Positron lifetime changes during the fatigue of Cu
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AND
J. G. BYRNE
P o s i t r o n l i f e t i m e and X - r a y p a r t i c l e s i z e m e a s u r e m e n t s w e r e made d u r i n g c y c l i c fatigue of e a c h of c o l d - r o l l e d and v a r i o u s l y a n n e a l e d Cu. F a t i g u e cy cl i n g of i n i t i a l l y h a r d Cu c a u s e d the a v e r a g e p o s i t r o n l i f e t i m e and X - r a y p a r t i c l e s i z e to change in opposite s e n s e s , the f o r m e r d e c r e a s i n g and the l a t t e r i n c r e a s i n g , i n d i c a t i n g d e c r e a s i n g defect d e n s i t y and
an increase in the average subgrain size. Fatigue cycling of Cu initially in the annealed condition caused the mean positron lifetime to increase and the X-ray particle size to decrease, indicating an increase in the defect density and a decrease in the average subgrain size. Vacancy generation and condensation are verified as early fatigue mechanisms during fatigue in the absence of significant change of the dislocation cell structure.
I. INTRODUCTION EARLIER measurements I of mean positron lifetime changes caused by cyclic fatigue of steel prompted the present investigation of a more simple material, Cu, under similar circumstances. Each of initially hard and initially soft copper was chosen as the counterpart of the initially hard and initially soft 4340 steel in the previous investigation. I It had been found for 4340 steel that fatigue softening of initially hard (Rc51) steel was accompanied by an easily discernible decrease (40 ps) in the mean positron lifetime and that fatigue h a r d e n i n g of i n i t i a l l y soft (Rc27) s t e e l was a c c o m p a n i e d by a s i m i l a r magnitude i n c r e a s e in m e a n p o s i t r o n l i f e t i m e . A m a j o r i n t e r p r e t a t i o n a l difficu l t y e x i s t e d , h o w e v e r , in that e a c h of the two m i c r o s t r u c t u r e s studied was c o m p l e x and quite d i f f e r ent f r o m the o t h e r , p r e c l u d i n g any c l e a r i d e n t i f i c a t i o n of p o s i t r o n t r a p p i n g c e n t e r s . A s i m i l a r study of Cu p r e s e n t s a b e t t e r likelihood of i s o l a t i n g the p o s i t r o n t r a p p i n g c e n t e r s s i n c e only one p h a s e is involved and the p r i n c i p a l d i f f e r e n c e in s t a r t i n g m a t e r i a l s is s o l e l y dislocation substructure.
2. EXPERIMENTAL Commercially pure (99.95 pct) Cu was provided by the Kennecott Copper Corporation. The main impurity was 500 ppm 02. The minor impurities were 30 ppm Fe, 15 ppm S, 9 ppm Ag and 3 ppm of each of Ni, Pb and Sb. The Cu p r o c e s s i n g c o n s i s t e d of hot r o l l i n g f r o m a t h i c k n e s s of 0.22 m to 0.02 m, s u r f a c e s c a l p i n g of 4 to 8 m m p e r s u r f a c e and cold r o l l i n g to a f i n al t h i c k n e s s of 1 m m . F a t i g u e s p e c i m e n s w e r e m a c h i n e d f o r constant m a x i m u m a l t e r n a t i n g s t r e s s c a n t i l e v e r bending in a Sontag S F - 2 - U m a c h i n e o p e r a t i n g at 30
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