ-quenched U-7.5 wt pct Nb-2.5 wt pct Zr alloy was studied as a function of strain rate and compared to equilibrium stress-strain tests performed by allowing the strain to reach a maximum value at incrementally increasing stresses. Equilibrium stress-strain tests were also performed on prestressed samples. Sheet tensile specimens were held at various states of strain in an X-ray diffractometer to determine crystal structural changes during deformation. Prestressed tensile bars were sectioned and examined metallographically and with the X-ray diffractometer. Two linear regions were observed in the equilibrium stress-strain tests: a low stress region with a slope of 5.3 to 5.5 x 10⁶ psi, and a region above 40,000 psi with a slope of 3.3 x 10⁶ psi. Finite strain rates tended to increase both slopes. The diffractometer experiments yielded plots of lattice parameter vs strain which showed a shift from a bcc structure of the γ^s phase, to a bct structure of the γ₀ phase between 1 and 3 pct deformation. It is postulated that this is a thermoelastic martensite transformation. A semiempirical equation was developed which describes the equilibrium stress-strain behavior of this alloy in terms of a stress induced phase transformation.     

Thermal strain in the mushy zone for aluminum alloys

The parameters in a recently developed constitutive equation for macroscopic thermal strain in the mushy zone have been determined for the commercial alloys A356, AA2024, AA6061, and AA7075 in addition to an Al-4 wt pct Cu alloy. The constitutive equation for macroscopic thermal strain in the mushy zone reflects that there is no thermal strain in the solid part of the mushy zone at low solid fractions and that the thermal strain in the mushy zone approaches thermal strain in the fully solid material as the solid fraction increases toward 1. The development of thermal strain in the mushy zone is determined by combining experimentally measured contraction of a cast sample with thermomechanical stimulations. Experiments were performed at cooling rates in the range from 2 to 5.5 °C/s. The solid fractions when the tested alloys start to contract,g _s ^th, are in the range from 0.63 to 0.94. Grain refinement increasesg _s ^th for all the tested alloys. For most of the tested alloys the thermal strain in the mushy zone increases rapidly to the same level as thermal strain in fully solid material once the solid fraction becomes higher thang _s ^th.     

The role of thermal stabilization in martensite transformations

The variation of the kinetics of the martensite transformation with carbon content and martensite habit plane has been investigated in several Fe−Ni based alloys. Transformation in an Fe-25 wt pct Ni-0.02 wt pct C alloy exhibits predominantly athermal features, but some apparently isothermal transformation also occurs. In a decarburized alloy, on the other hand, the observed kinetic features, such as the dependence ofM _s on cooling rate, were characteristic of an isothermal transformation. In contrast, Fe-29.6 wt pct Ni-10.7 wt pct Co alloys with carbon contents of 0.009 wt pct C and 0.003 wt pct C transform by burst kinetics to {259}_γ plate. At both these carbon levels, theM _b temperatures of the Fe−Ni−Co alloys are independent of cooling rate. It is proposed that the change in kinetic behavior of the Fe-25 pct Ni alloy with the different carbon contents is due to the occurrence of dynamic thermal stabilization in the higher carbon alloy. Dynamic thermal stabilization is relatively unimportant in the Fe−Ni−Co alloys which transform by burst kinetics to {259}_γ plate martensite. P. J. FISHER, formerly with the University of New South Wales D. J. H. CORDEROY, formerly with the University of New South Wales     

Microstructure and yield strength of UDIMET 720LI alloyed with Co-16.9 Wt Pct Ti

We proposed a new method for developing Ni-base turbine disc alloy for application at temperatures above 700 °C by mixing a Ni-base superalloy U720LI with a two-phase alloy Co-16.9 wt pct Ti in various contents. The microstructure and phase stability of the alloys were analyzed using an optical microscope, a scanning electron microscope, energy-dispersive spectroscopy, and an X-ray diffractometer. The yield strength was studied by compression tests at temperatures ranging from 25 °C to 1200 °C. The results show that all the alloys had a dendritic structure. Ni₃Ti (η) phase was formed in the interdendritic region in the alloys with the addition of Co-16.9 wt pct Ti, and its volume fraction increased with the increase in the addition of Co-16.9 wt pct Ti. The results of exposure at 750 °C show that the addition of Co-16.9 wt pct Ti to U720LI had a great effect on suppressing the formation of σ phase due to the reduced Cr content in the γ matrix. Compared to U720LI, the alloys with the addition of Co-16.9 wt pct Ti possessed higher yield strength. The solid-solution strengthening of γ and γ′ and higher volume fraction of γ′ were assumed to cause this strength increase.     

