Mechanical deformation of dendrites by fluid flow during the solidification of undercooled melts

Mechanical interactions between growing dendrites and their parent melt are normally considered to be of little significance. During conventional solidification processing this is undoubtedly true. However, during the solidification of undercooled melts the twin conditions required to produce mechanical damage to dendrites, high flow velocities and very fine dendrites, may exist. This is most likely in strongly partitioning alloy systems where the tip radius experiences a local minimum at undercoolings in the range of 50-100 K. In this paper we present a model for the skin stress resulting from fluid flow around a family of realistically shaped dendrites. We find that within a narrow undercooling range about the minimum in the tip radius, mechanical deformation of the growing dendrite is likely. Experimental evidence is presented from the Cu-3wt%Sn and Cu-O alloy systems that appear to show evidence of extensively deformed dendritic structures consistent with mechanical damage. Other mechanisms for causing dendritic bending during growth are considered and shown to be unlikely in this case. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.