Numerical analysis of constitutional supercooling during directional solidification of alloys

Some limitations of Tiller’s morphological stability criterion are discussed in the present study. This criterion assumes a purely diffusive regime in the melt as well as a planar solid–liquid interface and a constant solidification rate. But experimental works in agreement with previous numerical modeling have shown a significant decrease of the growth rate and a variable interface curvature during the concentrated semiconductor alloys solidification. The mathematical expression of the morphological stability criterion was derived by using Tiller’s equation, predicting the solute distribution in the liquid. The numerical computations performed in this study show a significant disagreement between the numerical results and Tiller’s formula. Numerical modeling conducted in conditions when the supercooling should occur, show that the Tiller’s stability criterion cannot predict the moment of interface destabilization. The interface destabilization is numerically observed when some fluctuations appear in the liquid solutal profiles and cause the appearance of a supercooled zone inside the liquid at small distance from the interface. The present numerical results are not in contradiction with the basic elements of the classical constitutional supercooling theory, providing only that the stability criterion cannot predict the moment of the interface destabilization.