Metallurgy of Superalloys
The microstructure of most precipitation strengthened nickel-base superalloys consists of the gamma matrix, and of intermetallic γ' precipitates. The γ-phase is a solid solution with a face-centered crystal (fcc) lattice and randomly distributed different species of atoms. By contrast, the γ'-phase has an ordered crystalline lattice of type LI2. Modern alloys typically contain about 70% by volume fraction of cube-like γ' precipitates whose edge length is about 0.5 μm.
In pure Ni3Al phase atoms of aluminium are placed at the vertices of the cubic cell and form the sublattice A. Atoms of nickel are located at centers of the faces and form the sublattice B. The phase is not strictly stoichiometric. There may exist an excess of vacancies in one of the sublattices, which leads to deviations from stoichiometry. Sublattices A and B of the γ'-phase can solute a considerable proportion of other elements. The alloying elements are dissolved in the γ-phase as well. The γ'-phase hardens the alloy through an unusual mechanism called the yield strength anomaly. Dislocations dissociate in the γ'-phase, leading to the formation of an anti-phase boundary. It turns out that at elevated temperature, the free energy associated with the anti-phase boundary (APB) is considerably reduced if it lies on a particular plane, which by coincidence is not a permitted slip plane. One set of partial dislocations bounding the APB cross-slips so that the APB lies on the low-energy plane, and, since this low-energy plane is not a permitted slip plane, the dissociated dislocation is now effectively locked. By this mechanism, the yield strength of γ'-phase Ni3Al actually increases with temperature up to about 1000 °C, giving superalloys their currently unrivalled high-temperature strength.
Initial material selection for blade applications in Gas Turbine engines included alloys like the Nimonic series alloys in the 1940s. The early Nimonic series incorporated γ' Ni3(Al,Ti) precipitates in a γ matrix, as well as various metal-carbon carbides (e.g. Cr23C6) at the grain boundaries for additional grain boundary strength. Turbine blade components were forged until vacuum induction casting technologies were introduced in the 1950s. This process significantly improved cleanliness, reduced defects, and increased the strength and temperature capability of the material.
Modern superalloys were developed in the 1980s with the advent of single crystal, or monocrystal, solidification techniques (see Bridgman technique) for superalloys that enable grain boundaries to be entirely eliminated from a casting. Because the material contained no grain boundaries, carbides were unnecessary as grain boundary strengthers and were thus eliminated. Additionally, the volume fraction of the γ' precipitates increased to about 50-70%. The first generation superalloys incorporated increased Aluminium, Titanium, Tantalum, and Niobium content in order to increase the γ' volume fraction in these alloys. Examples of first generation superalloys include: PWA1480, René N4 and SRR99.
The second and third generation superalloys introduced about 3 and 6 weight per cent Rhenium, respectively for increased temperature capability. Examples of second generation superalloys include PWA1484, CMSX-4 and René N5. Third generation alloys include CMSX-10, and René N6. Fourth, Fifth, and even Sixth generation superalloys have been developed which incorporate Ruthenium additions, making the already costly Re-containing alloys more expensive.
The current trend is to avoid very expensive and very heavy elements. A possible remedy to this is Eglin steel, a budget material with compromised temperature range and chemical resistance. It does not contain rhenium or ruthenium and its nickel content is limited. To reduce fabrication costs, it was chemically designed to melt in a ladle (though with improved properties in a vacuum crucible). Also, conventional welding and casting is possible before heat-treatment. The original purpose was to produce high-performance, inexpensive bomb casings, but the material has proven widely applicable to structural applications, including armor.
In addition, it is often beneficial for grain boundaries that the nickel-base superalloy contains carbides (or boron or zirconium) for improvements in creep strength. Where the carbides (e.g. MC where M is a metal and C is a carbon atom) are precipitated at the grain boundaries, they act to pin the grain boundaries and improve the resistance to sliding and climbing and migration that would occur during creep diffusion. However if they precipitate as a continuous grain boundary film, the fracture toughness of the alloy may be reduced, together with the ductility and rupture strength.
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