Complexity and Evolution of a Three-Phase Eutectic Pattern Uncovered by 4D X-Ray Nano-Tomography

Exploiting the microstructure of multi-phase materials is critical to develop advanced materials with novel properties [1-2]. It is especially important to evaluate the microstructure evolution at elevated temperatures to understand the stability of hetero-interfaces. Currently, 2D characterization of such complex and interconnected microstructures provides a limited understanding of the underlying structure and dynamics of multi-phase systems upon coarsening in three dimensions.<br/><br/>To overcome these challenges, we analyze in 4D (three spatial dimensions plus time) the crystallographic orientation of hetero-interfaces in a multi-phase eutectic. To our knowledge, this is the first-ever assessment of the time-evolution of interfacial crystallography in 3D and at the nanoscale. We accomplish this feat by fusing data from synchrotron-based X-ray nano-tomography (TXM) and electron backscatter diffraction (EBSD). As a proof-of-concept of our approach, we examine the dynamics of an Al-Ag₂Al-Al₂Cu three-phase eutectic during isothermal annealing. With the aid of an innovative total variation regularization tomographic reconstruction algorithm and machine learning based segmentation, we identify conclusively the coexisting coarsening mechanisms [3] within this three-phase eutectic. Despite the fact that the length-scales increase with the cube-root of time, we find the microstructure is not self-similar. Based on our fused dataset, we observe instead a preferential alignment of hetero-interfaces towards low misfit habit planes as annealing proceeds. Ultimately, a competition may develop between neighboring hetero-interfaces with varying misfits, and hence, interfacial energies. The results uncover the hidden role of topology in determining which hetero-interfaces will survive and which others will recede and disappear during annealing. By integrating EBSD with TXM, researchers may gain new insights that may not be afforded by either technique alone.<br/><br/><b>References</b><br/>[1] J. Tang <i>et al.</i>, <i>Nature Communications</i>, 2019<br/>[2] R. Elliot, <u>Eutectic Solidification Processing Crystalline and Glassy Alloys</u>, 1983<br/>[3] A. Ardell, <i>Journal of the European Ceramics Society</i>, 1999