Spherical ternary eutectic cells formed during free fall

We report a kind of spherical ternary eutectic cell formed within an undercooled liquid Al74.6Cu17.2Mg8.2 alloy under containerless and reduced-gravity conditions during free fall. A morphological transition from lamellar to anomalous eutectic growth occurs at large degrees of undercooling. The ternary ( + + S) eutectic cell exhibits perfectly spherical characteristics because of the radial symmetry of the temperature and concentration fields inside a falling droplet. For the smallest droplet undercooled by 283 K (0.36TE), the anomalous ternary eutectic grows radially from a preferentially nucleated site of phase and forms only one spherical eutectic cell occupying the whole droplet. The crystalline orientation relationship is determined as [image omitted] and [101]//[110] between the and phases and [image omitted] and [101]//[013]S between the and S phases. The volume fractions of each phase within the ( + + S) ternary eutectic depend strongly on the degree of undercooling.