To optimise properties (often of a coating--metal system), nickel based superalloys are, after solution treatment, heat treated at two different temperatures within the γ/γ' phase field. The higher temperature heat treatment precipitates coarser particles of γ'. 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 of γ'. The net result is a bimodal distribution of γ', as illustrated in this figure (courtesy R. J. Mitchell).
The solution heat treatment temperature determines not only the amount of γ' that dissolves, but also the grain size of the γ. The size becomes coarser if all the γ' is dissolved, since there is then no pinning effect of the precipitate particles on the movement of the γ/γ boundaries. The picture on the left has been heat treated at a sub-solvus temperature, that on the right at a super-solvus temperature. (Image courtesy of R. J. Mitchell).
The The three-dimensional shape of the grains has recently been determined by Michael Uchic.
nickel superalloys nickel superalloys
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. A transmission electron micrograph showing the oxide dispersion in a mechanically-alloyed nickel based superalloy is shown below.
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 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., localised 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 circumstances if its temperature exceeds 400oC.
Single crystal
Directionally solidified
columnar grains
Equiaxed polycrystalline
Engine materials (source: Michael Cervenka)
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.
superalloy disc
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 atomisation in an inert gas; the extent of chemical segregation cannot exceed the size of the powder. After atomisation, 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.
Powder metallurgical aeroengine disc. Image provided by M. Hardy of Rolls-Royce.
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.
turbocharger
Turbocharger of nickel based superalloy Inconel 713C, Ni-2Nb-12.5Cr-4.2Mo-0.8Ti-6.1Al-0.12C-0.012B-0.1Zr wt%.
Melt Processing
The superalloys contain reactive elements such as aluminium and titanium. It is necessary therefore to melt the alloys under vaccum, with the added advantage that detrimental trace elements are removed by evaporation. Vaccum induction melting is commonly used because the inductive stirring encourages homogenisation and helps expose more of the liquid to the melt-vaccum interface. This in turn optimises the removal of undesirable gases and volatile impurities.
Many alloys are then vaccum arc remelted in order to achieve a higher purity and better solidification microstructure. The ingot is made an electrode (a). 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 mould. There is a liquid pool (b) where further purification occurs by the floatation of solid impurities. The solidified metal (c) has a desirable directional-microstructure.
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 mould. The image on the left shows a wax assembly for making the mould used in the casting of single-crystal nickel based superalloy turbine blades. The wax is removed from the solid ceramic and molten metal poured in to fill the mould. The actual process is more complicated because of the intricate shape of the blade, with its cooling channels and other features. The image on the right shows the ceramic mould for the simultaneous casting of several blades. The metal is poured in from the top and feeds into the blade sections from the bottom.
More information on nickel based superalloys.
Photograph of "superalloy", courtesy Franck Tancret.
Acknowledgment
I am grateful to Sammy Tin for arranging access to several of the micrographs.
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