Superalloy

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 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 industry, eg. for turbine blades for jet engines.

Examples of superalloys are Hastelloy, Inconel, Monel, Haynes alloys, Incoloy, MP98T, TMS alloys, and CMSX single crystal alloys.

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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) 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 protecting the rest of the component. Oxidation or corrosion resistance is provided by elements such as aluminium and chromium.

Chemical development

Creep resistance is dependent on slowing the speed of dislocations within the crystal structure. The gamma prime phase [Ni₃(Al,Ti)] present in nickel and nickel-iron superalloys presents a barrier to dislocations. 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. Cobalt base superalloys do not have a strengthening secondary phase like 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.

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.

Single-crystal superalloys (SC superalloys) are formed as a single crystal, 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 used where there is a need for high temperature strength and corrosion/oxidation resistance.

The largest applications of superalloys are the following: aircraft and industrial gas turbines; rocket engines; space vehicles; submarines; nuclear reactors; military electric motors, chemical processing vessels, and heat exchanger tubing.

Many of the industrial nickel-based superalloys contain alloying elements, including chromium, aluminium, and titanium, also molybdenum, tungsten, niobium, tantalum and cobalt.

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 matrix, the gamma phase, and of intermetallic γ' precipitates. The γ-phase is a solid solution with a face-centered crystal lattice and randomly distributed different species of atoms.

By contrast, the γ'-phase has an ordered crystalline lattice of type L₁2. 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.

Diffusion coatings

Products from superalloys, which are subjected to high working temperatures and corosive atmosphere (like first stages of turbine blades of the jet engines) are coated with various kinds of diffusion coatings. Mainly, two kinds of coating processes are applied: pack cementation process and gas phase coating. Both of them are CVD coatings. In most cases, after the coating process, near-surface regions of parts are enriched with aluminum, the matrix of the coating is nickel aluminide.

Pack cementation process

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

Gas phase coating

This process is carried out at higher temperatures: about 1080°C. The coating material is usually loaded on special trays without physical contact with parts. The coating mixture contains active coating material and activator, but, usually does not contain thermal ballast. Like in the pack cementation process, the gaseous aluminum 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.

Superalloys in the future

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 our understanding of the basic material science behind these nanoparticle formations, we’ll then be able to expand our research into other aspects of superalloys, like nickel-based alloys.” Tina Nenoff says.

References

Levitin, Valim (2006). High Temperature Strain of Metals and Alloys: Physical Fundamentals. WILEY-VCH. ISBN 978-3-527-31338-9. 

Sims, Chester T.; Stolloff, Norman S., Hagel, William C. [1987]. Superalloys II: High Temperature Materials for Aerospace and Industrial Power. John Wiley & Sons.