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A N D D. H. B A L D W I N

Growth o r i e n t a t i o n s a r e given for 50 g r a m and 2 g r a m single c r y s t a l s of tin grown at a c c e l e r a t i o n l e v e l s of 1 through 7-g. A c c e l e r a t i o n g r a d i e n t s produce a p r e f e r r e d growth o r i entation effect not p r e v i o u s l y o b s e r v e d for tin. Convection c u r r e n t s at a p p r o x i m a t e l y 5-g encourage multiple nucleation and subsequent random orientation of growth direction. The m a c r o m o s a i c s u b s t r u c t u r e in the 99.95 pct tin does not obey the (RG) -x growth r e l a t i o n s h i p . The shape of the growth i n t e r f a c e changes smoothly as the a c c e l e r a t i o n force i n c r e a s e s , with the d i r e c t i o n and magnitude of the change dependent on growth orientation. D e f o r m a tion effects such as r e c r y s t a l l i z a t i o n and twinning a r e o b s e r v e d at a c c e l e r a t i o n l e v e l s g r e a t e r than 2-g. A {431}twin plane was obtained for c r y s t a l s grown at 6 - g ' s . T H E effects of a c c e l e r a t i o n on solidification have been studied p r i m a r i l y through a n a l y s e s of fluid flow and phase segregation.* Recent developments in the Space P r o g r a m , however, have led to a m o r e extensive a n a l y s i s of p a r a m e t e r s influencing solidification such as the p e r t u r b a t i o n s induced by the p r e s e n c e (or absence) of gravity. It was the purpose of this r e s e a r c h to enhance the g r a v i t a t i o n a l a c c e l e r a t i o n to such an extent that its effect on nucleation and c r y s t a l growth was magnified. This p a p e r d i s c u s s e s p r e f e r r e d orientation effects, int e r f a c e and s u b s t r u c t u r e changes, and the a p p e a r a n c e of a new twinning plane in single c r y s t a l s of tin grown at specific a c c e l e r a t i o n s . EXPERIMENTAL P R O C E D U R E An apparatus was designed and assembled to directionally solidify single crystals under various acceleration levels. Instrumentation consisted of a D.C. magnetic torque motor, low-noise electrical slip ring, dual pen temperature recorder, programmable power supply, and a period sensing network. A rotating vacuum feed through connected the the growth cannister to a vacuum p u m p and an inert gas purge system. A schematic representation of the apparatus is shown in Fig. I. The crystal growth cannister is shown in Fig. 2. A temperature gradient was obtained by placing a copper heat sink at the base of the crucible with the programmable power supply controlling the cool-down rate. Single crystals seeded to determine the growth orientation, were initiatedby limiting the m a x i m u m temperature such that the melting point was reached in the region of the small tip. Crystals were also grown from a fully molten charge for analysis of the orientation mechanism. A free swinging aluminum growth cannister and a counterweight were suspended from the centrifuge a r m s by a pivot to ensure that the acceleration forc