Influence of differential dislocation mobility on the fatigue behavior of alloyed
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r e c e n t y e a r s , i n t e r e s t in the y ' - p h a s e h a s b e e n foc u s s e d on the a n o m a l o u s t e m p e r a t u r e d e p e n d e n c e of its y i e l d s t r e n g t h , which shows a d r a m a t i c i n c r e a s e with t e m p e r a t u r e up to a m a x i m u m at ~ 760~ Attempts to e x p l a i n this b e h a v i o r have b e e n b a s e d on p o s t u l a t e d t e m p e r a t u r e d e p e n d e n t c h a n g e s in i) l a t t i c e i n t e r a c t i o n s with d i s l o c a t i o n s , s'7 it) s l i p m o d e , 4 iii) d i s l o c a t i o n / v a c a n c y i n t e r a c t i o n s , 5 and iv) d i f f e r e n t i a l d i s l o c a t i o n m o b i l i t y . 6,s In t h i s a r t i c l e it is shown that the r e s p o n s e of y ' to c y c l i c s t r a i n i n g is c o n s i s t e n t with the c o n c e p t of d i f f e r e n t i a l d i s l o c a t i o n m o b i l i t y in 7 ' , w h a t e v e r i t s p r e cise origin. EXPERIMENTAL DETAILS A 7 ' a l l o y of c o m p o s i t i o n 7 6 . S N i - 1 6 . 2 A l - 6 . 6 W - 0 . T H f (at. pct) was v a c u u m m e l t e d , d i r e c t i o n a l l y s o l i d i f i e d and m a c h i n e d into 37.5 m m long r i d g e l e s s s p e c i m e n s with gage s e c t i o n 3 m m in d i a m and 13 m m long. The s p e c i m e n a x i s was t a k e n p a r a l l e l to the [100] g r o w t h d i r e c t i o n of the c a s t i n g . The a l l o y m i c r o s t r u c t u r e c o n s i s t e d of a s m a l l a m o u n t of y p h a s e in a 7 ' m a t r i x , F i g . 1. S t r e s s c o n t r o l l e d t e n s i o n - t e n s i o n fatigue e x p e r i m e n t s (+35 MN/m2 to the m a x i m u m t e n s i l e s t r e s s ) w e r e done on a s - c a s t s a m p l e s u s i n g a Sonntag fatigue m a c h i n e at 30 Hz; the u l t r a s o n i c fatigue t e s t s w e r e done at 20 kHz and w e r e s i m i l a r to t h o s e d e s c r i b e d previously. 9 Strain controlled tension-compression fatigue e x p e r i m e n t s w e r e done by u s i n g a h y d r a u l i c MTS m a c h i n e with s p e c i a l g r i p s t h a t c o n t a i n e d r i d g e s for s t r a i n m e a s u r e m e n t . 1~ T h e s e t e s t s w e r e c o n d u c t e d a t an " a v e r a g e " p l a s t i c s t r a i n r a n g e i n i t i a l l y of ep = 0.005, that i s a f t e r e v e r y 100 c y c l e s , when the h y s t e r e s i s loop w a s a l m o s t c l o s e d the p l a s t i c s t r a i n r a n g e was r e s e t to Ep = 0.01. T e n s i l e s p e c i m e n s w e r e of the s a m e g e o m e t r y a s the fatigue s p e c i m e n s with the e x c e p t i o n of t h o s e u s e d to d e t e r m i n e the m i c r o y i e l d b e h a v i o r . In t h i s c a s e s a m p l e s w e r e m a c h i n e d with J. E. DOHERTY, A. F. GIAMEI, and B. H. KEAR are with Materials Engineering and Research Laboratory, Pratt and Whitney Aircraft, Middletown, Conn. 96457. Manuscript submitted April 3, t975. METALLURGICAL TRANSACTIONS A
r i d g e s and the g a g e s e c t i o n length w a s i n c r e a s e d to 2
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