Now Showing in 3D: Quasicrystal Growth and Dissolution

Now Showing in 3D: Quasicrystal Growth and Dissolution

Isochrones of the solid-liquid interface in the ten-fold plane of a single Al-Co-Ni decagonal quasicrystal during growth (left) and dissolution (right). (Scale bar: 50 µm, with 40 s increments)

Assistant Professor Ashwin Shahani and his team recently captured the real-time growth and dissolution of decagonal (2D) and icosahedral (3D) quasicrystals. Quasicrystals are structures that exhibit long-range order and non-crystallographic symmetry. Hence, quasicrystals do not have repeating unit cells, making this the main difference between quasicrystals and conventional, periodic crystals. The emergence of quasicrystalline structure from a parent liquid phase has been stimulating the curiosity of researchers worldwide since their discovery in the 1980s. Even so, the growth of quasicrystals remains a mystery. This is due to the lack of experimental investigations with which to test the various theories of quasicrystal formation.

To fill in the gaps in our understanding, Professor Shahani and his team of researchers conducted real-time X-ray imaging experiments on a stable decagonal quasicrystal (in the Al-Co-Ni system) and an icosahedral quasicrystal (in the Al-Pd-Mn system) at the Advanced Photon Source in Argonne National Laboratory (Lemont, Illinois). In situ and high flux X-ray tomography technique was employed to visualize the morphological evolution in real time at temperature during slow cooling. By virtue of the high spatial and temporal resolutions afforded by the synchrotron experiment, Professor Shahani and his team were able to observe the growth, equilibration, and dissolution of a single quasicrystal from the parent liquid phase and keep track of the transient morphological evolution.

Interestingly, the ten-fold symmetry of the 2D decagonal quasicrystal was maintained during growth, which indicates nearly the same growth rate of the ten facets belonging to the decagonal quasicrystal (see image at left). In this mobility-limited regime, growth occurs via cluster attachments and rearrangements at the growth front. However, the ten-fold symmetry was broken upon dissolution (right). Therefore, the growth and melting processes of the decagonal quasicrystal do not have time-reversal symmetry, and quasicrystal melting is not locally-controlled. In addition, Professor Shahani and his team derived the kinetic coefficient of the Al-Co-Ni decagonal quasicrystal, which is proportional to the growth rate of crystals, during the growth experiment. The kinetic coefficient of decagonal quasicrystal was calculated based on analysis of X-ray attenuation through the sample as solidification proceeds. The calculated kinetic coefficient of the Al-Co-Ni decagonal quasicrystal was significantly smaller than kinetic coefficients of periodic crystals (both simple and intermetallic). The smaller kinetic coefficient points to the presence of large attaching clusters or “building blocks” that contribute to a sluggish growth rate. This observation supports the theory of cluster-based quasicrystal growth.

The morphology of quasicrystals during growth may be a function of the supersaturation.  For instance, Professor Shahani and his team also investigated the growth of a 3D icosahedral quasicrystal and concluded that the growth shape of the icosahedral quasicrystal (a pentagonal dodecahedron) differed from its equilibrium shape (a truncated dodecahedron). Near-equilibrium is defined when the interfacial velocity of each facet is nearly zero. This study provided the first experimental evidence that the two shapes are not necessarily the same, consistent with theories of faceting in bond-oriented solids. 

Shahani (right) and graduate student Insung Han (left) discuss cluster attachments on an atomic model of a 2D decagonal quasicrystal.

“Aperiodic short-range order transcends quasicrystals.  For instance, metallic glasses are also built up of icosahedral clusters,” said Shahani.  “Thus, the key insights gained in these works may also generally advance our understanding of glass formability and complex crystallization.  Our next steps are to identify the atomic origins of cluster attachments leading to quasicrystal growth.” 

The papers regarding the quasicrystal growth were recently published in Scientific Reports and Scripta Materialia.

Story by Insung Han, Materials Science and Engineering