On Friday, I wrote about the development of amorphous or “glassy” metal alloys, in which the atoms are packed together with no regular crystal structure. At the microscopic level, almost all metals are made of crystal grains, which can be bigger or smaller depending on how the metal has been heat treated. Amorphous alloys, recall, are extremely unusual among metals because they can be cast without forming crystal grains at all.
At the opposite end of the spectrum are single crystal (SC or SX) alloys, which are mixtures of metals that can be cast in such a way that the entire object is essentially a single giant “grain,” i.e. one continuous crystal. Among minerals like quartz, large objects composed of a single crystal are fairly common; you also may have seen the large ingots of artificially-grown single-crystal silicon, known as “boules,” from which wafers are cut to make microchips and other semiconductor devices.
But when it comes to metals, single-crystal objects are outside of most people’s experience. Forming single-crystal metal objects requires both special alloys and special casting techniques. The alloys are almost always nickel-based, with as many as nine minor metal components including five or more percent chromium, cobalt, tungsten, tantalum, aluminum, and/or rhenium. The casting method is known as “directional solidification,” and involves carefully cooling a cast metal part starting at one end to guarantee a particular orientation of its crystal structure. That orientation is chosen, naturally, based on expected stresses in the finished part.
The primary application for single crystal superalloys is the manufacture of jet engine turbine blades, which must endure tremendous forces at extremely high temperatures for prolonged periods of time. Under such conditions, metals with a grain structure tend to “creep,” or slowly deform, along grain boundaries. Because single-crystal alloy parts have no grain boundaries, however, they are highly resistant to this kind of wear.
If you’re interested in reading more, check out the excellent online primer on nickel-based superalloy technology maintained by The University of Cambridge’s Dr. Harry Bhadeshia.
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