Ordering of the fibrous eutectic microstructure of Al-Al{sub 3}Ni due to accelerated solidification conditions

Directional solidification experiments with eutectic Al-Al{sub 3}Ni alloys were performed in which the solidification front was accelerated or decelerated continuously. The temperature gradient at the solid/liquid interface could be kept constant. At a fixed position in all samples processed, the absolute velocity was, however, the same, independent of the velocity with which the solidification started. The microstructure perpendicular to the growth direction was analyzed with respect to average fiber distances, number of nearest neighbors and the distribution of these quantities. An order parameter is defined to describe the deviation from a perfect hexagonal arrangement. Inasmuch as the acceleration changes from negative to positive values, the arrangement of the fibers comes closer to a perfect hexagonal lattice. The experimental observations are described and interpreted qualitatively.