Influence of precipitates on the mechanical response and substructure evolution of shock-loaded and quasi-statically deformed Al−Li−Cu alloys

The mechanical response and substructure evolution of two Al−Li−Cu alloys (Al-2.90 wt pct Li-1.00 pct Cu-0.12 pct Zr and Al-2.30 pct Li-2.85 pct Cu-0.12 pct Zr) subjected to shock-loading (strain rate έ> 10⁶ s^-1), Split-Hopkinson-Pressure-Bar compression (έ ~ 5 × 10³ s^-1), and quasi-static compression (έ ~ 1.5 × 10^-3 s^-1) were examined. The strain levels achieved in these three deformation paths were desined to be comparable,i.e., all ∼15 pct. Both alloys were either naturally or artificially aged to yield an underaged or overaged condition. Various precipitates, such as theδ' andT ₁ phase, of different sizes and volume fractions were dispersed in the matrix and at the grain boundaries. The substructure in all of the shock-loaded, Split-Hopkinson-Pressure-Bar, and quasi-static compression samples was characterized by localized slip bands and microbands with the exception of the overaged alloys. The density of dislocations and dislocation loops was higher, independent of the aging condition, in the shock-loaded specimens. Well-defined cell structures were not observed in any of the samples, independent of strain rate. The influence of precipitates, shearable or not, on the substructure development in Al−Li−Cu alloys during shock-loading was seen to be pronounced, even though the size and volume fraction of precipitates was small and low, respectively. Flow stress measurements showed that the shock-loaded samples have flow strengths 3 to 8 pct higher than the quasi-statically deformed samples. This small, but reproducible, strength increment, for alloys deformed to equivalent strains at low and high rates, indicates that the Al−Li alloys studied have a small rate sensitivity. Based upon comparison of the results of the shock-loaded and quasi-static samples, it is concluded that the fundamental deformation mechanisms and substructure evolution in all three loading paths are not drastically different, corroborating previous investigations.     

The constitution and phase stability of overlapping melt trails in Ag- Cu alloys produced by continuous laser melt quenching

Coatings consisting of overlapping trails melted with a scanning CW CO₂ laser have been produced on Ag-Cu alloys with the following compositions: Cu 17 at. pct Ag; Cu 37 at. pct Ag; Cu 61.7 at. pct Ag; Cu 71.8 at. pct Ag; and Cu 82 at. pct Ag. The laser beam was scanned at a velocity of 34 cm s¹ and with an intensity of 3.6 MW cm^-2. Selected trails were examined by X-ray diffractometry, optical microscopy, and scanning electron microscopy in the as-irradiated condition and after annealing for various periods of time in the temperature range 100 to 450 ° C. Time-temperature-transformation diagrams based on the annealing studies are presented. Significant amounts of the metastable extended solid solution (γ) were observed in the Ag-rich alloy trails. The silver rich terminal solid solution (α) was also detected, formed probably by solid state precipitation. An α’ phase with lattice parameter lying between that ofy and α was also observed in the Cu 61.7 at. pct Ag alloy. A metastable equilibrium diagram has been constructed and is employed to interpret these observations. The most striking microstructural feature of the trails are bands marking sequential positions of the melt-solid interface. We propose that these bands are evidence for planar, oscillating - steady-state, interface motion. The observation of a periodic cellular breakdown of the planar interface in the Cu 61.7 at. pct Ag alloy is attributed to a diffusional instability previously predicted by Baker and Cahn.     

Some Effects of Parent Phase Aging on the Martensitic Transformation in a Cu- AI- Ni Shape Memory Alloy

Transmission electron microscopy observations have been carried out for a Cu-14 pct Al-4 pct Ni (wt pct) alloy aged in the thin foil state in an electron microscope. It was found that large cuboidal precipitates of theγ ₂ phase and many small domains of a highly ordered phase form in the DO₃ matrix during aging. The small ordered domains form preferentially on matrix antiphase boundaries as well as within the antiphase domains. The formation ofγ ₂ and the highly ordered phase, both of which are rich in alloy content, depletes the matrix of solute and thus raises the transformation temperaturesM _s andM _f. The small domains of the highly ordered phase prevent the propagation and reversion of martensite plates, leading to higherM _s-M_f andA _fins-A_f temperature intervals.     

