Superalloys, development

As mentioned in the preceding text, pack aluminizing is commonly carried out on nickel- and cobalt-base superalloys. Diffusion-coated superalloys develop an aluminide (NiAl or CoAl) outer layer with enhanced corrosion resistance. It is estimated that more than 90% of all coated gas turbine engine hot section blades and vanes made from superalloys are coated by pack cementation and related processes. Detailed information on protective diffusion coatings for superalloys can be found in Ref 24. 

Any major materials development programme, such as that on the eutectic superalloys, can only be undertaken if a successful outcome would be cost effective. As Fig. 20.10 shows, the costs of development can be colossal. Even before a new material is out of the laboratory, 5 to 10 million pounds (8 to 15 million dollars) can have been spent, and failure in an engine test can be expensive. Because the performance of a new alloy cannot finally be verified until it has been extensively flight-tested, at each stage of development risk decisions have to be taken whether to press ahead, or cut losses and abandon the programme. 

Until the late 1990s, Waspaloy was still the best alloy available for the majority of hot gas turboexpanders used in industry and, until recently, it continued to offer the many special characteristic needed for hot gas expander applications. However, a new development followed in 2000 when the Ebara Corporation (Japan) released data on a nickel-base superalloy. 

The evolution of superalloys has been splendidly mapped by an Ameriean metallurgist, Sims (1966, 1984), while the more restrieted tale of the British side of this development has been told by Pfeil (1963). I have analysed (Cahn 1973) some of the lessons to be drawn from the early stages of this story in the eontext of the methods of alloy design it really is an evolutionary tale... the survival of the fittest, over and over again. The present status of superalloy metallurgy is eoneisely presented by MeLean (1996). 

Aluminium diffusion calorising, aluminisingY protects steels against oxidation at elevated temperatures, and the more recently developed processes for aluminising and chromaluminising superalloys are widely used to increase the life and operating temperature of aircraft gas turbine vanes, etc. 

Aeronautics also stimulated the development of superalloys, largely based on nickel. They hold their strength (hardness) at very high... 

Vacuum atomization is a commercial batch process)180 The development of vacuum atomization started in the mid 1960 s, concurrent with the development of inert gas atomization. In 1970, a patent for the vacuum atomization method was issued to Homogeneous Metals, Inc. Using vacuum atomization, this company routinely produces superalloy powders of fine size without great consumption of argon, giving powders free of inert gas filled porosity. Wentzell1 801 has made detailed description of this proprietary process. 

The arrangement of the melting and vacuum spray chambers is critical for guiding the liquid metal to eject into the vacuum chamber. Difficulties exist in precisely controlling the expulsion of the liquid metal into the vacuum chamber. Therefore, flaky droplets may be formed in vacuum atomization. Although vacuum atomization was developed mainly for the production of high-purity nickel and cobalt based superalloy powders, it is also applied to atomize the alloys of aluminum, copper and iron. 

This process, originally designated as RSR (rapid solidification rate), was developed by Pratt and Whitney Aircraft Group and first operated in the late 1975 for the production of rapidly solidified nickel-base superalloy powders.[185][186] The major objective of the process is to achieve extremely high cooling rates in the atomized droplets via convective cooling in helium gas jets (dynamic helium quenching effects). Over the past decade, this technique has also been applied to the production of specialty aluminum alloy, steel, copper alloy, beryllium alloy, molybdenum, titanium alloy and sili-cide powders. The reactive metals (molybdenum and titanium) and... 

Hydrogen has to be safely stored and transported. Therefore, hydrogen resistant alloys for containers, such as FeNiCr stainless steel, FeNiCr strengthened with N and Mn, y -precipitated strengthening superalloy, have been developed at our institute. 

Earlier work on systems such as Ni-Al-Cr reported in Sanchez et al. (1984b) used FP methods to obtain information on phases for which there was no experimental information. In the case of Ni-base alloys, the results correctly reproduced the main qualitative features of the 7 — 7 equilibrium but cannot be considered accurate enough to be used for quantitative alloy development. A closely related example is the work of (Enomoto and Harada 1991) who made CVM predictions for order/disorder (7 — 7 ) transformation in Ni-based superalloys utilising Lennard-Jones pair potentials. 

In addition to the appearance of numerous new alloys, sometimes called superalloys, recent developments in this field include many relatively new processes and methodologies, such as electron beam refining, rapid solidification, single-ciystal superalloys, and metallic glasses, among others. Some of these are described in separate articles in this encyclopedia. 

Iron-niekcl base superalloys were developed primarily from the stainless steels. In the United Slates, these alloys included 19-9 DL. lb-25-6, and A-286, Luler, higher nickel contents were employed to take advantage of the superior oxidation resistance of nickel and the beneficial effects of y -forming elements. All iron-nickel base superalloys rely on solid solution hardening lo some extent. 

The SiC whisker-reinforced alumina composite, a model for engineered materials, has opened new vistas for tool material development. Whereas SiC whisker-reinforced alumina is used extensively for the machining of nickel-base superalloys, SiC whiskers react chemically with steel, causing rapid wear on the rake face. Attempts are underway to replace SiC whiskers with less reactive whiskers such as TiC or TiN. 

Poster 29. M. Morinaga, Y. Murata, R. Hashizume, A. Yoshinari and T. Kiyono (Nagoya University, The Kansai Electric Power Company, Inc., Hitachi Co. Ltd.) Design and Development of Ni-Based Single Crystal Superalloys for Gas Turbine Blades Using the Alloying Parameters Obtained by DV-Xa Method... 

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