Cyclic behavior and delayed softening of commercial purity titanium

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W H I L E the c y c l i c d e f o r m a t i o n b e h a v i o r and the f a c t o r s influencing the fatigue life of t i t a n i u m have r e c e i v e d c o n s i d e r a b l e attention in the l i t e r a t u r e , ~-9 the v a r i a t i o n of s t r e s s a m p l i t u d e with c y c l e s d u r i n g s t r a i n c o n t r o l l e d t e s t s has only r e c e n t l y b e e n r e p o r t e d . ~~ P l u m t r e e and c o - w o r k e r s ~~ found t h a t on s t r a i n c y c l i n g a n n e a l e d e l e c t r o l y t i c g r a d e t i t a n i u m at s m a l l t o t a l s t r a i n a m p l i t u d e s (viz +0.25 pct), i n i t i a l l y h a r d e n ing t o o k p l a c e . T h i s w a s followed by a s a t u r a t i o n p e r i o d w h e r e the s t r e s s r e m a i n e d a p p r o x i m a t e l y c o n s t a n t to f r a c t u r e . In s a m p l e s c y c l i c a l l y o r m o n o t o n i c a l l y p r e s t r a i n e d to a s t r e s s l e v e l h i g h e r than t h e a n t i c i p a t e d s a t u r a t i o n s t r e s s , the c y c l i c s t r e s s r e s p o n s e d e c r e a s e d r a p i d l y to b e c o m e c o n s t a n t at the s a t u r a t i o n s t r e s s . T h i s s t r e s s l e v e l was s i g n i f i c a n t l y h i g h e r t h a n t h a t f o r the a n n e a l e d m a t e r i a l c y c l e d at the s a m e s t r a i n a m p l i t u d e . A t high t o t a l s t r a i n a m p l i t u d e s (e.g. • pct), which r e s u l t e d in s a t u r a t i o n s t r e s s e s h i g h e r than the p r e s t r a i n e d s t r e s s l e v e l s , both the p r e s t r a i n e d and the a n n e a l e d s a m p l e s continuously h a r d e n e d to a p p r o x i m a t e l y the s a m e s t r e s s p r i o r to f r a c t u r e . H i g h - p u r i t y t i t a n i u m i s known to have a high s t a c k i n g fault e n e r g y and should b e h a v e l i k e a w a v y - s l i p mode m a t e r i a l . The a n o m a l o u s d e p e n d e n c e of s a t u r a t i o n s t r e s s e s on p r i o r s t r a i n h i s t o r y at t h e low s t r a i n a m p l i t u d e s has b e e n a t t r i b u t e d to the p r e s e n c e of a g r e a t e r n u m b e r of twins in the p r e s t r a i n e d s a m p l e s . It was a l s o found that p r e s t r a i n i n g d e c r e a s e d t h e fatigue life. At low c y c l i c s t r a i n a m p l i t u d e s , u s i n g l o w e r p u r i t y J. I. DICKSON and J. DUCHER are Associate Professor and Research Associate, respectively, Ddpartment de Gdnie Mdtallurgique, l~cole Polytechnique, Campus de L'Universitd de Montrdal, Montrdal, Qudbec, Canda and A. PLUMTREE is Associate Professor, Department of Mechanical Engineering, University of Waterloo, Waterloo, Ontario, Canada. Manuscript submitted September 19, 1975. METALLURGICAL TRANSACTIONS A

a n n e a l e d t i t a n i u m , Munz 12'13 o b s e r v e d s t r e s s r e s p o n s e s w h i c h w e r e i n i t i a l l y m o r e o r l e s s c o n s t a n t followed b y a d e c r e a s e in s t r e s s w e l l b e f o r e f r a c t u r e . The s t a r t

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