A study of microsegregation in Al-Cu using a novel single-pan scanning calorimeter

A single-pan scanning calorimeter has been developed that eliminates the smearing of latent heat that occurs in a conventional two-pan heat-flux differential scanning calorimeter (DSC). In the new calorimeter, accurate enthalpy/temperature data was obtained in pure Al without smearing, and excellent sensitivity to new phases was obtained in a multicomponent Al alloy (LM25). The calorimeter has been used to investigate microsegregation in an Al-4.45 wt pct Cu alloy. The enthalpy/temperature data fell between that calculated, assuming no mixing in the solid (Scheil) and complete mixing in the solid (equilibrium solidification). The amount of segregation agreed well with that calculated using a diffusion-based model of microsegregation. The difficulty of getting the fraction solid from the enthalpy data is discussed, and it is concluded that it is not possible to do so without using a microsegregation model. In addition, it is concluded that it is wrong to assume that the enthalpy of an alloy can be given by a specific heat term and a constant latent heat term that depend on fraction liquid as is assumed in most casting models. This article is based on a presentation given in the symposium “Fundamentals of Solidification,” which occurred at the TMS Fall meeting in Indianapolis, Indiana, November 4–8, 2001, under the auspices of the TMS Solidification Committee.     

A combined solubility product/new PHACOMP approach for estimating temperatures of secondary solidification reactions in superalloy weld metals containing Nb and C

Solubility data are combined with the new PHACOMP calculation procedure and solute redistribution equations to propose a simple method for estimating temperatures of the secondary solidification reactions that occur in superalloy weld metals. The variation in the primary solid composition with temperature is calculated with the Scheil equation for substitutional elements, while the lever law is used for C. The calculated compositions are used to determine the temperature-dependent variation in the solubility parameter and the metal d-level parameter, which is used in the new PHACOMP routine. The solubility parameter and metal d-level profiles are compared to temperature-dependent critical values to predict temperatures of secondary solidification transformations involving carbides and topologically close-packed phases such as Laves. The general approach is applied to experimental Ni-base and Fe-base alloys containing systematic variations in Nb, Si, and C. The procedure correctly predicts formation of the NbC phase prior to the Laves phase and reveals the effect of nominal alloy content on reaction temperature. Calculated reaction temperatures are in reasonable agreement with experimentally measured values, particularly when considering the relatively large range of material constants that are needed to make the calculations.     

Influence of solid/liquid interfaces on the microstructure and stress-rupture life of the single-crystal nickel-base superalloy NASAIR 100

The [001] oriented single crystals of nickel-base superalloy NASAIR 100 with the planar, cellular, coarse-dendritic, and fine-dendritic solid/liquid (S/L) interfaces were prepared, respectively, and their microstructure and stress-rupture behavior at 1050 °C were investigated in both as-cast and solution heat-treated conditions. It was found that in as-cast single crystals of NASAIR 100, microsegregation and γ/γ′ eutectic produced in the solidification process increased, γ′ size decreased, and γ′ shape tended progressively to be cuboidal, with the successive transition of the S/L interface from planar to cellular, then to coarse-dendritic, and finally to fine-dendritic morphology. Furthermore, the solution temperature required to dissolve all as-cast γ′ and most of the γ/γ′ eutectic increased with the aforementioned successive transition of S/L interfaces. The reprecipitated γ′, after solution heat treatment (SHT), was usually fine and cuboidal. However, some W-rich phase was present in the heat-treated dendritic single crystals. Both the planar and the cellular single crystals of NASAIR 100 exhibited no superiority in stress-rupture life, irrespective of the heat-treatment conditions. Instead, the single crystals with dendritic morphology possessed excellent stress-rupture lives, after heat treatment at 1320 °C for 4 hours, followed by air cooling (AC). Perfect γ′ rafts with high-average aspect ratios formed during the stress-rupture tests of dendritic single crystals; in contrast, irregularly coarsening structures appeared in both the planar and cellular single crystals. The microstructure and solution behavior were illustrated in detail. Furthermore, the microstructural factors to affect the high-temperature stress-rupture life of the single crystals of NASAIR 100 were also analyzed.     

