Will Giant Crystals Toughen Up Giant Turbines?
Ordinary metal alloys look uniform, but under the microscope they're a multicrystalline crazy quilt. That's because as molten alloys cool, crystals branch out from many points like frost on a window. At high temperatures, alloys are weakest where different crystals abut. To make stronger aircraft-engine parts, scientists at United Technologies Corp.'s Pratt & Whitney Co. in the late 1970s controlled the cooling of molten-nickel alloys so each part was a huge crystal grown from one "seed." That's now the norm for jet-engine parts.
Howmet Corp. in Greenwich, Conn., a unit of France's Pechiney that makes single-crystal parts for jet engines, hopes to use the technique for the much bigger blades and vanes used in power-generation turbines. Single-crystal parts would allow turbines to run hotter, improving efficiency. Blades in a big power turbine are a foot or longer, compared with just a few inches for a jet engine. That much molten metal tends to deform ceramic molds and cool unevenly, so multiple crystals form. Now, Howmet is developing rigid, more durable molds and better furnace controls that guide the crystal to grow from a single seed. It has made parts as long as two feet but not yet in large quantities.