Interdiffusion in multiphase,Al-Co-Cr-Ni-Ti diffusion couples

The interdiffusion in Co matrix/Al particle alloys and Co/Ni-based substrates was studied using electron probe microanalysis and was simulated with the software DICTRA. Alloys were prepared by mixing elemental powders and furnace melting under an inert atmosphere. The phases involved in the study were γ (Ni-based or Co-based), β-CoAl, and γ′. The alloys were single-phase (γ) as well as two-phase (γ + β and γ + γ′). Several equilibrium points in the Al-Co-Cr system were measured and compared with the calculated diagram at 1100 °C. The diffusion couples were prepared to produce combinations of selected alloys and were subjected to annealing at 1100 °C for times up to 72 h. The diffusion calculations made with DICTRA were performed using the TCNI1 thermodynamic database together with mobility data collected from different literature sources. A literature survey on diffusion data of this system was performed, and comparisons with available data were made. The validity of the selected mobility data was checked with the composition profiles measured on some single-phase γ/γ diffusion couples. In the γ + β/γ and γ/γ + γ′ diffusion couples, a regression of the dispersed phase (β or γ′) was observed due to the interdiffusion of Al. From combined experimental and theoretical results, the effects of temperature and coating thickness were determined as an input for a coating lifetime prediction model. Paper presented at Calphad XXXII, La Malbaie, Québec, Canada, May 25–30, 2003.     

Thermomechanical treatment of complexly alloyed nickel alloys

The results of a study of dispersion-hardening high-temperature nickel alloys éI698, éP742, and éP741 after thermomechanical treatment (TMT) are considered. The alloys differ from the éI437BU type by a higher content of the strengthening γ′-phase and considerably higher temperatures of recrystallization and complete dissolution of the γ′-phase. The alloys studied are used for disks of aircraft engines. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 5–8, August, 1997.     

INFLUENCE OF Ge ON THE ANTIFERROMAGNETIC TRANSITION AND γ → ε MARTENSITIC TRANSFORMATION OF Fe-24Mn ALLOYS

The aims of the work were to study the effect of Ge (0-6wt. %) on the paramagnetic-antiferromagnetic transition and martensitic transformation of Fe-Mn alloy using the susceptibility, micro structure examination, X-ray diffraction (XRD) and latticeparameter measurement. Ge lowers the Néel temperature, TN, and enhances the magnetic susceptibility x, changing the Pauli paramagnetism above TN to paramagnetismstate obeying the Curie Weiss law, which is essentially similar to that of γ-Fe-Mnalloys containing Al or Si; Ge depresses γ-ε martensitic transformation, whichattribute to Ge increasing the stacking fault energy; Moreover, Ge increases the lattice parameter of γ phase, and low content Ge increases the lattice parameter of γphase more than that of high Ge content. Comparing Ge(4s24p2) with Si(3s23p2) and Al(3s2 3p1), which have the same outer-shell of electron structures, we found that their effects on the martensitic transformation of Fe-Mn alloy are completely different. The result suggests the     

Metastable eutectic formation in Ni- Al alloys

Directional solidification experiments have been carried out at low rates in alloys near the N13AI composition in the Ni-AI binary alloy system. Results confirm an earlier study at high rates showing that the eutectic in this system occurs between the β and γ′ phases. It is shown that under certain composition and growth conditions a metastable eutectic between the β and γ phases forms in these alloys. Analysis of the decomposition of this metastable eutectic into the γ′ phase and additional composition measurements are used to provide an improved version of the phase diagram near the N13AI composition.     

Metastable eutectic formation in Ni- Al alloys

Directional solidification experiments have been carried out at low rates in alloys near the N13AI composition in the Ni-AI binary alloy system. Results confirm an earlier study at high rates showing that the eutectic in this system occurs between the β and γ′ phases. It is shown that under certain composition and growth conditions a metastable eutectic between the β and γ phases forms in these alloys. Analysis of the decomposition of this metastable eutectic into the γ′ phase and additional composition measurements are used to provide an improved version of the phase diagram near the N13AI composition.     

Strengthening behavior of beta phase in lamellar microstructure of TiAl alloys

β phase can be introduced to TiAl alloys by the additions of β stabilizing elements such as Cr, Nb, W, and Mo. The β phase has a body-centered cubic lattice structure and is softer than the α₂ and γ phases in TiAl alloys at elevated temperatures, and hence is thought to have a detrimental effect on creep strength. However, fine β precipitates can be formed at lamellar interfaces by proper heat treatment conditions and the β interfacial precipitate improves the creep resistance of fully lamellar TiAl alloys, since the phase interface of γ/β retards the motion of dislocations during creep. This paper reviews recent research on high-temperature strengthening behavior of the β phase in fully lamellar TiAl alloys.     

