Peering Inside the Solidification Process of Metals

X-ray tomography and 3D EBSD help researchers better understand the formation of complex microstructures through crystallization

The complex pathway by which metals and alloys form solids via crystallization is still not fully understood by the scientists and engineers who have studied these materials for decades.

Dr. Ashwin Shahani and his group at the University of Michigan, USA, leverage a wide range of synchrotron- and laboratory-based methods, including X-ray tomography and diffraction, to better understand the growth dynamics of crystals.

In an article published in the journal Small, Dr. Shahani and his team investigate the origins of certain eutectics that grow into a unique and fascinating spiral geometry.

Saman Moniri, ChE PhD Student, and Ashwin Shahani, Assistant Professor of Materials Science and Engineering, prepare a sample of a spiral structure in the G. G. Brown Building on North Campus of the University of Michigan in Ann Arbor, MI on January 22, 2020.

MSE Assistant Professor Ashwin ShahaniÕs group is working to map out the process of creating spiral structures in materials. This could lead to a wide range of new applications in optical and mechanical applications. 

Photo: Joseph Xu/Michigan Engineering, Communications & Marketing

Dr. Ashwin Shahani (right) and his graduate student, Saman Moniri, preparing metal alloy samples for advanced multimodal characterization.

Why research spiral eutectics? What are the applications of such materials?

Eutectics in general are in situ composite materials (mixtures of two or more phases) that offer significant improvements in mechanical and optical properties as compared to monolithic materials. The unique properties of the eutectic stem from the self-organization of the phases into rods, lamellae, labyrinths, and even spirals.  It has been widely recognized that the formation of the latter (from a parent liquid phase) is poorly understood, with a number of competing proposals.

My philosophy is – understanding the solidification of microstructure is the key to controlling it.  

X-ray tomography was used to investigate the morphology of the Zn-Mg 3D spirals. Could you describe your findings and explain the need for including this technique?

X-ray tomography using ZEISS Xradia 810 Ultra provided a wealth of information on the morphology of the spirals in 3D. It was the only technique that offered a sufficient spatial resolution (down to tens of nanometers) to discriminate between the two phases in the eutectic microstructure. Using X-ray tomography we learned that the spirals are hierarchical structures, organized into “cones” where the two eutectic phases wrap together inside the cone. Past work in the literature suggests that spirals are in the shape of DNA helices, but our work indicates that this is not necessarily the case – thus broadening our understanding of the complexity of such structures.

Left: Hrishikesh Bale from ZEISS loading a sample into the ZEISS Xradia Ultra X-ray microscope. Right: 3D rendering of the reconstructed nanoscale X-ray tomography dataset obtained from a Zn-Mg spiral eutectic sample. A few spirals can clearly be seen on the surface. The backdrop is an SEM image of the sample showing clusters of such spirals.

In your publication, 3D EBSD was used to generate a 3D orientation map. What were you able to learn from this?

Here too, 3D EBSD generated with ZEISS Crossbeam FIB-SEM allowed us to validate one of the hypotheses regarding spiral formation. We learned that the eutectic phases are single crystals within each cone, which goes against one viewpoint from the literature, that spiraling arises from rotations in crystal structure. We were also able to quantify fully the crystallography of the spirals in 3D, bringing us one step closer to discovering their growth mechanism. 

Integrated EDS and EBSD mapping in 3D carried out using a fully automated ZEISS ATLAS 5 3D workflow on the Crossbeam 550 FIB-SEM. EDS mapping provides elemental information and EBSD provides the local crystallographic orientation. 3D-EBSD/EDS datasets generated by Tobias Volkenandt, ZEISS.

Were any of your results particularly surprising or interesting? How so?

Great Q — almost everything was a surprising inversion of our conventional wisdom!  Through integrated, multimodal imaging we were able to trace the origin of a spiral to a single defect (a screw dislocation).  That such defects can exert such a strong influence on eutectic nucleation has never been seen before, and offers some fresh ideas for how we can design large-area spiral patterns for technological applications.  

3D rendered movie of the spiral structure observed in Zn-Mg eutectics. Video highlights one specific faceted spiral domain (yellow-red scheme) with a pyramidal geometry. All other spirals encountered within the volume showed a consistent geometry, but had randomly oriented central pyramidal axes.

Learn More

Read Dr. Shahani’s publication: “Multi‐Step Crystallization of Self‐Organized Spiral Eutectics

Get more information on the ZEISS technology used in this publication:

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