The mechanisms of formation and prevention of channel segregation during alloy solidification

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NH~C1 is very large (~-30 pct) while there is actually a small expansion for the NH~C1-H20 eutectic solidification. By comparison the primary lead solid solution undergoes a much smaller contraction on freezing ( ~ 3 pct), and there is a small contraction on passing through the eutectic. However, conbined with the relative volume fractions at the eutectic temperatures the primary and total contractions on freezing are not dissimilar, i . e . , - 4 . 8 pct v s 3.5 pct and 4.4 pct vs 3.7 pct, respectively, in each system. METALLURGICALTRANSACTIONS A

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I

100

I

I

50

60

1 80

u Ld rr

60 40 ~40

20

ยง

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-20 I

H20

I

10

I

20

I

30

WEIGHT ~

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40

I

NH4CI

(a)

J0

Pb-Sn

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20

50

40

Lead-Tin

AtOrTt,s ~etcertlacje Tm 70 50 60

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8O

90

L

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Fig. 2--Type of rotational or precessional movementemployed.

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~.-,--r~:~.--~---r

.... 619

Comparison of the dynamic and kinematic viscosity coefficients also shows relatively small differences, the latter being the more important in that it appears in the Reynolds number (vr/v), and with measured flow rates, v, in the aqueous system and mean channel radii, Re, in either system (see later), the Reynolds numbers are thought not to exceed 102, which is well within the range for streamline flow (~103), despite the fact that the channels or interdendritic capillaries are irregular in section. Otherwise, the major difference between the two systems is probably in the thermal conductivities (Table I), and the much lower values for the nonmetallic phases mean that the overall temperature gradients through the growth front will be much steeper in the aqueous system and that local gradients around perturbations will also be correspondingly larger as heat is conducted away less rapidly; this thermal inertia is indicated by a much larger Prandtl number (u/Dh~.,.~,) than for the metallic system, implying that as far as heat flow is concerned, perturbations will be less easily damped. II.

EXPERIMENTAL

A. NH4CI-H20 System The base chilled arrangement s was essentially that described by Copley et al. 4 and consisted of a 75 mm diameter plexiglas cylinder, height 160 mm, sealed by an ' O ' ring onto a copper base which was cooled by a reservoir of liquid nitrogen at - 1 9 6 ~ Salt solutions were poured into this METALLURGICAL TRANSACTIONS A

' ~ 1 7 6..............,---1-- t - - - - Pb I0 c D,MothnL

20

30

40 50 60 We,ght Percenloge T,n

(a-Sn)] 70

80

90

Sn

(b) Fig. 3--Partial phase diagrams for the systems NH4CI-H20 and Pb-Sn.

mold with a constant superheat of 35 K above the liquidus, and subsequent events then recorded photographically from the side and above. Salt solutions containing 25 wt pet, 30 wt pet, 35 wt pet NHaC1 were used; these would have liquid fractions at the eutectic temperature of - 9 5 pct, - 9 0 pet, and - 8 4 pet, respectively. As the solution freezes upward from the chilled base it becomes resolved into three zones: (a) supernatant bulk liquid, (b)primary dendrites with interdendritic liquid, and (c) primary de