Rod-lamella transition in directionally solidified Ni-W eutectic alloy

The morphological transition of rods to lamellae with increasing growth rate in a Ni-W eutectic alloy was investigated. The eutectic alloy was directionally solidified in a Bridgman-type apparatus at the eight steps of growth rates from 1.0 to 16 mm/h. The change in morphology with increasing growth rate appeared in the following two different ways of lamella formation: (1) Under the condition of the planar solid/liquid interface, the eutectic grain was uniformly comprised of rods or lamellae (blades). In this case, the deviation of the γ-phase 〈100〉 direction from the growth direction controlled the morphology. (2) When cellular interface was formed at relatively higher growth rates, both rods and radial lamellae coexist simultaneously in a cell. We confirmed that, as a common factor in the above two ways, the driving force of lamellae formation should be influenced by the angle between the γ-phase 〈100〉 direction and the local growth direction defined as being perpendicular to the solid/liquid interface.