Effects of stress coarsening on coherent particle strengthening

We have determined the effects of stress coarsening of coherent, γ’ [Ni₃ (Al,X)] precipitates on the yield behavior of nickel-base super alloy single crystals. In so doing, we have also determined the influence of coherent precipitate shape on yield strength, and the strengthening role of dislocation substructures innickel-base superalloys. The γ’ morphologies considered are large plates, long rods, and the familiar cuboids. These morphologies were obtained by annealing the crystals under applied tensile stress, applied compressive stress, and zero stress conditions, respectively. The <100> tensile test axes were perpendicular to the γ’ plates and parallel to the γ’ rods. Between room temperature and 1400°F, the γ’ rod and γ’ plate morphologies are found to increase the yield strength of the crystals over that due to the γ’ cuboids by about 10 ksi (10 pct) and 30 ksi (35 pct), respectively. Above 1400°F, the yield strengths of the crystals are found to be essentially independent of precipitate morphology. The increase in yield strength in going from crystals with γ’ cuboids to those with γ’ rods is shown to be due to dislocation networks, which are produced as a natural consequence of annealing under stress. A simple analysis of the interaction between dislocations and the precipitates show that the increase in yield strength in going from γ’ rods to γ’ plates is due to dislocation line tension and consistent with the difference in the shape of these coherent and ordered precipitates. The results of this investigation are comforting in that they show creep applications should not degrade the strength of nickel-base superalloy single crystals. The results are also discussed with respect to polycrystalline nickel-base superalloys and composite structures of γ and γ’ phases. Formerly Senior Research Associate, Pratt and Whitney Aircraft, Middletown, Conn. Formerly Senior Research Assistant, Pratt and Whitney Aircraft     

Fracture of single crystals of the nickel-base superalloy PWA 1480E in helium at 22 °C

The fracture behavior of single crystals of the PWA 1480E nickel-base superalloy was studied using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Notched single crystals with seven different crystal growth orientations near [100], [110], [111], [013], [112], [123], and [223] were tensile tested at 22 °C in a helium atmosphere at 34 MPa. Gamma prime particles were orderly and closely aligned with the cube edges along the [100], [010], and [001] directions of theγ matrix. The cuboid morphology of theγ’ precipitate was not influenced by the crystal growth orientation. The specimen with the [110] orientation was the strongest, while the crystal with the [100] orientation was the weakest. A stereoscopic technique, combined with the use of planary’ morphologies, was applied to identify the cleavage plane orientation. All specimens failed predominately by {lll}-type cleavage which originated from combined slip on various {111} planes. In most cases, deformation was found to occur inhomogeneously in intense slip bands lying on {111} planes and aligned parallel to the different slip directions. Both SEM and TEM studies indicated that {lll}-type slip was the controlling factor during cleavage fracture of single crystals of the PWA 1480E nickel-base superalloy. Formerly Graduate Student, Auburn University     

The effect of orientation and sense of applied uniaxial stress on the morphology of coherent gamma prime precipitates in stress annealed nickel-base superalloy crystals

The coarsening of coherent γ′[Ni₃(Al, Ti)] precipitates in single crystals of a representative nickel-base superalloy, Udimet-700, is shown to be affected by a uniaxial stress applied during annealing. Depending on the sense of the applied stress and its crystallographic orientation, stress annealing results in oriented cuboidal, plate, or parallelepiped shaped γ′ precipitates. A general thermodynamic analysis of the effect of stress annealing on precipitate morphology is presented that takes into account free energy changes due to changes in bulk precipitation strain, effective modulus, coherency strain energy, and the total interphase boundary area. The analysis correctly predicts the observed γ′ precipitate morphologies as a function of stress axis orientation, stress sense, the lattice misfit of the precipitate phase, and the elastic constants of the matrix and precipitate phases. The analysis also shows that stress induced morphological changes can be completely precluded, as may be desired to optimize mechanical behavior, only if the elastic constants of the matrix and precipitate phases are equal. Changes in morphology due to changes in bulk precipitation strain, which in the case of Udimet-700 is shown to be the dominant effect, can be eliminated by alloying for zero lattice misfit or, in single crystals, by stressing parallel to < 111> . Applications to long-term creep behavior and to the fabrication of composite structures are discussed. Formerly with Advanced Materials Research and Development Laboratory, Pratt & Whitney Aircraft     

The room temperature fatigue behavior of nickel-base superalloy crystals at ultrasonic frequency

