, Volume 43, Issue 2, pp 391-404
Date: 07 Dec 2011

Mechanism of Dendritic Branching

Abstract

Theories of dendritic growth currently ascribe pattern details to extrinsic perturbations or other stochastic causalities, such as selective amplification of noise and marginal stability. These theories apply capillarity physics as a boundary condition on the transport fields in the melt that conduct the latent heat and/or move solute rejected during solidification. Predictions based on these theories conflict with the best quantitative experiments on model solidification systems. Moreover, neither the observed branching patterns nor other characteristics of dendrites formed in different molten materials are distinguished by these approaches, making their integration with casting and microstructure models of limited value. The case of solidification from a pure melt is reexamined, allowing instead the capillary temperature distribution along a prescribed sharp interface to act as a weak energy field. As such, the Gibbs-Thomson equilibrium temperature is shown to be much more than a boundary condition on the transport field; it acts, in fact, as an independent energy field during crystal growth and produces profound effects not recognized heretofore. Specifically, one may determine by energy conservation that weak normal fluxes are released along the interface, which either increase or decrease slightly the local rate of freezing. Those responses initiate rotation of the interface at specific locations determined by the surface energy and the shape. Interface rotations with proper chirality, or rotation sense, couple to the external transport field and amplify locally as side branches. A precision integral equation solver confirms through dynamic simulations that interface rotation occurs at the predicted locations. Rotations points repeat episodically as a pattern evolves until the dendrite assumes a dynamic shape allowing a synchronous limit cycle, from which the classic repeating dendritic pattern develops. Interface rotation is the fundamental mechanism responsible for dendritic branching.

Dr. Martin Eden Glicksman is a recognized expert on the solidification of metals and semiconductors, atomic diffusion processes, the energetics and kinetics of network structures, grain growth, phase coarsening, and microstructure evolution. He has co-authored over 300 technical papers, reviews, and monographs, and has authored two major textbooks: Diffusion in Solids (Wiley Interscience, 2000) and Principles of Solidification (Springer USA, 2011). He is a Fellow of the Metallurgical Society, the American Society for Materials International, American Association for the Advancement of Science, and the American Institute for Aeronautics and Astronautics, and a member of the American Physical Society. For his research accomplishments on solidification, he received the Rockwell medal, Case-Western University’s van Horn Award, the Metallurgical Society’s Chalmers Medal, ASM’s International Gold Medal and Honorary Membership, and the Alexander von Humboldt Senior Research Prize. Professor Glicksman’s experiments aboard Space Shuttle Columbia led to his receiving NASA’s Award for Technical Excellence, and AIAA’s 1998 National Space Processing Medal. In 2010 he was awarded the Sir Charles Frank Prize of the International Organization for Crystal Growth for his fundamental contributions to dendritic crystal growth. He is a member of the National Academy of Engineering, and serves as Chairman of the Materials Engineering Section of the National Academy of Engineering for 2011–2012.