Hysteresis in the composite-to-dendrite transition in off-eutectic Sn-Pb alloys
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ent than the r a t e which p r o d u c e s the c r i t i c a l condition for d e n d r i t e d i s a p p e a r a n c e .
EXPERIMENTAL
PROCEDURE
AND
RESULTS
Experiments were carried out on Sn-Pb alloys prepared from metal having quoted purities of 99.999 pet. The ingots were solidified vertically in the apparatus described in detail elsewhere. 9'I~ The drive mechanism was modified so that the r a t e of growth could be changed at a c o n s t a n t a c c e l e r a t i o n . The r a t e of motion of the c a r r i a g e was d e t e r m i n e d by s u c c e s s i v e l y m e a s u r i n g the time required for it to advance 0.127 cm. These readings were taken automatically and stored on tape throughout each experiment. The experiments were terminated by quenchingand the quench interface provided a sharp reference point for determining the rate at any point along the ingot. We have previously found9 that at the high gradients and the relatively slow rates used here the rate of interface motion equals the rate of carriage motion. Sincethe accelerations used in this study were very small this assumption was also employed here. The temperature gradient was measured with a 0.25 mm OD thermocouple in the manner previously described. I~ Experiments were carried out on two entirely different pieces of equipmentby two different operators. The alloy was contained in vycor tubes having an ID of 5 mm or quartz tubes with a 3 mm ID. In some cases the inside of the tubes was coated with a very thin layer of graphite or MoS2. In a few cases a 3 mm ID tube was used inside of a 6 mm tube with the inside of the 6 mm tube being coated with graphite. This will be referred to as a double tube. Two main types of experiments were made, accelerating and decelerating. Table I presents the results for the deceleration experiments. These runs were started at a rate sufficiently high to produce dendrites and then decelerated at constant rates as shown in Table I. The position at which the dendrites disappeared was determined by metallographic examination of transverse sections of the ingots. The disappearance r a t e , Rd, was then d e t e r m i n e d f r o m the r a t e vs d i s t a n c e data c o l l e c t e d d u r i n g the e x p e r i m e n t . As the c r i t i c a l r a t e for d i s a p p e a r a n c e was a p p r o a c h e d the a r e a f r a c t i o n d e n d r i t e s in the t r a n s v e r s e s e c t i o n s g r a d u a l l y d i m i n i s h e d to z e r o . T h i s effect, a l s o r e p o r t e d by M o l l a r d and F l e m i n g s , I h a s s o m e i n t e r e s t i n g i m p l i c a t i o n s which we a r e e x a m i n i n g in detail and will d i s c u s s in a f u t u r e p a p e r . C h e m i c a l a n a l y s e s w e r e VOLUME 3, JULY 1972-1893
Table I. Summary of Deceleration Data
Conditions A
B
C
Composition, At. Pct*
Temp. Grad., ~
Rate Disappearance, Rd,/~/sec
Starting Rate, p/sec
Rate Decel p/sec: X 104
Tube Coating
21.2, 21.2, 21.3 20.9 20.7 20.9, 21 20.9 20.7, 21 20.6 14.4 14.9, 15.5 15.0
121 • 1 121 1
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