3D Characterisation of a Rosette in an Aluminium-Silicon Alloy

Small rounded pocket with very fine multi-phase structure are quite commonly observed in aluminium alloys. Usually, their appearance is due to small liquid droplets having been entrapped within dendrite arms during primary deposition of (Al) phase. These droplets are sometimes given the name of rosette. In this present work, transmission electron microscopy (TEM) and 3D characterization of rosettes from an aluminium - silicon - iron alloy have been performed. For this, focused ion beam (FIB) milling has been used to extract thin foils for TEM examination and to prepare series of images for tomography. The observed rosettes contained an intricate mixture of silicon, an iron-rich phase and (Al), thus corresponding to a three-phase eutectic. The iron-rich phase showed up sometimes with a so-called Chinese script appearance, but it was definitely indexed as beta-Al9Fe2Si2 phase. The reconstruction revealed the eutectic Si network along with the beta-Al9Fe2Si2 phase developing within the rosette. 3D reconstruction also showed the presence of solid-state precipitation of Si and beta phase. Further observation clearly showed that beta phase could not act as a nucleation site for eutectic silicon.