Nickel Based Superalloys

A Superalloy 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. Superalloys can be based on iron, cobalt or nickel, the latter being best suited for aeroengine applications.

How They Work

The essential solutes in nickel based superalloys are aluminium and/or titanium, with a total concentration which is typically less than 10 atomic percent. This generates a two-phase equilibrium micro-structure which is largely responsible for the elevated-temperature strength of the material and its incredible resistance to creep deformation. The amount of this depends on the chemical composition and temperature.

The Ni-Al-Ti ternary phase diagrams for the nickel - aluminium - titanium system show this two phase fields created. For a given chemical composition, the fraction of one phase decreases as the temperature is increased. This phenomenon is used in order to dissolve this phase 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.

It has a primitive cubic lattice in which the nickel atoms are at the face-centres and the aluminium or titanium atoms at the cube corners. This atomic arrangement has the chemical formula Ni₃Al, Ni₃Ti or Ni₃(Al,Ti). In addition to aluminium and titanium, niobium, hafnium and tantalum partition preferentially into it. Dislocations in the lattice nevertheless find it difficult to penetrate. The order interferes with dislocation motion and hence strengthens the alloy.

The small misfit between the two-phase lattices is important for two reasons. Firstly, when combined with the orientation relationship, it ensures a low interfacial energy. The ordinary mechanism of precipitate coarsening is driven entirely by the minimization of total interfacial energy. A coherent or semi-coherent interface therefore makes the micro-structure stable, a property which is useful for elevated temperature applications.

The magnitude and sign of the misfit also influences the development of microstructure under the influence of a stress at elevated temperatures. The positive misfit can be controlled by altering the chemical composition, particularly the aluminium to titanium ratio. A negative misfit stimulates the formation of rafts, essentially layers of the phase in a direction normal to the applied stress. This can help reduce the creep rate if the mechanism involves the climb of dislocations across the precipitate rafts.

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 superalloys contain essentially an inter-metallic compound based on the formula Ni₃(Al,Ti), are particularly resistant to temperature. It is the presence of the two-phases which is responsible for the fact that the strength of nickel based superalloys is relatively insensitive to temperature.

When greater strength is required at lower temperatures (e.g. turbine discs), alloys can be strengthened using another phase. This phase occurs in nickel superalloys with significant additions of niobium or vanadium; the composition of this phase is then Ni₃Nb or Ni₃V. Its particles are in the form of discs with the other phases The crystal structure is based on a body-centred tetragonal lattice with an ordered arrangement of nickel and niobium atoms. Strengthening occurs therefore by both a coherency hardening and order hardening mechanism.

Alloy Compositions

Commercial superalloys contain more than just Ni, Al and Ti. Chromium and aluminium are essential for oxidation resistance small quantities of yttrium help the oxide scale to cohere to the substrate. Polycrystalline superalloys 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 aluminium are also solid-solution strengtheners, both in gamma and gamma-prime.

There are, naturally, limits to the concentrations that can be added without inducing precipitation. It is particularly important to avoid certain embrittling phases. There are no simple rules governing the critical concentrations; it is best to calculate or measure the appropriate part of a phase diagram.

The single-crystal superalloys 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 gamma and makes the lattice misfit more negative. Atomic resolution experiments have shown that the Re occurs as clusters in the gamma phase. It is also claimed that rhenium reduces the overall diffusion rate in nickel based superalloys.

The properties of superalloys 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 sequence. However, although this sequence is maintained in the TCP phases, the atoms are not close-packed, hence the adjective 'topologically'. 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 superalloys is much reduced. Chromium does protect against oxidation, but oxidation can also be prevented by coating the blades.

Microstructure and Heat Treatment

To optimize properties (often of a coating--metal system), nickel based superalloys are, after solution treatment, heat treated at two different temperatures within the phase field. The higher temperature heat treatment precipitates coarser particles. The second lower temperature heat treatment leads to further precipitation, as expected from the phase diagram. This latter precipitation leads to a finer, secondary dispersion. The net result is a bimodal distribution

The solution heat treatment temperature determines not only the amount of prime-phase that dissolves, but also the grain size. The size becomes coarser if all the prime-phase is dissolved, since there is then no pinning effect of the precipitate particles on the movement of the phase boundaries.

Oxide Dispersion Strengthened Superalloys

Oxide dispersion strengthened superalloys can be produced starting from alloy powders and yttrium oxide, using the mechanical alloying process. The yttria 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. The oxide dispersion in a mechanically-alloyed nickel based superalloy is ODS alloy MA6000

Applications of nickel based superalloys

Turbine Blades

A major use of nickel based superalloys is in the manufacture of aeroengine turbine blades. A single-crystal blade is free from phase 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 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 grains which are a remnant of the solidification process. Subsequent heat treatment can therefore be used to achieve a controlled and fine-scale precipitation. 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 prime grains 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 400°C, with titanium blades in the colder regions. This is because there is a danger of titanium igniting in special circumstances if its temperature exceeds 400°C.

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.

One difficulty is that cast alloys have a large columnar grain structure and contain significant chemical segregation; the latter is not completely eliminated in the final product. This can lead to scatter in mechanical properties. One way to overcome this is to begin with fine, clean powder which is then consolidated. The powder is made by atomization in an inert gas; the extent of chemical segregation cannot exceed the size of the powder. After atomization, some discs are made from powder which is hot-isostatically pressed, extruded and then forged into the required shape. The process is difficult because of the need to avoid undesired particles introduced, for example, from the refractories used in the atomisation process, or impurities picked up during solidification. Such particles initiate fatigue; the failure of an aeroengine turbine disc can be catastrophic.

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.

Melt Processing

The nickel based superalloys contain reactive elements such as aluminium and titanium. It is necessary therefore to melt the alloys under vacuum, with the added advantage that detrimental trace elements are removed by evaporation. Vacuum induction melting is commonly used because the inductive stirring encourages homogenization and helps expose more of the liquid to the melt-vacuum interface. This in turn optimizes the removal of undesirable gases and volatile impurities.

Many alloys are then vacuum arc remelted in order to achieve a higher purity and better solidification micro-structure. The ingot is made an electrode. An arc burns in the vacuum, thereby heating the front end of the electrode. Droplets are formed which then trickle through the vacuum and become purified. The molten metal is contained by a water-cooled copper mold. There is a liquid pool where further purification occurs by the flotation of solid impurities. The solidified metal has a desirable directional-micro-structure.

The diagram for electroslag refining looks similar to that for vacuum arc remelting, except that the melt pool is covered by a 10 cm thick layer of slag (lime, alumina and flourite). The ingot is again an electrode in contact with the slag. The slag has a high electrical resistivity and hence melts, the temperature being in excess of the melting point of the metal electrode. The tip of the electrode melts, allowing metal to trickle through the slag into the liquid sump at the bottom. This refines the alloy.

It is common for alloys destined for critical applications to go through two or more of these melting processes.

Casting of Blades

Nickel based superalloy blades are generally made using an investment casting process. A wax model is made, around which a ceramic is poured to make the mold. The wax is removed from the solid ceramic and molten metal poured in to fill the mold. The actual process is more complicated because of the intricate shape of the blade, with its cooling channels and other features.