The orientation and temperature dependence of the yield stress of Ni 3 (Al, Nb) single crystals

  • PDF / 3,193,179 Bytes
  • 10 Pages / 612 x 792 pts (letter) Page_size
  • 57 Downloads / 214 Views

DOWNLOAD

REPORT


S. CHIN, AND

D. P. POPE

The flow s t r e s s of Ni3(A1, Nb) s i n g l e c r y s t a l s has been m e a s u r e d a s a function of o r i e n t a tion in the t e m p e r a t u r e r a n g e 77 to 910 K. While the i n c r e a s i n g flow s t r e s s b e h a v i o r is s i m i l a r to that o b s e r v e d in o t h e r N i ~ l - b a s e d a l l o y s , the a b s o l u t e v a l u e of the s t r e s s was found to be much h i g h e r . A l s o , the effect of o r i e n t a t i o n changes was to p r o d u c e much g r e a t e r changes in the t e m p e r a t u r e at which the peak flow s t r e s s o c c u r s than has b e e n p r e v i o u s l y o b s e r v e d . The o p e r a t i v e s l i p s y s t e m s w e r e a n a l y z e d by two s u r f a c e s l i p t r a c e a n a l y s i s . P r i m a r y o c t a h e d r a l s l i p was found to be p r e d o m i n a n t at t e m p e r a t u r e s below the p e a k s t r e s s t e m p e r a t u r e , while p r i m a r y cube s l i p is p r e v a l e n t above the p e a k t e m p e r a t u r e . The a n o m a l o u s i n c r e a s e in the flow s t r e s s of Ni3(A1, Nb) with i n c r e a s i n g t e m p e r a t u r e is g e n e r a l l y c o n s i s t e n t with the t h e r m a l l y a c t i v a t e d c r o s s - s l i p of a/2(l10> d i s l o c a t i o n s f r o m {111} p l a n e s onto {100} p l a n e s . The c r o s s - s l i p i s shown to be a i d e d not only by the r e s o l v e d s h e a r s t r e s s on the {100} c r o s s - s l i p p l a n e but a l s o by the s t r e s s tending to c o n s t r i c t the a / ( l 1 2 > Shockley p a r t i a l d i s l o c a t i o n s on the p r i m a r y glide p l a n e . I. INTRODUCTION

IN

many n i c k e l b a s e a l l o y s the high t e m p e r a t u r e s t r e n g t h is p r o v i d e d by the y ' - p r e c i p i t a t e b a s e d on Ni~A1, a p h a s e with the L12 s t r u c t u r e which is known to be o r d e r e d up to 1320~ A g r e a t d e a l of i n t e r e s t has b e e n shown in this p h a s e b e c a u s e i t s flow s t r e s s i n c r e a s e s with i n c r e a s i n g t e m p e r a t u r e . 2-4 Studies of the flow p r o c e s s e s giving r i s e to t h i s phenomenon a r e h a m p e r e d by the fact that s t o i c h i o m e t r i c Ni3A1 cannot be p r o d u c e d in s i n g l e c r y s t a l l i n e f o r m ; 5 t h e r e f o r e , t e r n a r y a d d i t i o n s a r e m a d e which f a c i l i t a t e c r y s t a l growth. Although s u c h m o d i f i c a t i o n s of the c o m p o s i t i o n a l t e r the a b s o l u t e s t r e n g t h of the i n i t i a l NiaA1 p h a s e , they do not s i g n i f i c a n t l y a l t e r the g e n e r a l flow s t r e s s v s t e m p e r a t u r e behavior.S, 7 F o r t h i s r e a s o n the m o r e r e c e n t f u n d a m e n t a l s t u d i e s have b e e n p e r f o r m e d on t e r n a r y s y s t e m s of the type Ni3 (A1,X), w h e r e X is u s u a l l y W, T i o r Nb. In the p a r t i c u l a r c a s e of p o l y c r y s t a l l i n e Ni3(A1,Nb), T h o r n t o n e t alS