This is an interesting video GE put together for one of their latest gas turbine power generators. What got me at the end is how small the power generator is compared to the emission control equipment on the exhaust side. And since I haven't yet finished my first cup of coffee it had a little bit of the turbo-encabulator vibe going.

One of the really cool things that they mention is the single crystal superalloy turbine blades. These turbine blades are made of a nickel superalloy and the mold is designed so that the blade is composed of a single crystal.

If you take a look at the blade to the left it is an unfinished casting. The rectangular block and the spiral (called a pigtail) are not part of the finished blade. When the metal is poured into the mold the far end of the block is cooled first, so that is where the metal begins to solidify. As the metal solidifies it starts to form crystals that line up with the axis of the turbine blade. The pigtail basically chokes off all those parallel crystals so that only one crystal makes it into the turbine blade. Once the blade has completely solidified the spiral and block will get machined off of the end.

Here is a representation of what that looks like. In this picture each color represents a crystal. By the time the metal has solidified through the pigtail there is only a single crystal left in the finished part.

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Making the blades out of a single crystal gives them a high creep strength that is needed to withstand the high temperatures and loads that the blades see in the high pressure turbines. As a metal structure has a load applied to it when it is heated the metal will creep. This is a slow deformation over time. In a jet engine this will eventually lead to catastrophic failure if the creep allows the turbine blade to make contact with other parts.

To make this go planelopnik have a couple of Rafales aboard the USS Harry Truman. The Rafales use single crystal HPT blades.

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