Partial phase diagram of the Ti-Al binary system

The equilibrium temperature-composition coordinates of the β/(β + α), (β + α)/α,α/(α + γ), and (α γ @#@)/γ phase boundaries were determined for the binary Ti-Al phase diagram through the temperature range 1150 to 1400 °C by means of diffusion couples with subsequent EPMA examination. This was supplemented with microstructural examination of a two-phase alloy. No peritectoidal decomposition reaction of the type α→ β + γ reported by Murray [88Mur] at 1280 °C was found. Results indicate that the a phase persists to temperatures well above 1400 °C. The phase boundaries from the present work are in agreement with the Mc Cullough [88Mcc] binary phase diagram.     

Experimental Phase Diagram Investigations in the Ni-Rich Part of Al-Fe-Ni and Comparison with Calculated Phase Equilibria

The phase equilibria in the section Ni₃Fe-Ni₃Al and phase boundaries of γ′(Ni₃Al) at 1000 °C were studied by a combination of powder X-ray diffraction (XRD), differential thermal analysis (DTA), and electron probe microanalysis (EPMA). The existence of a continuous solid solution at 450 °C was confirmed by a linear decrease of the lattice parameter from Ni₃Al to Ni₃Fe. The phase boundaries of γ′ with γ and the B2-type phase were determined at 1000°C by EPMA. A vertical section of the phase diagram from Ni₃Al to Ni₃Fe above 450 °C, including the liquidus temperatures, is proposed based on the DTA investigations. The invariant four-phase equilibrium U: L + γ′ = γ + B2 is found to occur at 1366 ± 1 °C. The experimental data are compared with a calculated phase diagram obtained by extrapolation from the corresponding binary data sets.     

Oxidation Behavior of Nickel-Base Single-Crystal Superalloy with Rhenium-Base Diffusion Barrier Coating System at 1,423 K in Air

The oxidation behavior of the nickel-base single-crystal superalloy TMS-82+ coated with a duplex Re(W)–Cr–Ni/Ni(Cr)–Al layer was investigated in air at 1,150 °C for up to 100 h. The coating layer was formed by electroplating Re(Ni) and Ni(W) films on the alloy, followed by Cr-pack cementation at 1,300 °C, and as a result, forming a continuous Re(W)–Cr–Ni diffusion-barrier layer. A Ni film containing fine Zr particles was then electroplated on the duplex layer, followed by Al pack cementation at 1,000 °C for 1 and 5 h to form an Al reservoir layer with a duplex Ni₂Al₃/γ-Ni layer, which changed quickly to γ-Ni phase containing (10∼13)at.% Al for the 1 h Al-pack coat and a mixture of γ′-Ni₃Al and β-NiAl phases for the 5 h Al-pack coat during high-temperature oxidation. A protective α-Al₂O₃ scale formed during oxidation at 1,150 °C in air, and parabolic rate constants of 7.4 × 10⁻¹¹ and 6.6 × 10⁻¹⁰ kg² m⁻⁴ s⁻¹ were obtained for the 1 h- and 5 h-Al pack-coatings, respectively. There was little change in the structures of the superalloy substrate after oxidation at 1,150 °C in air for up to 100 h. It was found that the Re(W)–Cr–Ni layer remained stable, acting as a diffusion barrier between the alloy substrate and Al reservoir layers.     

Formation of an <Emphasis Type="Italic">L</Emphasis>1<Subscript>0</Subscript> superstructure in austenite upon the α → γ transformation in the invar alloy Fe-32% Ni

Structure of a metastable austenitic invar alloy Fe-32% Ni preliminarily quenched for martensite and subjected to α → γ transformation using slow heating to various temperatures (430–500°C) with the formation of variously oriented nanocrystalline lamellar austenite, which was subjected to an additional annealing at 280°C (below the calculated temperature of ordering of the γ phase), has been studied electron-microscopically. An electron diffraction analysis revealed the presence of an L1₀ superstructure in the disperse nickel-enriched nanocrystalline γ phase both after annealing at 280°C and in the unannealed alloy immediately after α → γ transformation upon slow heating to 430°C.     

