This was a super fun read, and it felt nice to take the time to carefully read it to understand the very complex processes it was describing. That said, can someone explain how the pigtail accomplishes the following?: > "The crystals grow in a straight line in the direction that the mould is being withdrawn, but because of the pigtail’s twisted shape, all but the fastest-growing crystals are eliminated."
By the principle of elimination by bottleneck. As the crystals grow into the pigtail, the lagging crystals hit the already-formed leading ones or the mold surface, leaving a primary, single crystal grain to exit the pigtail and grow across the blade proper. This would be almost impossible to happen without the pigtail because multiple crystal grains would grow in parallel across the blade.
That's the basis of single-crystal casting. The solidification process relies on operating very close to the melting point of the material. The idea is to create an alloy mixture that's ready to crystallize, then starting the process by dropping in a crystal and letting the liquid crystallize in a very controlled way. The heated zone is moved through the liquid, as in zone refining (or the solid is pulled from the heated zone, or gas cooling is used) so that the crystal growth takes place in only a narrow layer.
This is decades old, and one of the reasons jet engines last a long time now.
[1] http://www.asminternational.org/documents/10192/6019788/amp1... [2]