Structure/property/continuum synthesis of ductile fracture in inconel alloy 718
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to the c r a c k p l a n e due to s h e a r s t r a i n s . F o r this mode of f r a c t u r e , it is not n e c e s s a r y to have l a r g e n o r m a l , Eyy, s t r a i n s ; the s h e a r s t r a i n s r e s u l t i n g f r o m a s h a r p c r a c k a r e s u f f i c i e n t to p r o d u c e void growth. A t h e o r e t i c a l m o d e l d e s c r i b i n g this f r a c t u r e m o d e has b e e n p r o p o s e d by McCUntock, K a p l a n and B e r g . a T h e y c o n s i d e r e d the m a t e r i a l to be d i v i d e d into s e p a r a t e s q u a r e s , e a c h c o n t a i n i n g a c i r c u l a r hole w h e r e the s q u a r e and hole a r e d e f o r m e d i n d e p e n d e n t l y by the s h e a r s t r a i n s . When the hole m e e t s the s i d e s of the s q u a r e , c o a l e s c e n c e and f r a c t u r e is a s s u m e d . The o b j e c t i v e of this r e s e a r c h was to i n v e s t i g a t e the p r o c e s s of d i m p l e d r u p t u r e in a s i n g l e a l l o y heat t r e a t e d to d i f f e r e n t f r a c t u r e t o u g h n e s s e s u s i n g s m a l l geometry compact tension samples. The fractographic s t u d i e s e m p l o y e d h e r e i n p r o d u c e d an u n d e r s t a n d i n g of the s t e p s involved in the d i f f e r e n t m o d e s of d i m p l e d r u p t u r e . It was then p o s s i b l e to apply the a p p r o p r i a t e m o d e l s to p r e d i c t m a t e r i a l f r a c t u r e t o u g h n e s s and c o m p a r e the r e s u l t s to m e a s u r e d f r a c t u r e toughness.
II. M A T E R I A L AND M A T E R I A L MICROSTRUCTURE The m a t e r i a l s e l e c t e d f o r this i n v e s t i g a t i o n was Inc o n e l 718, a p r e c i p i t a t i o n h a r d e n e d s u p e r a l l o y . Inc o n e l 718 is a n i c k e l b a s e a l l o y with l a r g e a d d i t i o n s of C r , F e and Nb. T h e a c t u a l c o m p o s i t i o n is given b e low in weight p e r c e n t . T h e r e a r e s e v e r a l p h a s e s C o m p o s i t i o n of I n c o n e l 718 (wt pct) Ni bal
Cr 18.6
Fe 18.5
Nb Mo 5.0 3.1
A1 0.4
Ti 0.9
Si 0.3
C 0.04
Mn 0.20
p r e s e n t in I n c o n e l 718, a s i l l u s t r a t e d by the c o m p o s i t e of m i c r o g r a p h s in F i g . 1. E f f e c t i v e h a r d e n i n g i s a s s o c i a t e d with c o h e r e n c y s t r a i n s p r o d u c e d by the p r e c i p i t a t i o n of a D022 o r d e r e d b o d y - c e n t e r e d t e t r a gonal p h a s e , denoted ,,9-14 and i n d i c a t e d by the open c i r c l e in F i g . l ( a ) . The 7" p h a s e is b a s e d on Ni3Nb and p o s s e s s e s a d i s c l i k e m o r p h o l o g y having the following c u b e - c u b e r e l a t i o n s h i p to the p a r e n t fcc y m a t r i x : [001]v"l }(001>v and {100}V"{ }{100}7.11,la'ls T h e a l l o y a l s o c o n t a i n s l e s s e r a m o u n t s of s p h e r i c a l V', i d e n t i f i e d by the s q u a r e in F i g . l(a); V' is an o r d e r e d fee L12 p r e c i p i t a t e which has a Nia (Ti,AI)
[SSN 0360-2133/78/0510-0
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