The recrystallization of 0.18 mm doped tungsten wire, swaged and drawn to a ... At 2100~ these mechanisms produced a fine-grained, partially recrystallized.
The Recrystallization of Heavily-Drawn Doped Tungsten Wire DAVID
B. SNOW
The r e c r y s t a l l i z a t i o n of 0.18 m m doped tungsten w i r e , s w a g e d and d r a w n to a t r u e s t r a i n of 7.7 at t e m p e r a t u r e s of < 0.47 Tin, was i n v e s t i g a t e d by light m i c r o s c o p y and by t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y . The a s - d r a w n s t r u c t u r e of the w i r e c o n s i s t e d of g r e a t l y e l o n g a t e d , r i b b o n - s h a p e d g r a i n s which had a p r o n o u n c e d (110) f i b e r t e x t u r e , T h e s e cont a i n e d w e l l - d e v e l o p e d , e l o n g a t e d c e i l s with few t r a n s v e r s e b o u n d a r i e s . The o n s e t of s u b b o u n d a r y and g r a i n b o u n d a r y m i g r a t i o n , t o g e t h e r with the f o r m a t i o n of p o t a s s i u m - c o n t a i n i n g bubbles in r o w s o r i e n t e d p a r a l l e l to the d r a w i n g d i r e c t i o n , was o b s e r v e d a f t e r a n n e a l i n g at 800~ for 1 h. At h i g h e r t e m p e r a t u r e s , the s p a c i n g and the m i s o r i e n t a t i o n of the l o n g i t u d i n a l s u b b o u n d a r i e s i n c r e a s e d , and new t r a n s v e r s e s u b b o u n d a r i e s w e r e f o r m e d . Both s u b b o u n d a r y and g r a i n b o u n d a r y m~gration w e r e s t r o n g l y i n h i b i t e d by the bubble r o w s , a s w e l l a s by the u n i f o r m i t y of the d e f o r m a t i o n and the w e l l - d e v e l o p e d t e x t u r e . At 2100~ t h e s e m e c h a n i s m s p r o d u c e d a f i n e - g r a i n e d , p a r t i a l l y r e c r y s t a l l i z e d s t r u c t u r e (1.2 ~ m a v e r a g e l o n g i t u d i n a l b o u n d a r y spacing) without change in the d e f o r m a tion t e x t u r e . A t 2150~ and above, l a r g e g r a i n s of high a s p e c t r a t i o , which a l s o r e t a i n e d the (110) d r a w i n g t e x t u r e , w e r e f o r m e d by e x a g g e r a t e d g r a i n growth. T h i s p r o c e s s was i n i t i a t e d by a v e r y s m a l l population of g r a i n s which had a c q u i r e d the n e c e s s a r y s i z e a d v a n t a g e d u r i n g the growth of the f i n e - g r a i n e d s t r u c t u r e .
DURINGthe
p a s t 15 y e a r s , c o n s i d e r a b l e e f f o r t has been devoted to the study of c o m m e r c i a l l y doped t u n g s t e n w i r e due to i t s t e c h n o l o g i c a l i m p o r t a n c e a s an i n c a n d e s c e n t l a m p f i l a m e n t m a t e r i a l . S p e c i a l a t t e n t i o n has b e e n d i r e c t e d t o w a r d the m e c h a n i s m s by which the, r e c r y s t a l l i z a t i o n and s u b s e q u e n t c r e e p r e s i s t a n c e of t h i s m a t e r i a l a r e influenced by s m a l l a m o u n t s of d e l i b e r a t e l y i n t r o d u c e d p o t a s s i u m , s i l i con, and a l u m i n u m (total n o m i n a l l y < 500 at ppm). R e cent s t u d i e s 1-3 have c o n f i r m e d t h a t a n n e a l e d d o p e d t u n g s t e n w i r e , f a b r i c a t e d by s t a n d a r d c o m m e r c i a l p r a c t i c e , 4 c o n t a i n s r o w s of b u b b l e s a l i g n e d p a r a l l e l to the w i r e a x i s . T h e m a t e r i a l which f o r m s and s t a b i l i z e s t h e s e bubbles has been i d e n t i f i e d a s p o t a s s i u m , 5-~ e n t r a p p e d in the t u n g s t e n d u r i n g s i n t e r i n g and e l o n g a t e d by s u b s e q u e n t m e c h a n i c a l d e f o r m a t i o n . T h e s e bubble r o w s a r e p a r t i c u l a r l y e f f e c t i v e