Morphological instability due to double diffusive convection in directional solidification: the pit formation

Deep interface depression or "pit formation", as a result of solute accumul ation, due to double-diffusive convection in the directional solidification of succinonitrile (SCN) containing ethanol in an ampoule is investigated b y a fully nonlinear numerical simulation. The calculated results are consis tent with previous observations (Schaefer and Coriell, Metal. Trans. 15A (1 984) 2109), and the instability margin falls between the convective and mor phological boundaries at a low growth rate. For a high growth rate, the glo bal interface depression becomes deep due to significant release of the hea t of fusion; in this case, the critical concentration can be lower than the convective value. Near the instability margin, the pit forms at the center of the interface and is soon followed by constitutional supercooling. Also , the pit shape is affected significantly by the convective solute transpor t and thus the how structures. Such pit formation, results from the nonline ar coupling of double-diffusive convection and the interface deformation, a nd although differs from the traditional mechanisms, it could be an importa nt route to interface breakdown. (C) 2000 Published by Elsevier Science B.V .