Phenomena and mechanism on superplasticity of duplex stainless steels

The superplasticity of Fe-24Cr-7Ni-3Mo-0.14N duplex stainless steel after being solution treated at 1350°C followed by 90% cold rolling was investigated at 850°C with a strain rate ranging from 10^-3-10^-1s^-1. The microstructure of duplex stainless steel consists of a matrix γ phase having low angle grain boundaries and a σ phase as second phase particles before the deformation at 850°C. It is well known that the constituent phases in duplex stainless steel is changed following α→α+γ→α+γ+σ→γ+σ through phase transformation during deformation at 850°C. The final microstructure of duplex stainless steel consisted of 70 vol.% of γ and 30 vol.% of the σ phase. A maximum elongation of 750% was obtained at 850°C with a strain rate of 3.16xl0^-3s^-1. The dislocation density within matrix γ grains was low and a significant strain-induced grain growth was observed during the deformation. The misorientation angles between the neighboring γ grains increased as the strain increased, thus the low angle grain boundaries were transformed into high angle grain boundaries suitable for sliding by dynamic recrystallization during the deformation at 850°C. The grain boundary sliding assisted by dynamic recrystallization is considered to be a controlling mechanism for superplastic deformation at 850°C.     

Deformation and phase transformations during the cyclic oxidation of Ni−Al and Ni−Pt−Al

The reversible high-temperature γ′ to β phase transformation may be critical to explaining the unusual high-temperature oxidation behavior of (Ni,Pt)Al alloys and coatings. During high-temperature, high-frequency (1 h) cyclic oxidation in dry, flowing O₂, unprecedented macroscopic deformation was observed in two-phase (γ′+β) cast specimens of Hf-doped Ni−Al at 1,150°C and Hf-doped Ni−Pt−Al at 1,100° and 1,150°C, Outside of this two-phase field or when the cycle frequency was decreased to 100h, no deformation was observed. Using high-temperaturex-ray diffraction in an inert environment, the β-to-γ′ phase ratio was observed to increase above 1,000°C, causing a 2.5% volume change. The addition of platinum appeared to lower the transformation temperature consistent with the deformation observed in castalloys and rumpling of simple and platinum-modified aluminide coatings.     

Phase stability among the α(A1), β(A2), and γ(D83) phases in the Cu-Al-X system

The phase equilibria among the α(A1), β(A2), and γ(D8₃) phases in the Cu-Al-X systems (X: Ti, V, Mn, Fe, Co, Ni, Zn, Sn, and Sb) at 700 and 800 °C were investigated, and the effects of these alloying elements on the phase boundaries, homogeneous solid solution ranges, and tie-lines among the α, β, and γ phases were accurately determined by the diffusion couple method. The phase stabilities among α, β, and γ phases are discussed in terms of the partition coefficient. In addition, a modified Guillet’s method, which is used to estimate the effect of alloying elements on microstructure, is proposed by taking into account the change of equivalent coefficient with the phase fraction, and applied to predict the effect of alloying elements on the microstructure of the Cu-Al base alloy.     

Diffusion and phase transformation on interface between substrate and NiCrAlY in Y-PSZ thermal barrier coatings

NiCrAlY/Y₂O₃-Y-PSZ (yttria-partially stabilized zirconia) thermal barrier coatings were developed on a superalloy (Ni-10Co-9Cr-7W-5Al, wt.%) surface. The superalloys were first coated with a bond coat of Ni-19Cr-8Al-0.5Y (wt.%) alloy that was deposited by low-pressure plasma spraying and then covered with a top coat of ZrO₂-8wt.%Y₂O₃ by air plasma spraying. The microstructure near the interface was analyzed using an optical microscope, a scanning electron microscope, microhardness measurements, and x-ray diffraction, and the phases of composition were measured using an electron probe microanalyzer after exposure at 1100°C for different times in air or a vacuum. The reaction processes also were simulated using diffusion-controlled transformation (DICTRA) software in which diffusion was considered as being only the γ phase, and the γ′ phase was treated as spheroidal particles in γ. From the authors’ results, it can be concluded that a γ′-phase layer is observed at the interface between substrate and bond coat, and its thickness increases with increasing exposure times in air at 1100 °C. This layer showed good cohesion with the substrate and bond coat. It can also be concluded that the formation of the γ′-phase layer can be predicted from DICTRA simulation. The simulation also shows the same trend of the composition profiles as experimental data.     

