text: In pure NiAl phase Al atoms are placed at the vertices of the cubic cell and form sublattice A. Ni atoms are located at centers of the faces and form sublattice B. The phase is not strictly stoichiometric. An excess of vacancies in one of the sublattices may exist, 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. The γ phase hardens the alloy through the yield strength anomaly. Dislocations dissociate in the γ phase, leading to the formation of an anti-phase boundary. 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 effectively locked. By this mechanism, the yield strength of γ phase NiAl increases with temperature up to about 1000 °C.
  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' Ni(Al,Ti) precipitates in a γ matrix, as well as various metal-carbon carbides (e.g. CrC) 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.
  Modern superalloys were developed in the 1980s. First generation superalloys incorporated increased Al, Ti, Ta, and Nb content in order to increase the γ volume fraction. Examples include: PWA1480, René N4 and SRR99. Additionally, the volume fraction of the γ precipitates increased to about 50–70% with the advent of monocrystal solidification techniques that enable grain boundaries to be entirely eliminated. Because the material contains no grain boundaries, carbides are unnecessary as grain boundary strengthers and were thus eliminated.
  Second and third generation superalloys introduce about 3 and 6 weight percent rhenium, for increased temperature capability. Re is a slow diffuser and typically partitions the γ matrix, decreasing the rate of diffusion (and thereby high temperature creep) and improving high temperature performance and increasing service temperatures by 30 °C and 60 °C in second and third generation superalloys, respectively. Re promotes the formation of rafts of the γ' phase (as opposed to cuboidal precipitates). The presence of rafts can decrease creep rate in the power-law regime (controlled by dislocation climb), but can also potentially increase the creep rate if the dominant mechanism is particle shearing. Re tends to promote the formation of brittle TCP phases, which has led to the strategy of reducing Co, W, Mo, and particularly Cr. Later generations of Ni-based superalloys significantly reduced Cr content for this reason, however with the reduction in Cr comes a reduction in oxidation resistance. Advanced coating techniques offset the loss of oxidation resistance accompanying the decreased Cr contents. Examples of second generation superalloys include PWA1484, CMSX-4 and René N5. 
  Third generation alloys include CMSX-10, and René N6. Fourth, fifth, and sixth generation superalloys incorporate ruthenium additions, making them more expensive than prior Re-containing alloys. The effect of Ru on the promotion of TCP phases is not well-determined. Early reports claimed that Ru decreased the supersaturation of Re in the matrix and thereby diminished the susceptibility to TCP phase formation. Later studies noted an opposite effect. Chen, et al., found that in two alloys differing significantly only in Ru content (USTB-F3 and USTB-F6) that the addition of Ru increased both the partitioning ratio as well as supersaturation in the γ matrix of Cr and Re, and thereby promoted the formation of TCP phases.
  The current trend is to avoid very expensive and very heavy elements. An example 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). 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.