A new "Jackson-Hunt' model for monotectic composite growth

Directional solidification of monotectic alloys can lead under certain cond itions of growth velocity and temperature gradient in the melt to composite microstructures with a rodlike appearance. For a theoretical description m ost researchers applied the Jackson and Hunt model of rod eutectic growth a nd predicted a relation between the mean rod distance R and the solidificat ion velocity nu(0), as nu(0)R(2) = const. similar to eutectics. The compari son between theory and experiments always led to discrepancies not yet reso lved. In the approach presented here we propose an additional mode of mass transport in front of the zone coupled growth, since in our mind the main d ifference between monotectic and eutectic solidification is the liquid phas e state of the (rod) L-2 phase growing simultaneously within a nearly perfe ctly pure solid matrix. We assume that the thermocapillary effect causes co nvection at the surface of the liquid L-2 phase. This Marangoni convection induces a flow field in front of the solidification front and has a strong influence on the solute transport, depending on the local temperature gradi ent and the Peclet number. We find a new relation between R and nu(0) in th e case of small Peclet numbers and discuss some consequences on the stabili ty of composite growth. (C) 1999 Elsevier Science B.V. All rights reserved.