Grain growth in lr- 0.3 Pct W alloys
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a l l o y c u r r e n t l y used f o r t h e fuel c l a d d i n g in i s o t o p e h e a t s o u r c e s o p e r a t i n g a b o v e 1573 K (1300°C) is I r - 0 . 3 p c t W . I n t h e M u l t i - H u n d r e d - W a t t h e a t s o u r c e , 1 Pu23802 f u e l s p h e r e s a r e c l a d w i t h I r - 0 . 3 p c t W , n o m i n a l l y 0.64 m m (0.025 in.) t h i c k , w h i c h i s d e s i g n e d to c o n t a i n t h e fuel u n d e r b o t h o p e r a t i n g a n d accident conditions. T h e most s e v e r e accident condit i o n t h a t t h e cladding is e x p e c t e d t o s u r v i v e is i m p a c t a t 85 m / s (280 i f / s ) a t 1623 K ( 1 3 5 0 ° C ) . T h e resistance of Ir-0.3 pct W t o brittle intergranular fracture u n d e r high t e m p e r a t u r e impact c o n d i t i o n s i s a d v e r s e l y a f f e c t e d by l a r g e g r a i n s i z e s .2 I n t h i s study, g r a i n growth d a t a o n I r - 0 . 3 pct W were gathered t o d e t e r m i n e t h e e f f e c t s of h i g h t e m p e r a t u r e e x p o s u r e d u r i n g fabrication, a s s e m b l y , ground testing, and o p e r a tion of the h e a t s o u r c e . T h i s i n f o r m a t i o n , a l o n g w i t h i m p a c t r e s i s t a n c e vs g r a i n s i z e d a t a , w i l l h e l p d e s i g n e r s p r e d i c t t h e i m p a c t d u c t i l i t y o f the f u e l c l a d d i n g a s a function of fabrication and operational history. D a t a w e r e c o l l e c t e d on h e a t t r e a t m e n t s a t 1583 K (1310°C), 1688 K (1415°C), a n d 1803 K (1530°C) f o r t i m e s f r o m 1 / 6 t o 2000 h . G r a i n s i z e w a s m e a s u r e d by t h e l i n e i n t e r c e p t t e c h n i q u e . T h e d a t a f i t t h e g r a i n growth equation:
L u r e u n d e r i m p a c t c o n d i t i o n s is o f t e n a s s o c i a t e d w i t h such l a r g e g r a i n s . T h e r e f o r e , in a d d i t i o n to t h e a v e r a g e g r a i n s i z e , t h e l a r g e s t g r a i n in e a c h s a m p l e was m e a s u r e d to d e t e c t p o s s i b l e a b n o r m a l g r a i n g r o w t h . T h e recrystallization temperature of I r - 0 . 3 pct W v a r i e s considerably depending upon t h e s o u r c e o f t h e i r i d i u m p o w d e r . T h e r e c r y s t a l l i z a t i o n c u r v e s of v a r i o u s b a t c h e s of I t - 0 . 3 p c t W f a b r i c a t e d into s h e e t r a n g e o v e r a b o u t 1 7 5 ° C as s h o w n in Fig. 1. R e c r y s t a l l i z a t i o n was m e a s u r e d by m a k i n g a v i s u a l e s t i m a t e o f the a m o u n t of r e c r y s t a l l i z e d g r a i n s in m e t a l l o g r a p h i c s a m p l e s . I n
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d 2 - d 2 = kt
with k = ko e x p ( - Q / R T ) ,
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w h e r e d i s the g r a i n d i a m e t e r a t t i m e t , do i s t h e g r a i n d i a m e t e r a t t = O , ko i s a c o n s t a n t , R i s t h e g a s c o n s t a n t , T is t h e a b s o l u t e t e m p e r a t u r e , a n d Q is a n a c t i vation e n e r g y
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