Formation of three-phase eutectic grains on primary phases: Observations from correlative imaging

Abstract

We investigate the interaction between three-phase eutectic and a primary phase during solidification of an off-eutectic Al-Ag-Cu alloy through correlative imaging. In situ synchrotron X-radiography reveals the locations and orientations of primary Al2Cu rods, with respect to the surrounding eutectic. We also identified an unusually high and anisotropic eutectic growth velocity, often exceeding 100 μm/s when the growth front was parallel to the surface of a primary Al2Cu rod. By combining these results with three-dimensional (3D) focused ion beam tomography, scanning electron microscopy, and electron backscatter diffraction, we demonstrate that this anomaly results from two-dimensional (2D) dendrites, or ‘fingers’, of Al and Ag2Al that spread along the primary Al2Cu rod surfaces; meanwhile, protrusions of Al2Cu grow in between pockets of the fingers and into the surrounding eutectic, preserving their crystallographic orientations and morphology in the process. We propose a competitive growth relationship between the three phases of the eutectic to explain these features as well as the diversity of two- and three-phase microstructures obtained at the onset of eutectic solidification.