Effects of carbon content and ausaging on γ ↔ α′ transformation behavior and reverse-transformed structure in Fe-Ni-Co-Al-C alloys

The effects of carbon content and ausaging on austenite γ ↔ martensite (α′) transformation behavior and reverse-transformed structure were investigated in Fe-32Ni-12Co-4Al and Fe-(26,28)Ni-12Co-4Al-0.4C (wt pct) alloys. TheM _s temperature, the hardness of γ phase, and the tetragonality of α′ increase with increasing ausaging time, and these values are higher in the carbon-bearing alloys in most cases. The γ → α′ transformation behavior is similar to that of thermoelastic martensite; that is, the width of α′ plate increases with decreasing temperature in all alloys. The αt’ → γ reverse transformation temperature is lower in the carbon-bearing alloys, which means that the shape memory effect is improved by the addition of carbon. The maximum shape recovery of 84 pct is obtained in Fe-28Ni-12Co-4Al-0.4C alloy when the ausaged specimen is deformed at theM _s temperature and heated to 1120 K. There are two types of reverse-transformed austenites in the carbon-bearing alloy. One type is the reversed y containing many dislocations which were formed when the γ/α′ interface moved reversibly. The plane on which dislocations lie is (01 l)_γ if the twin plane is (112)_α′. The other type of reverse-transformed austenite exhibits γ islands nucleated within the α′ plates.     

Effect of B on the microstructure and mechanical properties of mechanically milled TiAl alloys

The present study is concerned with γ-(Ti₅₂Al₄₈)_100−x B _x (x=0, 0.5, 2, 5) alloys produced by mechanical milling/vacuum hot pressing (VHPing) using melt-extracted powders. Microstructure of the as-vacuum hot pressed (VHPed) alloys exhibits a duplex equiaxed microstructure of α₂ and γ with a mean grain size of 200 nm. Besides α₂ and γ phases, binary and 0.5 pct B alloys contain Ti₂AlN and Al₂O₃ phases located along the grain boundaries and show appreciable coarsening in grain and dispersoid sizes during annealing treatment at 1300 °C for 5 hours. On the other hand, 2 pct B and 5 pct B alloys contain fine boride particles within the γ grains and show minimal coarsening during annealing. Room-temperature compressing tests of the as-VHPed alloys show low ductility, but very high yield strength >2100 MPa. After annealing treatment, mechanically milled alloys show much higher yield strength than conventional powder metallurgy and ingot metallurgy processed alloys, with equivalent ductility to ingot metallurgy processed alloys. The 5 pct B alloy with the smallest grain size shows higher yield strength than binary alloy up to the test temperature of 700 °C. At 850 °C, 5 pct B alloy shows much lower strength than the binary alloy, indicating that the deformation of fine 5 pct B alloy is dominated by the grain boundary sliding mechanism. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.     

Microstructure and its development in Cu-Al-Ni alloys

The microstructure of as-cast Cu-AI-Ni alloys, based on copper containing 9 to 10 wt pct Al and up to 5 wt pct Ni, has been examined. The development of the microstructure on continuous cooling has also been investigated. For alloys with 9.2 to 9.3 wt pct Al, and less than 1 wt pct Ni, the as-cast microstructure consists of proeutectoid α solid solution, α + γ₂ eutectoid, and martensitic β. If the nickel content is more than 2.5 wt pct, the α + γ₂ eutectoid is replaced by α + β ₂ ^′ eutectoid, and no martensitic β is observed in the as-cast alloys. The morphologies of the β ₂ ^′ and γ₂ eutectoid phases are similar; both have the Kurdjumov-Sachs (K-S) orientation relationship with the a phase. Two eutectoid reactions, involving β to α + γ₂ and β to α + β′₂, have been observed in an alloy containing 9.7 wt pct Al and 2.7 wt pct Ni. When both eutectoid reactions occur, the Nishiyama-Wassermann (N-W) orientation relationship exists between γ₂ or β ₂ ^′ and the α phase. During continuous cooling, proeutectoid α solid solution is the first phase to precipitate from the high-temperature β phase. The β to α + β ₂ ^′ eutectoid reaction starts at higher temperatures than the β to α + γ₂ reaction. Tempering of the as-cast alloys results in the elimination of the martensitic β. Y.S. SUN formerly Research Associate with the Manchester Materials Science Centre.     