EFFECT OF Cr AND Al CONTENT ON THE STACKING FAULT ENERGY IN γ—Fe—Mn ALLOYS

The effects of Cr and Al content were investigated on the stacking fault energy in austenitic Fe-31Mn-(0-7.26)Cr-0.96C and Fe-31Mn-(0-8.68)Al-0.85C alloys by the thermodynamic analysis.The results show that the additions of chromium or aluminum increase the non-magnetic component of the stacking fault energy in the γ-Fe-Mn alloys,and the effect of aluminum is larger than that of chromium.The change in the magnetic entropy caused in the antiferromagnetic transition increases the free energy difference between the γ and ε phases in the γ-Fe-Mn alloys.The effects of chromium and aluminum on the magnetic component were discussed on the basis of the influence of both upon the antiferromagnetic transition in the γ-Fe-Mn alloys.     

The use of precious-metal-modified nickel-based superalloys for thin gage applications

Precious-metal-modified nickel-based superalloys are being investigated for use in thin gage applications, such as thermal protection systems or heat exchangers, due to their strength and inherent oxidation resistance at temperatures in excess of 1,050°C. This overview paper summarizes the Air Force Research Laboratory (AFRL) interest in experimental two-phase γ-Ni + γ′-Ni₃Al superalloys. The AFRL is interested in alloys with a based composition of Ni-15Al-5Cr (at. %) with carbon, boron, and zirconium additions for grain-boundary refinement and strengthening. The alloys currently being evaluated also contain 4–5 at.% of platinum-group metals, in this case platinum and iridium. The feasibility of hot rolling these alloys to a final thickness of 0.12–0.25 mm and obtaining a nearly fully recrystallized microstructure was demonstrated.     

Determination of the Fe-rich portion of the Fe-Ni-S phase diagram

The low-temperature, Fe-rich portion of the Fe-Ni-S phase diagram was determined from Fe-Ni-S alloys (2.5,5,10,20, and 30 wt.% Ni, 10 wt % S, balance Fe) heat treated at 100 °C intervals from 900 to 300 °C. The microstructure and microchemistry of the phases in the heat treated Fe-Ni-S alloys were studied using a high-resolution field-emission gun (FEG) scanning electron microscope (SEM), electron probe microanalyzer (EPMA), and analytical electron microscope (AEM). Tieline compositions were obtained by determining the average phase composition and by measuring compositional profiles across interphase interfaces with the EPMA and AEM. At 600 °C and below, at least one phase was <1 Μm in size requiring the use of the AEM for analysis. The measured α + FeS, γ+ FeS, and α + γ + FeS boundaries in the Fe-rich corner of the Fe-Ni-S isotherms are consistent with previous studies. However, two new phases were observed for the first time coexisting with γ and FeS phases: FeNiγ′′ (∼52 wt.% Ni) at 600 and 500 °C and Ni ₃ Fe, ordered Ll ₂,γ′ (∼64 wt.% Ni) at 400 °C. New ternary isotherms are given at 600,500, and 400 °C that include the newly determined γ+γ′′ + FeS and the γ + γ′ + FeS three-phase fields. The effects of S on the phase boundaries of the α + γ phase field and the application of the Fe-Ni-S phase diagram to explain the microstructure and microchemistry of the metallic phases of stony meteorites are also discussed.     

Effect of the aging regime on the hardness and amount of γ-phase in high-temperature nickel alloys KhN56VMTYu and KhN77TYuR

It is known that the properties of high-temperature nickel alloys depend considerably on the amount of γ'-phase, which is determined, first of all, by the aging, regime. In order to evaluate the properties of these alloys it is expedient to establish the relation between the parameters of the aging regime and the amount of γ'-phase or, which is even more important, between the aging parameters and the mechanical properties. The present paper concerns high-temperature nickel alloys KhN56VMTYu and KhN77TYuR, which are used to establish the relations between the amount of γ'-phase, the hardness, and its increment due to aging and the aging temperature. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 27–30, November, 1997.     

