CONVECTION-INDUCED NOVEL OSCILLATING MICROSTRUCTURE FORMATION IN PERITECTIC SYSTEMS

Critical experiments have been carried out to examine the formation of banded microstructures in the peritectic Sn-Cd system in both diffusi ve and convective regimes. It is shown that in directional solidificat ion studies by a conventional method in 3-6 mm diameter tubes, a new o scillatory structure forms which is induced by convection. In this osc illatory mode the two phases form an interconnected network which appe ar as bands only on specific cross-sections. A successive sectioning h as revealed that this microstructure consists of a large tree-like str ucture of the primary phase that is surrounded by the peritectic phase . A new experimental technique is developed to obtain diffusive growth by directionally solidifying in very thin samples. As the diameter of the sample was reduced, convection effects were reduced and the oscil latory structure was found to disappear for compositions in the hyperp eritectic region. Only in the hypoperitectic composition range, where band formation is predicted by the diffusive model, discrete bands hav e been observed to form in thin samples of diameter 0.6 mm or less. (C ) 1998 Elsevier Science B.V. All rights reserved.