Lamellar to Rod Eutectic Transition in the Hypereutectic Nickel-aluminum Alloy

Directional solidification experiments were carried out on the hypereutectic Ni-25 at.% Al alloy to examine the effect of growth velocity on the eutectic microstructure. The growth velocity was varied from 1 to 20 μm/s at a constant temperature gradient of 10.0 K/mm. The microstructural observations of unidirectionally solidified samples show that the lamellar eutectic growth was observed in the sample solidified at a constant velocity of 1 μm/s and the rod eutectic growth at velocities higher than 10 μm/s. A microstructural transition from lamellar to rod eutectics was achieved at the intermediate velocity. The lamellar to rod eutectic transition was shown to result from the compositional change due to the presence of strong convection in the melt. The undercooling-spacing curves showed that the average eutectic spacings for the lamellar and the rod structures were 1.6 times larger than that in the minimum undercooling for a given velocity.