Superalloy - Process Development

Process Development

The historical developments in superalloy processing have brought about considerable increases in superalloy operating temperatures. Superalloys were originally iron based and cold wrought prior to the 1940s. In the 1940s investment casting of cobalt base alloys significantly raised operating temperatures. The development of vacuum melting in the 1950s allowed for very fine control of the chemical composition of superalloys and reduction in contamination and in turn led to a revolution in processing techniques such as directional solidification of alloys and single crystal superalloys.

Within gas turbine engines many forms of superalloys are present. Polycrystalline Ni-base superalloys are used for the disks of the high pressure turbine, which can be created using powder metallurgy or casting technology. Turbine blades can be polycrystalline, have a columnar grain structure, or be a single crystal. Polycrystalline blades are formed using casting technology into a ceramic mold. Columnar grain structured blades are created using directional solidification techniques and have grains parallel to the major stress axes.

Single-crystal superalloys (SX or SC superalloys) are formed as a single crystal using a modified version of the directional solidification technique, so there are no grain boundaries in the material. The mechanical properties of most other alloys depend on the presence of grain boundaries, but at high temperatures, they would participate in creep and must be replaced by other mechanisms. In many such alloys, islands of an ordered intermetallic phase sit in a matrix of disordered phase, all with the same crystalline lattice. This approximates the dislocation-pinning behavior of grain boundaries, without introducing any amorphous solid into the structure.