Superalloy

A superalloy, sometimes called a heat-resistant superalloy or a high-performance alloy, is an alloy with the ability to operate at a high fraction of its melting point. Key characteristics of a superalloy include mechanical strength, thermal creep deformation resistance, surface stability, and corrosion and oxidation resistance.
The crystal structure is typically face-centered cubic austenitic. Examples of such alloys are Hastelloy, Inconel, Waspaloy, Rene alloys, Incoloy, MP98T, TMS alloys, and CMSX single crystal alloys. They are broadly grouped into three families: nickel-based, cobalt-based, and iron-based.
Superalloy development relies on chemical and process innovations. Superalloys develop high temperature strength through solid solution strengthening and precipitation strengthening from secondary phase precipitates such as gamma prime and carbides. Oxidation or corrosion resistance is provided by elements such as aluminium and chromium. Superalloys are often cast as a single crystal in order to eliminate grain boundaries, trading in strength at low temperatures for increased resistance to thermal creep.
The primary application for such alloys is in aerospace and marine turbine engines. Creep is typically the lifetime-limiting factor in gas turbine blades.
Superalloys have made much of very-high-temperature engineering technology possible.

Chemical development

Because these alloys are intended for high temperature applications their creep and oxidation resistance are of primary importance. Nickel -based superalloys are the material of choice for these applications because of their unique γ' precipitates. The properties of these superalloys can be tailored to a certain extent through the addition of various other elements, common or exotic, including not only metals, but also metalloids and nonmetals; chromium, iron, cobalt, molybdenum, tungsten, tantalum, aluminium, titanium, zirconium, niobium, rhenium, yttrium, vanadium, carbon, boron or hafnium are some examples of the alloying additions used. Each addition serves a particular purpose in optimizing properties.
Creep resistance is dependent, in part, on slowing the speed of dislocation motion within a crystal structure. In modern Ni-based superalloys, the γ'-Ni₃ phase acts as a barrier to dislocation. For this reason, this γ;' intermetallic phase, when present in high volume fractions, increases the strength of these alloys due to its ordered nature and high coherency with the γ matrix. The chemical additions of aluminum and titanium promote the creation of the γ' phase. The γ' phase size can be precisely controlled by careful precipitation strengthening heat treatments. Many superalloys are produced using a two-phase heat treatment that creates a dispersion of cuboidal γ' particles known as the primary phase, with a fine dispersion between these known as secondary γ'. In order to improve the oxidation resistance of these alloys, Al, Cr, B, and Y are added. The Al and Cr form oxide layers that passivate the surface and protect the superalloy from further oxidation while B and Y are used to improve the adhesion of this oxide scale to the substrate. Cr, Fe, Co, Mo and Re all preferentially partition to the γ matrix while Al, Ti, Nb, Ta, and V preferentially partition to the γ' precipitates and solid solution strengthen the matrix and precipitates respectively. In addition to solid solution strengthening, if grain boundaries are present, certain elements are chosen for grain boundary strengthening. B and Zr tend to segregate to the grain boundaries which reduces the grain boundary energy and results in better grain boundary cohesion and ductility. Another form of grain boundary strengthening is achieved through the addition of C and a carbide former, such as Cr, Mo, W, Nb, Ta, Ti, or Hf, which drives precipitation of carbides at grain boundaries and thereby reduces grain boundary sliding.

Element	Composition range	Purpose
Ni, Fe, Co	50-70%	These elements form the base matrix γ phase of the superalloy. Ni is necessary because it also forms γ'. Fe and Co have higher melting points than Ni and offer solid solution strengthening. Fe is also much cheaper than Ni or Co.
Cr	5-20%	Cr is necessary for oxidation and corrosion resistance; it forms a protective oxide CrO.
Al	0.5-6%	Al is the main γ' former. It also forms a protective oxide AlO, which provides oxidation resistance at higher temperature than CrO.
Ti	1-4%	Ti forms γ'.
C	0.05-0.2%	MC and MC carbides are the strengthening phase in the absence of γ'.
B,Zr	0-0.1%	Boron and zirconium provide strength to grain boundaries. This is not essential in single-crystal turbine blades, because there are no grain boundaries.
Nb	0-5%	Nb can form γ'', a strengthening phase at lower temperatures.
Re, W, Hf, Mo, Ta	1-10%	Refractory metals, added in small amounts for solid solution strengthening. They are heavy, but have extremely high melting points.

