The Absence of an AIIotropic Transformation

in Pure Vanadium at High Temperatures GEORGE BARNES

F i e l d - e l e c t r o n e m i s s i o n techniques show that an a l l o t r o p i c t r a n s f o r m a t i o n does not occur in vanad ium at high t e m p e r a t u r e s if the s a m p l e s a r e of high pu r i t y . Such pur i ty can be ob- ta ined by hea t ing for a su f f i c ien t ly long t ime in a high vacuum o r by zone re f in ing .

T H E l i t e r a t u r e i nd i ca t e s that t h e r e have been d i f f e r - ences in opinion as to whe ther an a l l o t r o p i c fo rm of vanad ium ex i s t ed at high t e m p e r a t u r e s , l - 4 al though a low t e m p e r a t u r e t r a n s i t i o n at about 230 K f rom bcc to t e t r a g o n a l s t r u c t u r e has been d i s c o v e r e d . 5 This a r t i c l e c o n f i r m s the r e s u l t s of the m o r e r e c e n t s tud ies on v a - nadium at high t e m p e r a t u r e s which show that no high t e m p e r a t u r e a l l o t r o p e e x i s t s in vanad ium of high pur i ty .

The f i e l d - e l e c t r o n m i c r o s c o p e 6 i s an e x t r e m e l y u s e - ful tool in the s tudy of the c r y s t a l l i n e s t r u c t u r e of m e t a l s b e c a u s e i t p r e s e n t s the o b s e r v e r with a v i s u a l i m a g e of an e l e c t r o n - e m i s s i o n p a t t e r n f rom the s i ng l e - c r y s t a l t ip of a needle of the m a t e r i a l in ques t ion . C o r - r e l a t i o n s be tween the l ight and d a r k a r e a s of the p a t t e r n and the c r y s t a l d i r e c t i o n s within the s ingle c r y s t a l can be made.7 'a

To an e x p e r i e n c e d o p e r a t o r , the g e n e r a l a p p e a r a n c e of the p a t t e r n can fu rn i sh i n fo rma t ion as to whe the r o r not t h e r e i s much i m p u r i t y on the s u r f a c e . In g e n e r a l , if the p a t t e r n changes smooth ly and g r a d u a l l y f rom the d a r k a r e a s (holes) to the b r igh t a r e a s , the su r f ace is r e l a t i v e l y f r ee of i m p u r i t i e s , but if the in tens i ty of the l ight f rom the p a t t e r n changes s h a r p l y o r if l ight a r e a s a r e s e t off by s h a r p , d a r k b o r d e r s , as in F ig . 1, one can be s u r e that t he re a r e s u r f a c e , if not vo lume, i m - p u r i t i e s . P a n k o w 9 has r e p o r t e d the e f fec t s of some s u r - face i m p u r i t i e s on the f i e l d - e m i s s i o n p a t t e r n s of vanad ium.

The f i e l d - e l e c t r o n and f i e l d - i o n e m i s s i o n m i c r o s c o p e not only enable the o b s e r v e r to ident i fy the c r y s t a l s t r u c t u r e of the e m i t t e r m a t e r i a l , but they p e r m i t him to s ee v i s u a l l y , by m e a n s of the f ie ld e m i s s i o n p a t t e r n , any changes in the c r y s t a l s t r u c t u r e while they a r e taking place.X~ If a suf f ic ien t ly fine con t ro l of e m i t t e r t e m p e r a t u r e i s p o s s i b l e , the t r a n s i t i o n f rom one c l i o - t r op i c phase to ano ther can be o b s e r v e d in the fo rm of a s h a r p l ine o r phase boundary moving a c r o s s the pa t - t e rn a s r e p o r t e d in Ref. 11.

with the p r y o m e t e r . Once the tubes we re c a l i b r a t e d ( t e m p e r a t u r e vs f i l amen t c u r r e n t c u r v e s obta ined) , high vo l t ages we re app l ied in o r d e r to o b s e r v e the e m i s s i o n p a t t e r n s . As expec ted , p a t t e r n s c o r r e s p o n d i n g to bcc s t r u c t u r e we re obta ined . The f i r s t two f igu re s show the vanad ium f i e l d - e m i s s i o n p a t t e r n a s the s u r f a c e i s c leaned by hea t ing f rom r o o m t e m p e r a t u r e , F ig . 1, to about 1250 K, F ig . 2.

