Cavitation-Induced erosion of Ti-6Al-4V
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M. F. PREZKOP,
D. E. MIKKOLA,
AND
D. A. KOSS
C a v i t a t i o n - i n d u c e d e r o s i o n has b e e n e x a m i n e d in Ti-6A1-4V in the m i l l a n n e a l e d , s o l u t i o n - t r e a t and aged, and beta a n n e a l e d c o n d i t i o n s . Weight loss data show only s m a l l d i f f e r e n c e s b e t w e e n heat t r e a t m e n t s with the s o l u t i o n - t r e a t and aged m i c r o s t r u c t u r e exhibiting the lowest weight loss r a t e s . S e q u e n t i a l m i c r o g r a p h s of the s a m e s p e c i m e n a r e a as a function of e r o s i o n time show that i n i t i a l f r a c t u r e o c c u r s along the ~-~3 i n t e r faces and a l o n g c r y s t a l l o g r a p h i c slip b a n d s in the o~-phase. The e a r l y s t a g e s of e r o s i o n a r e also dependent on the o r i e n t a t i o n of the W i d m a n s t a t t e n colonies in the beta a n n e a l e d condition. T h e s e o b s e r v a t i o n s s t r o n g l y s u g g e s t that fatigue f r a c t u r e is i m p o r t a n t , at l e a s t in the e a r l y s t a g e s of the c a v i t a t i o n e r o s i o n p r o c e s s . D e p r e s s i o n of the softer o~phase also o c c u r s at s h o r t e x p o s u r e t i m e s , and this f a c i l i t a t e s f r a c t u r e and r e m o v a l of the exposed m a t e r i a l ; i.e., /3-phase or t e m p e r e d m a r t e n s i t e . E x a m i n a t i o n of the eroded s u r f a c e s in the l a t e r s t a g e s where c o n s i d e r a b l e m a t e r i a l has b e e n r e m o v e d shows little e v i d e n c e of the u n d e r l y i n g m i c r o s t r u c t u r e , despite the d i s t i n c t d i f f e r e n c e s in the m i c r o s t r u c t u r e s of the s a m p l e s t e s t e d .
THE
e r o s i o n of alloys by c a v i t a t i o n has r e c e i v e d c o n s i d e r a b l e a t t e n t i o n in r e c e n t y e a r s . Much of this effort has b e e n d i r e c t e d toward e x a m i n i n g and c o r r e l a t i n g weight loss data with v a r i o u s m e c h a n i c a l p r o p e r t i e s . As a r e s u l t , b a s e d p r i m a r i l y on m a c r o scopic o b s e r v a t i o n s , m a n y t h e o r i e s and r a t i o n a l i z a tions have e m e r g e d to account for c a v i t a t i o n - i n d u c e d e r o s i o n . In an effort to u n d e r s t a n d the c a v i t a t i o n e r o s i o n p r o c e s s , the p r e s e n t study r e p o r t s o b s e r v a tions of c a v i t a t i o n - i n d u c e d e r o s i o n as this p r o c e s s r e l a t e s to alloy m i c r o s t r u c t u r e and m e t a l r e m o v a l on a m i c r o s c o p i c s c a l e . T h e r e have b e e n p r e v i o u s s t u d i e s r e l a t i n g e r o s i o n b e h a v i o r to m i c r o s t r u c t u r e . F o r example, the c a v i t a t i o n - i n d u c e d e r o s i o n of a p u r e A1~ and a s i n g l e phase C u - A u alloy 2 have b e e n e x a m i n e d in detail, as has the i n f l u e n c e of a c a v i t a t i o n induced phase t r a n s f o r m a t i o n on the e r o s i o n
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