Effect of alloying elements on the electrochemical polarization behavior and passive film of Fe-Mn base alloys in various aqueous solutions

The corrosion properties of austenitic Fe-Mn, Fe-Mn-Al, Fe-Mn-Cr and Fe-Mn-Al-Cr alloys with compositions of 23-30 wt% Mn, 2.8-8.2 wt% Al and 4.9-6.9 wt% Cr in various aqueous solutions of pH −0.8 to 15.3 and the passivating mechanism induced by the presence of Al, Cr, or Al and Cr have been studied using electrochemical measurements and Auger electron spectroscopic/X-ray photoelectron spectroscopic analysis. Binary Fe-Mn alloys can be passivated only in 10-50% NaOH solutions, and alloying of binary Fe-Mn alloy with Al or Cr or combination of Al and Cr seems not so obviously beneficial to corrosion resistance in HNO₃ or Na₂SO₄ solutions. All of the experimental Fe-Mn based alloys and steels for comparison cannot passivate in either 10% HCl or 3.5% NaCl solution. The Fe-Mn based alloys containing Al or Cr or Al and Cr can passivate in 10-50% HNO₃ or 1 mol l⁻¹ Na₂SO₄ solutions and rainwater. In general, Fe-Mn based alloys can passivate in oxidizing acid, neutral and basic solution, but cannot passivate in reducing acid or solution containing active Cl⁻ ions. In the passive film formed on the surface of Fe-Mn base alloys in various aqueous solutions, bound water and hydroxides are present at the surface of the film, while mixed oxides of Al, Cr, Mn and Fe are located in the inner part. The resistance to corrosion is imparted by a barrier film of bound water, hydroxides and oxides of Al, Cr or Fe, while the Mn oxides in passive film reduce the corrosion resistance.     

New Co-based γ-γ′ high-temperature alloys

New cobalt-based alloys containing ordered L1₂ precipitates have been investigated. With additions of Cr, Mo, Ni, Re, Ta, and V to the ternary Co-Al-W system, two phase γ-γ′ microstructures have been established. Solidus and liquidus temperatures are 100°C–150°C higher than advanced nickel-based single-crystal alloys strengthened with the L1₂ phase. An anomalous rise in flow stress with temperature is observed. Single crystals have been solidified and partitioning during solidification is limited in the ternary system, suggesting a high resistance to convective instabilities. Oxidation at 900°C results in the formation of cobalt oxide. Following oxidation, an inner layer of Al₂O₃ is observed in uncoated Cr-containing alloys and Cr₂O₃ is observed in alloys subjected to chromization.     

Primary creep: Secondary gamma prime and the rhenium effect

Creep at low temperatures and high stresses in some second-generation nickel-based superalloys can result in large primary creep strains exceeding 5% in fewer than 5 hours. Several possibilities have been given for the appearance of large primary creep strains including the presence of secondary γ′ and the addition of rhenium to these alloys. In light of the recent surge in cost of rhenium, this article explores secondary γ′ and the rhenium effect in the first-and second-generation alloys PWA 1480 and PWA 1484, respectively. In addition, it will be shown that selecting higher aging temperatures can aid in the reduction of primary creep strains in PWA 1484.     

Pt-Al-Cr-Ni superalloys: Heat treatment and microstructure

In this study; the microstructure of precipitation-strengthened platinum-based superalloys after homogenization heat treatment and various cooling regimes was investigated. The study focused on the lattice misfit between the γ matrix and γ′ precipitate phase as well as the precipitate morphologies. Water quenching led to a γ single-phase structure, whereas air cooling resulted in the precipitation of 200 nm sized γ′ cubes with 30% volume fraction. Further investigations found that chromium in Pt-Al-Cr-Ni alloys had a strong γ′ promoting effect. For more information, contact M. Wenderoth, University Bayreuth, Metallic Materials, Ludwig-Thoma-Str. 36b, 95447 Bayreuth, Germany; +49-921-555-567; fax +49-921-555-561; e-mail markus.wenderoth@uni-bayreuth.de.     

High temperature mechanical properities and creep crack initiation of DS CM186LC for nozzle guide vane

The second generation DS alloy, CM186LC is used in the as-cast and double aged condition which has creep-rupture properities equivalent to the first generation single crystal alloys CMSX-2 and CMSX-3. In production, cast vane components have to be subjected to a brazing treatment for joining into pairs. The effect of the brazing treatment and modified brazing treatment (heat treatment) on mechanical properities at high temperature was studied in accordance with microstructure. Brazing treatments gave no effect on tensile properities and creep failure mode of DS CM186LC, although a small decrease in stress-rupture life was observed. Creep failure was related to the solidified microstructure. Creep cracks began at the grain boundary normal to the applied stress, especially at the γ/γ’ eutectic phase on grain boundaries. Most of γ/γ’ eutectics which had solidified at the last stage of casting, had microporosity which became a crack initiation site during creep. MC carbide reaction with the matrix γ was observed in the creep failed specimens.     

