MACROSEGREGATION AND CONVECTION IN THE HORIZONTAL BRIDGMAN CONFIGURATION .2. CONCENTRATED ALLOYS

Numerical simulations of the horizontal Bridgman solidification process are performed for a rectangular cavity filled with a low Prandtl concentrated alloy. Interesting information about the influence of the concentration on the flow and on the dopant distribution in the melt and in the crystal is obtained. Two cases are considered: the first deals with buoyancy driven flows in confined cavities and the second with surface tension driven flows in open cavities. The effects of the solutal Grashof (Gr(s)), the solutal Marangoni (Ma(s)) and the Schmidt (Sc) numbers are described for an aspect ratio A = 4 (A = L/H where L is the length and H is the height of the cavity), a segregation coefficient k = 0.087 (corresponding to an alloy of Ge-Ga or GaAs-In), a growth rate V(f) = 2.7 10(-5) m/s and a Prandtl number Pr = 0.015. The results related to macrosegregation are shown when half of the cavity is solidified. For the confined cavity, depending on the relative densities of the alloy constituents, the solutal buoyancy forces may either augment (Gr(s) greater-than-or-equal-to 0) or oppose (Gr(s) les-than-or-equal-to 0) the thermal buoyancy forces. The consequent changes of the flow will affect the crystal quality, particularly for Gr(s) less-than-or-equal-to 0 where a solutal counter-rotative roll may appear. A similar behaviour is observed for the open cavity. Nevertheless, in this case, the changes affecting the flow and consequently the crystal structure have different characteristics. A very attractive phenomenon is the sudden expansion of the solutal counter-rotative roll, For this latter case a qualitative agreement is found between the numerical prediction and the experimental results of Tosello.