Super alloys :-
are metallic alloys for elevated temperature service, usually based on group VIIA elements of the periodic table, and are generally for elevated temperature applications where the resistance to deformation and stability are prime requirements. The common super alloys are based on nickel, cobalt or iron…..etc .
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is a metallic alloy which can be used at high temperatures, often in excess of 0.7 of the absolute melting temperature. Creep and oxidation resistance are the prime design criteria, it can be based on iron, cobalt or nickel, the latter being best suited for aeroengine applications .
There are three very important categories for any super alloys :-
The Ni-Al-Ti ternary phase diagrams show the γ and γ' phase field. For a given chemical composition, the fraction of γ' decreases as the temperature is increased. This phenomenon is used in order to dissolve the γ' at a sufficiently high temperature (a solution treatment) followed by ageing at a lower temperature in order to generate a uniform and fine dispersion of strengthening precipitates.
The γ-phase is a solid solution with a cubic-F lattice and a random distribution of the different species of atoms. Cubic-F is short for face-center cubic (F.C.C)
By contrast, γ' has a cubic-P (primitive cubic) lattice in which the nickel atoms are at the face-centers and the aluminum or titanium atoms at the cube corners. This atomic arrangement has the chemical formula Ni3Al, Ni3Ti or Ni3(Al , Ti). However, as can be seen from the (γ+γ')/γ' phase boundary on the ternary sections of the Ni, Al, Ti phase diagram, the phase is not strictly stoichiometric. There may exist an excess of vacancies on one of the sub lattices which leads to deviations from stoichiometry; alternatively, some of the nickel atoms might occupy the Al sites and vice-versa. In addition to aluminum and titanium, niobium, hafnium and tantalum partition preferentially into γ'.
Strength versus Temperature
The strength of most metals decreases as the temperature is increased, simply because assistance from thermal activation makes it easier for dislocations to surmount obstacles. However, nickel based super alloys containing γ', which essentially is an inter metallic compound based on the formula Ni3(Al, Ti), are particularly resistant to temperature.
Ordinary slip in both γ and γ' occurs on the {111}<110>. If slip was confined to these planes at all temperatures then the strength would decrease as the temperature is raised. However, there is a tendency for dislocations in γ' to cross-slip on to the {100} planes where they have a lower anti-phase domain boundary energy. This is because the energy decreases with temperature. Situations arise where the extended dislocation is then partly on the close-packed plane and partly on the cube plane. Such a dislocation becomes locked, leading to an increase in strength. The strength only decreases beyond about 600oC whence the thermal activation is sufficiently violent to allow the dislocations to overcome the obstacles.
To summaries, it is the presence of γ' which is responsible for the fact that the strength of nickel based super alloys is relatively insensitive to temperature. When greater strength is required at lower temperatures (e.g. turbine discs), alloys can be strengthened using another phase known as γ''. This phase occurs in nickel super alloys with significant additions of niobium (Inconel 718) or vanadium; the composition of the γ'' is then Ni3Nb or Ni3V. The particles of γ'' are in the form of discs with (001)γ''||{001}γ and [100]γ''||<100>γ
The crystal structure of γ'' is based on a body-center tetragonal lattice with an ordered arrangement of nickel and niobium atoms. Strengthening occurs therefore by both a coherency hardening and order hardening mechanism. The lattice parameters of γ'' are approximately a=0.362 nm and c=0.741 nm
Alloy Compositions
Commercial super alloys contain more than just Ni, Al and Ti. Chromium and aluminum are essential for oxidation resistance small quantities of yttrium help the oxide scale to cohere to the substrate. Polycrystalline super alloys contain grain boundary strengthening elements such as boron and zirconium, which segregate to the boundaries. The resulting reduction in grain boundary energy is associated with better creep strength and ductility when the mechanism of failure involves grain decohesion.
There are also the carbide formers (C, Cr, Mo, W, C, Nb, Ta, Ti and Hf). The carbides tend to precipitate at grain boundaries and hence reduce the tendency for grain boundary sliding.
Elements such as cobalt, iron, chromium, niobium, tantalum, molybdenum, tungsten, vanadium, titanium and aluminum are also solid-solution strengtheners, both in γ and γ'.
There are, naturally, limits to the concentrations that can be added without inducing precipitation. It is particularly important to avoid certain embrittling phases such as Laves and Sigma. There are no simple rules governing the critical concentrations; it is best to calculate or measure the appropriate part of a phase diagram.