in inh i b i t i n g g r a i n b o u n d a r y m i g r a t i o n and in c o n t r o l l i n g the s h a p e of g r a i n s f o r m e d by e x a g g e r a t e d g r a i n growth. In addition, the u n u s u a l l y l a r g e d e g r e e of r e t a i n e d d e f o r m a t i o n e n e r g y 7-~ and p r o n o u n c e d t e x t u r e 1'%x~ which a r e c h a r a c t e r i s t i c of h e a v i l y - d r a w n BCC w i r e can be e x p e c t e d to affect r e c r y s t a l l i z a t i o n . T h i s i n v e s t i g a t i o n a t t e m p t s to f u r t h e r c h a r a c t e r i z e the p r o c e s s e s of s t r u c t u r a l change a c c o m p a n y i n g the a n n e a l i n g of 0.18 m m doped tungsten w i r e , a s i z e u t i l i z e d in c o m m e r c i a l i n c a n d e s c e n t l a m p f i l a m e n t s . E X P E R I M E N T A L PROCEDURE S p e c i m e n s w e r e p r e p a r e d f r o m a 200 m length of G e n e r a l E l e c t r i c doped t u n g s t e n w i r e (Type 218), swaged and d r a w n to a d i a m e t e r of 0.18 m m (0.007 in.) at t e m p e r a t u r e s which w e r e s u c c e s s i v e l y d e c r e a s e d f r o m 0.47 to 0.28 T m. T h e w i r e was w a r m w o r k e d to a t r u e s t r a i n of 7.7 without i n t e r m e d i a t e a n DAVID B. SNOW is Research Metallurgist, General Electric Co., Lamp Business Division, Refractory Metals R & D Subsection, Cleveland, OH 44117. Manuscript submitted August 11, 1975. METALLURGICAL TRANSACTIONS A
n e a l s . A n o m i n a l c h e m i c a l a n a l y s i s of t h i s w i r e in the a s - d r a w n s t a t e has b e e n p r e v i o u s l y p u b l i s h e d . 6 S p e c i m e n s 15 cm in length w e r e c l e a n e d in a s o l u t i o n of 30 pct s o d i u m h y d r o x i d e , and w e r e s u b s e q u e n t l y a n n e a l e d in a t u n g s t e n boat in a v a c u u m of a p p r o x i m a t e l y 1.3 • 10 -3 P a (10 -5 m m Hg) f o r 1 h. A n n e a l s w e r e cond u c t e d at t e m p e r a t u r e s of 400 to 2000~ in i n c r e m e n t s of 200~ and at 2050 to 2250~ in i n c r e m e n t s of 50~ A d d i t i o n a l s p e c i m e n s of the s a m e w i r e w e r e obtained which had b e e n s e l f - r e s i s t a n c e a n n e a l e d at 2450~ f o r v a r y i n g t i m e s in 1.3 • 10 -4 P a v a c u u m . L o n g i t u d i n a l s e c t i o n s of t h e s e s p e c i m e n s w e r e e x a m i n e d by light m i c r o s c o p y and by t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y at 100 KV. RESULTS As-Worked Structure The g e n e r a l c h a r a c t e r i s t i c s of the a s - w o r k e d s t r u c t u r e w e r e s i m i l a r to t h o s e d e s c r i b e d in o t h e r s t u d i e s of h e a v i l y - d r a w n BCC w i r e J ' n The g r a i n s w e r e d e f o r m e d into g r e a t l y elongated, r i b b o n - l i k e s h a p e s , folded to v a r y i n g d e g r e e s about t h e i r l o n g i t u d i n a l a x i s and a l i g n e d n e a r l y p a r a l l e l to (110). The r e l a t i o n s h i p of t h i s d i s t i n c t i v e g r a i n s h a p e and c r y s t a l l o g r a p h y to the d e f o r m a t i o n m o d e i m p o s e d on e a c h g r a i n by w i r e d r a w i n g has b e e n p r e v i o u s l y d i s c u s s e d . 12 The d e f o r m a t i o n s u b s t r u c t u r e within the g r a i n s cons i s t e d of n a r r o w , w e l l - d e v e l o p e d c e l l w a l l s , m o s t of which w e r e o r i e n t e d c l o s e to the d r a w i n g (longitudinal) d i r e c t i o n ( F i g . 1). T r a n s v e r s e c e l l w a l l s w e r e i n f r e quently o b s e r v e d , in c o n t r a s t to the r e l a t i v e l y e q u i a x e d c e l l s o b s e r v e d in a s - d r a w n i r o n w i r e . 8 M o s t of the c e l l w a l l s a r e n a r r o w , s h a r p l y defined, l o w - a n g l e b o u n d a r i e s . H o w e v e r , t h e i r low m i s o r i e n t a t i o n d o e s not i m p l y a s i m p l e s t r u c t u r e , a s they a r e c o m p o s e d of c o m p l e x , high d e n s i t y t a n g l e s of d i s l o c a t i o n s . T h e m e a n s p a c i n g of the longitudinal c e l l w a l l s was 0.14 ~m, measured from transmission electron microg r a p h s . The s p a c i n g of l o n g i t u d i n a l h i g h - a n g l e g r a i n VOLUME 7A, JUNE 1976-783
