(651f) Emergence of Faceted Spiral Patterns during Eutectic Crystallization

Engineered self­‑organization, including nonlinear pattern formation in crystallization, presents a bottom‑up approach to design new multi‑phase materials with potentially exotic morphologies and superior properties. Particularly appealing are eutectic systems – mixtures of two or more distinct solid phases that emerge from a parent liquid phase – which exhibit outstanding electromechanical properties because their microstructures act as natural composite materials. Eutectics comprise a wide range of pattern‑forming systems, dictated by a number of factors including material properties and growth conditions. In only a few documented cases, the solid phases arrange in intricate spiraling patterns, somewhat akin to a DNA helix. The intrinsic chirality of spiral eutectics offers a new strategy for rapid fabrication – or templates for additive manufacturing – of large‑area photonic crystals. Unfortunately, our progress is hindered largely by the lack of mechanistic understanding of spiral eutectic crystallization.

Herein, we demonstrate the two‑step formation pathway of faceted spiral eutectics upon directional solidification in the Zn‑Mg alloy system. These two‑phase Zn‑MgZn₂ microstructures are periodic, metastable, and intrinsically chiral. We trace the emergence of such structures from the parent liquid through a correlative and multiscale investigation encompassing 3D measurements – namely X‑ray nano‑tomography aided by machine learning and electron backscatter diffraction – along with in situ synchrotron X‑ray diffraction and atomic‑resolution electron microscopy. Altogether, the results reveal the morphological and heteroepitaxial relationships between the eutectic phases and the origin of spiral growth. The results also identify a thus‑far neglected nucleation mechanism involving crystallographic defects within 'hidden' polytetrahedral phases, which provide a template for spiral eutectic crystallization. Our findings provide the necessary benchmark data for simulations of complex crystallization patterns, thus expanding the horizon for the bottom‑up design of next‑generation alloys.