The influence of grain size on the high cycle fatigue crack initiation of a metastable beta Ti alloy

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The Influence of Grain Size on the High Cycle Fatigue Crack Initiation of a Metastable Beta Ti Alloy R. CHAIT AND T. S. DeSISTO High cycle fatigue b e h a v i o r for a given m a t e r i a l is u s u a l l y d i s p l a y e d on an S-N c u r v e (applied s t r e s s - t o t a l n u m b e r of c y c l e s to f a i l u r e ) curve. Here, both c r a c k i n i t i a t i o n and c r a c k p r o p a g a t i o n c o n t r i b u t e to the total t i m e to f r a c t u r e . Crack i n i t i a t i o n under high cycle f a tigue loading (usually s t r e s s controlled) has r e c e i v e d i n c r e a s e d a t t e n t i o n e s p e c i a l l y in the a r e a of high s t r e n g t h s t e e l s . For e x a m p l e , an i n v e s t i g a t i o n of quenched and t e m p e r e d s t e e l s showed that c r a c k n u c l e a t i o n s i t e s a r e located at m a r t e n s i t e b o u n d a r i e s . ~ In a n o t h e r study conducted on 4340 steel,2 i n c l u s i o n s p r o v i d e d s i t e s for fatigue c r a c k initiation. With r a p i d l y quenched s t e e l s c o n t a i n i n g a m i c r o s t r u c t u r e of both f e r r i t e and m a r t e n s i t e , fatigue c r a c k s f o r m e d within the f e r r i t e . 3 Recent a t t e n t i o n has also b e e n given to fatigue c r a c k i n i t i a t i o n p r o c e s s of m i l d s t e e l u n d e r high cycle fatigue loading. 4 Regarding high s t r e n g t h t i t a n i u m a l l o y s , Lucas and Konieczny ~ showed i m p r o v e d fatigue s t r e n g t h for the T i - 6 A1-4 V and the T i - 6 A1-6 V-2 Sn a l p h a / b e t a a l l o y s that p o s s e s s a fine alpha g r a i n s i z e . B e n s o n et a l ~ have e x a m i n e d the fatigue c r a c k i n i t i a t i o n p r o c e s s i n notched s p e c i m e n s of T i - 6 A1-4 V and have concluded that c r a c k s f o r m e d by a slip band m e c h a n i s m in the hcp alpha g r a i n s except at s t r e s s l e v e l s n e a r the fatigue l i m i t w h e r e they f o r m e d at the i n t e r f a c e b e t w e e n alpha and bcc b e t a g r a i n s . Another f a m i l y of high s t r e n g t h t i t a n i u m alloys is the m e t a s t a b l e b e t a group where r e t e n t i o n of the beta phase at r o o m t e m p e r a t u r e m a k e s p o s s i b l e the c a p a c i t y for through h a r d e n i n g of a thick s e c t i o n upon aging. As a r e s u l t , t h r o u g h - t h e s e c t i o n s t r e n g t h p r o p e r t i e s of m e t a s t a b l e b e t a alloys R. CHAIT and T. S. DeSISTOare SupervisoryMaterials Engineers, Army Materials and MechanicsResearch Center, Watertown, MA 02172. Manuscript submitted June 22, 1976. METALLURGICALTRANSACTIONSA

a r e m o r e u n i f o r m when c o m p a r e d to those obtained f r o m equal s e c t i o n s i z e s of a l p h a / b e t a a l l o y s . 7 In a d dition, a t t r a c t i v e s t r e n g t h v a l u e s a c c o m p a n i e d by exc e l l e n t f r a c t u r e toughness and t e n s i l e p r o p e r t i e s have b e e n obtained. 7's While s o m e S-N c u r v e s do ex