• Physics 18, 171
Large-scale obstacles to crystal growth can throw the whole lattice off kilter, but quasicrystals can accommodate them without losing their atomic-scale order.
When a growing crystal encounters an obstacle, the orderly array of atoms may have to adjust in ways that create lattice defects or large-scale rearrangements. But a research team has found through experiments that peculiar materials called quasicrystals can take such disruptions in stride [1] The quasicrystalline lattice, which is orderly but not periodic, can accommodate obstacles without sacrificing its order, thanks to a type of rearrangement unique to quasicrystals. The work suggests the possibility of making quasicrystalline metal alloys that are more durable than conventional alloys.
Quasicrystals, discovered in 1984, are typically compounds composed of metals such as aluminum, nickel, and manganese. X-ray diffraction seems to show that their atomic lattices have symmetries that aren’t permitted in conventional crystals, such as pentagonal or decagonal symmetry. But these symmetries can exist in small regions because quasicrystals are not conventional crystals—you can’t shift the atomic lattice in space and then superimpose it exactly on the original lattice.
If a regular crystal encounters something that interferes with its orderly lattice as it grows—for example, an impurity atom—the disruption of the periodicity can propagate long distances through the crystal. That can lead to large-scale defects such as dislocations or grain boundaries between lattices with different orientations. Such defects can act as weak spots that are vulnerable to failure.
Previous work has shown that, because quasicrystals don’t have perfect periodic order, they can adjust to such disruptions by rearranging their lattice locally, with no longer-range knock-on consequences [2, 3]. Sharon Glotzer and colleagues at the University of Michigan–Ann Arbor wondered how far this accommodation in quasicrystals can be pushed. Could it allow the growing material to adapt its structure even to large, extended obstacles?
Quasicrystal structures are often described in terms of geometric “tiles” (say, with atoms located at the corners) packed together in two dimensions. For example, one can place two types of rhombus-shaped tiles so that they fill up a surface without gaps, and this pattern produces the “forbidden” symmetries of quasicrystals. If one of these tiles is forced to change its orientation—say, by an impurity atom in the lattice—just the rearrangement of a few neighboring tiles may suffice to fit the new structure. That collective reshuffling of the atoms comprising the tiles is called a phason. Because of phasons, “quasicrystals have a kind of structural flexibility that normal crystals do not—they can adjust their arrangement to locally fix mismatches, absorbing disruptions or strains,” says team member Ashwin Shahani.
To study how big a disruption phasons could accommodate, the Michigan team looked at the effect of 10-µm-diameter pores threading through a decagonal quasicrystal of aluminum, cobalt, and nickel (Al79Co6Ni15). Such pores are often created when metals and alloys cool from the liquid state, as they relieve the strain that results from the smaller volume of the solid relative to the liquid. The researchers observed the quasicrystal using x-ray microtomography, a technique that combines x-ray images of a sample positioned in many different orientations to create a 3D picture.
They found that as the quasicrystals grew, the pores could be accommodated without any signs of flaws or defects. “We saw the crystal front smoothly wrapping around the pores, with no persistent ‘dents’ or irregularities left behind, suggesting that any internal disruptions are quickly resolved,” Shahani says.
In molecular-dynamics simulations, Glotzer’s team found that growth around a pore initially creates a defect where the two growth fronts collide on the far side of a pore. But phason-type rearrangements can rapidly “heal” such disruptions.
“The self-healing behavior we observed suggests that quasicrystals could, in theory, form materials that are more tolerant of obstacles like pores,” Shahani says. Such obstacles “are often unavoidable in large-scale casting and manufacturing.” He says this could make them useful in applications requiring high durability in harsh conditions, such as exposure to mechanical wear or corrosion. He adds that phason-driven healing might take place even after the material has solidified, though such healing would be slower.
Michael Schmiedeberg of the University of Erlangen-Nuremberg in Germany says that the work provides an “important major step” in showing how phasons, previously known to be able to repair local defects incurred during growth, can also perform this role for very large disruptions and obstacles. “The resulting growth and repair mechanisms could lead to material properties that might become important for applications, such as adaptive or even self-healing materials,” he says.
–Philip Ball
Philip Ball is a freelance science writer in London. His latest book is How Life Works (Picador, 2024).
References
- K. L. Wang et al., “Defect-free growth of decagonal quasicrystals around obstacles,” Phys. Rev. Lett. 135, 166203 (2025).
- K. Nagao et al., “Experimental observation of quasicrystal growth,” Phys. Rev. Lett. 115, 075501 (2015).
- M. Schmiedeberg et al., “Dislocation-free growth of quasicrystals from two seeds due to additional phasonic degrees of freedom,” Phys. Rev. E 96, 012602 (2017).