The alpha prime to delta reversion transformation in Pu-Ga alloys

The α′→δ reversion transformation in stabilized δ phase Pu-Ga alloys containing 0.5 wt.% and 0.6 wt.% Ga was studied using dilatometry and differential scanning calorimetry. The α′→δ reversion was observed to occur as a series of transformation bursts spread over a 20°C to 40°C temperature range. The enthalpy of the endothermic α′→δ reversion transformation was estimated to be approximately 0.56 J/g for the 0.6 wt.% Ga alloy. Lower gallium was found to raise the α′→δ reversion temperature in specimens cooled to −155°C from 44–48°C for 0.6 wt.% Ga to 92°C for 0.5 wt.% Ga. Repeated cycling to −155°C caused the reversion onset temperature to decrease by ∼10°C. Raising the minimum hold temperature from −150°C to −125°C caused the reversion onset temperature to decrease by 5–10°C. Annealing at 300–400°C was required to erase all traces of the effects of cold exposure; material heated to only 200°C after thermal cycling displayed decreased reversion onset temperatures during subsequent thermal cycling. For more information, contact D.S. Schwartz, Los Alamos National Laboratory, NMT Division, Los Alamos, New Mexico 87545.     

Detection of the ɛ phase and the Headley-Brooks orientation relationships upon α → γ transformation in the Fe-32% Ni alloy

The structure of an Fe-32% Ni alloy preliminarily quenched for martensite and subjected to α→ γ transformation upon a slow heating to different temperatures (430–500°C) has been studied by the electron-microscopic method. There has been observed an intermediate ɛ phase with an hcp lattice and rarely encountered Headley-Brooks bcc/fcc orientation relationships, which differ from the Kurdjumov-Sachs relationships. The networks of reflections of the ɛ phase have been observed in electron-diffraction patterns of the Fe-32% Ni alloy after both a slow heating to 430°C without annealing and a slow heating to 500°C with a subsequent annealing at 280°C; the Headley-Brooks relationships between the α matrix and the γ phase, which are typical for increased temperatures of phase transformations, have been observed in the samples after a slow heating to 500°C with annealing.     

Phase stability among the α(A1), β(A2), and γ(D83) phases in the Cu-Al-X system

The phase equilibria among the α(A1), β(A2), and γ(D8₃) phases in the Cu-Al-X systems (X: Ti, V, Mn, Fe, Co, Ni, Zn, Sn, and Sb) at 700 and 800 °C were investigated, and the effects of these alloying elements on the phase boundaries, homogeneous solid solution ranges, and tie-lines among the α, β, and γ phases were accurately determined by the diffusion couple method. The phase stabilities among α, β, and γ phases are discussed in terms of the partition coefficient. In addition, a modified Guillet’s method, which is used to estimate the effect of alloying elements on microstructure, is proposed by taking into account the change of equivalent coefficient with the phase fraction, and applied to predict the effect of alloying elements on the microstructure of the Cu-Al base alloy.     

Formation of a Rhenium-base Diffusion-barrier-coating System on Ni-base Single Crystal Superalloy and its Stability at 1,423 K

A diffusion-barrier-coating system having a duplex structure comprised of an inner Re(W)–Cr–Ni layer and an outer Ni-aluminide layer was formed on a fourth generation, single-crystal Ni-base superalloy by using electroplating of Re(Ni) and Ni(W) films, Al- and Cr- (high-Cr and low-Cr) pack cementations, and a combination of the two treatments. With the ReW-high-Cr coating, fine needle- or plate-like precipitates formed in the alloy substrate below the inner Re(W, Cr, Ni) layer, while there was little of this precipitate with the ReW-low-Cr pack-cementation coating. The inner, Re-base alloy layer in the ReW-high-Cr coating was identified to be a σ-(Re,Cr,W,Ni) phase, while the inner layer of the ReW-low-Cr was a mixture of σ-(Re,Cr,W,Ni) and δ-Re(Cr,W,Ni) phases. After heating the coated alloys at 1,150 °C for 100 h in air, the outer Al reservoir layer became β-NiAl containing (31–33)Al with α-Cr particles and fine precipitates of γ′-Ni₃Al with both the ReW-high-Cr and ReW-low-Cr treatments. In the case of the ReW-high-Cr coating there were numerous light-colored, needle-like precipitates formed deep in the alloy substrate under the inner layer, while in the case of the ReW-low-Cr coating γ′ appeared near the inner layer. It was found that the inner, Re-base alloy layer acted as a diffusion barrier, and that its structure was maintained with little change in composition after 100 h of oxidation at 1,150 °C. K. Z. Thosin is from Indonesian Institute of Sciences, LIPI.     

