When a molten material cools quickly, parts of it may have enough time to grow into orderly crystals. But if the cooling rate is too fast for the entire melt to crystallize, the remaining material ends up in a non-crystalline state known as a glass, with atoms caught in place essentially as a frozen liquid.
Recently, a group of researchers at the National Institute of Standards and Technology (NIST) came across an unexpected reversal of this usual sequence of events.
After cooling a molten alloy of aluminum, iron, and silicon, they found that glassy nodules of a non-crystalline solid phase formed first, growing slowly enough to organize and select some chemical species, rejecting other species into the surrounding melt that, on further cooling, coalesced into crystals.
Investigation of the new phase at the U.S. Department of Energy Office of Science's Advanced Photon Source (APS) suggests that it may be an example of a novel structure, theoretically possible but not seen until now, that is isotropic, with infinite rotational symmetry, but lacking any discrete translational symmetry.
Direct visual evidence (see the figure) indicates that the nodules formed first, followed by metallic crystals radiating around them. Closer inspection revealed that the radiating white bands are crystalline aluminum with small amounts of iron and silicon. The nodule composition is approximately Al13Fe3Si4.
The researchers concluded that the nodules formed by nucleation and displaced excess aluminum as they grew; eventually, the aluminum concentration in the surrounding material was high enough that the remaining melt crystallized, pushing residual iron and silicon into the gaps.
Electron microscopy showed that the material in the nodules is isotropic, leading the researchers to call it "q-glass," despite the fact that it does not form as glasses normally do.
To investigate its structure further, researchers from NIST and Argonne National Laboratory went to X-ray Science Division (XSD) beamline 1-ID-C at the Argonne APS to obtain high-energy x-ray scattering data with which to compare samples of the q-glass to two other known phases of Al-Fe-Si with composition similar to that of the q-glass: the crystalline cubic ?-phase and the quasicrystalline icosahedral phase.
Diffraction of 80-keV x-rays yielded data that the team analyzed using pair distribution function (PDF) methods, which show a series of peaks corresponding to distances between pairs of atoms.