Previous investigations have invariably observed strain rate related deformation effects as the fatigue frequency is raised to the ultrasonic range. Through room temperature tests on strain rate insensitive nickel-base superalloy single crystals of Mar-M200, we have shown that another effect of increasing the fatigue frequency to the ultrasonic range is in the suppression of the deleterious influence of environment. It was found that above a stress amplitude of 30,400 psi the fatigue lives of crystals ultrasonically fatiguedin air increase with decreasing stress in a manner which is functionally similar to, that of crystals conventionally fatiguedin vacuum. Similarly, the fracture surfaces of ultrasonically fatigued crystals have a dimpled appearance over most of their areas which is characteristic of locally ductile failure and identical to, the appearance of crystals failed at conventionally frequency in vacuum. These results, along with a kinetic analysis of gaseous adsorption, indicate that the major effect of increasing the fatigue frequency to the ultrasonic, range is in the suppression of the influence of oxygen in enhancing the rate of crack propagation. In addition, the short test times involved in running large numbers of cycles have allowed for the determination of the fatigue limit in a nickel-base superalloy. This is the first indication of no-fail behavior in this type of alloy.     

Comparison of fatigue deformation and fracture in a dispersion-strengthened and a conventional nickel-base superalloy

INCONEL alloy MA 753 is a dispersion strengthened nickel-base superalloy made by mechanical alloying which combines γ’ precipitation hardening and yttria dispersion strengthening with good oxidation and sulfidation resistance. At temperatures up to 1227 K (1750°F), the fatigue strength of MA 753 is greater than that of a conventional wrought superalloy which has a composition close to that of the MA 753 matrix. Fatigue strength at elevated temperatures is strongly dependent on testing frequency. This behavior is correlated with the strain rate dependence of tensile strength. Fatigue crack initiation sites and propagation modes in MA 753 are discussed as a function of temperature and microstructure and compared to those in the conventional superalloy. The transition from transgranular to intergranular fracture mode in MA 753 occurs at a higher temperature than found in conventional nickel-base superalloys. While the γ’ precipitate controls the fatigue strength at low and intermediate temperatures, the oxide dispersoid and carbides also affect deformation in this temperature range. At elevated temperatures, fatigue deformation is controlled by the dispersoid and carbides.     

Comparison of fatigue deformation and fracture in a dispersion-strengthened and a conventional nickel-base superalloy

INCONEL alloy MA 753 is a dispersion strengthened nickel-base superalloy made by mechanical alloying which combines γ’ precipitation hardening and yttria dispersion strengthening with good oxidation and sulfidation resistance. At temperatures up to 1227 K (1750°F), the fatigue strength of MA 753 is greater than that of a conventional wrought superalloy which has a composition close to that of the MA 753 matrix. Fatigue strength at elevated temperatures is strongly dependent on testing frequency. This behavior is correlated with the strain rate dependence of tensile strength. Fatigue crack initiation sites and propagation modes in MA 753 are discussed as a function of temperature and microstructure and compared to those in the conventional superalloy. The transition from transgranular to intergranular fracture mode in MA 753 occurs at a higher temperature than found in conventional nickel-base superalloys. While the γ’ precipitate controls the fatigue strength at low and intermediate temperatures, the oxide dispersoid and carbides also affect deformation in this temperature range. At elevated temperatures, fatigue deformation is controlled by the dispersoid and carbides. Trademark of The International Nickel Company, Inc.     

Influence of precipitate morphology on intermediate temperature creep properties of a nickel-base superalloy single crystal

The influence of γ’ precipitate morphology on the-creep behavior of the single crystal nickel-base superalloy NASAIR 100 at 760-°C was investigtated. As-heat treated crystals with cuboidal γ’ particles and crystals given an additional pre-rafting treatment to form a continuous lamellar structure were creep tested at stress levels which produced rupture lives ranging from 40 to 2500 hours. At high applied stresses, the crystals with cuboidal γ’ had both lower minimum creep rates and longer rupture lives than the crystals with lamellar γ. At lower stress levels, the initially cubic γ’ material maintained a lower crep rate, but exhibited a similar rupture life compared to the pre-rafted crystals. Examination of the microstructures which developed during creep indicated that dislocations could shear the semi-coherent γ’ rafts relatively easily compared to the coherent cuboidal γ’. In tests at lower applied stresses, slow directional coarsening of the initially cuboidal γ’ resulted in the development of a lamellar structure similar to that in the pre-rafted material, such that the rupture lives of the two materials were similar.     

