Pore formation in iron
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D. A. H I R S C H F E L D AND F. W E I N B E R G There are currently three general m e c h a n i s m s for the n u c l e a t i o n of p o r e s in m e t a l s . T w o of t h e s e a r e h e t e r o g e n e o u s n u c l e a t i o n on s o li d p a r t i c l e s ~,2 and h o m o g e n e o u s n u c l e a t i o n in the i n t e r d e n d r i t i c liquid d u r i n g s o l i d i f i c a t i o n . 3 Both c o n s i d e r the c o n c e n t r a t i o n of the g a s e o u s component d u r i n g c o o l i n g and s o l i d i f i c a t i o n in the a n a l y s i s . T h e t h i r d - - n o n n u c l e a t i n g m e c h a n i s m s - r e s u l t s f r o m the a n a l y s i s by C a m p b e l l and o t h e r s of p o r e f o r m a ti o n , % ~ which c o n c l u d e s that both h o m o g e n e o u s and h e t e r o g e n e o u s n u c l e a t i o n a r e not probable. Other mechanisms are therefore required. In this i n v e s t i g a t i o n a s e t of s i m p l e e x p e r i m e n t s was c a r r i e d out to t r y to c l a r i f y which of the a b o v e m e c h a n i s m s is a p p l i c a b l e to p o r e f o r m a t i o n in i r o n . Th e e x p e r i m e n t s c o n s i s t e d of s u p e r c o o l i n g p u r e i r o n , then e x a m i n i n g the s o l i d i f i e d m a t e r i a l f o r p o r o s i t y . If p o r e f o r m a t i o n can only o c c u r by h e t e r o g e n e o u s n u c l e a t i o n on n u c l e i which a r e the s a m e as that on which so l i d n u c l e a t e s , then the r e s u l t a n t p o r o s i t y should be m a r k edly r e d u c e d in i r o n which can be s t r o n g l y s u p e r cooled. T h e e x p e r i m e n t s c o n s i s t e d of m e l t i n g 25 g of A r m c o iron* by induction, u n d e r A r g o n , with the m e t a l
*Analysis:C-0.02 pct, Mn 0 15 pct, Si-0.09 pct, Cu-0.125 pct, N2-0.01 pct, and O2-0.010 pct. s e p a r a t e d f r o m the c o n t a i n i n g c r u c i b l e by s i l i c a f l o u r . T h e m e l t t e m p e r a t u r e was obtained with a s h i e l d e d P t - P t 10 p ct Rh t h e r m o c o u p l e which t o u c h e d the u p p e r m e t a l s u r f a c e . A f t e r s e v e r a l h e a t i n g and c o o l i n g c y c l e s , the liquid m e t a l was o b s e r v e d to s u p e r c o o l by as m u c h as 300~ as it cooled r a p i d l y and s o l i d i f i e d . E x p e r i m e n t s w e r e a l s o c a r r i e d out in which c a r b o n ( l am p b l ack ) and oxygen, in the f o r m of o x i d i z e d A r m c o i r o n , w e r e p l a c e d in d r i l l e d h o l e s in p r e v i o u s l y s u p e r c o o l e d i r o n s a m p l e s . T h e s e s a m p l e s w e r e then r e m e l t e d , w e l l m i x e d , and cooled in the s a m e m a n n e r as the p u r e i r o n . A f t e r the s a m p l e s had c o o l e d , they w e r e s e c t i o n e d v e r t i c a l l y and p o l i s h e d . T h e s i z e and d i s t r i b u t i o n of the p o r e s on the p o l i s h e d s u r f a c e w e r e m e a s u r e d at 100 t i m e s m a g n i f i c a t i o n u s i n g a Q u an
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