Damping studies of aging in Al-Cu
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AND
S.K.
GUPTA
P r e c i p i t a t i o n in a n A1-3 pct Cu a g e - h a r d e n i n g a l l o y has b e e n m o n i t o r e d at 130~ by u l t r a sonic p u l s e - e c h o m e a s u r e m e n t s of a t t e n u a t i o n and sonic v e l o c i t y at 15 to 75 mHz and r e s o n a n c e b a r d a m p i n g and d y n a m i c m o d u l u s m e a s u r e m e n t s at 80 kHz. The u s e of two i n t e r n a l f r i c t i o n m e a s u r e m e n t t e c h n i q u e s allows s e p a r a t i o n of the total d a m p i n g loss into its c o m p o n e n t p a r t s . D a m p i n g and m o d u l u s defect r e m a i n c o n s t a n t with t i m e up to 100 h r , w h e r e u p o n they begin to i n c r e a s e and r e a c h a m a x i m u m at 250 h r . The o b s e r v e d t i m e d e p e n d e n c e is a s s o c i a t e d with s c a t t e r i n g l o s s e s by p r e c i p i t a t e p a r t i c l e s and with i n c r e a s ing dislocation damping as dtslocattous become increasingly mobile during the depletion of their solute atmospheres by growing solute-rich precipitates.
DAMPING experiments provide a measure of the mechanical energy dissipated by processes occurring within a vibrating body. This information in turn, is used to establish the atomistic details of the processes. Since the processes are studied indirectly, the damping data must be subjected to very careful interpretation if unambiguousinformation concerning the loss processes is to be obtained. It is for this reason that the t y p i c a l e x p e r i m e n t is c a r r i e d out in a r a n g e of f r e q u e n c y , s t r a i n a m p l i t u d e , and t e m p e r a t u r e , at which the loss m e c h a n i s m of p r i m a r y i n t e r e s t is l a r g e s t , so that o t h e r d i s s i p a t i v e p r o c e s s e s can be n e g l e c t e d . In m a n y c a s e s of i n t e r e s t , h o w e v e r , it is i m p o s s i b l e to a d j u s t e x p e r i m e n t a l c o n d i t i o n s so that a s i n g l e loss p r o c e s s can be i s o l a t e d . F o r e x a m p l e , it has been known for s o m e t i m e 1 that i n t e r n a l f r i c t i o n and d y n a m i c m o d u l u s m e a s u r e m e n t s a r e a s e n s i t i v e tool for the study of p r e c i p i t a t i o n in s o l i d s . Chiou et al.2 have pointed out the a d v a n t a g e s of such m e a s u r e m e n t s o v e r X - r a y s t u d i e s , r e s i s t i v i t y m e a s u r e m e n t s , and s e v e r a l o t h e r e x p e r i m e n t a l p r o c e d u r e s . E v e n in the s i m p l e s t c a s e of p r e c i p i t a t i o n of a phase in a s u p e r s a t u r a t e d m a t r i x , the p r e s e n c e of s e v e r a l c o n c u r r e n t and i n t e r a c t i n g loss p r o c e s s e s m a k e s i n t e r p r e t a t i o n of d a m p i n g data difficult. In a polycrystalline material, various losses associated with g r a i n b o u n d a r i e s e x i s t at f r e q u e n c i e s f r o m 1 Hz to hundreds of mHz. The growing precipitate itself is responsible for scattering losses which chan
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