A superalloy, or high-performance alloy, is an alloy that exhibits excellent mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Superalloys typically have a matrix with an austenitic face-centered cubic crystal structure. A superalloy's base alloying element is usually nickel, cobalt, or nickel-iron. Superalloy development has relied heavily on both chemical and process innovations and has been driven primarily by the aerospace and power industries. Typical applications are in the aerospace, industrial gas turbine and marine turbine industry, e.g. for turbine blades for hot sections of jet engines.

Examples of superalloys are Hastelloy, Inconel, Waspaloy, Rene alloys (e.g. Rene 41, Rene 80, Rene 95, Rene 104), Haynes alloys, Incoloy, MP98T, TMS alloys, and CMSX single crystal alloys.

Introduction

Superalloys are metallic materials for service at high temperatures, particularly in the hot zones of gas turbines. Such materials allow the turbine to operate more efficiently by withstanding higher temperatures. Turbine Inlet Temperature (TIT), which is a direct indicator of the efficiency of a gas turbine engine, depends on the temperature capability of 1st stage high pressure turbine blade made of Ni base superalloys exclusively.

One of the most important superalloy properties is high temperature creep resistance. Other crucial material properties are fatigue life, phase stability, as well as oxidation and corrosion resistance.

Superalloys develop high temperature strength through solid solution strengthening. Oxidation and corrosion resistance is provided by the formation of a protective oxide layer which is formed when the metal is exposed to oxygen and encapsulates the material, and thus protecting the rest of the component. Oxidation or corrosion resistance is provided by elements such as aluminium and chromium. By far the most important strengthening mechanism is through the formation of secondary phase precipitates such as gamma prime and carbides through precipitation strengthening.

Chemical development

Creep resistance is dependent on slowing the speed of dislocations within the crystal structure. In Ni-base superalloys the gamma prime phase [Ni₃(Al,Ti)] present acts as a coherent barrier to dislocation motion and is a precipitate strengthener. Chemical additions such as aluminum and titanium promote the creation of the gamma prime phase. The gamma prime phase size can be precisely controlled by careful precipitation hardening heat treatments. Many superalloys have a two phase heat treatment which creates a dispersion of square gamma prime particles known as the primary phase with a fine dispersion between these known as secondary gamma prime. Many other elements, both common and exotic, (including not only metals, but also metalloids and nonmetals) can be present; chromium, cobalt, molybdenum, tungsten, tantalum, aluminium, titanium, zirconium, niobium, rhenium, carbon, boron or hafnium are just a few examples. Cobalt base superalloys do not have a strengthening secondary phase like gamma prime.

[edit] Process development

The historical developments in superalloy processing have brought about considerable increases in superalloy operating temperatures. Superalloys were originally iron based and cold wrought prior to the 1940s. In the 1940s investment casting of cobalt base alloys significantly raised operating temperatures. The development of vacuum melting in the 1950s allowed for very fine control of the chemical composition of superalloys and reduction in contamination and in turn led to a revolution in processing techniques such as directional solidification of alloys and single crystal superalloys.

Within gas turbine engines many forms of superalloys are present. Polycrystalline Ni-base superalloys are used for the disks of the high pressure turbine which can be created using powder metallurgy or casting technology. Turbine blades can be polycrystalline, have a columnar grain structure, or being single crystal. Polycrystalline blades are formed using casting technology into a ceramic mold. Columnar grain structured blades are created using directional solidification techniques and have grains parallel to the major stress axes.

Single-crystal superalloys (SC superalloys) are formed as a single crystal using a modified version of the directional solidification technique, so there are no grain boundaries in the material. The mechanical properties of most other alloys depend on the presence of grain boundaries, but at high temperatures, they would participate in creep and must be replaced by other mechanisms. In many such alloys, islands of an ordered intermetallic phase sit in a matrix of disordered phase, all with the same crystalline lattice. This approximates the dislocation-pinning behavior of grain boundaries, without introducing any amorphous solid into the structure.

Applications

Superalloys are commonly used in gas turbine engines in regions that are subject to high temperatures which require high strength, excellent creep resistance, as well as corrosion and oxidation resistance. In most turbine engines this is in the high pressure turbine, blades here can face temperatures approaching if not beyond their melting temperature. Thermal barrier coatings (TBCs) play an important role in blades allowing them to operate under such conditions, protecting the base material from the thermal affects as well as corrosion and oxidation. Additional applications of superalloys include the following: gas turbines (commercial and military aircraft, power generation, and marine propulsion); space vehicles; submarines; nuclear reactors; military electric motors; chemical processing vessels, and heat exchanger tubing.

