Solidification behavior of undercooled Cu70Ni30 alloy melt

The solidification behavior of undercooled Cu70Ni30 alloy in cylindrical crucible was investigated. By controlling the nucleation point at the top of the specimen, a directionally solidified dendrite structure in the undercooling range of 90-185 K has been obtained even if no longitudinal temperature gradient existed in the melt prior to nucleation. The dendrite growth velocity as a function of undercooling was measured by high speed cinematography. The results indicate that the dendrite growth velocity increases slowly with increasing undercooling when the directional solidification occurs. However, as the directional solidification breaks down above 185 K, the dendrite growth velocity rises up quickly. It is demonstrated that the breakdown of the directional dendrite growth at high undercoolings results from the destabilization of the dendrite growth, while this destabilization is caused by the solidification contraction stress occurring in the rapid dendrite growth. At higher undercoolings the destabilization even leads to the formation of a number of new crystals, and the recrystallization of the deformed solid during or immediately after solidification can make the grain size decrease further. On the assumption that only the thermal diffusion fields around the dendrites overlap, and that the dendrite tip can be represented by a paraboloid of revolution, a semi-quantitative theoretical model has been established. (C) 2000 Published by Elsevier Science S.A. All rights reserved.