"
~. " Fig. 1--As-drawn 0.18 mm doped tungsten wire. The alternately wide and narrow cell boundary spacing shown here was infrequently observed. b o u n d a r i e s ( b o u n d a r i e s with > 10 deg m i s o r i e n t a t i o n about a (110) c o m m o n to both g r a i n s ) was e s t i m a t e d to be t h r e e to four t i m e s g r e a t e r than this, b a s e d on the f r e q u e n c y with which d i f f e r e n t d i f f r a c t i o n p a t t e r n s w e r e o b s e r v e d while t r a v e r s i n g a s - d r a w n s p e c i m e n s b e n e a t h a 25 ~ m SAD a p e r t u r e . Selected area diffraction patterns from large areas of a s - d r a w n thin foils r e v e a l e d that m o s t of the g r a i n s had a c o m m o n (110) a x i s a l i g n e d within 1 deg of the drawing direction. Isolated grains were infrequently o b s e r v e d in which (110) d e v i a t e d f r o m the d r a w i n g d i r e c t i o n by as m u c h as 2 to 10 deg. T h e s e o b s e r v a tions of the a s - d r a w n w i r e t e x t u r e a r e in a g r e e m e n t with e a r l i e r data obtained by f i e I d - i o n m i c r o s c o p y , ~3-1S X - r a y d i f f r a c t i o n z'%~~ and t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y , n The fine s t r u c t u r e of the c o n t r a s t f r o m within the c e l l s did not, h o w e v e r , s u p p o r t the m o d e l of t w i s t e d g r a i n s p r o p o s e d f o r a s - d r a w n 0.76 m m doped t u n g s t e n w i r e .11 The fine s t r i a t i o n s r e p o r t e d in Ref. 11 to be a m a n i f e s t a t i o n of t w i s t e d g r a i n s w e r e a l s o d e t e c t e d in this i n v e s t i g a t i o n , but no r e l a t i v e Kikuchi line d i s p l a c e m e n t was o b s e r v e d in d i f f r a c t i o n p a t t e r n s taken f r o m n e i g h b o r i n g l o c a t i o n s within a s t r i a t e d s u b g r a i n (Fig. 2). C o n s e q u e n t l y , it is p r o b a b l e that such s t r i a t e d c o n t r a s t is an a r t i f a c t p r o d u c e d by s u r f a c e etching, p e r h a p s a s s o c i a t e d with {110} p l a n e s o r i e n t e d p e r p e n d i c u l a r to the d r a w i n g d i r e c t i o n : and d o e s not i n d i c a t e a c r y s t a l l o g r a p h i c m i s a l i g n m e n t of the l a t t i c e . E x a m i n a t i o n of a s - d r a w n s p e c i m e n s at high m a g n i f i c a t i o n r e v e a l e d the p r e s e n c e of v e r y n a r r o w r e g i o n s of an e l o n g a t e d s e c o n d p h a s e aligned p a r a l l e l to the d r a w i n g d i r e c t i o n (Fig. 3). O b s e r v a t i o n of s i m i i a r f e a t u r e s in < 0.7 m m doped t u n g s t e n w i r e have b e e n r e c e n t l y r e p o r t e d . ~ - ~ T h e s e n a r r o w (-< 10 nm) r e t i o n s of s e c o n d phase could be r e a d i l y d i s t i n g u i s h e d f r o m the longitudinal c e l l w a l l s by t h e i r p r o n o u n c e d l i n e a r i t y , and by t h e i r change in c o n t r a s t when the f o i l was t i l t e d J 6 S t e r e o s c o p i c e x a m i n a t i o n showed that they w e r e e n c l o s e d within the foil ( F i g . 4), and thus w e r e not c r a c k s o r e l e c t r o e t c h e d s u r f a c e a r t i f a c t s .
