Cavitation erosion of age-hardenable aluminum alloys
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s e q u e n c e of events which leads to c a v i t a t i o n induced f a i l u r e of m e t a l s v a r i e s a c c o r d i n g to t h e i r c r y s t a l s t r u c t u r e , m i c r o s t r u c t u r e and d e f o r m a t i o n mode.l,2 The work r e p o r t e d in this p a p e r is the second p a r t of a broad, s y s t e m a t i c study a i m e d at u n d e r s t a n d ing these f a c t o r s . The d e f o r m a t i o n and e r o s i o n of p u r e FCC m e t a l s by c a v i t a t i o n in d i s t i l l e d w a t e r has been d i s c u s s e d in an e a r l i e r p u b l i c a t i o n , s The p r e s e n t work d e s c r i b e s the effect of v a r y i n g the c o m p o s i t i o n and heat t r e a t m e n t of a g e - h a r d e n a b l e a l u m i n u m alloys on t h e i r r e s p o n s e to cavitation. T h r e e alloy s y s t e m s , n a m e l y A1-Cu, A1-Mg and A1Z n - M g - C u , were chosen for study. The A1-Cu alloys a r e r a p i d l y p r e c i p i t a t i n g and t h e i r s t r u c t u r e and " s t a t i c " m e c h a n i c a l p r o p e r t i e s as a function of heat t r e a t m e n t a r e well documented. 4 The A1-Mg s y s t e m , on the other hand, exhibits m o r e s l u g g i s h r e a c t i o n k i n e t i c s and its m e c h a n i c a l s t r e n g t h is c o n s i d e r e d to be d e r i v e d l a r g e l y f r o m solid s o l u t i o n s t r e n g t h e n i n g and s t r a i n aging instead. 4 The q u a t e r n a r y alloy is a high p u r i t y analogue of the c o m m e r c i a l 7 0 7 5 - a l u m i n u m alloy. EXPERIMENTAL PROCEDURE Alloys of the c o m p o s i t i o n s given in T a b l e I were cold r o l l e d to a t h i c k n e s s of 2 m m and cut into 13 • 13 m m coupons. T h e s e w e r e m e c h a n i c a l l y polished and heat t r e a t e d u n d e r the conditions given in T a b l e I. The t i m e s n e c e s s a r y to obtain peak h a r d n e s s of the A1-Cu alloys at the heat t r e a t i n g t e m p e r a t u r e (220~ were d e t e r m i n e d by m o n i t o r i n g t h e i r m i c r o h a r d n e s s v a l u e s . A1-4 pct Cu was studied i n two o v e r a g e d conditions: " O v e r - a g e d I " with c o a r s e , widely spaced CuA12 p r e cipitates and " O v e r - a g e d I I " in which the CuA12 p a r S. VAIDYAis Graduate Research Assistant, Department of Materials Science,State Universityof New York at Stony Brook, NY 11794 and C. M. PREECE, formerly Associate Professor at S.U.N.Y., Stony Brook, is now a member of the Technical Staff at Bell Laboratories, Murray Hill, NJ 07974. Manuscript submitted June 22, 1977, METALLURGICALTRANSACTIONSA
t i c l e s were f i n e r and m o r e c l o s e l y d i s p e r s e d . A c c e l e r a t e d c a v i t a t i o n e r o s i o n was produced in the s y s t e m i l l u s t r a t e d in Fig. 1 and d e s c r i b e d p r e v i ously. 3's The f r e q u e n c y and a m p l i t u d e of the v i b r a t i n g horn were 20 kHz and 40 /Jan, r e s p e c t i v e l y , ( c o r r e s p o n d i n g to an a v e r a g e cavitation s t r e s s
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