• PDF / 1,792,728 Bytes
• 9 Pages / 612 x 783.28 pts Page_size
• 5 Downloads / 275 Views

DOWNLOAD

REPORT

---

IN

r e c e n t y e a r s , a new d i m e n s i o n in the t e c h n o l o g y of c a s t n i c k e l - b a s e s u p e r a l l o y s has e m e r g e d . It h a s b e e n d e m o n s t r a t e d t h a t t h e s e a l l o y s m a y be d i r e c t i o n a l l y s o l i d i f i e d b y e n s u r i n g that the h e a t flow d u r i n g s o l i d i f i c a t i o n i s u n i d i r e c t i o n a l and that t h e r e is no n u c l e a t i o n in the m e l t a h e a d of the a d v a n c i n g i n t e r f a c e . The b a c k ground to t h i s d e v e l o p m e n t and s o m e of the a d v a n t a g e s to be gained a r e s u m m a r i z e d in a r e c e n t r e v i e w b y V e r Snyder and Shank.* The d i r e c t i o n a l s o l i d i f i c a t i o n a l l o w s c o m p l e x s h a p e s to b e p r e c i s i o n c a s t , g e n e r a l l y with a c r y s t a l l o g r a p h i c growth d i r e c t i o n , e i t h e r in the f o r m of c o l u m n a r g r a i n s (with a w i r e t e x t u r e ) o r a s m o n o c r y s t a l s . In the f o r m e r c a s e , the t r a n s v e r s e g r a i n b o u n d a r i e s a r e e l i m i n a t e d although t r a n s v e r s e s e g m e n t s of d e n d r i t e and g r a i n b o u n d a r i e s r e m a i n . In the l a t t e r c a s e , o n l y the d e n d r i t e b o u n d a r i e s r e m a i n . In g e n e r a l , c o l u m n a r g r a i n and m o n o c r y s t a l s p e c i m e n s have shown s i m i l a r c r e e p r u p t u r e b e h a v i o r in t e r m s of i n c r e a s e d r u p t u r e life and d u c t i l i t y c o m p a r e d with the e q u i a x e d s t r u c t u r e . X - s The e n h a n c e d d u c t i l i t y a p p e a r s to b e the s i n g l e d o m i n a n t p r o p e r t y i m p r o v e m e n t . * H o w e v e r , the l o w e r modulus in the d i r e c t i o n m a y a l s o s i g n i f i c a n t l y r e d u c e the s t r e s s r a n g e in s t r a i n c o n t r o l l e d low c y c l e fatigue and a c c o u n t for r e p o r t e d i m p r o v e m e n t s in life. In t h e r m a l shock t e s t i n g , i m p r o v e d r e s i s t a n c e to c r a c k i n i t i a t i o n h a s b e e n d e m o n s t r a t e d in d i r e c t i o n a l l y s o l i d i f i e d m a t e r i a l . 4,5 Howe v e r , when c r a c k p r o p a g a t i o n is i n t e r d e n d r i t i c , the c r a c k s m a y grow a s r a p i d l y p e r p e n d i c u l a r l y to the growth d i r e c t i o n a s p a r a l l e l to that d i r e c t i o n . 6 An a d d i t i o n a l p o s s i b l e a d v a n t a g e in d i r e c t i o n a l s o l i d i f t c a t i o n is i m p l i c i t in the w o r k of H a r r i s o n and T i l l y . s T h e s e a u t h o r s showed that a s e r i o u s l o s s in r u p t u r e life o c c u r r e d a s the s p e c i m e n c r o s s - s e c t i o n of c o n v e n t i o n a l l y cast Mar-M246 was decreased. In directionally solidified specimens, this loss of rupture life was not D. A. WOODFORD and J. J. FRAWLEY are associated with Corporate Research and Development and Materials Technol