Carbide reinforcement in two directionally solidified alloyed nickel eutectic alloys

Two directionally solidified carbide-reinforced alloyed nicel eutectics were evaluated; an alloy consisting of monocarbide fibers in a single phase matrix and one containing monocarbide fibers in a two-phase γ-γ′ matrix. The mechanical properties and microstructures of these alloys are compared to those of two directionally solified alloys having the same nominal matrix compositions, but not containing carbide fibers. The calculated strengths of the monocarbide fibers in the γ′-containing eutectic alloy are 1,400,000 psi (9650 mn/m²) and 243,000 psi (1680 mn/m²) at room temperature and 1000°C, respectively, while those in the single phase γ matrix eutectic at the same temperatures are 590,000 psi (4060 mn/m²) and 298,000 psi (2050 mn/m²). At room temperature, the lower strength of fibers from the γ matrix alloy is believed to result from stress concentrations induced by the presence of growth facets on the fibers. The lower apparent strength at 1000°C of fibers from the γ′-containing eutectic alloy is related to nucleation of needles believed to be M₂₃C₆ on the monocarbide fibers during deformation. These needles appear to act as stress raisers and cause early failure of fibers.     

Solidification of undercooled Sn-Sb peritectic alloys: Part I. Microstructural evolution

The droplet emulsion technique, which involves dispersal of a bulk liquid alloy into a collection of fine droplets (5 to 30μm), was applied to Sn-Sb alloys to yield high levels of controlled undercooling. The maximum undercooling levels achieved varied from 179 °C for pure Sn to 113 °C for a Sn-16 at. pct Sb alloy. Analysis of hypoperitectic alloy samples (alloys with an Sb content less than that of the liquid at the peritectic temperature) indicates that solute trapping occurs during solidification at the levels of undercooling and cooling rate investigated, yielding nearly homogeneousβ-tin solid solutions with compositions approaching those of the bulk alloys. With increasing undercooling and/or cooling rate, hyperperitectic alloys exhibit a transition from a highly segregated structure consisting of faceted primary intermetallic phase and cellularβ to a structure consisting primarily of a supersaturated tin-rich solid solution. Lattice constant measurements confirm that virtually complete supersaturation ofβ-tin was achieved in emulsion samples cooled at 200 °C s^s−1 for compositions up to approximately 20 at. pct Sb. The development and characteristics of subsequent solid-state precipitation were used to guide the interpretation of the often complex solidification reaction sequences in the hyperperitectic alloys. The formation of supersaturatedβ-tin solid solutions in the undercooled samples is related to the appropriate metastable phase equilibria and the development of solute trapping. Formerly Graduate Student, Department of Materials Science and Engineering, University of Wisconsin-Madison     

Effects of cooling rate and γ′ morphology on creep and stress-rupture properties of a powder metallurgy superalloy

The 650 °C creep and stress-rupture (S/R) behavior of powder metallurgy (PM) RENé 95 alloy was investigated under various cooling rates from two subsolvus solution temperatures. The cooling conditions were chosen to alter the morphology of the cooling γ′ (γ′_c), which constituted a high volume fraction (≃0.85) of all γ′ precipitates. The dispersion characteristics of γ _c ^′ changed from very fine at the highest cooling rate to progressively coarser with reduced cooling rate. At small γ _c ^′ (≃0.05 μm), a marked increase in creep and S/R resistance was observed, whereas at larger γ _c ^′ (≃0.1 μm), a marked decrease was observed. Also, for a givenγ _c ^′ size, both properties benefited from a higher solution temperature. The change in properties with the size of γ _c ^′ was associated with a change in deformation mechanism. The operative mechanism was essentially determined by the mean surface-to-surface spacing(l _s ) of the γ _c ^′ precipitates. Atl _s > 0.05 μm, the Orowan mechanism of dislocation expansion and looping became favorable, which led to higher primary creep strain and steady-state rate and also reduced S/R life. Atl _s < 0.05 μm, dislocation motion was significantly restricted, and the low dislocation density and inadequate dislocation sources were responsible for greatly suppressed primary creep strain and steady-state rate, and in some cases, for an incubation period in the creep curve. In addition, the S/R life increased significantly at lowl _s . Atl _s ≃ 0.05 μm, a mixed mode of deformation was observed due to inherent distribution of particle spacing in the structure, and this gave rise to intermediate creep and S/R resistance. Formerly Manager, Research and Development Department, Cameron Forge Company.     

