| High-Temperature Alloys Research: Still Going Strong |
By G.E. Fuchs, University of Florida
Posted Date: 7/2/2008
 The following article is excerpted from the July 2008 edition of JOM, which features a series of papers on high-temperature alloys. To read the full collection of articles, visit the JOM web site.
In this issue of JOM, the High Temperature Alloys Committee and the Seven Springs International Symposium Committee have compiled seven papers to provide a snapshot of the dynamic world of high-temperature alloys research. The search for structural materials that can be used at higher temperatures, under increased stresses, in more aggressive environments, and for longer times is a never-ending battle. In particular, the development of the gas turbine engine has been paced by the development of high-temperature materials, and more specifically, superalloys. The goal to achieve higher efficiencies, increased performance, and decreased emissions in gas turbines has and will continue to require improved materials.
The papers in this issue are broken down into two categories: processing and properties of high-performance coatings and alloys and recent development of high-temperature alloys. Most of the papers are on nickel-based superalloys, the most commonly used materials in the hot sections of gas turbines. Also included are two examples of other high-temperature materials.
Within the five high-performance coatings and alloys papers, the development of new alloys and the effects of composition and processing on the properties of high-temperature alloys are discussed. M.P. Brady et al. describe in "The Development of Alumina-Forming Austenitic Stainless Steels for High-Temperature Structural Use" the development of a new family of Al2O3-forming austenite stainless steels developed for use in aggressive environments at temperatures in the range of 600–900°C. These austenitic stainless steels would be expected to have much lower raw material costs than the nickel-based and cobalt-based alloys, so interest in them is great.
The search for higher-temperature materials to replace nickel-based superalloys continues. Refractory metal alloys and intermetallic compounds are frequently considered for high-temperature applications. M.R. Middlemas and J.K. Cochran's paper "Dense, Fine-Grain Mo-Si-B Alloys from Nitride-Based Reactions" reports on the formation of three-phase Mo-Si-B alloys from reactions of Mo, Si3N4, and BN powders.
"The Three-Dimensional Reconstruction of the Dendritic Structure at the Solid-Liquid Interface of a Ni-Based Single Crystal" by J. Madison et al. describes attempts to better understand the solidification front during directional solidification, and the effects of composition and processing conditions. An increased casting yield and lower cost would be expected by better understanding and controlling the directional solidification process.
K. Kawagishi et al. report on a new bond coat system that is thermodynamically stable to resist the formation of secondary reaction zones within the nickel-based superalloy substrate in "A Concept for the EQ Coating System for Nickel-Based Superalloys." Currently, the airfoil alloy substrate and coatings are considered airfoil systems since these structures must function together for long periods of times at elevated temperatures.
Fourth- and fifth-generation single-crystal nickel-based superalloys contain ruthenium for increased stability, and the mechanism for suppression of topologically close-packed (TCP) phases are described by R.A. Hobbs et al. in "TCP Suppression in a Ruthenium-Bearing Single-Crystal Nickel-Based Superalloy." The cost of ruthenium is rising, and understanding the ruthenium addition effect is key to developing alloys that do not rely on this alloying addition, or can utilize lower-level ruthenium additions.
As noted above, there is a great deal of interest in the development of new alloys that do not rely on rhenium and ruthenium. In "Primary Creep: Secondary Gamma Prime and the Rhenium Effect," B.K. Wilson and G.E. Fuchs report attempts to better understand the effects of rhenium additions and secondary gamma-prime on single-crystal nickel-based superalloys.
The forging of wrought nickel-based superalloys requires an understanding of the evolution of the microstructure during the forging operations. The paper "Automatic Feature Extraction in Micrographs of Forged Superalloys" by E. Berhuber et al. describes methodologies to determine the metallurgical parameters of the forging of these alloys using an automatic, image-based feature extraction.
It should also be noted that these articles are representative of the papers to be presented at the upcoming 11th International Symposium on Superalloys to be held September 14–18 at the Seven Springs Mountain Resort in Champion, Pennsylvania. Additional information can be found at www.tms.org/Meetings/specialty/superalloys2008/home.html.
G.E. Fuchs is with the Materials Science & Engineering Department at the University of Florida and is the JOM advisor from the High-Temperature Alloys Committee of the TMS Structural Materials Division.
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COMMUNITY CENTERS
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MODERATOR'S CHOICE:
TOP DIGITAL RESOURCES
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Superalloys Cast Alloys Table
A thorough overview containing basic descriptions, typical applications, links to supplier data sheets and selected technical papers for typical cast superalloys.
Superalloys Wrought Alloys Table
A thorough overview containing basic descriptions, typical applications, links to supplier data sheets and selected technical papers for typical wrought superalloys.
Physical Metallurgy and Microstructure of Superalloys
A primer describing the major phases present in the nickel-based superalloys: gamma (the matrix phase), gamma' (the strengthening precipitate), carbides, borides and the deleterious, topologically close-packed phases.
Superalloy Processing Summary
Extensive links to articles, digital resources and video clips covering processing of superalloys.
Superalloy Producers
A compilation of links to superalloy producers.
Superalloy University Research
A compilation of links to superalloy research programs at universities around the world.
Design Study in Nickel-Based Superalloy Castings: Floatwall Panels for a Jet Engine Combustor
An online design study of casting design issues floatwall panels for a jet engine combustor.
The Superalloys
Extensive superalloy website covering the metallurgy and processing of superalloys
Coatings for High Temperature Applications
Extensive covering historical information, bond coats and thermal barrier coatings.
Metallic Materials Properties Development and Standardization (MMPDS)
Link to the full text of the chapter on “Heat Resistant Alloys” in this handbook of reliable aircraft materials data.
Handbook of Materials Selection
Link to the full text of the “Selection of Superalloys for Design” chapter in this resource for materials properties, their evaluation and industrial applications.
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SUPERALLOYS PROCEEDINGS ARCHIVE |
The Superalloys Conference Proceedings Archive contains nearly 1,000 technical articles from 14 superalloys conferences, including the International Symposium on Superalloys (1968 to 2004) and the International Special Emphasis Symposium on Superalloys 718, 625, 706, and Various Derivatives (1989 to 2005). Many papers from these volumes may be downloaded at no charge. The Superalloys Conference Proceedings Archive is made possible through the support of the International Symposium on Superalloys Committee.
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