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CATEGORIES (articles) > Chassis & Bodywork Construction > Metal working > Superalloy explained

Superalloy explained


A super-alloy, or high-performance alloy, is an alloy with superior mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Superalloys usually are face-centered cubic and austenitic. 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-188, MP98T, TMS-63, TMS-71, and TMS-75.


Introduction

Superalloys are metallic materials for service at high temperatures. Particularly in the hot zones of modern gas turbines used in airplanes, such materials are needed to improve fuel efficiency, which requires the alloys to withstand higher temperatures and stresses. One of the most important requirements is resistance against high temperature creep. Other crucial material properties are crack resistance, stiffness, as well as an ability to resist oxidation and an acceptable density.

The availability of such superalloys led during past decades to a steady increase in the turbine entry temperatures and the trend is expected to continue.


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 finally controlled by annealing. 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.


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 intermetalic 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.


Uses in the real world

The largest applications of superalloys are the following: aircraft and industrial gas turbines; rocket engines; space vehicles; submarines; nuclear reactors; military electric motors.

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


Scientific facts

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 majority of nickel-base superalloys consists of matrix, i.e. the γ-phase, and of particles of the hardening γ'-phase. 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.






CATEGORIES (articles) > Chassis & Bodywork Construction > Metal working > Superalloy explained

 
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