EFFECT OF WEAK CONVECTION ON LAMELLAR SPACING OF EUTECTICS

An asymptotic expansion method is proposed to solve the convection-dif fusion equation which governs the directional solidification of eutect ic alloys under the condition of weak convection at low Peclet number. The so-called weak convection is defined here as the condition For wh ich the velocity of convective flow and its gradient ahead of the S/L interlace are both relatively small. Our analysis shows that such weak convective how will give rise to an increase of eutectic lamellar spa cing since the convection produces a slight shift of solute concentrat ion profile at the interface and a decrease of eutectic growth underco oling. Our analytical results are compared with other models and have been applied to some practical cases such as weak forced convection in duced by a rotating disk and natural convection induced by solutal and thermal differences in solidification processing. Calculations for tw o typical eutectic alloys, Pb-Sn and Al-Cu, are performed in order to reveal schematically the effect of weak convective flow on eutectic gr owth. (C) 1998 Acta Metallurgica Inc.