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

The $6,000 single-crystal blade as the showcase application of investment casting. Tom walks the full chain: ceramic-shell mold → polycrystalline cast → temperature-gradient furnace → pigtail seed-crystal selection → CAT-scan wall-thickness inspection. Used to teach how casting can produce the most thermodynamically demanding part in modern engineering.

[Tom holds up a single-crystal turbine blade, cut in section.] And that's how we start to make these $6,000 turbine blades. When they do it, they have a little ceramic mold the shape of the inside of this thing — this one's been cut in two so you can see inside. When Pratt and Whitney sold this, it would have been a solid part. They make it by investment casting and they end up with a polycrystalline grain.

Physical demonstration. A 20-year-old single-crystal turbine blade ("no grain boundaries"), worth $6,000–$7,000, ~100 per wheel. Used to motivate the grain-boundary discussion that frames Backofen vs. Grant's research programs.

What was happening: Professor Backofen in the '60s was looking at the effects of fine grain size. Professor Grant was looking at the effects of no grain size, or very large grains, because he was a high-temperature materials person. He was the person looking at things like turbine blades for jet engines. [Tom holds up a turbine blade.] This turbine blade goes back 20 years. It's a single crystal, no grain boundaries. Grain boundaries destroy the creep high-temperature properties. The grain boundaries are like butter and the thing just slides and deforms. If this was a good blade it would be worth about $6,000 or $7,000, and there's a hundred of them on every wheel of every turbine engine. That's why the engines cost five or ten million bucks. Grant was looking at how to get rid of grain boundaries in structural materials for high-temperature properties. Backofen was going in the opposite direction, looking at how to get very fine grain size, and how things deformed. We'll talk about that.

$6,000/blade × 100 blades/disc = $600K replacement; engine companies make 40% of profit on 20% of volume from blades/vanes.

They don't oxidize. When I was 19 years old, between my freshman and sophomore years, I got a job working in the Naval Air Rework Facility in Norfolk, Virginia rebuilding engines as an engineering student that summer. The TF30 engines were coming back from Vietnam, and we had to rebuild them and send them back. The engines had 500 hours on them — that was the lifetime of an engine, 500 hours, and you had to rebuild it every 500 hours. Today, 30,000 hours on a commercial engine before you have to rebuild it. So it's not just operating temperature, it's operating lifetime that has improved over this time, in terms of oxidation resistance and other things. From 1972 to 1990 or 1995, you're up to 30,000 hours. We went from better and better alloys to single crystals — single-crystal turbine blades that cost six thousand dollars a blade. You've got a hundred blades on a disc, so six hundred thousand dollars for a replacement set of blades on one disc in one engine. That's why the engines cost five or ten million dollars — it's the blades. Engine companies make maybe twenty percent of their volume on the blades, but forty percent of the profit is on the blades. The vanes are the most valuable part.