:
[11ol,
\
O "
-
9
O
9
.
QG"
-.
F i g 2 - - S t r i a t e d g r a m m s p e c i m e n a n n e a l e d at 1400~ The c r y s l i g h t c h a n g e in the p o s i t i o n of t h e K~kuchi p a t t e r n s h o w s t h a t t h e d i f f e r e n c e in l a t t i c e o r i e n t a t i o n b e t w e e n t h e c i r c l e d a r e a s is ~0.1 deg
~[l[
b "
0o2 p
-:nr
"-
Fig
3--Elongated potassium (arrows) in as-drawn 0.18 mm doped tungsten wire. Area shown is in lower right of Fig. 1. 2
Structural Coarsening The only s u b s t a n t i a l s t r u c t u r a l change which could be d e t e c t e d a f t e r a n n e a l i n g at 400 o r 600~ was an in7 8 4 - V O L U M E 7A, JUNE 1976
Fig. 4--Bright field stereo view of elongated potassium (arrows) m as-drawn 0.18 mm doped tungsten wire. Twobeam, g = 110, parallel to the drawing direction. METALLURGICAL TRANSACTIONS A
.ll]l
I
!
I
I
I
1.2
1.0 :L
Z
0,8
o F i g . 5 - - I n c r e a s e in s p a c i n g o f l o n g i t u d i nal boundaries of all misorientations with increasing annealing temperature as measured from transmission elect r o n micrographs.
Z Q
0.6
2 Z
o
0.4
O Z t,j
0.2 Z p a r a l l e l to the d r a w ing d i r e c t i o n , in o r d e r to c o n s i d e r the p o s s i b l e effect of c o i n c i d e n c e b o u n d a r i e s on the r e c r y s t a l l i z a t i o n of d o p e d t u n g s t e n w i r e , s i n c e a l a r g e p o p u l a t i o n of such b o u n d a r i e s i s p r e s e n t in any < l l 0 ) - t e x t u r e d BCC w i r e 9 T h e s e c o n d , and m o r e i m p o r t a n t , r e c r y s t a l l i z a t i o n inhibiting c h a r a c t e r i s t i c of the a s - d r a w n s t r u c t u r e of 0.18 m m doped t u n g s t e n w i r e was the p r e s e n c e of an e l o n g a t e d , d i s p e r s e d s e c o n d p h a s e . T h i s c o n s i s t e d of p o t a s s i u m , e n t r a p p e d d u r i n g the s i n t e r i n g of the ingot in h y d r o g e n , and s u b s e q u e n t l y e l o n g a t e d d u r i n g s w a g ing and d r a w i n g . 18 The high s t r a i n and d e c r e a s e d d r a w i n g t e m p e r a t u r e a s s o c i a t e d with t h i s w i r e s i z e m a k e s it p r o b a b l e that no p o r o s i t y was a s s o c i a t e d with the e l o n g a t e d p o t a s s i u m p r i o r to a n n e a l i n g . T h e s e a r e a s of e l o n g a t e d p o t a s s i u m w e r e , h o w e v e r , the p r e c u r s o r s of the bubble r o w s , which f o r m e d by e x p a n s i o n and s u b s e q u e n t s p h e r o i d i z a t i o n when the w i r e w a s h e a t e d to r e l a t i v e l y low t e m p e r a t u r e s , and thus r e s t r i c t e d the s t r u c t u r a l c o a r s e n i n g of the w i r e f r o m i t s i n c e p t i o n . T h e r e was no e v i d e n c e for s t r o n g i n t e r a c t i o n b e t w e e n d i s l o c a t i o n s o r c e l l w a l l s and the e l o n g a t e d p o t a s s i u m d u r i n g d r a w i n g . T h i s i s of conc e r n s i n c e d i s p e r s i o n s t r e n g t h e n i n g of t h i s s o r t could l e a d to s u b s t r u c t u r a l r e f i n e m e n t and h o m o g e n i z a t i o n , a s p r o p o s e d for o t h e r m a t e r i a l s c o n t a i n i n g a d i s p e r s e d s e c o n d p h a s e , 33'34 and