Extrinsic effects in the dynamics and selection of cellular arrays

Abstract

In addition to convection in the liquid phase, which can be made negligible in microgravity, other extrinsic effects should be suppressed, or at least controlled, in order to be able to actually relate the dynamics and selection of cellular arrays that result from the pure morphological instability of a planar solidification front to the processing parameters: growth velocity V, thermal gradient G, and solute concentration C0. Therefore, we successively analyzed the influence of four major extrinsic sources of parasitic effects in cellular solidification and, when complementary, in Benard convection with a free surface. The container imposes confinement and wall conditions. Cellular doublets or spatiotempora l chaos are favored by the presence of grain boundaries. When the solute is volatile, gas pores may nucleate, which then provoke capillary convection and whose growth can either suppress morphological instability or form localized duplexes, upon coupling with a cellular or dendritic envelope. Finally, the experimental procedure, essentially through the initial conditions that are applied, determines the initial transient and sometimes prevents the asymptotic state to be reached, possibly by dynamically locking the system in an intermediate configuration, characterized by a significant amount of noise.