From irregular to regular eutectic growth in the Al-Al3Ni system: In situ observations during directional solidification

• Published in: Acta materialia Vol. 280; no. C; p. 120314
• Main Authors: Chao, Paul, Aramanda, Shanmukha Kiran, Xiao, Xianghui, Bottin-Rousseau, Sabine, Akamatsu, Silvère, Shahani, Ashwin J.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Ltd 01.11.2024; Elsevier
• Subjects: Condensed Matter; Crystalline anisotropy; Eutectic solidification; In situ experiments; Materials Science; Physics; Solidification microstructures; x-ray synchrotron radiation; Solidification microstructures; Eutectic solidification; In situ experiments; x-ray synchrotron radiation; Crystalline anisotropy
• ISSN: 1359-6454
• DOI: 10.1016/j.actamat.2024.120314

We investigate the irregular eutectic growth dynamics of the Al-Al3Ni alloy, in which one of the solid phases (Al3Ni) grows faceted from the liquid. Leveraging in situ optical microscopy and synchrotron transmission x-ray microscopy, we address the question of the degree of coupling between Al and Al3Ni at the growth front and that of the shape of the microstructures left behind in the bulk solid during directional solidification. Real-time optical observations bring evidence for a morphological transition from a eutectic-grain dependent, irregular eutectic growth at low solidification velocity V (typically 1μms−1), to a weakly anisotropic, regular growth at higher V (reaching 10μms−1). Unprecedented x-ray nano-imaging of the solid–liquid interface, and 3D characterization of the growth patterns, were made possible by a new DS setup at Brookhaven National Laboratory’s NSLS-II. At low V, the leading tips of partly faceted Al3Ni crystals are observed to grow not far ahead of the Al growth front. Correlating in situ images and postmortem 3D tomographic reconstructions reveals that the presence of faceted and non-faceted regions of Al3Ni crystals in the solid is a direct consequence of coupling and decoupling during DS, respectively. Upon increasing V, the lead distance of Al3Ni vanishes, and the shape of Al3Ni ceases to be governed by faceted growth. These observations cast light onto the basic mechanisms (faceted growth, diffusive coupling, and the dynamics of trijunctions) governing a faceted to rod-like transition upon increasing V in the Al3Ni system, with broad implications to a large class of irregular eutectics. [Display omitted]