would thus m a k e an a d d i t i o n a l c o n t r i b u t i o n to r e c r y s t a l l i z a t i o n i n h i b i t i o n . H o w e v e r , the bubble d i s p e r s i o n in doped t u n g s t e n w i r e e x e r t s i t s influence by r e s t r i c t i n g b o u n d a r y m i g r a t i o n , and not by m o d i f y i n g the d e f o r m a t i o n s t r u c t u r e . T h e o b s e r v a t i o n of b u b b l e s in v a r i o u s s t a g e s of s h a p e change a f t e r a n n e a l i n g at the r e l a t i v e l y low t e m p e r a t u r e of 800~ (0.3 T m) for 1 h r a i s e s a q u e s tion a s to the m e c h a n i s m of t h e i r i n i t i a l e x p a n s i o n . P r e v i o u s s u g g e s t i o n s that p o t a s s i u m v a p o r p r e s s u r e alone w a s s u f f i c i e n t to f o r m b u b b l e s by p l a s t i c d e f o r m a t i o n 35'36 a p p e a r to be i n a p p l i c a b l e in the p r e s e n t c a s e , s i n c e at 800~ t h i s p r e s s u r e is < 2 A T M . , and is thus u n l i k e l y to o v e r c o m e the r e s t r a i n i n g effect of s u r f a c e t e n s i o n (Appendix). In any event, the d i s l o c a METALLURGICAL TRANSACTIONS A
t i o n l o o p s and t a n g l e s that would be c r e a t e d in the m a t r i x s u r r o u n d i n g e a c h bubble by the o p e r a t i o n of t h i s m e c h a n i s m w e r e not o b s e r v e d . It i s m u c h m o r e p r o b a b l e that the i n i t i a l e x p a n s i o n of the p o t a s s i u m - c o n t a i n i n g r i b b o n s into c y l i n d r i c a l b u b b l e s o c c u r r e d b y the a c c u m u l a t i o n of v a c a n c i e s f r o m s o m e n e a r b y s o u r c e , a i d e d by the v o l a t i l i z a t i o n of the p o t a s s i u m . A f t e r t h i s expansion, the c y l i n d r i c a l b u b b l e s s p h e r o i d i z e d into r o w s due to the a c t i o n of s u r f a c e t e n s i o n . The c o n c e p t of bubble f o r m a t i o n by v a c a n c y c a p t u r e i s s u p p o r t e d by the o b s e r v a t i o n that b u b b l e s f i r s t a p p e a r e d a f t e r a n n e a l i n g at the l o w e s t t e m p e r a t u r e w h e r e e x t e n s i v e v a c a n c y m o b i l i t y has b e e n r e p o r t e d in doped tungsten w i r e , 700 to 900~ (Ref. 37).* Moon and Koo ~ concluded that bubble f o r *An earher report of vacancy moblhty during the Stage III recovery of tungsten (300 to 400~ 38 appears to have been erroneous. More recent investigations attribute this stage to the migratmn and anmlulatmn of self-mterstmals 39,40
m a t i o n in doped t u n g s t e n w i r e was c o n t r o l l e d by v a cancy diffusion f r o m the s u b b o u n d a r i e s t h r o u g h the c e l l i n t e r i o r s to the p o t a s s i u m . T h i s m e c h a n i s m a p p e a r s to be i n o p e r a b l e in the t e m p e r a t u r e r a n g e at which bubble f o r m a t i o n w a s o b s e r v e d in the p r e s e n t i n v e s t i g a t i o n . E x t r a p o l a t i o n of the diffusion d a t a of P a w e l and Lundy 41 to 800~ y i e l d s a l a t t i c e s e l f - d i f f u s i o n coefficient f o r tungsten of 5 • 10 -29 e m 2 / s . Cons e q u e n t l y , the v a c a n c y flux f r o m a s u b b o u n d a r y o r g r a i n b o u n d a r y s o u r c