The influence of grain boundaries on the fracture mode of Zirconium and Zircaloy-2
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The mode of f r a c t u r e w a s e x a m i n e d in Z i r c a l o y - 2 sheet s p e c i m e n s p u l l e d in t e n s i o n o v e r a r a n g e of s t r a i n r a t e s (10 -8 - 10 -1 m i n -1) at 350~ As the s t r a i n r a t e d e c r e a s e d f r o m 10 -1 to 10 -4 m i n -1, the mode of f r a c t u r e changed f r o m t r a n s g r a n u l a r d u c t i l e s h e a r to a mixed mode of i n t e r g r a n u l a r and d u c t i l e s h e a r f a i l u r e . T h i s w a s a c c o m p a n i e d by the r i d g i n g of g r a i n b o u n a r i e s on the f r a c t u r e s u r f a c e . With d e c r e a s i n g g r a i n s i z e this r i d g i n g b e c a m e m o r e evident. It w a s not o b s e r v e d on the f r a c t u r e s of high p u r i t y iodide z i r c o n i u m . This c o r r e l a t e s with the e a r l i e r o b s e r v a t i o n that the f r a c t u r e s t r a i n d e c r e a s e s with d e c r e a s i n g s t r a i n r a t e for Z i r c a l o y - 2 but not for iodide z i r c o n i u m . No d i r e c t e v i d e n c e could be found for any g r a i n b o u n d a r y s t r e n g t h e n ing p h e n o m e n o n . The m i c r o s c o p i c f r a c t u r e b e h a v i o r w a s , h o w e v e r , found to c o r r e l a t e with the m a c r o s c o p i c f r a c t u r e o r i e n t a t i o n . The r i d g i n g of the b o u n d a r i e s i n c r e a s e s a s the f r a c t u r e path shifts f r o m a d i r e c t i o n with a high s h e a r c o m p o n e n t to one that is n e a r l y n o r m a l to the t e n s i l e axis.
DESPITE
the l i m i t e d work on the i d e n t i f i c a t i o n of the f r a c t u r e c h a r a c t e r i s t i c s of z i r c o n i u m and its a l l o y s , it is now w e l l e s t a b l i s h e d that a - Z r with a hcp s t r u c t u r e g e n e r a l l y fails in t e n s i o n in a d u c t i l e m o d e . 1-a C1ea v age and i n t e r g r a n u l a r f a i l u r e a r e u s u a l l y not o b s e r v e d . T h e r e a r e two exceptions to t h i s g e n e r a l t r e n d , howe v e r ; m i c r o c r a c k s o r b r i t t l e f a i l u r e s have b e e n o b s e r v e d i n h y d r i d e d z i r c o n i u m , 1,3-7 and also when f r a c t u r e is i n i t i a t e d by twinningz o r i n c l u s i o n s , s T h i s p a p e r d e a l s with a n o t h e r " n o n d u c t i l e " mode of f r a c t u r e that o c c u r s in Z i r c a l o y - 2 when t e s t e d in t e n s i o n at slow strain rates. D u r i n g r e c e n t s t u d i e s of the effects of t e x t u r e 9 and a l l o y i n g 1~ on the m e c h a n i c a l b e h a v i o r of Z i r c a l o y - 2, the t e n s i l e d u c t i l i t y was o b s e r v e d to d e c r e a s e r o u g h l y by o n e - h a l f when the s t r a i n r a t e was r e d u c e d by two o r d e r s of m a g n i t u d e . T h i s o c c u r r e d at a t e s t t e m p e r a t u r e of 350~ a p p r o x i m a t e l y 0.3 of the a b s o l u t e m e l t ing t e m p e r a t u r e of z i r c o n i u m . T h e f r a c t o g r a p h y of t h o s e t e n s i l e s p e c i
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