tim's journal

Single-crystal superalloy turbine blades

I met Morgan, one of Professor Melosh's students, this morning. In his prior life at Georgia Tech, he did a project on modeling fatigue in superalloy turbine blades at GE. Amazingly, they can now make 30-lb single crystal nickel-based superalloy turbines. These are used in generators and are expected to have useful lives exceeding 40 years. The secret of making such large single crystals is in the turn. You start with a number of crystal nuclei and grow these in the same direction. When they hit a turn, the fastest growing crystals grow faster, since they have first access to the media. Eventually, if you turn enough times, there will only be one. You then grow this into a mold. Cool huh? This corkscrew mechanism is called a grain selector. Here's a description from an article on monitoring growth using x-ray diffraction:

During casting experiments, a metal alloy charge is placed in the crucible of the mold. After a 1 Pa vacuum has been established, the gate valve is opened, and the ram raises the mold into the hot zone of the furnace. The alloy melts, filling the mold. Solidification of polycrystals initiates at the base of the mold in the starter block where the alloy contacts the water-cooled ram. The fastest-growing grain reaches the grain selector (a corkscrew-shaped section of the mold) first and blocks growth of all other crystals with differing orientation. If the thermal conditions are correct, growth of a single-crystal continues upward in the mold as the mold is slowly withdrawn (150 mm/h) vertically from the furnace. Polished and etched sections of the completed castings showed a dendritic structure with several phases and an absence of grain boundaries.

I wonder if this technique is being used to grow holographic crystals. Turbine blades like the ones I talked about are used in power plants and in jet planes.