`Single-crystal turbine blades (GE/Pratt & Whitney)`

Student-driven aside on why we don't grow single-crystal structural metal. Tom explains creep resistance motivation, growth rate (~1-2 mm/hour), $7,000 per blade, Pratt & Whitney / GE Christmas-tree casting trees vs. Rolls-Royce's small-furnace approach.

Student: A lot of small companies are working on crystal technologies, where they grow one crystal that is one single grain — the point being it's stronger and has special properties for electronics.

For electronics, and for high temperature in engines.

Student: Is it a stupid question to say, what about trying to create that within the metal? What prevents you from creating a single grain crystal in any arbitrary large size of metal?

We do it. I'll bring in my single-crystal turbine blade if I can remember. We make single-crystal turbine blades if you want creep resistance at high temperature, so it doesn't flow like Silly Putty does at room temperature. If you don't want it changing shape at high temperature, you want a single crystal. Fine grain size at elevated temperatures is the kiss of death. They make single crystals in silicon because grain boundaries destroy the electronic properties. I just bought some outdoor solar-powered lights for my house, and you can see the solar cells have grains that are an inch across. You can see them just looking at it. It's like a piece of galvanized steel bucket — you see the huge grains. Wherever you have a grain boundary, you're losing electrons, and you're not getting the solar efficiency. So you'd love single-crystal silicon for solar cells, but that's too expensive in general.

Student: It's cost-prohibitive to make large-volume single-grain crystal metal?

To make single-crystal metal, we grow it in a furnace at about a millimeter an hour, maybe two. It's slow. But when you're growing a turbine blade for $7,000, and you're growing twenty-four of them at a time — Rolls-Royce grows two at a time, but they have lots of small furnaces, whereas Pratt & Whitney and General Electric use two or three dozen on one sprue. It's like a Christmas tree with turbine blades on the end, you just break them all off.