Alumina-base plasma-sprayed materials part I: Phase stability of alumina and alumina-chromia

Aluminum oxide is a relatively cheap, abundant material that is widely used for plasma- spray applications. This material, however, exists in many crystallographic modifications with different properties. In addition, most of these modifications are metastable and cannot be used in applications employed at elevated temperatures. Usually γ, δ, or other phases form after spraying, while α phase (corundum) is often the most desirable phase due to high corrosion resistance and hardness. This paper first reviews the method of α stabilization in the as- sprayed materials offered in literature. Then, as an example, it summarizes the results of an extensive study of chromia additions to alumina. Chromia was chosen because of its complete solid solubility in alumina and its crystal lattice type, which is similar to that of alumina. It was demonstrated that the addition of approximately 20 wt% chromia results in the formation of one solid solution of (Al- Cr)₂O₃ in the α- modification. Finally, this paper discusses the thermal stability of various alumina phases. Phase change routes of heating for different starting alumina modifications are discussed, and a case study of alumina- chromia is presented. Both types of as-sprayed structures, a mixture of α, δ, and γ phases, and 100% (Al- Cr)₂O₃ were annealed up to 1300 °C and the phase composition checked. At lower temperatures and shorter holding times, the amount of α phase decreases while another metastable θ phase appears, and the fraction of γ + δ, if present, increases. At temperature above 1100 °C, the amount of α phase increases again.     

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.     

The incoherent γ/γ′ solvus in Ni-Al alloys

Analysis of the kinetics of solute (Al) depletion during the coarsening of γ′ (Ni ₃ Al)precipitates has been used in earlier investigations to provide values of the coherent solubility of the γ phase in binary Ni-Al alloys. As demonstrated in recent experimental investigations, the coherent solubility is a function of the total concentration of Al in the two-phase alloy. Using the model of Ardell and Maheshwari to analyze the data on coherent equilibrium, the authors estimated the incoherent equilibrium solubility of the γ′ phase over the temperature range 400 to 800 ° C. The calculated incoherent solubility limits are 3 to 5 % smaller than previously published measurements, the differences decreasing as the temperature increases. The authors argue that the calculations in this article provide the first estimates of the incoherent solvus of the γ′ phase in Ni-Al alloys.     

Complete and Incomplete Wetting of Ferrite Grain Boundaries by Austenite in the Low-Alloyed Ferritic Steel

Low-carbon low-alloyed ferritic steels are the main material for the production of high-strength pipes for the transportation of oil and gas. The formation of brittle carbide network during the lifetime of a pipeline could be a reason for a catastrophic failure. Among other reasons, it can be controlled by the morphology of grain boundary (GB) carbides. The microstructure of a low-alloyed ferritic steel containing 0.09 at.% C and small amounts of Si, Mn, Nb, Cu, Al, Ni, and Cr was studied between 300 and 900 °C. The samples were annealed very long time (700 to 4000 h) in order to produce the equilibrium morphology of phases. The (α-Fe)/(α-Fe) GBs can be either completely or incompletely wetted (covered) by the γ-Fe (austenite) above the temperature of eutectoid transition. The portion of (α-Fe)/(α-Fe) GBs completely wetted by γ-Fe is around 90% and does not change much between 750 and 900 °C. The (α-Fe)/(α-Fe) GBs can be either completely or incompletely wetted (covered) by the Fe₃C (cementite) below the temperature of eutectoid transition. The portion of (α-Fe)/(α-Fe) GBs completely wetted by Fe₃C changes below 680 °C between 67 and 77%. The formation of the network of brittle cementite layers between ductile ferrite grains can explain the catastrophic failure of gas- and oil-pipelines after a certain lifetime.     

High-Temperature Deformation Behavior of Ti-6Al-4V Alloy without and with Hydrogenation Content of 0.27 wt.%

Isothermal compression of Ti-6Al-4V alloy without and with hydrogenation content of 0.27 wt.% was carried out on Gleeble-1500D thermal simulation machine at deformation temperature between 760 and 1000 °C and strain rate from 0.001 to 1 s⁻¹. The experimental results show that hydrogenation can decrease the deformation temperature or increase the strain rate of Ti-6Al-4V alloy. The apparent activation energy was determined to be 667 kJ mol⁻¹ for isothermal compression of the Ti-6Al-4V alloy without hydrogenation content of 0.27 wt.% in the α + β phase region (760-960 °C), and this value was about 655 and 199 kJ mol⁻¹ for the alloy with 0.27 wt.% of hydrogenation content in the α + β phase region (760-840 °C) and β phase region (840-960 °C), respectively. Constitutive equation was developed for the high-temperature deformation of Ti-6Al-4V alloy both without and with hydrogenation content of 0.27 wt.%.     

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.     

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.