Alloying element effects in nickel based super alloys. The "M" in M23C6 stands for a mixture of metal atoms. Click on chart to enlarge.
Nominal chemical compositions, wt%. MA/ODS ≡ mechanically alloyed, oxide dispersion-strengthened.
PM ≡ powder metallurgical origin. The alloy names are proprietary. SX ≡ single crystal.
The single-crystal super alloys are often classified into first, second and third generation alloys. The second and third generations contain about 3 wt% and 6wt% of rhenium respectively. Rhenium is a very expensive addition but leads to an improvement in the creep strength. It is argued that some of the enhanced resistance to creep comes from the promotion of rafting by rhenium, which partitions into the γ and makes the lattice misfit more negative. Atomic resolution experiments have shown that the Re occurs as clusters in the γ phase. It is also claimed that rhenium reduces the overall diffusion rate in nickel based super alloys.
The properties of super alloys deteriorate if certain phases known as the topologically close-packed (TCP) phases precipitate. In these phases, some of the atoms are arranged as in nickel, where the close-packed planes are stacked in the sequence ...ABCABC.. However, although this sequence is maintained in the TCP phases, the atoms are not close-packed, hence the adjective 'topologically'. TCP phases include σ μ. Such phases are not only intrinsically brittle but their precipitation also depletes the matrix from valuable elements which are added for different purposes. The addition of rhenium promotes TCP formation, so alloys containing these solutes must have their Cr, Co, W or Mo concentrations reduced to compensate. It is generally not practical to remove all these elements, but the chromium concentration in the new generation super alloys is much reduced. Chromium does protect against oxidation, but oxidation can also be prevented by coating the blades
Oxide Dispersion Strengthened Super alloys
Oxide dispersion strengthened super alloys can be produced starting from alloy powders and yttrium oxide, using the mechanical alloying process. The yttrium becomes finely dispersed in the final product. It is also a very stable oxide, making the material particularly suitable for elevated temperature applications. However, mechanical alloying is a very difficult process so such alloys have limited applications. A transmission electron micrograph showing the oxide dispersion in a mechanically-alloyed nickel based super alloy is shown below
Applications of nickel based super alloys
Turbine Blades
A major use of nickel based superalloys is in the manufacture of aeroengine turbine blades. A single-crystal blade is free from γ/γ grain boundaries. Boundaries are easy diffusion paths and therefore reduce the resistance of the material to creep deformation. The directionally solidified columnar grain structure has many γ grains, but the boundaries are mostly parallel to the major stress axis; the performance of such blades is not as good as the single-crystal blades. However, they are much better than the blade with the equiaxed grain structure which has the worst creep life.
One big advantage of the single-crystal alloys over conventionally cast polycrystalline superalloys is that many of the grain boundary strengthening solutes are removed. This results in an increase in the incipient melting temperature (i.e., localized melting due to chemical segregation). The single-crystal alloys can therefore be heat treated to at temperatures in the range 1240-1330°C, allowing the dissolution of coarse γ' which is a remanent of the solidification process. Subsequent heat treatment can therefore be used to achieve a controlled and fine-scale precipitation of γ'. The primary reason why the first generation of single-crystal superalloys could be used at higher temperatures than the directionally solidified ones, was because of the ability to heat-treat the alloys at a higher temperature rather than any advantage due to the removal of grain boundaries. A higher heat-treatment temperature allows all the γ' to be taken into solution and then by aging, to precipitate in a finer form.
Superalloy blades are used in aeroengines and gas turbines in regions where the temperature is in excess of about 400oC, with titanium blades in the colder regions. This is because there is a danger of titanium igniting in special cir cumstances if its temperature exceeds 400oC.
Turbine Discs
Turbine blades are attached to a disc which in turn is connected to the turbine shaft. The properties required for an aeroengine discs are different from that of a turbine, because the metal experiences a lower temperature. The discs must resist fracture by fatigue. Discs are usually cast and then forged into shape. They are polycrystalline.
Turbochargers
An internal combustion engine generally uses a stoichiometric ratio of air to fuel. A turbocharger is a device to force more air into the engine, allowing a correspondingly greater quantity of fuel to be burned in each stroke. This boosts the power output of the engine.
The turbocharger consists of two components, a turbine which is driven by exhaust gases from the engine. This in turn drives an air pump which forces more air into the engine. The typical rate of spin is 100-150,000 rotations per minute. Because the turbocharger is driven by exhaust gasses, it gets very hot and needs to be oxidation resistant and strong.