Measurement of the lattice misfit of the nickel-base superalloy SC16 by high-energy synchrotron radiation

High-energy, high-resolution synchrotron radiation diffraction was successfully used to measure the lattice misfit in the single-crystal nickel-base superalloy SC16. A three-peak model, one belonging to the precipitate phase γ′ and two to the matrix γ, was used to fit the diffraction peaks. Room-temperature (RT) contour plots and a temperature scan up to 1170 K revealed that the misfit evolves together with the measured thermal expansion difference between the γ and γ′ phases. This suggests that the origin of the misfit lies in the different thermal expansion coefficient of the two phases. The misfit was found to be positive at RT and to evolve toward negative values as the temperature increases.     

Behavior of nickel-base superalloy single crystals under thermal-mechanical fatigue

The thermal-mechanical fatigue behavior of AM1 nickel-base superalloy single crystals is studied using a cycle from 600 °C to 1100 °C. It is found to be strongly dependent on crystallo-graphic orientation, which leads to different shapes of the stress-strain hysteresis loops. The cyclic stress-strain response is influenced by variation in Young’s modulus, flow stress, and cyclic hardening with temperature for every crystallographic orientation. The thermalmechanical fatigue life is mainly spent in crack growth. Two main crack-initiation mechanisms occur, depending on the mechanical strain range. Oxidation-induced cracking is the dominant damage mechanism in the lifetime of interest for turbine blades.     

Growth of the solid-liquid region during one-dimensional solidification of binary alloys. Part II

The solidification behavior of finite slabs of Fe-Si, Fe-P, and Fe-S alloys is calculated assuming: negligible diffusion in solid; complete diffusion in the liquid; equal solid and liquid densities; and constant temperature at the cooling surface. The effects of alloy parameters,e.g., solute distribution ratio, eutectic temperature and composition, and the initial concentration of solute in the molten alloys on the growth of the solid-liquid region (mushy zone) are explored. It is found that the thickness of the mushy zone relative to the solidified shell becomes greater with decreasing solute distribution ratio.     

Thermochemistry of alloys of transition metals: Part III. Copper-Silver, -Titanium,Zirconium, and -Hafnium at 1373 K

The enthalpies of mixing of liquid copper with liquid silver and with solid titanium, zirconium, and hafnium have been measured by high temperature reaction calorimetry at 1371 to 1373 K. A least squares treatment of the data for copper-silver alloys yields the following expression for the molar enthalpy of mixing: ΔH_mix = ϰ_Agϰ_Cu(17.66 − 5.46 ϰ_Ag) kJ mol⁻¹. The enthalpies of solution of solid titanium, zirconium, and hafnium in dilute solutions in liquid copper are all exothermic; the following values were found: -2.0 kJ mol⁻¹ for Ti, -52.5 kJ mol⁻¹ for Zr, and -46.3 kJ mol⁻¹ for Hf. These values are all significantly less exothermic than predicted by the semiempirical theory of Miedema. The enthalpies of formation of congruent melting intermetallic phases in the systems Cu-Ti, Cu-Zr, and Cu-Hf were measured by drop calorimetry or by solution calorimetry in liquid copper. The enthalpies of formation of the solid alloys have been compared with corresponding data for the liquid alloys.     

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).     

The temperature independent plateau stress of solid solution crystals

A calculation of the plateau stress in solid solution crystals is presented assuming an arbitrarily oriented dislocation loop of lengthL, that moves under an applied stress. At high concentrations of solute atoms the dislocation segment does not interact with an individual solute atom but instead with all the solute atoms along the dislocation segment within a certain radius. The macroscopic flow stress is assumed to be determined by the maximum force that is encountered when a dislocation is moved over a distance equal to the distance between the position at zero stress and the critical position of an activated Frank-Read source. If the dislocation segment is assumed to be large compared to atomic distances, the interaction with groups of atoms will lead to an athermal process and therefore can explain the origin of the temperature independent flow stress in solid solution crystals. From this model the flow stress can be calculated with the help of statistical methods similar to those used in calculations of the movement of Bloch walls in magnetic materials. Besides the proper temperature dependence of the plateau stress the above model yields a dependence of the plateau stress upon the square root of the solute concentration, a result that is in good agreement with the measurements on silver, gold, and copperbased alloys. A linear relation between the solid solution hardening parameter dT/d√c and the strength of the solute atoms is obtained which is confirmed by the experimental results on copper-based alloys.     

