MICROSTRUCTURE OF RAPIDLY SOLIDIFIED EUTECTIC AND HYPEREUTECTIC AL-CU ALLOYS

The microstructures of melt-spun eutectic Al-33 wt % Cu and hypereutectic Al-38 wt % Cu alloys have been investigated by a combination of optical and transmission electron microscopy. A variety of different microstructures are formed in a sequence of layers through the thickness of the melt-spun ribbons. From the chilled surface in contact with the copper drum to the unchilled surface in contact with the atmosphere, the different layered microstructures consist of: (a) columnar supersaturated alpha-grains, with fine scale theta" internal precipitates exhibiting the oriention relationship {001}theta" parallel-to {001}alpha and < 100 > theta" parallel-to < 100 > alpha; (b) a particulate degenerate eutectic structure of randomly oriented 0.04 to 0.08-mu-m sized theta-particles embedded in a 1-mu-m grain size alpha-matrix; (c) a continuous degenerate eutectic structure of irregular 0.1-mu-m thick alternating alpha and theta-lamellae; and (d) equiaxed grains of 1 to 1.5-mu-m sized central primary dendritic theta-particles, surrounded by regular lamellar or continous degenerate alpha/theta-eutectic. The evolution of the different microstructures can be explained as a consequence of decreasing liquid undercooling and solidification rate as the solid-liquid interface advances through the thickness of the melt-spun ribbons. In particular, the transitions from particulate degenerate to continuous degenerate and then to regular lamellar eutectic structure are associated with a decreasing difference in alpha and theta growth rates. A regular lamellar eutectic spacing of 25 to 30 nm corresponds to a solidification rate of approximately 80 mm/s near the unchilled surface of the melt-spun ribbons. In addition to the sequence of layered microstructures, equiaxed grains are found near the chilled surface of melt-spun hypereutectic Al-38 wt % Cu ribbons, containing an unusual banded structure of regularly spaced alternating alpha and theta-bands normal to the radial growth direction of the equiaxed grains.