FIGURE 18 Progress in temperature capability of nickel-base superalloys, due to compositional and process innovations. From Gell and Duhl.31 Reprinted with permission.

channel above the water-cooled solidification “starter block” permits only a single growing grain to pass through, normally with a >001< crystallographic direction advancing parallel to the longitudinal axis of the turbine blade. However, seeding also permits the selection of other orientations for the study of anisotropic properties.

Ordinarily, the optimal composition of superalloys is complicated by a menu of alloying elements to help stabilize the grain boundaries at elevated temperatures. Since this is not necessary with monocrystals, the compositional variations can be focused on property control of the base alloy—e.g., improvement of creep strength as a function of temperature. The progress along these lines has been truly remarkable. Figure 1831 indicates that the single-crystal processing of superalloy turbine blades has led to a quantum jump in high-temperature operating capability, and the way is now open to substantial further improvements, with corresponding benefits in engine performance and fuel efficiency.

MICROSTRUCTURAL REFINEMENT
Strength and Toughness

Notwithstanding the sharp advance in gas turbine technology resulting from the single-crystal processing of turbine blades, other metallurgical fronts will see a strong push toward finer and finer microstructures, both in grain size of the matrix phase and in particle size of the embedded precipitates. This is one of the promising aspects of RSP, as described earlier. Extending



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