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

TL;DR

This study uses in situ microscopy and x-ray imaging to explore how the growth patterns of Al-Al3Ni eutectic change from irregular to regular as solidification velocity increases, revealing the mechanisms behind this transition.

Contribution

It provides new real-time 3D imaging evidence of the morphological transition and coupling dynamics during directional solidification of Al-Al3Ni alloys.

Findings

Transition from irregular to regular eutectic growth with increasing velocity

Faceted Al3Ni crystals grow ahead of Al at low velocities

Coupling and decoupling of phases influence microstructure morphology

Abstract

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 1 ${\mu}ms^{-1}$), to a weakly anisotropic, regular growth at higher V (reaching 10 ${\mu}ms^{-1}$). Unprecedented x-ray nano-imaging of the solid-liquid interface, and 3D characterization of the growth patterns, were made possible by a new Directional Solidification (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 shed light on the basic mechanisms (faceted growth, diffusive coupling, and the dynamics of trijunctions) governing the transition from faceted to rod-like growth upon increasing V in the Al3Ni system, with broad implications for a large class of irregular eutectics.