Galvano-thermomagnetic properties of aligned bi-ag eutectic alloy
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T y p i c a l scanning e l e c t r o n m i c r o g r a p h s obtained f r o m the s a m e a r e a of the s u r f a c e of a c o p p e r s p e c i m e n a f t e r i n c r e a s i n g n u m b e r s of s t r a i n c y c l e s a r e shown in F i g . 2. The p a r t i c u l a r s p e c i m e n w a s m a c h i n e d f r o m a drawn e l e c t r o l y t i c copper r o d (99.9 pct Cu). A f t e r m a c h i n i n g the s p e c i m e n was e l e c t r o p o l i s h e d and s l i g h t l y e t c h e d in the gage s e c t i o n to r e v e a l g r a i n b o u n d a r i e s . The e l e c t r o n m i c r o g r a p h s of the a s - e t c h e d s u r f a c e r e v e a l e d e l o n g a t e d g r a i n b o u n d a r i e s and o c c a s i o n a l e t c h - h i l l s . F o r the d e s c r i b e d e x p e r i m e n t the s t r a i n a m p l i t u d e w as c a l c u l a t e d f r o m the m e a s u r e d d i s p l a c e m e n t to be 5 • 10 -4, which is e q u i v a l e n t to a s t r e s s a m p l i t u d e of a p p r o x i m a t e l y 15 k p / m m 2. A f t e r each fatigue t e s t i n t e r v a l the s p e c i m e n s u r f a c e w a s e x a m i n e d o v e r it s e n t i r e c i r c u m f e r e n c e . The f i r s t fatigue d a m a g e , c o n s i s t i n g of a s h o r t s t r e t c h of a s u r f a c e slip m a r k i n g , could be d e t e c t e d a f t e r 4 • 104 s t r a i n c y c l e s as i n d i c a t e d by A in Fig. 2(a). The two e t c h - h i l l s B and C m ay be used as r e f e r e n c e m a r k s in the s u b s e quent m i c r o g r a p h s . F i g . 2(b) shows the s a m e a r e a a f t e r 6 • 104 c y c l e s , the i n c r e a s e in the n u m b e r of the s u r face m a r k i n g s is c l e a r l y v i s i b l e . F r o m the v a r i o u s o r i e n t a t i o n s of the s u r f a c e slip m a r k i n g s one m a y deduce that s e v e r a l slip s y s t e m s had been a c t i v a t e d . A f t e r 8 • 104 c y c l e s , a s u r f a c e c r a c k b e c a m e v i s i b l e in the r e gion of the m o s t s e v e r e d a m a g e , at point D in F i g . 2(c). T h i s is m o r e c l e a r l y shown at h i g h e r m a g n i f i c a t i o n in F i g . 2(d). The c r a c k r a p i d l y p r o p a g a t e d along r e g i o n s of s e v e r e d a m a g e during the s u b s e q u e n t s t r a i n c y c l e s and e x t e n d e d a l m o s t c o m p l e t e l y a r o u n d the c i r c u m f e r e n c e of the s p e c i m e n a f t e r 1.2 • l0 s s t r a i n c y c l e s . Th e c o n c e n t r a t i o n of the s u r f a c e d a m a g e n e a r the c e n t e r of the gage s e c t i o n and the extent of the s u r f a c e c r a c k is i l l u s t r a t e d in the l o w - m a g n i f i c a t i o n c o m p o s i t e e l e c t r o n m i c r o g r a p h in Fig. 2(e), which c o v e r s a l m o s t 73 of the specimen circumference. The scanning e l e c t r o n m i c r o g r a p h s shown in F i g . 2 c l e a r l y r e v e a l the c h a n g e s of the s p e c i m e n s u r f a c e t o p o g r a p h y a s s o c i a t e d with the fatigue d a m a g e a
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