e , through a s u b s t a n t i a l l y d i s l o c a t i o n - f r e e c e l l i n t e r i o r , to the p o t a s s i u m would be i n s u f f i c i e n t to p r o d u c e bubble f o r m a t i o n . An a l t e r n a t i v e v a c a n c y supply is s u g g e s t e d by a r e c e n t r e p o r t of the g e n e r a t i o n of v a c a n c i e s in tungs t e n d u r i n g r a p i d d e f o r m a t i o n , ~2 which i m p l i e d that a c o n s i d e r a b l e e x c e s s c o n c e n t r a t i o n of v a c a n c i e s m i g h t e x i s t in t u n g s t e n w i r e d r a w n at 700 to 800~ There is no c l e a r e v i d e n c e that e x c e s s v a c a n c i e s m a k e a s i g n i f i c a n t c o n t r i b u t i o n to bubble f o r m a t i o n s i n c e they should have a l l o w e d t h i s p r o c e s s to be c o m p l e t e d m o r e r a p i d l y than w a s o b s e r v e d . H o w e v e r , s o m e c o n t r i b u tion of v a c a n c i e s f r o m this s o u r c e cannot be e n t i r e l y r u l e d out. A m o r e p r o b a b l e m e c h a n i s m for v a c a n c y t r a n s p o r t to the p o t a s s i u m i s s u g g e s t e d by the m i c r o s t r u c t u r e of the a s - d r a w n w i r e ( F i g . 3); in which m a n y of the a r e a s of e l o n g a t e d p o t a s s i u m w e r e l o c a t e d v e r y c l o s e to a c e l l w a l l o r g r a i n b o u n d a r y , o r in s o m e c a s e s w e r e a c t u a l l y in c o n t a c t with t h e m . Since both c e l l w a l l (subboundary) and g r a i n b o u n d a r y m i g r a t i o n o c c u r r e d in a s u b m i c r o n s c a l e at 800~ it i s p r o b a b l e that m a n y s e g m e n t s of both t y p e s of b o u n d a r y c a m e s u f f i c i e n t l y c l o s e ( a p p r o x i m a t e l y 1 nm) to s o m e of the p o t a s s i u m r i b b o n s d u r i n g t h i s a n n e a l to a c t a v a c a n c y s o u r c e s f o r bubble f o r m a t i o n . T h i s m o d e l is c o n s i s t e n t with the o b s e r v a t i o n that bubble e x p a n s i o n was not c o m p l e t e d until the 1400~ a n n e a l , at which t e m p e r a t u r e the a v e r a g e s p a c i n g b e t w e e n l o n g i t u d i n a l bounda r i e s had a p p r o x i m a t e l y doubled. Thus the a n n e a l i n g t e m p e r a t u r e at which bubble f o r m a t i o n was o b s e r v e d to be c o m p l e t e d was a l s o that at which a l l of the e l o n g a t e d p o t a s s i u m w a s b r o u g h t into c l o s e p r o x i m i t y with a b o u n d a r y v a c a n c y s o u r c e . The f o r m a t i o n of p o t a s s i u m - s t a b i l i z e d b u b b l e s at a t e m p e r a t u r e c o i n c i d e n t with the o n s e t of s t r u c t u r a l c o a r s e n i n g r e s t r i c t e d the s u b s e q u e n t m i g r a t i o n of both s u b b o u n d a r i e s and h i g h - a n g l e g r a i n b o u n d a r i e s to a METALLURGICAL TRANSACTIONS A
s u f f i c i e n t d e g r e e so that a n n e a l i n g at t e m p e r a t u r e s a s high a s 2100~ p r o d u c e d a s t r u c t u r e in which the average spacing between longitudinal grain boundaries was < 1.5 ~ m . Although both t h e i r s i z e and s p a t i a l d i s t r i b u t i o n w e r e inhomogeneous, m o s t of the b u b b l e s w e r e
