Superalloy
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A superalloy, or high-performance alloy, is an alloy able to withstand extreme temperatures that would destroy conventional metals like steel and aluminum. Superalloys exhibit excellent mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Superalloys typically have a 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, Haynes alloys, Incoloy, MP98T, TMS alloys, and CMSX single crystal alloys.
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[edit] 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.
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 two mechanisms, Solid solution strengthening and Precipitation strengthening. Oxidation and corrosion resistance is provided by the formation of a tenacious oxide scale.
[edit] Chemical development
Creep resistance is dependent on slowing the speed of dislocations within the crystal structure. The body centered cubic 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, can be present; chromium, molybdenum, tungsten, aluminium, zirconium, niobium, rhenium, carbon or silicon are just a few examples.
[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.
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.
[edit] Applications
Super alloys 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.
[edit] 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 crystals 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.
[edit] Superalloys in the future
The availability of superalloys led during past decades to a steady increase in the turbine entry temperatures and the trend is expected to continue.
[edit] External links
[edit] 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.
