Microstructural Changes of a γ-TiAl Based Alloy with a Fully Lamellar Microstructure Due to Annealing at Elevated Temperatures

2002 ◽  
Vol 753 ◽  
Author(s):  
M. Beschliesser ◽  
F. Appel ◽  
H. Kestler ◽  
H. Clemens
Keyword(s):  
Chemical Composition ◽  
Thermodynamic Equilibrium ◽  
Elevated Temperatures ◽  
Volume Fraction ◽  
Sheet Material ◽  
Lamellar Microstructure ◽  
Microstructural Changes ◽  
Fully Lamellar ◽  
Equilibrium Calculations ◽  
Thermodynamic Equilibrium Calculations

ABSTRACTThe changes of a fully lamellar microstructure caused by annealing at 700 and 800°C for up to 10,000 hours in air have been investigated for a Ti-46.5at.%Al-4at.%(Cr,Nb,Ta,B) sheet material. Changes in morphology as well as in chemical composition and volume fraction of constitutional phases were recorded by means of transmission electron microscopy. Thermodynamic equilibrium calculations performed with the software package ThermoCalc provide baseline information upon both the chemical composition and the amount of constitutional phases of the investigated multi-component alloy. It could be demonstrated that due to high cooling rates which are required to adjust a “defect-free” undisturbed fully lamellar microstructure, composition and amount of the α2 phase are far from the thermodynamic equilibrium. However, during high temperature exposure the composition as well as the amount of α2 phase move towards the values predicted by the calculation. Furthermore, former perfectly shaped γ/α2 lamellae interfaces are disturbed, α2 lamellae decompose and new phases appear instead. Dislocations emitted from dissolving α2 lamellae move into the γ lamellae. Mechanisms responsible for decomposition of lamellae and formation of new phases are explained by thermodynamic equilibrium calculations and diffusion considerations. Finally, results of tensile tests conducted at room temperature and 700°C on long-term annealed specimens are presented and discussed. This work is a basis for understanding microstructural changes and their impact on mechanical properties of γ-TiAl based alloys with a fully lamellar microstructure during service at elevated temperatures.

A Mechanism of Polycrystallization in Fully Lamellar Ti-48Al-8Nb Single Crystal Alloy Aged at Elevated Temperatures

2002 ◽  
Vol 753 ◽  
Author(s):  
Yukinori Yamamoto ◽  
Masao Takeyama ◽  
Takashi Matsuo
Keyword(s):  
Growth Rate ◽  
Diffusion Coefficient ◽  
Single Crystal ◽  
Grain Growth ◽  
Elevated Temperatures ◽  
Volume Diffusion ◽  
Activation Enthalpy ◽  
Lamellar Microstructure ◽  
Aged Sample ◽  
Fully Lamellar

ABSTRACTPolycrystallization mechanism of a fully lamellar microstructure during aging at 1473 and 1273 K has been examined using Ti-48Al-8Nb fully lamellar single crystal, which consists mostly of γ/γ interfaces (variant, perfect-twin and pseudo-twin boundaries). After a certain period of aging, a few γ grains are formed within the lamellae and the lamellar microstructure collapses rapidly to become a γ grained microstructure at both temperatures. An EBSP analysis for aged sample revealed that most of the grains follow the orientation of variant domains in the lamellar microstructure. A kinetic analysis of the grain growth during aging revealed that the activation enthalpy of the growth rate is estimated to be 390 kJ/mol, which is very close to that for volume diffusion coefficient of Al and Nb in γ-TiAl. Based on the results, it is concluded that the formation of the grains is attributed to coarsening of variant domains within the lamellar plates and coalescence of the same variant domains across the lamellae, leading to a γ grained microstructure following the orientation of variant domains. These reactions also make the number of the variant domains decrease during aging, which remains only two variant domains with perfect-twin relationship.

scholarly journals Thermodynamic characterization of Mexico City aerosol during MILAGRO 2006

2007 ◽  
Vol 7 (3) ◽  
pp. 9203-9233 ◽  
Author(s):  
C. Fountoukis ◽  
A. Nenes ◽  
A. Sullivan ◽  
R. Weber ◽  
T. VanReken ◽  
...  
Keyword(s):  
Gas Phase ◽  
Mexico City ◽  
Thermodynamic Equilibrium ◽  
Phase State ◽  
Molar Ratio ◽  
Inorganic Species ◽  
Equilibrium Calculations ◽  
Thermodynamic Equilibrium Model ◽  
Thermodynamic Equilibrium Calculations

Abstract. Fast measurements of aerosol and gas-phase constituents coupled with the ISORROPIA-II thermodynamic equilibrium model are used to study the partitioning of semivolatile inorganic species and phase state of Mexico City aerosol sampled at the T1 site during the MILAGRO 2006 campaign. Overall, predicted semivolatile partitioning agrees well with measurements. PM2.5 is insensitive to changes in ammonia but is to acidic semivolatile species. Semi-volatile partitioning equilibrates on a timescale between 6 and 20 min. When the aerosol sulfate-to-nitrate molar ratio is less than 1, predictions improve substantially if the aerosol is assumed to follow the deliquescent phase diagram. Treating crustal species as "equivalent sodium" (rather than explicitly) in the thermodynamic equilibrium calculations introduces important biases in predicted aerosol water uptake, nitrate and ammonium; neglecting crustals further increases errors dramatically. This suggests that explicitly considering crustals in the thermodynamic calculations are required to accurately predict the partitioning and phase state of aerosols.