The thermodynamic modeling of precious-metal-modified nickel-based superalloys

Thermodynamic modeling of precious-metal-modified Ni-based super-alloys (PMMS) was performed in this study using the CALPHAD approach. With this approach, the effects of platinum-group metals (PGMs) such as platinum, iridium, and ruthenium on the properties of nickel-based superalloys and their interplay with other alloying elements were understood from a thermodynamic and phase equilibrium point of view. Thermodynamic database containing PGMs was developed on the basis of the PanNi¹ database for multi-component nickel alloys. The database was first validated with available experimental data. It was then used to understand phase stability and phase transformation temperatures, such as liquidus, solidus, and γ′ precipitation temperature, of PGM modified nickel-based superalloys. The effects of alloying elements on the formation of strengthening γ′ precipitate and their partitioning in γ and γ′ were also discussed.     

The 1400°C isothermal section of the Ti-Al-Nb ternary system

The 1400‡C isothermal section of Ti-Al-Nb system was determined using the diffusion couple technique, along with optical microscopy and electron probe microanalysis (EPMA). Equilibrated alloys were employed to identify the phase regions using XRD. The isothermal section consists of seven single-phase regions including Β(ΒTi,Nb), α(αTi), γ(TiAl), η((Ti,Nb)Al₃), σ(Nb₂Al), δ(Nb₃Al), and γ₁. The detailed studies on the γ₃₁ phase were made by XRD, DTA, and DSC.     

Development of low thermal expansion superalloy

Alloy 903 and Alloy 909 are well-known Fe-Co-Ni-Al-Ti-Nb alloys with controlled low thermal expan-sion, but they have some properties that can be improved. To improve stress-accelerated grain boundary oxidation embrittlement of Alloy 903 and instability of theγ phase of alloy 909, two new alloys with good stress-rupture ductility, high creep-rupture strength, high tensile strength at high temperature, and good controlled thermal expansion were developed. These property improvements were accomplished by the combination of optimizing the Fe-Co-Ni ratio of the matrix and stabilizing theγ phase with the addition of aluminum.     

RETRACTED ARTICLE: Dislocation-particle interaction in precipitation strengthened Ll<Subscript>2</Subscript>-ordered Ni<Subscript>3</Subscript>Al

Transmission electron microscope investigation has been performed on the particle-dislocation interactions in Ni₃Al-based intermetallics containing various types of fine precipitates. In an Ll₂-ordered Ni₃Al alloy with 4 mol.% of chromium and 0.2–0.5 mol.% of carbon, fine octahedral precipitates of M₂₃C₆ type carbide, which has a cube-cube orientation relationship with the matrix, appear during aging. Typical Orowan loops are formed in Ni₃Al containing fine dispersions of M₂₃C₆ particles. In the alloys with appropriate titanium content, fine precipitates of coherent disordered γ are formed during aging. The γ precipitates are initially spherical or rounded cubic in shape and grow into platelets as aging proceeds. Loss of coherency is initiated by the introduction of dislocations at the γ/γ′ interface and results in step formation at the dislocations. The γ precipitates become globular after the loss of coherency. In the γ′ phase hardened by the precipitation of the disordered γ phase, dislocations are attracted into the disordered γ phase and cut through the particles during deformation at any stage of aging. In Ni₃Al containing a fine dispersion of disordered γ, superdislocations are strongly attracted to the disordered particles and dissociate on the (111) plane in the γ particles, while they dissociate on the (010) plane in the matrix. It is shown by comparison that the strengthening due to attractive interaction is more effective than that due to repulsive interaction. The roles of the variation of the interaction modes and of the dissociation of superdislocations in the matrix and particles are discussed in connection with the optimum microstructures of Ll₂-ordered intermetallics as high temperature structural materials.     

Mechanism and kinetics of coagulation of γ′-phase in nickel alloys at temperatures of hot plastic deformationin nickel alloys at temperatures of hot plastic deformation

It is known that hot plastic deformation of hard-to-deform complexly alloyed heat-resistant nickel alloys is easier to conduct at temperatures of the double-phase region than at those of the single-phase region. In heating such alloys before deformation and in the process of deformation cooling of them the amount of strengthening γ^′-phase and its sizes and distribution change, which exerts a substantial effect on their susceptibility to hot plastic deformation in the double-phase range. For this reason, knowledge of the laws of variation of the amount, dispersity, and distribution of the strengthening γ^′-phase in heating to the deformation temperature is very important, because it makes it possible to control these processes. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 11–13, March, 1997. Deceased.