When is a crystal not a crystal? When it’s a quasi-crystal, a paradoxical form of metal recently found in some 3D printed metal alloys by [A.D. Iams et al] at the American National Institute for Standards and Technology (NIST).
As you might remember from chemistry class, crystals are made up of blocks of atoms (usually called ‘unit cells’) that fit together in perfect repetition — baring dislocations, cracks, impurities, or anything else that might throw off a theoretically perfect crystal structure. There are only so many ways to tessellate atoms in 3D space; 230 of them, to be precise. A quasicrystal isn’t any of them. Rather than repeat endlessly in 3D space, a quasicrystal never repeats perfectly, like a 3D dimensional Penrose tile. The discovery of quasicrystals dates back to the 1980s, and was awarded a noble prize in 2011.
Penrose tiling– the pattern never repeats perfectly. Quasicrystals do this in 3D. (Image by Inductiveload, Public Domain)
Quasicrystals aren’t exactly common in nature, so how does 3D printing come into this? Well, it turns out that, quite accidentally, a particular Aluminum-Zirconium alloy was forming small zones of quasicrystals (the black spots in the image above) when used in powder bed fusion printing. Other high strength-alloys tended to be very prone to cracking, to the point of usability, and this Al-Zr alloy, discovered in 2017, was the first of its class.
You might imagine that the non-regular structure of a quasicrystal wouldn’t propagate cracks as easily as a regular crystal ..
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