Phase formation

Adding elements is usually helpful because of solid solution strengthening, but can result in unwanted precipitation. Precipitates can be classified as geometrically close-packed, topologically close-packed, or carbides. GCP phases usually benefit mechanical properties, but TCP phases are often deleterious. Because TCP phases are not truly close packed, they have few slip systems and are brittle. Also they "scavenge" elements from GCP phases. Many elements that are good for forming γ' or have great solid solution strengthening may precipitate TCPs. The proper balance promotes GCPs while avoiding TCPs.
TCP phase formation areas are weak because they:

have inherently poor mechanical properties
are incoherent with the γ matrix
are surrounded by a "depletion zone" where there is no γ'
usually form sharp plate or needle-like morphologies which nucleate cracks

The main GCP phase is γ'. Almost all superalloys are Ni-based because of this phase. γ' is an ordered L1, which means it has a certain atom on the face of the unit cell, and a certain atom on the corners of the unit cell. Ni-based superalloys usually present Ni on the faces and Ti or Al on the corners.
Another "good" GCP phase is γ. It is also coherent with γ, but it dissolves at high temperatures.
Phase Classification Structure Composition Appearance Effect
γ matrix disordered FCC Ni, Co, Fe and other elements in solid solution The background for other precipitates The matrix phase, provides ductility and a structure for precipitates
γ' GCP L1 Ni cubes, rounded cubes, spheres, or platelets The main strengthening phase. γ' is coherent with γ, which allows for ductility.
Carbide Carbide FCC m''C, mC, and mC string-like clumps, like strings of pearls There are many carbides, but they all provide dispersion strengthening and grain boundary stabilization.
γ GCP D0 NiNb very small disks This precipitate is coherent with γ'. It is the main strengthening phase in IN-718, but γ dissolves at high temperatures.
η GCP D0 NiTi may form cellular or Widmanstätten patterns The phase is not the worst, but it is not as good as γ'. It can be useful in controlling grain boundaries.
δ not close-packed orthorhombic NiNb acicular The main issue with this phase is that it's not coherent with γ, but it is not inherently weak. It typically forms from decomposing γ'', but sometimes it's intentionally added in small amounts for grain boundary refinement.
σ TCP tetrahedral FeCr, FeCrMo, CrCo elongated globules This TCP is usually considered to have the worst mechanical properties. It is never desirable for mechanical properties.
μ TCP hexagonal FeNb, CoTi, FeTi globules or platelets This phase has typical TCP issues. It is never desirable for mechanical properties.
Laves TCP rhombohedral coarse Widmanstätten platelets This phase has typical TCP issues. It is never desirable for mechanical properties.
Families of superalloys
Ni-based
History
The United States became interested in gas turbine development around 1905. From 1910-1915, austenitic stainless steels were developed to survive high temperatures in gas turbines. By 1929, 80Ni-20Cr alloy was the norm, with small additions of Ti and Al. Although early metallurgists did not know it yet, they were forming small γ' precipitates in Ni-based superalloys. These alloys quickly surpassed Fe- and Co-based superalloys, which were strengthened by carbides and solid solution strengthening.
Although Cr was great for protecting the alloys from oxidation and corrosion up to 700 °C, metallurgists began decreasing Cr in favor of Al, which had oxidation resistance at much higher temperatures. The lack of Cr caused issues with hot corrosion, so coatings needed to be developed.
Around 1950, vacuum melting became commercialized, which allowed metallurgists to create higher purity alloys with more precise composition.
In the 60s and 70s, metallurgists changed focus from alloy chemistry to alloy processing. Directional solidification was developed to allow columnar or even single-crystal turbine blades. Oxide dispersion strengthening could obtain very fine grains and superplasticity.

Phase	Classification	Structure	Composition	Appearance	Effect
γ	matrix	disordered FCC	Ni, Co, Fe and other elements in solid solution	The background for other precipitates	The matrix phase, provides ductility and a structure for precipitates
γ'	GCP	L1	Ni	cubes, rounded cubes, spheres, or platelets	The main strengthening phase. γ' is coherent with γ, which allows for ductility.
Carbide	Carbide	FCC	m''C, mC, and mC	string-like clumps, like strings of pearls	There are many carbides, but they all provide dispersion strengthening and grain boundary stabilization.
γ	GCP	D0	NiNb	very small disks	This precipitate is coherent with γ'. It is the main strengthening phase in IN-718, but γ dissolves at high temperatures.
η	GCP	D0	NiTi	may form cellular or Widmanstätten patterns	The phase is not the worst, but it is not as good as γ'. It can be useful in controlling grain boundaries.
δ	not close-packed	orthorhombic	NiNb	acicular	The main issue with this phase is that it's not coherent with γ, but it is not inherently weak. It typically forms from decomposing γ'', but sometimes it's intentionally added in small amounts for grain boundary refinement.
σ	TCP	tetrahedral	FeCr, FeCrMo, CrCo	elongated globules	This TCP is usually considered to have the worst mechanical properties. It is never desirable for mechanical properties.
μ	TCP	hexagonal	FeNb, CoTi, FeTi	globules or platelets	This phase has typical TCP issues. It is never desirable for mechanical properties.
Laves	TCP	rhombohedral		coarse Widmanstätten platelets	This phase has typical TCP issues. It is never desirable for mechanical properties.