Effects of composition and microstructure on fatigue of γ/γ′-δ type eutectic alloys

Room temperature tension-tension fatigue tests were performed on two lamellar γ/γ′-δ alloys, one with 0 pct Cr and one with 6 pct Cr. The 6 pct Cr alloy was solidified at 3 cmJh while the 0 pct Cr alloy was solidified at 3 cm/h and 5.7 cm/h. Fatigue testing was done on both alloys in the as-directionally solidified condition and on the 0 pct Cr alloy after heat treatment. Increasing the growth speed of the 0 pct Cr alloy increased the fatigue life of the material at stresses above the 10⁷ cycle fatigue limit. Partial solution treating and aging of the 0 pct Cr alloy,R = 3 cm/h, increased the fatigue life relative to the as-directionally solidified material at high stresses, to the same extent as increasing the growth speed. Full solution treatment and aging of the 0 pct Cr alloy,R = 5.7 cm/ h, caused a reduction in the fatigue life relative to the as-directionally solidified material. Fatigue cracking tended to be faceted in the 6 pct Cr alloy as opposed to the more ductile failure of the 0 pct Cr alloy. Microstructural perfection, grain size and shape, interlamellar spacing, longitudinal cracking, and longitudinal and transverse ductility all are believed to have influenced the fatigue resistance of the alloys.     

Refining by fractional solidification

A new process is described for purifying metals by fractional solidification. The process comprises isothermal compression of solute-rich interdendritic liquid from semi-solid starting material. Purification is measured by “refining ratio”, ˉC_c/C_o (where ˉC_c is wt pct solute of the refined “cake” and C_o is initial pct solute). Refining ratio depends on partition ratio,K, initial fraction solid, amount of liquid retained in the “cake”, and extent of diffusion in the solid. For intermediate fractions solid and low retained liquid, ˉC_c/C_o ≈ k. Refining experiments were conducted on Sn-Pb alloys. Semi-solid samples were isothermally compressed against a filter under controlled strain to pressures up to 21 MPa (2900 psi). The pct solute of the liquid squeezed from the sample was that given by the liquidus curve of the phase diagram. The amount of liquid removed depended on fraction liquid initially present and applied pressure, and not on plunger speed in the range of 10⁻³ to 10⁻¹f cmJ.s. At 21 MPa (2900 psi) and fractions solid up to 0.7, over 95 pct of the interdendritic liquid present was removed so that fraction liquid in the final “cake” (“cake wetness”) was reduced to about 0.04. Refining ratios within 80 pct of the theoretical minimum were obtained. Refining ratios obtained ranged from 0.1 at 0.08 to 0.4 at 0.8 fraction solid. This paper is based on doctoral thesis work of A. L. Lux.     

Microanalysis of precipitate free zones (PFZ) in Al-Zn-Mg and Cu-Ni-Nb alloys

In order to better understand the formation of Precipitate Free Zones (PFZ), microanalysis was conducted on heat treated Al-2.2 at. pct Zn-4.7 at. pct Mg and Cu-30 at. pct Ni-0.9 at. pct Nb alloys. In both the alloys, no appreciable solute depletion at the grain boundaries was observed in the as-quenched condition. After aging, marked solute depletion was observed in the PFZ of both the alloys. In the Al-Zn-Mg alloy, the PFZ were supersaturated with respect toη andT phases up to 4 h of aging at 473 K. In the Cu-Ni-Nb alloy, the PFZ were supersaturated only with respect to theβ phase but not the metastable γ″ phase. Based on the results, the factors affecting the formation and growth of PFZ are discussed.     

Lattice imperfections studied by x-ray diffraction in deformed aluminum-base alloys: Al-Ge alloy

In the present analysis, which is part of a series on a study that has been undertaken on aluminum-base alloys, a detailed X-ray diffraction study of deformation^[1–5] is made on aluminum-base germanium alloys in four different compositions: Al-3.10 at. pct Ge, Al-3.80 at. pct Ge, Al-4.16 at. pct Ge, and Al-4.60 at. pct Ge. The alloys were prepared from spectroscopically pure metals supplied by Johnson-Matthey and Co. Ltd., London, by melting them in graphite crucibles sealed under vacuum in quartz capsules. The alloys were homogenized for 15 days at 400 °C in the face-centered cubic phase (Figure 1), and cold working was performed by careful hand filing at room temperature. The diffractometer samples were prepared in the usual manner,^[3,4] and X-ray diffraction profiles were recorded in a Siemens Kristalloflex-4 X-ray diffractometer using Cu K_α radiation. A portion of the powder obtained by hand filing from each alloy was annealed at 400 °C to relieve strain and was taken as standard for line shift, line asymmetry, and line shape analyses in light of recent developments.^[3,4,6–8] The microstructural parameters, such as coherent domain size (D _e, microstrain 〈∈_L〉, stacking faults α′ and a" (both intrinsic and extrinsic), deformation twin fault β, dislocation density ρ, and stacking fault energy parameter γ/μ, were determined by adopting the same method of analysis and following the same equations that were used before.^[3–7]