T h r e e of the four tubes evacua ted dur ing the f i r s t vacuum " r u n " showed an a l l o t r o p i c t r a n s f o r m a t i o n on e a r l y a t t e m p t s to o p e r a t e them at a t e m p e r a t u r e of about 1820 K which is a p p r o x i m a t e l y that r e p o r t e d in Ref. 1. Unfor tuna te ly , e v e r y a t t emp t to d e t e r m i n e v i s u - a l ly o r to photograph the high t e m p e r a t u r e s t r u c t u r e was u n s u c c e s s f u l b e c a u s e each t ime the phase boundary was seen to move a c r o s s the p a t t e r n , an a r c d e s t r o y e d the needle b e f o r e t h e r e was t ime to pho tograph the pa t - t e rn (which t akes about 30 s) o r to ident i fy the h igh- t e m p e r a t u r e c r y s t a l s t r u c t u r e v i sua l l y .

EXPERIMENTAL PROCEDURE AND RESULTS

Field-emitter needles were fabricated from 99.5 pct pure vanad ium wi r e and p laced ins ide convent iona l evacua ted f i e l d - e l e c t r o n e m i s s i o n tubes .8 The e m i t t e r t e m p e r a t u r e as a funct ion of e m i t t e r - f i l a m e n t c u r r e n t was obta ined by m e a n s of an op t i ca l p y r o m e t e r be fo re an a t t emp t was made to o b s e r v e f i e l d - e m i s s i o n pa t - t e r n s . This p r o c e d u r e enab le s an o p e r a t o r to r e a d f i l a - men t c u r r e n t s , which can be done quickly f rom a m e t e r , r a t h e r than a t t empt ing to d e t e r m i n e each t e m p e r a t u r e

GEORGE BARNES is Professor, Department of Physics, University of Nevada, Reno, Nev. 89507.

Manuscript submitted December 29, 1971. Fig. 1- -Fie ld-e lec t ron emiss ion pat tern f rom a vanadium 0aec) emi t t e r just above room temperature .

METALLURGICAL TRANSACTIONS VOLUME 4, FEBRUARY 1973-549

Fig. 2 - - E m i s s i o n pa t t e rn of the e m i t t e r of Fig. 1 when the t e m - p e r a t u r e is r a i s e d to about 1250 K.

Fig. 4 - - E m i s s i o n pa t t e rn of the s a m e e m i t t e r at 1630 K.

Fig. 3 - - E m i s s i o n pa t t e rn of an e m i t t e r made of z o n e - r e f i n e d va- nadium at 1325 K. The pa t t e rn c o r r e s p o n d s to a bcc s t r u c t u r e , but with a d i f fe ren t o r i en ta t ion f r o m that in Figs . 1 and 2. Fig, 5 - -Emis s ion pa t t e rn of the s a m e e m i t t e r at 1900 K.

550-VOLUME4, FEBRUARY 1973 METALLURGICAL TRANSACTIONS

I t w a s s o o n d i s c o v e r e d t h a t v a n a d i u m n e e d l e s w h i c h w e r e d u l l e d by a n a r c c o u l d b e r e s h a r p e n e d w h i l e s t i l l i n t h e i r t u b e s b y h e a t i n g to a t e m p e r a t u r e o v e r 1800 K, and s e v e r a l v a n a d i u m e m i t t e r s w e r e s o t r e a t e d and r e - u s e d . H o w e v e r , a f t e r the e m i t t e r s h a d b e e n h e a t e d a f ew t i m e s a b o v e t h e t e m p e r a t u r e a t w h i c h t he t r a n s f o r - m a t i o n h a d o c c u r r e d , no f u r t h e r c h a n g e in t he c r y s t a l s t r u c t u r e cou ld be o b s e r v e d . O ne s u c h e m i t t e r w a s t a k e n to o v e r 1900 K ( j u s t b e l o w the m e l t i n g t e m p e r a - t u r e ) a n d a n o t h e r h e l d a t 1900 K f o r 7 r a i n w i t h o u t u n - d e r g o i n g a c h a n g e .