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).     

The physical state of mannitol after freeze-drying: effects of mannitol concentration, freezing rate, and a noncrystallizing cosolute

The objectives of this study were to (1) measure the effects of freezing rate and mannitol concentration on the physical state of freeze-dried mannitol when mannitol is present as a single component, (2) determine the relative concentration threshold above which crystalline mannitol can be observed by X-ray powder diffraction in the freeze-dried solid when a variety of noncrystallizing solutes are included in the formulation, and (3) measure the glass transition temperature of amorphous mannitol and to determine the degree to which the glass transition temperature of freeze-dried solids consisting of mannitol and a disaccharide is predicted by the Gordon-Taylor equation. Both freezing rate and mannitol concentration influence the crystal form of mannitol in the freeze-dried solid when mannitol is present as a single component. Slow freezing of 10% (w/v) mannitol produces a mixture of the alpha and beta polymorphs, whereas fast freezing of the same solution produces the delta form. Fast freezing of 5% (w/v) mannitol results primarily in the beta form. The threshold concentration above which crystalline mannitol is detected in the freeze-dried solid by X-ray diffraction is consistently about 30% (w/w) when a second, noncrystallizing solute is present, regardless of the nature of the second component. The glass transition temperature of amorphous mannitol measured from the quench-cooled melt is approximately 13 degreesC. Accordingly, mannitol is an effective plasticizer of freeze-dried solids when the mannitol remains amorphous. Glass transition temperatures of mixtures of mannitol and the disaccharides sucrose, maltose, trehalose, and lactose are well predicted by the Gordon-Taylor equation with values of k in the range of 3 to 4.     

Surface recrystallization of a single crystal nickel-base superalloy

The recrystallization behavior of a single crystal nickel-base superalloy was investigated by shot peening and subsequent annealing. Two kinds of recrystallization microstructures, which are intensively dependent on the annealing temperature, are shown in the nickel-base superalloy after shot peening and subsequent annealing. Surface recrystallized grains are obtained when the superalloy is anparticles occurs. Cellular recrystallization is observed after annealing at lower temperatures. Cellular structures induced by high diffusivity of the shot-peened alloy annealed at 1050℃ accords with the Johnson-Mehl-Avrami-Kolmogorov equation. The low Avrami exponent is caused by the inhomogeneous distribution of stored energy, the decreasing of stored energy during recovery, and the strong resistance of boundary migration yb γ' particles.     

The discontinuous precipitation of a liquid phase in MoNi induced by diffusional coherency strain

When a liquid phase sintered MoNi alloy is heat-treated at a temperature lower than that used for sintering where the solid and liquid phases coexist, the liquid films and grain boundaries between the grains migrate, leaving behind a new solid solution somewhat depleted of Ni. Since some liquid is produced during this migration, it resembles a discontinuous precipitation of the liquid phase. It is demonstrated experimentally that the driving force for this discontinuous precipitation arises from the coherency strain produced by Ni atom diffusion out of the grains. When two solute atom species simultaneously diffuse into or out of crystals in a ternary system, the resulting coherency strain depends on the size and concentration of the diffusing atoms and can be varied independently of the free energy of mixing. In particular, the coherency strain can be reduced to zero when the strain effects of the two atomic species exactly cancel each other. In this study, series of 90Mo10Ni alloy (by wt%) have been prepared by liquid phase sintering at temperatures between 1400 and 1520°C in order to produce solid MoNi grains of varying Ni concentration that are in equilibrium with the surrounding liquid matrix. The migration of liquid films and grain boundaries in the sintered specimens is induced by heat-treating them at 1400°C after adding various amounts of Fe to the liquid matrix. The migration does not occur when the estimated coherency strain is close to 0, although the free energy of mixing is finite. This result is a definitive demonstration that the driving force for the steady state migration is the coherency strain energy. The evidence that the grain boundaries exist in the sintered specimens and remain as grain boundaries during the migration is discussed.