Void formation, void growth and tensile fracture in Ti-6AI-4V

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HOLDEN,Ogden and Jaffee ~ in a study of the

mec h a n i c a l p r o p e r t i e s of a-~3 T i - M n alloys showed that the d u c t i l i t y of equiaxed a (Eot) s t r u c t u r e s was higher than that for W i d m a n s t ~ t t e n plus g r a i n b o u n d a r y a (W + GBa) s t r u c t u r e s . L a t e r , in an i n v e s t i g a t i o n to d e t e r m i n e the r e a s o n for this d i f f e r e n c e , G r e e n f i e l d and M a r g o l i n 2 s t u d i e d an a - f l T i - 5.25A1- 5 . 5 V - 0 . 9 F e ( T i - 5 - 5 ) alloy. F o r both Eot and W + G B a s t r u c t u r e s they found that f r a c t u r e depended on void n u c l e a t i o n and growth, f r a c t u r e o c c u r r i n g w h e n e v e r the c r i t i c a l c r a c k s i z e - s t r e s s r e l a t i o n s h i p was r e a c h e d for the T i- 5- 5 alloy. It was found that void growth o c c u r r e d m o r e r a p i d l y at the g r a i n b o u n d a r i e s of the W + G B a s t r u c t u r e s than in the case of Eot. F o r the W + G B a s t r u c t u r e s the c r i t i c a l c r a c k s i z e - s t r e s s r e l a t i o n s h i p was r e a c h e d at a lower s t r a i n p r i o r to f r a c t u r e . (The void is cons i d e r e d a c r a c k when it r e a c h e s the c r i t i c a l size for fracture.) T h i s study was u n d e r t a k e n to study the m e c h a n i s m of t e n s i l e f a i l u r e of the Ti-6A1-4V alloy in a n a t t e m p t to t m d e r s t a n d why the T i - 5 - 5 alloy can be heat t r e a t e d to higher s t r e n g t h s while m a i n t a i n i n g or exceeding the Ti-6A1-4V ductility. I. E X P E R I M E N T A L PROCEDURE ELI Ti-6A1-4V, c o n t a i n i n g 0.078 wt pct oxygen, 12.2 m m r o u n d b a r stock, which was a v a i l a b l e f r o m an e a r l i e r i n v e s t i g a t i o n , 3 was used for the c u r r e n t s t u d i e s . T h e t3-transus was d e t e r m i n e d as 1245 K. T h e m a t e r i a l was heat t r e a t e d to p r o d u c e e i t h e r a W + G B a or an E a s t r u c t u r e , the fi phase of which t r a n s f o r m e d to m a r t e n s i t e on quenching. W + G B a s t r u c t u r e was p r o duced by i n i t i a l l y h e a t i n g to 1266 K, then f u r n a c e cooling to 1199 K, hold for v a r i o u s t i m e s , W.Q. E a s t r u c t u r e s w e r e p r o d u c e d b y d i r e c t l y h e a t i n g to 1199 K. F o r both types of s t r u c t u r e t i m e s at 1199 K v a r i e d f r o m 1 to 150 h to c o n t r o l the size of p r i m a r y a p a r t i HAROLD MARGOLINis Professor and YASHWANTMAHAJAN is Graduate Student in the Department of Physical and Engineering Metallurgy, Polytechnic Institute of New York, Brooklyn, NY 11201. Manuscript submitted April 28, 1977. METALLURGICALTRANSACTIONSA

c l e s . Aging t e m p e r a t u r e s were v a r i e d f r o m 755 to 1033 K to obtain y i e l d s t r e n g t h s f r o m 110 to 140 k s i (748 to 965 MN/m2). M e a s u r e m e n t of p a r t i c l e s i z e s and volume f r a c t i o n s was c a r r i e d out with a H u r l b u t c o u n t e r . G e n e r