Metallurgy of superalloys

The superalloys of the first generation were intended for operation up to 700 °C (973 K). The up-to-date superalloys of the fourth generation are used as single or Monocrystals and are extra alloyed, especially with ruthenium. They can operate up to 1100 °C (1373 K).

The structure of most precipitation strengthened nickel-base superalloys consists of the gamma matrix, and of intermetallic γ' precipitates. The γ-phase is a solid solution with a face-centered crystal (fcc) lattice and randomly distributed different species of atoms.

By contrast, the γ'-phase has an ordered crystalline lattice of type LI₂. In pure Ni₃Al phase atoms of aluminium are placed at the vertices of the cubic cell and form the sublattice A. Atoms of nickel are located at centers of the faces and form the sublattice B. The phase is not strictly stoichiometric. There may exist an excess of vacancies in one of the sublattices, which leads to deviations from stoichiometry. Sublattices A and B of the γ'-phase can solute a considerable proportion of other elements. The alloying elements are dissolved in the γ-phase as well. The γ'-phase hardens the alloy through an unusual mechanism called the yield stress anomaly. Dislocations dissociate in the γ'-phase, leading to the formation of an anti-phase boundary. It turns out that at elevated temperature, the free energy associated with the anti-phase boundary (APB) is considerably reduced if it lies on a particular plane, which by coincidence is not a permitted slip plane. One set of partial dislocations bounding the APB cross-slips so that the APB lies on the low-energy plane, and, since this low-energy plane is not a permitted slip plane, the dissociated dislocation is now effectively locked. By this mechanism, the yield strength of γ'-phase Ni₃Al actually increases with temperature up to about 1000 °C, giving superalloys their currently unrivalled high-temperature strength.

In addition, it is often beneficial for a grain boundary containing nickel-base alloy to contain carbides for improvements in creep strength. Where the carbides (e.g. MC where M is a metal and C is a carbon atom) are precipitated at the grain boundaries, they act to pin the grain boundaries and improve the resistance to sliding and migration that would occur during creep diffusion. However if they precipitate as a continuous grain boundary film, the fracture toughness of the alloy may be reduced, together with the ductility and rupture strength.

Coating of superalloys

Superalloy products that are subjected to high working temperatures and corrosive atmosphere (such as high pressure turbine region of jet engines) are coated with various kinds of coating. Mainly two kinds of coating process are applied: pack cementation process and gas phase coating. Both are a type of CVD. In most cases, after the coating process near-surface regions of parts are enriched with aluminium, the matrix of the coating being nickel aluminide.

 Pack cementation process

The pack cementation process is carried out at lower temperatures, about 750 °C). The parts are loaded into boxes that contain a mixture of powders: active coating material, containing aluminium, activator (chloride or fluoride), and thermal ballast, like aluminum oxide). At high temperatures the gaseous aluminium chloride (or fluoride) is transferred to the surface of the part and diffuses inside (mostly inward diffusion). After the end of the process the so-called "green coating" is produced, which is too thin and brittle for direct use. A subsequent diffusion heat treatment (several hours at temperatures about 1080 °C) leads to further inward diffusion and formation of the desired coating.

Gas phase coating

This process is carried out at higher temperatures, about 1080 °C. The coating material is usually loaded onto special trays without physical contact with the parts to be coated. The coating mixture contains active coating material and activator, but usually does not contain thermal ballast. As in the pack cementation process, the gaseous aluminium chloride (or fluoride) is transferred to the surface of the part. However, in this case the diffusion is outwards. This kind of coating also requires diffusion heat treatment.

Research and development of new superalloys

The availability of superalloys during past decades has led to a steady increase in the turbine entry temperatures and the trend is expected to continue. Sandia National Laboratories is studying a new method for making superalloys, known as radiolysis. It introduces an entirely new area of research into creating alloys and superalloys through nanoparticle synthesis. This process holds promise as a universal method of nanoparticle formation. By developing an understanding of the basic material science behind these nanoparticle formations, there is speculation that it might be possible to expand research into other aspects of superalloys.

There may be considerable disadvantages in making alloys by this method. About half of the use of superalloys is in applications where the service temperature is close to the melting temperature of the alloy. It is common therefore to use single crystals. The above method produces polycrystalline alloys which will suffer from unacceptable level of creep.

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