A crystallographic study of fatigue damage in titanium
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is now well e s t a b l i s h e d that fatigue c r a c k i n g in ductile m e t a l s is c l o s e l y r e l a t e d with e x t r u s i o n s and i n t r u s i o n s which a r e f o r m e d in s l i p b a n d s . 1-~ F a t i g u e d e f o r m a t i o n was g e n e r a l l y o b s e r v e d to c o n c e n t r a t e at b o u n d a r i e s of a g r a i n at higher s t r a i n a m p l i t u d e , while at lower s t r a i n a m p l i t u d e d e f o r m a t i o n was widely d i s p e r s e d over the g r a i n s . 4,5 Most of t h e s e s t u d i e s w e r e c a r r i e d out on m e t a l s and a l l o y s p o s s e s s i n g a face c e n t e r e d cubic c r y s t a l s t r u c t u r e . P u b lished fatigue s t u d i e s for m e t a l s and alloys having bcc and hcp c r y s t a l s t r u c t u r e s a r e much m o r e l i m i t e d . P a r t i c u l a r l y , few s t u d i e s a p p e a r in the l i t e r a t u r e on hexagonal c l o s e packed m e t a l s w h e r e m e c h a n i c a l t w i n n i n g can b e c o m e an i m p o r t a n t mode of d e f o r m a t i o n . It was shown that in p u r e t i t a n i u m f a tigue d e f o r m a t i o n and c r a c k i n i t i a t i o n o c c u r r e d at d e f o r m a t i o n t w i n s . 6-13 T i t a n i u m is known to have a high s t a c k i n g fault e n e r g y , 14,1~ thus wavy s l i p l i n e s might also be expected on the s u r f a c e of a s p e c i m e n d e f o r m e d at r o o m t e m p e r a t u r e . However, f r e s h twins were o b s e r v e d to be c o n t i n u o u s l y i n t r o d u c e d d u r i n g fatigue at high s t r a i n a m p l i t u d e s , and c r a c k s origi.nated p r e f e r e n t i a l l y on the twin b o u n d a r i e s . 8-~~ Twin b o u n d a r y d a m a g e was a l s o o b s e r v e d at e x i s t i n g twins where the applied s t r e s s was i n s u f f i c i e n t to n u c l e a t e f r e s h twins. 9 C o n t r a r y to t h e s e o b s e r v a t i o n s T u r n e r and R o b e r t s o b s e r v e d slip b a n d c r a c k i n g ; ~6 and MacDonald and Wood o b s e r v e d a " s l i p l e s s c r a c k i n g " , which o r i g i n a t e d and p r o p a g a t e d in a g e n e r a l d i r e c t i o n of m a x i m u m s h e a r . 17 It was f u r t h e r shown by D i c k s o n e t al that the m a j o r cyclic d e f o r m a t i o n mode was by slip, and that twins occupied l e s s than 10 pct of the s u r f a c e a r e a even at a t o t a l s t r a i n a m plitude of :L1 . 0 pct. ~8 M. SUGANO is Assistant Professor, Department of Basic Technology, Yamagata University, Japan. C. M. GILMORE is Associate Professor, School of Engineering and Applied Science, George Washington Unwersaty, Washington, DC 20052. Manuscript submitted July 6, 1979. METALLURGICAL TRANSACTIONS A
The p r e s e n t study was conducted to o b t a i n c r y s t a l l o g r a p h i c i n f o r m a t i o n on the d e f o r m a t i o n mode a p p e a r ing in c o m m e r c i a l p u r i t y t i t a n i u m d u r i n g t o r s i o n a l f a tigue b e c a u s e t h e r e does not
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