Formation of eutectic intermetallic rosettes by entrapment of liquid droplets during cellular columnar growth Original Research Article

We study the formation of α-Al-(Fe,Mn)-Si phase intermetallic particles in a directionally-solidified model Al–0.5Fe–1.0Mn–0.2Si (wt%) alloy. In this the aluminium matrix has a cellular-columnar grain structure. Secondary phases are expected to form from solute-partitioned liquid at the cell boundaries. However, spherical rosettes of eutectic α-phase can be found within the cells. Evidence is presented that these are formed from liquid droplets trapped when a cell splits and then coalesces. The intercellular regions of the as-cast alloy are primarily occupied by metastable Al6(Fe,Mn) eutectic particles. However, these may transform to the stable α-phase, either by a ternary peritectic transformation during solidification, or by the eutectoid 6-to-α transformation during the period in which the ingot cools to room temperature.