I t a p p e a r e d a s i f , w h a t e v e r i m p u r i t y h a d b e e n p r e s e n t a n d h a d c a u s e d a t r a n s i t i o n in t he v a n a d i u m b e f o r e h e a t - i n g , had b e e n r e m o v e d by e v a p o r a t i o n , l e a v i n g t he v a n a - d i u m in a s t a t e of h i g h e r p u r i t y . I t f u r t h e r a p p e a r e d t h a t e x t r e m e l y p u r e v a n a d i u m p r o b a b l y would not u n d e r g o a t r a n s i t i o n a t h i g h t e m p e r a t u r e s .

A s a m p l e of t he v a n a d i u m w i r e , p u r i f i e d i n a z o n e r e - f i n e r by m a k i n g s e v e r a l p a s s e s o v e r i t , d i d not show any t e n d e n c y to u n d e r g o a t r a n s f o r m a t i o n , e v e n t h o u g h two f i e l d - e m i t t e r n e e d l e s m a d e of i t w e r e h e a t e d to 1900 K a n d a n o t h e r to i t s m e l t i n g p o i n t . F i g . 3 s h o w s t he b c c p a t t e r n of a z o n e - r e f i n e d n e e d l e a t 1325 K, F i g . 4 , a t 1630 K, a n d F i g . 5 a t 1900 K. A s c a n b e s e e n , t he p a t - t e r n s h o w s no c h a n g e in t he c r y s t a l s t r u c t u r e of the v a n a d i u m in t h i s s e q u e n c e . I t s h o u l d b e n o t e d t h a t h e a t - i n g a n e m i t t e r c a u s e s i t to e m i t e l e c t r o n s by t h e r m i o n i c

e m i s s i o n a s w e l l a s by f i e l d e m i s s i o n . T h e a d d e d c u r - r e n t t e n d s to f i l l i n o r e v e n c o m p l e t e l y m a s k s o m e of t he d a r k h o l e s , b u t , a s c a n b e s e e n in F i g s . 3, 4, a n d 5, t h e u n d e r l y i n g s t r u c t u r e i s the s a m e . A t r o o m t e m p e r - a t u r e , t h i s e m i t t e r s t i l l h a d t h e s a m e c r y s t a l s t r u c t u r e .

CONC LUSION

T h e r e i s no a U o t r o p i c t r a n s f o r m a t i o n a t h i g h t e m p e r - a t u r e s in p u r e v a n a d i u m .

REFERENCES

1. A. U. Seybolt and H. T. Sumsion: AIME Trans., 1953, vol. 197, p. 292. 2. J. O. McCaldin andP. Dune: J. Metals, 1954, vol. 6, p. 619. 3. Handbook of the Rare Elements, M. A. Filyand and E. I. Semenova, eds.,

vol. II, p. 348, Boston Technical Publishers, Inc., Cambridge, Mass., 1970. 4. B. M. Vasyutinskiy, G. N. Kartmazov, Y. M. Smimov, and V. A. Finkel: Phys.

Metals Met., 1966, vol. 21, p. 134. 5. V. A. Finkel, V. I. Glamazda, and G. P. Kortun: Zhur. Eksper. i Teoret. Fiz.,

1969, vol. 57, p. 1065. Also Soy. Phys. JETP, 1970, vol. 30, p. 581. 6. E. W. Muller: Ergeb. Exakt. Naturwiss., 1953, vol. 27, p. 290. 7. M. H. Nichols: Phys. Rev., 1940, vol. 57, p. 297. 8. G. Barnes: Phys. Rev., 1955, vol. 97, p. 1579. 9. G. Pankow: Vierter Internationaler Kongress Fur Electronenmikroskipie,

Berlin, 10-17 Sept. 1958, Band 1 Physikalisch-Technischer Tell, Springer- Verlag, Berlin, 1960.

10. G. N. Bates and G. Barnes: Appl. Phys. Lett., 1967, vol. 11, p. 75. 11. G. Barnes: Proc. of the Colloque International sur L'etude des Transformations

Crlstallines, a Haute Temperature (au-dessus de 2000 K), to be published.

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