FIGURE 17 Single-crystal solidification process for casting superalloy turbine blades to eliminate grain boundaries. From Gell, Duhl, and Giamei.³⁰ Reprinted with permission.

The use of single crystals in metallurgical research and in microelectronic devices is well known, but it was only in 1982 that monocrystalline alloys first entered service as a critical structural component. It was a high-technology application in every sense of the word, following two decades of research and development on cast superalloy turbine blades.^29,30 As a result, single-crystal gas-turbine blades are now performing advantageously in both commercial and military aircraft engines.³⁰

There are high-temperature applications in which the grain boundaries and random grain orientations of polycrystalline superalloys are undesirable, in part because of thermal fatigue failure and creep rupture along such interfaces. This motivated the development of directional solidification during the 1970s to produce columnar grains with controlled orientation and with grain boundaries parallel to the main stress direction. The next major step was to eliminate the grain boundaries entirely; this was made possible and practical by the single-crystal casting process shown schematically in Figure 17.³⁰ The helical