Low-Cycle fatigue crack nucleation and early growth in Ti-17
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Low-cycle
fatigue is an i m p o r t a n t c o n c e r n in r o t a r y c o m p o n e n t s of l a n d - b a s e d t u r b i n e s and a i r c r a f t engines due to cyclic loads i m p o s e d by v i b r a t i o n , engine power changes, and t h e r m a l v a r i a t i o n s . Since c r u c i a l m a t e r i a l s a p p l i c a t i o n s a r e involved, an u n d e r s t a n d i n g of the m i c r o s t r u c t u r a l a s p e c t s of the l o w - c y c l e fatigue p r o c e s s is vital. T h i s u n d e r s t a n d i n g can lead to i m p r o v e d s e r v i c e p e r f o r m a n c e through changes in alloy c o m p o s i t i o n or t h e r m o m e c h a n i c a l p r o c e s s i n g techniques. The fatigue process of an uncracked engineering component can be considered to have three stages:1 crack nucleation and early growth at or near the surface, c r a c k propagation through a p l a s t i c r e g i m e , and c r a c k p r o p a g a t i o n through an e l a s t i c r e g i m e , as r e p r e s e n t e d in F i g . 1. The p u r p o s e of this i n v e s t i g a t i o n was to study the m i c r o s t r u c t u r a l a s p e c t s of the c r a c k n u c l e a t i o n and e a r l y growth stage for an e + 13 t i t a n i u m alloy (Ti-17) in t h r e e t h e r m o m e c h a n i c a l l y p r o c e s s e d f o r m s : /3 p r o c e s s e d , h i g h - t e m p e r a t u r e c~ + /3 p r o c e s s e d , and low t e m p e r a t u r e a +/3 p r o c e s s e d . The h i g h - t e m p e r a t u r e a +/3 p r o c e s s e d m a t e r i a l was s e l e c t e d to cont a i n a m i c r o s t r u c t u r a l flaw c o m m o n in t i t a n i u m a l l o y s (the s o - c a l l e d / 3 - f l e c k ) and its effect on fatigue was also studied. O b s e r v a t i o n s on fatigue c r a c k n u c l e a t i o n and e a r l y growth were made on the Surfaces of smooth l a b o r a t o r y s a m p l e s s u b j e c t e d to high- s t r a i n fatigue, as shown in Fig. 1. To avoid ambiguity, c r a c k n u c l e a tion is defined in this p a p e r as the f o r m a t i o n of c r a c k s d e t e c t i b l e u s i n g s c a n n i n g e l e c t r o n m i c r o s c o p y (of the o r d e r of one m i c r o n ) . E a r l y c r a c k growth is defined as the growth and linkage of these m i c r o c r a c k s p r i o r to the f o r m a t i o n of m a c r o c r a c k s (~30 m i l s ) .
and low t e m p e r a t u r e o~ + ~ p r o c e s s e d forging s e c t i o n s used in this i n v e s t i g a t i o n were supplied by the Mater i a l s and P r o c e s s e s Technology L a b o r a t o r y , G e n e r a l E l e c t r i c A i r c r a f t Engine Group. M a t e r i a l c o m p o s i t i o n s and t e n s i l e p r o p e r t i e s of the forgings a r e p r e s e n t e d in T a b l e s I and II r e s p e c t i v e l y . The /3 p r o c e s s e d m a t e r i a l had b e e n forged at 913~ (above the /3 t r a n s u s ) , s o l u t i o n i z e d at 802~ for 4 h, w a t e r - q u e n c h e d , aged at 635~ for eight h, and a i r -
ZONE L . ~
Fig. 1--Model of crack ini
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