Scanning electron microscope fractography of continuously cast high purity copper after high temperature creep
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D. B. BALLARD
Fracture surfaces produced by high temperature creep were studied using the scanning electron microscope. The material investigated was continuously cast high purity copper containing a nodal impurity segregation structure at which grain boundary voids are formed during creep. The observed void shape suggests that vacancies are supplied mainly via grain boundaries, and also by enhanced diffusion via segregation nodes; the vacancies seem to originate mainly at internal sources. The known distribution of potential nucleation sites was used to study the efficiency of the segregation structure in nucleating voids under various test conditions. Within the range of conditions employed, three different fracture modes were observed in separate regions of the stress-temperature plane. The regions are sequentially denoted A, B, and C as the temperature is increased at a given stress; they shift to lower temperatures as the stress is increased. In region A fracture is initiated by extensive cavitation along grain edges (line of ]unction of three grains); cavitation at the segregation structure seems to be of secondary importance. In region B formation and growth to coalescence of voids at segregation nodes governs fracture; the change of growth mechanisms with test conditions is discussed. In region C fracture is controlled by plastic instability. T H E m e c h a n i s m s which g o v e r n c a v i t a t i o n f r a c t u r e in high t e m p e r a t u r e c r e e p a r e u s u a l l y studied by e x a m i n a t i o n of m e t a l l o g r a p h i c s e c t i o n s of s p e c i m e n s which have been tested into third stage c r e e p or to f r a c t u r e . Such techniques r e p r e s e n t a t w o - d i m e n s i o n a l s a m p l i n g p r o c e d u r e of a t h r e e - d i m e n s i o n a l void p a t t e r n , and one has to solve the p r o p e r s t e r e o l o g i c a l p r o b l e m to obtain valid i n f o r m a t i o n on the t h r e e - d i m e n s i o n a l void d i s t r i b u t i o n . R e c e n t l y n u m e r o u s s t u d i e s have been u n d e r t a k e n with the a i m of r e a c h i n g a b e t t e r u n d e r s t a n d i n g of the k i n e t i c s of the void growth m e c h a n i s m s by studying the shape of voids p r i o r to f r a c t u r e with the aid of e l e c t r o n m i c r o s copy and v a r i o u s s p e c i m e n p r e p a r a t i o n methods. 1-7 The use of the s c a n n i n g e l e c t r o n m i c r o s c o p e e l i m i n a t e s many of the difficulties with p r i o r methods; p e r m i t s the o b s e r v a t i o n of the u n d i s t u r b e d f r a c t u r e , and s u r f a c e a r e a not e a s i l y a c c e s s i b l e that give i n s i g h t into the f r a c t u r e p r o c e s s . The p r e s e n t study is p a r t of an i n v e s t i g a t i o n of high t e m p e r a t u r e c r e e p f r a c t u r e of a d i r e c t i o n a l l y solidified high p u r i t y copper. T h i s m a t e r i a l c o n tained a nodal (cellular) s o l i d i f i c a t i o n s t r u c t u r
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