Crystallographic and fractographic studies of hydrogen- induced cracking in purified iron and iron- silicon alloys
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ALTHOUGH
h y d r o g e n - i n d u c e d c r a c k i n g in f e r r o u s alloys has b e e n studied e x t e n s i v e l y f r o m both the f u n d a m e n t a l and p r a c t i c a l points of view, its f r a c t o g r a p h i c and c r y s t a l l o g r a p h i c n a t u r e i s s t i l l a s u b j e c t of dispute. Recently, f r a c t o g r a p h i c s t u d i e s have r e vealed that p l a s t i c d e f o r m a t i o n can play an i m p o r t a n t r o l e in the p r o c e s s of hydrogen induced c r a c k growth 1 and that it is not sufficient to i n t e r p r e t hydrogen e m b r i t t l e m e n t as a s i m p l e e x t e n s i o n of cleavage f r a c t u r e . z The c r y s t a l l o g r a p h i c n a t u r e of t r a n s g r a n u l a r hydrogen induced c r a c k s , f r o m the r e s u l t s T e t e l m a n and R o b e r t s o n , 3,4 and Gell and R o b e r t s o n ~ who exa m i n e d i r o n - 3 pct s i l i c o n single c r y s t a l s , is g e n e r a l l y accepted as {100} c r a c k s , the u s u a l cleavage p l a n e s of ~ - i r o n f '7 T h e i r r e s u l t s were obtained by o b s e r v ing the r e l a t i v e p o s i t i o n s of etch pit a r r a y s of d i s l o c a tions and one s u r f a c e a n a l y s i s of c r a c k t r a c e s . More r e c e n t l y , conflicting data have q u e s t i o n e d the g e n e r a l i t y of this r e s u l t . B e r n s t e i n 8,9 f r o m a o n e - s u r face t r a c e a n a l y s i s in p u r i f i e d p o l y c r y s t a l l i n e iron, s u g g e s t e d i n s t e a d that hydrogen induced t r a n s g r a n u l a r c r a c k s , in the a b s e n c e of an e x t e r n a l s t r e s s , occur on the potential slip p l a n e s , {110} or {112}. 8,9 T e r a s a k i and Nakasato 2 also d e m o n s t r a t e d the e x i s t ence of a (101) c r a c k in hydrogen charged and ext e r n a l l y s t r e s s e d i r o n single c r y s t a l . In conflict with these r e s u l t s , K i t a j i m a m has c l a i m e d that the f r a c t u r e s u r f a c e s a r e m a c r o s c o p i c a l l y along {100} p l a n e s in both i r o n single c r y s t a l s and i r o n - 3 pct s i l i c o n single c r y s t a l s u n d e r conditions of h y d r o g e n c h a r g i n g and ext e r n a l s t r e s s i n g . F o r hydrogen c h a r g i n g only in i r o n 3 pct s i l i c o n p o l y c r y s t a l s , Kikuta e t al u deduced, f r o m the shape of facet pits d i r e c t l y on the f r a c t u r e s u r f a c e , that t h e r e was a t r a n s i t i o n of c r a c k plane F. NAKASATO,formerly at Carnegie-MellonUniversity is now at Sumitomo Metal Industries Ltd., Central Research Laboratories, Amagasaki, Japan. I. M. BERNSTEIN is Professor, Department of Metallurgy and Materials Science, Carnegie-MellonUniversity, Pittsburgh, PA 15213. Manuscript submitted February 27, 1978. METALLURGICALTRANSACTIONSA
f r o m {100} to {110} planes with i n c r e a s i n g c h a r g i n g t i m e of hydrogen. In light of these conflicting r e s u l t s obtained for s i m i l a r m a t e r i a l s
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