Freckle criteria for the upward directional solidification of alloys

Macrosegregation defect freckles result from the convective flow due to a d ensity gradient in the mushy zone. Several freckle criteria were proposed i n the literature for the evaluation of the freckle tendency of alloys and p rocesses. These criteria were examined using experimental data of the upwar d directional solidification of Pb-Sn, Pb-Sb, and Pb-Sn-Sb systems. It is f ound that the freckle criterion given by Flemings and co-workers gives the best description of the freckle tendency. A Rayleigh number is derived from Flemings' criterion. The critical value of the Rayleigh number is found to be Ra-cr approximate to 1.