On the mechanism of pore formation in metals
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S E V E R A L t y p e s of p o r e s can b e o b s e r v e d in s o l i d i fied metal alloys. These pores are normally caused by g a s d i s s o l v e d in the m e t a l m e l t . N o r m a l l y the q u a n t i t y of gas d e t e r m i n e s the s h a p e of the p o r e s . T h e s o l u b i l i t y of g a s in m e t a l m e l t s n o r m a l l y d e c r e a s e s with d e c r e a s i n g t e m p e r a t u r e , and if the gas content i s high the s o l u b i l i t y l i m i t s c a n be r e a c h e d d u r i n g c o o l i n g and g a s p r e c i p i t a t i o n s t a r t s b e f o r e t h e s o l i d i f i c a t i o n p r o c e s s . T h e s e g a s b u b b l e s float in t h e m e t a l m e l t and n o r m a l l y r e a c h t h e s u r f a c e of the m e t a l b e f o r e the s t a r t of the s o l i d i f i c a t i o n p r o c e s s . T h e s o l u b i l i t y of g a s is n o r m a l l y l o w e r in the s o l i d s t a t e than in the liquid and b e c a u s e of t h i s the s e g r e gation of t h e gas e l e m e n t s w i l l give g a s p r e c i p i t a t i o n d u r i n g the s o l i d i f i c a t i o n p r o c e s s . T h e t y p e of p o r e s thus f o r m e d a r e often e l o n g a t e d a s f o r i n s t a n c e the p o r e s in the r i m zone in a n u n k i l l e d ingot and in w e l d s with high h y d r o g e n content. T h e f o r m a t i o n of t h e s e p o r e s i s s o m e t i m e s c o m p a r e d with a n o r m a l e u t e c t i c r e a c t i o n and i s d i s c u s s e d e l s e w h e r e in the l i t e r a t u r e 1-3 but the e x p l a n a t i o n of the f o r m a t i o n p r o c e s s s e e m s to be i n c o r r e c t . T h e a i m of t h i s w o r k was to s t u d y t h e f o r m a t i o n p r o c e s s of t h i s t y p e of p o r e by unidirectionally controlled solidification experiments followed by a t h e o r e t i c a l a n a l y s i s . A n o t h e r t y p e of p o r e w h i c h often o c c u r s in i n g o t s and c a s t i n g s at the end of the s o l i d i f i c a t i o n p r o c e s s is t h a t of m i c r o p o r e s . T h e y a r e s u g g e s t e d to be c a u s e d by t h e s o l i d i f i c a t i o n s h r i n k a g e but it i s p o s s i b l e t h a t t h e f o r m a t i o n of t h e s e p o r e s i s r e l a t e d to the gas c o n t e n t of the l i q u i d . T h e t e n d e n c y of g a s p r e c i p i t a t i o n w i l l i n c r e a s e b e c a u s e the s u p e r s a t u r a t i o n i n c r e a s e s when the p r e s s u r e d e c r e a s e s in the i n t e r d e n d r i t i c a r e a at the end of the s o l i d i f i c a t i o n p r o c e s s . T h e f o r m a t i o n of t h i s t y p e of p o r e w i l l a l s o be d i s c u s s e d in this paper.
T h e e q u i p m e n t for the u n i d i r e c t i o n a l s o l i d i f i c a t i o n e x p e r i m e n t s i s shown in F i g . 1. T h e a l l o y w a s m e l t e d in a c r u c i b l e at the l o w e r end of the f u r n a c e . A w a t e r c o o l e d p r o b e w a s p l a c e d at the u p p e r p a r t of the f u r
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