Structures revealed in zone refined aluminum by tritium decoration
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A method is d e s c r i b e d for d e c o r a t i n g s t r u c t u r e s in solid a l u m i n u m b y p r e c i p i t a t i o n of h y d r o g e n onto t h e m . R a d i o a c t i v e hydrogen is e m p l o y e d so that the d e p o s i t i o n s i t e s can be r e v e a l e d by the u s e of a u t o r a d i o g r a p h i c t e c h n i q u e s . The study was conducted with z o n e r e f i n e d a l u m i n u m , and the p r i m a r y s t r u c t u r e that was r e v e a l e d w a s that r e m a i n i n g a f t e r 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 d u r ing the l a s t zone p a s s . The t r i t i u m was added to the s p e c i m e n at 600~ and its p r e c i p i t a t i o n w a s brought about by q u e n c h i n g the s a m p l e . A s little a s 0.05 ppm of t r i t i u m w a s sufficient to r e v e a l complex s t r u c t u r e s in the cast m e t a l that a r e a t t r i b u t e d to a s s o c i a t i o n of u n i d e n t i f i e d r e s i d u a l i m p u r i t y a t o m s with low angle s u b g r a i n b o u n d a r i e s that a r e thought to have r e s u l t e d f r o m polygonization o c c u r r i n g i m m e d i a t e l y b e h i n d the a d v a n c i n g s o l i d - l i q u i d i n t e r f a c e . W i d e a n g l e g r a i n b o u n d a r i e s w e r e a l s o d e c o r a t e d . New s t r u c t u r e s w e r e i n t r o d u c e d by v a r i o u s l e v e l s of s t r a i n that a c c o m p a n i e d d i f f e r e n t u p s e t t i n g o p e r a t i o n s . G r a i n and s u b g r a i n b o u n d a r i e s that developed d u r i n g s t r a i n a n n e a l s w e r e " a n t i s i t e s " for hydrogen d e p o s i t i o n . T h i s opposite b e h a v i o r is a t t r i b u t e d to the a b s e n c e of i m p u r i t y a t o m s in t h e s e " n e w " b o u n d a r i e s . An e x p l a n a t i o n for the b e h a v i o r of h y d r o g e n in a l u m i n u m is g i v e n in t e r m s of the c o n d e n s a t i o n of v a c a n c i e s v s t h e i r loss to v a c a n c y s i n k s . T h e s e p h e n o m e n a a r e shown to be involved in the e n l a r g e m e n t of h y d r o g e n - f i l l e d p o r e s in a l u m i n u m . H Y D R O G E N shows unique b e h a v i o r in some m e t a l s in that it i s e x t r e m e l y mobile when d i s s o l v e d , but can b e come e s s e n t i a l l y i m m o b i l i z e d when p r e c i p i t a t e d in the g a s e o u s f o r m . T h i s b e h a v i o r i s a s s o c i a t e d with the equilibrium H 2 ~ - 2 H ; ZXFa98 = + 97 k c a l ; g
_ [HI z
[//2]
[1]
If the s o l u b l e s p e c i e s is a t o m i c hydrogen, Relation [ 1] r e q u i r e s that its s o l u b i l i t y at a given t e m p e r a t u r e , S H , be g o v e r n e d by S H cc PN:. T h i s was c o n f i r m e d in the c a s e of a l u m i n u m by R a n s l e y and Neufeld, 1 who found Molten
log s = l o g p I / z - 2760/T + 1.356
[2]
Solid
log s = log p , / z _ 2080/T - 0.652
[ 3]
w h e r e s is in m l p e r 100 g , p is in t o r r , and T is a b s o lute d e g r e e s . The
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