scholarly journals Thermodynamic Equilibrium Calculations on the Oxidation Behavior of the Mo-Ru-Rh-Pd Alloys

2012 ◽  
Vol 11 (1) ◽  
pp. 30-36 ◽  
Author(s):  
Yuji OHISHI ◽  
Ken KUROSAKI ◽  
Tohru SUGAHARA ◽  
Aikebaier YUSUFU ◽  
Yusuke MASAHIRA ◽  
...  
Keyword(s):  
Thermodynamic Equilibrium ◽  
Oxidation Behavior ◽  
Equilibrium Calculations ◽  
Thermodynamic Equilibrium Calculations

Thermodynamic equilibrium calculations of the volatilization and condensation of inorganics during wood gasification

Fuel ◽  
2013 ◽  
Vol 107 ◽  
pp. 269-281 ◽  
Author(s):  
K. Froment ◽  
F. Defoort ◽  
C. Bertrand ◽  
J.M. Seiler ◽  
J. Berjonneau ◽  
...  
Keyword(s):  
Thermodynamic Equilibrium ◽  
Equilibrium Calculations ◽  
Thermodynamic Equilibrium Calculations

Microstructure of Rolled Ti-46.5Al-4(CrNb,Ta,B)

1998 ◽  
Vol 552 ◽  
Author(s):  
B. J. Inkson ◽  
H. Clemens
Keyword(s):  
Phase Field ◽  
Boron Content ◽  
Sheet Material ◽  
Lamellar Microstructure ◽  
Heat Treatments ◽  
Grain Structure ◽  
Α Phase ◽  
Heat Treated ◽  
B2 Phase ◽  
Fully Lamellar

ABSTRACTAn industrial Ti-46.5A1–4(CrNb,Ta) alloy, with and without boron additions, has been successfully rolled to sheet 1.5mm thick. The microstructure of the sheet material has been characterised as a function of boron content and subsequent heat treatments. It is observed that rod-like (Ti-Ta)B precipitates are effective in refining the grain structure in both as-rolled and subsequently heat treated material. As-rolled microstructures rapidly cooled from the α + γ phase field contain unstable α2, which transforms to B2 phase on annealing below the α + γ tranus. Annealing in the α-phase field can result in a designed fully lamellar microstructure.

Carbon Deposition Simulation in Porous SOFC Anodes: A Detailed Numerical Analysis of Major Carbon Precursors

2015 ◽  
Vol 12 (5) ◽  
Author(s):  
C. Schluckner ◽  
V. Subotić ◽  
V. Lawlor ◽  
C. Hochenauer
Keyword(s):  
Carbon Monoxide ◽  
Thermodynamic Equilibrium ◽  
Operating Conditions ◽  
Carbon Formation ◽  
Fuel Supply ◽  
Wide Range ◽  
Pure Methane ◽  
Equilibrium Calculations ◽  
Elementary Carbon ◽  
Thermodynamic Equilibrium Calculations

Solid oxide fuel cells (SOFCs) can be operated on a wide range of fuels, including hydrocarbons. Such a fuel supply includes the risk of carbon formation on the catalytically active nickel centers within the porous anodic substrate. Coking is very critical for the reliability and durability of the SOFCs. Thus, within this study, coking propensity of the most prominent carbon containing fuels was analyzed by thermodynamic equilibrium calculations for two fundamentally different types of carbon and detailed transient numerical simulations based on heterogeneous reforming kinetics. This approach is new to the literature and reveals the strengths and weaknesses of both fundamentally different approaches. It was shown that in thermodynamic equilibrium calculations, carbon formation is most likely due to pure methane. Carbon monoxide will form the least amounts of carbon in typical SOFC temperature ranges. Furthermore, based on a validated computational fluid dynamics (CFD) simulation model, detailed heterogeneous reaction kinetics were used to directly simulate elementary carbon adsorbed to the reactive substrate surface. The amounts, spatial and temporal distribution, of carbon atoms within the porous structure were identified between 600 °C and 1000 °C for a broad steam-to-carbon ratio range of 0.5–2. It was shown that most carbon is formed at the beginning of the substrate. A key finding was that steady-state results differ greatly from results within the first few seconds of fuel supply. An increment in temperature causes a significant increase in the amount of carbon formed, making the highest temperatures the most critical for the SOFC operation. Moreover, it was shown that mixtures of pure methane deliver the highest amounts of adsorbed elementary carbon. Equimolar mixtures of CH4/CO cause second highest surface coverages. Pure carbon monoxide blends led to least significant carbon formations. This work has shown the important contribution that thermodynamic equilibrium calculation results, as well as the outcomes of the detailed CFD simulations, allow to identify suitable operating conditions for the SOFC systems and to minimize the risk of coking on porous anodes.

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