Nickel Based Superalloys
H. K. D. H. Bhadeshia
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.
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 microstructure, consisting of gamma (γ) and gamma-prime (γ'). It is the γ' which is largely responsible for the elevated-temperature strength of the material and its incredible resistance to creep deformation. The amount of γ' depends on the chemical composition and temperature, as illustrated in the ternary phase diagrams below.
NiAlTi phase diagram NiAlTi phase diagram
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-centred cubic.
By contrast, γ' has a cubic-P (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). 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 sublattices which leads to deviations from stoichiometry; alternatively, some of the nickel atoms might occupy the Al sites and vice-versa. In addition to aluminium and titanium, niobium, hafnium and tantalum partition preferentially into γ'.
Crystal structure of γ Crystal structure of γ'
The γ phase forms the matrix in which the γ' precipitates. Since both the phases have a cubic lattice with similar lattice parameters, the γ' precipitates in a cube-cube orientation relationship with the γ. This means that its cell edges are exactly parallel to corresponding edges of the γ phase. Furthermore, because their lattice parameters are similar, the γ' is coherent with the γ when the precipitate size is small. Dislocations in the γ nevertheless find it difficult to penetrate γ', partly because the γ' is an atomically ordered phase. The order interferes with dislocation motion and hence strengthens the alloy.
The small misfit between the γ and γ' lattices is important for two reasons. Firstly, when combined with the cube-cube orientation relationship, it ensures a low γ/γ' interfacial energy. The ordinary mechanism of precipitate coarsening is driven entirely by the minimisation of total interfacial energy. A coherent or semi-coherent interface therefore makes the microstructure 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 misfit is said to be positive when the γ' has a larger lattice parameter than γ. The misfit can be controlled by altering the chemical composition, particularly the aluminium to titanium ratio. A negative misfit stimulates the formation of rafts of γ', 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.
The transmission electron micrographs shown below illustrate the large fraction of γ', typically in excess of 0.6, in turbine blades designed for aeroengines, where the metal experiences temperatures in excess of 1000oC. Only a small fraction (0.2) of γ' is needed when the alloy is designed for service at relatively low temperatures (750oC) and where welding is used for fabrication.
Transmission electron micrograph showing a large fraction of cuboidal γ' particles in a γ matrix. Ni-9.7Al-1.7Ti-17.1Cr-6.3Co-2.3W at%. Hillier, Ph.D. Thesis, University of Cambridge, 1984.
Transmission electron micrograph showing a small fraction of spheroidal γ' prime particles in a γ matrix. Ni-20Cr-2.3Al-2.1Ti-5Fe-0.07C-0.005 B wt%. Also illustrated are M23C6 carbide particles at the grain boundary running diagonally from bottom left to top right.
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 containing γ', which essentially is an intermetallic 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 summarise, it is the presence of γ' which is responsible for the fact that the strength of nickel based superalloys is relatively insensitive to temperature.
The yield strength of a particular superalloy containing only about 20% of γ'. The points are measured and the curve is a theoretical prediction. Notice how the strength is at first 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 superalloys 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-centred 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
X-ray Powder Diffraction
A comparison of the diffraction patterns indicated below reveals many more peaks from the γ'. The additional reflections are quite weak in intensity. They arise because the γ' lattice is primitive cubic, which means that planes such as {100} give rise to diffracted intensity, whereas the reflections from the corresponding {100} planes of γ have zero intensity (destructive interference with the {200} planes). The additional reflections from the γ' prime are termed superlattice reflections and are weak because they depend on the difference in scattering power between the Ni and Al atoms.
X-ray diffraction pattern from γ, for a particular set of diffraction conditions.
X-ray diffraction pattern from γ', for a particular set of diffraction conditions.
Movies
* Crystal structure of γ, cubic-F lattice.
* Crystal structure of γ', cubic-P lattice.
* Crystal structure of γ'', body-centred tetragonal lattice.
Electron Diffraction
The figures below show a superimposed electron diffraction pattern from γ, γ' and M23C6 carbide. The γ and γ' phases have their cubic-lattice edges perfectly aligned.
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 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 superalloys. 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 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 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 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 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 superalloys is much reduced. Chromium does protect against oxidation, but oxidation can also be prevented by coating the blades.
Microstructure and Heat Treatment
nickel superalloys
