Gravitationally Induced Convection During Directional Solidification of Off-Eutectic Mn-Bi Alloys
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GRAVITATIONALLY INDUCED CONVECTION DURING DIRECTIONAL SOLIDIFICATION OF OFF-EUTECTIC Mn-Bi ALLOYS
RON G. PIRICH Metals Science Laboratory, Research Department, Aerospace Corporation, Bethpage, New York, USA
Grumman
ABSTRACT The effects of thermal and solute gradient, gravity induced convection during vertical directional solidification, on longitudinal macrosegregation of Bi and Mn rich off-eutectic starting compositions, has been studied as a function of composition, growth velocity and gravity vector orientation. Since the morphology of these alloys is characterized by an aligned, rodlike permanent magnet composite when grown cooperatively, the magnetic properties were used to measure composition segregation and the transition from dendritic to composite growth. Severe macrosegregation was observed in all cases studied and the degree of convection inferred by modeling the observed composition segregation using a stagnant film approach. Morphological stability was found to follow a constitutional supercooling-type law for both Bi and Mn rich compositions.
INTRODUCTION A natural limitation of aligned eutectic materials produced by directional solidification is that frequently the volume fraction of the potentially useful phase is small [i] and [2] . This is true for eutectic Bi/MnBi in which the This restriction may be ferromagnetic MnBi phase is limited to 3.18 v/o [3]. overcome by moving to off-eutectic compositions, but this may result in rather severe macrosegregation in the presence of gravity under the conditions necessary for aligned
growth at off-eutectic
compositions [4] and [5]
.
The origin
of the gravitationally induced segregation may be thermal, solutal or both [6] and [7] . Early studies by Mollard and Flemings [8] suggested that the range of off-eutectic compositions for which cooperative growth was possible depended on little or no convection as well as constitutional supercooling criteria. Later work by Verhoeven et al [9] and [5] argued that cooperative growth even in the presence of convection was possible. The only theoretically possible solidification configuration, in the presence of gravity, for which both thermal and solutal convection may be eliminated is a vertical arrangement in which the furnace hot zone is at the top and the solute increases the liquid density for equilibrium partition ratios less than unity. For the Mn-Bi system, the Mn solute has a lower fluid density than Bi. Ideally, since Mn is rejected for Bi rich off-eutectic compositions, solutal convection should occur for a growth up orientation with respect to the gravity direction with hot zone above cold zone and thermal convection growth down with hot zone below cold zone. For Mn-rich compositions, since Bi is rejected, neither thermal or solutal convection should occur growth up and both thermal and solutal growth down. However, as has been pointed out by Sekerka and Coriell [i0] and [ii] , unless both the thermal and compositional contributions to the density are negative, thermal and/or solutal convection
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