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THE EFFECT OF STATIC TENSILE BIAS STRESS
UPON THE SOLUBILITY LIMIT OF HYDROGEN IN
NIOBIUM CRYSTALS USING THE ULTRASONIC
TECHNIQUE
O. Florencio, D. Pinatti, J. Roberts
To cite this version:
THE EFFECT OF STATIC TENSILE BIAS STRESS UPON THE SOLUBILITY LIMIT OF HYDROGEN IN NIOBIUM CRYSTALS USING THE ULTRASONIC TECHNIQUE
Departamento d e Engenharia d e M a t e r i a i s , U n i v e r s i d a d e Federal d e S ~ O C a r l o s , 13560
do
C a r l o s , S.P. B r a s i l~ k s u m g
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Des mesures pr&ises de l a v i t e s s e e t l ' a t t & w a t i o n ultrasoniques-4
f a i t e s sur des c r i s t a u x de Nb t r e s purs contenant 0,,200, e t 300 ppm d t h y d r o g ~ n e en poids r e l a t i f . Les parametres dans c e t t e etude sont l e nombre de cycles thermiques dans l e domaine de temp6ratures 77 a 340K e t p l u s i e u r s v a l e u ~ s de c o n s t r a i n t e s ( b i a s ) de tension homogene j u s q u 8 d = 21MPa. La temperature du solvus de 1 'hydrog8ne observge l o r s du premier c y c l e thermique e s t i n f e r i e u r e a c e l l e s observees 1 ors des cycles thermi ues suivants. Aucun e f f e t mesurable de l a c o n s t r a i n t e ( b i a s ) n 8 a
gtd
observ? sur l a temp$rature du solvus s a t u r g apres p l u s i e u r s cysles thermiques. Tous l e s r e s u l t a t s obtenus semblent pouvoir & r e expliques2
1 'a i d e des p r i n c i p e s k1 kmentai r e s de 1 a thermodynamique e t des d i s1 ocations.Abstract
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Precision u l t r a s o n i c a t t e n u a t i o n and v e l o c i t y measurements have been conducted on h i g h p u r i t y Nb c r y s t a l s c o n t a i n i n g 0, 200, and 300 ppm by weight H. Variables i n t h i s study have been t h e number o f thermal cycles i n t h e temperature range 77 t o 340°K and various homogeneous t e n s i l e b i a s s t r e s s values up t o = 21 MPa. The hydrogen solvus temperature observed upon t h e f i r s t thermal c y c l e i s lower than t h a t observed upon subsequent thermal cycles. No measurable e f f e c t o f b i a s s t r e s s upon t h e saturated solvus temperature a f t e r several thermal cycles has been noted. A l l r e s u l t s seem t o be explainable from elementary thermodynamic and d i s l o c a t i o n p r i n c i p l e s.
I
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INTRODUCTIONP r e c i s i o n u l t r a s o n i c v e l o c i t y change measurements versus temperature i n t h e Nb-H system w i t h and without s t a t i c b i a s s t r e s s might c l a r i f y t h e e f f e c t o f a homogeneous t e n s i l e b i a s s t r e s s upon the s o l u b i l i t y l i m i t o f H i n Nb under c o n d i t i o n s o f a closed system 11-51. From t h e d e t a i l e d discussion o f t h e e f f e c t o f b i a s s t r e s s on hydride p r e c i p i t a t i o n by Flanagan, Mason and Birnbaum
151
w i t h respect t o t h e Nb-H system, i f t h e p a r t i a l molar volume o f hydrogen i n t h e s o l i d , i s equal o r almost equal i n the s o l i d s o l u t i o n and hydride phases, and t h e system i s closed, then the solvus i s unaffected o r almost unaffected by a uniform b i a s stress. This r e s u l t s because t h e s t r e s s cannot do work on the system. The present i n v e s t i g a t i o n was undertaken i n order t o v e r i f y one way or t h e o t h e r t h e v a l i d i t y of t h i s concept.Present address :
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Departamento de Fisica Universidade, Estadual deMaringa, Caixa Postal 331,
87.
100 Maringa, Pr, Brasil+ + U s i n a Aqua Limpa, Monte Aprazivel, S.P. Brasil "'~epartment of Materials Science, William Marsh Rice University, Houston, Texas 77251, U.S.A.
JOURNAL
DE
PHYSIQUE A - 0 p p m - H - 300 PPm -H. Cod;ng down X - 300 P P-H, ~ Heating up I * Cycle 0 0 X X X ~ X x * X X X x x x X X X X X X A A‘
* A~ A A A At
**
6 % 4 4 ~ b b ~ C C ~ * ~ A b A . l * I & b I A AFig. 1. A a ( d b / ~ s ) versus T(OK) f o r c r y s t a l Nb-2 f o r 2 thermal c y c l e s w i t h and without H doping.
I 1
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EXPERIMENTAL METHODSHigh p u r i t y s i n g l e c r y s t a l s o f Nb were grown from Marz grade Nb (0.25 i n . i n dia. rod) from t h e M a t e r i a l s Research Corporat'on by t h e electron-beam f l o a t i n g zone method. M u l t i p l e passes i n vacuum o f
lo-'
t o r r w i t h T i g e t t e r i n g t o m a i n t a i n 0 l e v e l s low i n t h e atmosphere were growth c o n d i t i o n s . C r y s t a l s Nb-1 and Nb-2 were o r i e n t e d such t h a t t h e specimen a x i s made angles of 38O, 19' and 18' w i t h respect t o t h e C0011, [ O l l ] and[Till
c r y s t a l l o g r a p h i c d i r e c t i o n s r e s p e c t i v e l y . C r y s t a l Nb-3 had i t s specimen a x i s o r i e n t e d along t h e [ O l O ] d i r e c t i o n .P r e c i s i o n u l t r a s o n i c v e l o c i t y and a t t e n u a t i o n measurements were made u s i n g a s i n g l e quartz transducer according t o the "Pulse Echo Overlap Method'' as described by Papadakis
161,
wherein a t u n i n g accuracy o f one p a r t per m i l l i o n i s a t t a i n e d . The s p e c i f i c d e t a i l s o f annealing t h e specimens, hydrogenizing and dehydrogenizing t h e specimens, t h e d e t a i l e d u l t r a s o n i c technique ( a c t u a l equipment used) and p r e p a r a t i o n o f t h e f l a t surfaces f o r bonding on of t h e transducer etc. and s p e c i a l l y designed c r y o s t a t s t o enable data t o be taken i n the temperature range 77 t o 340°K a r e a v a i l a b l e i n 0. ~ l o r $ n c i o ' s Master o f Science t h e s i s 171.I 1 1
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EXPERIMENTAL RESULTSNb-1 contained e i t h e r 0 o r 200 pp H by weight whereas c r y s t a l s Nb-2 and Nb-3 contained e i t h e r 0 o r 300 ppm H by weight. Many temperature c y c l e s between 340 t o 150°K were made on t h e samples w i t h and w i t h o u t hydrogen and f o r specimen Nb-3 these cycles were made w i t h and w i t h o u t v a r i o u s t e n s i l e b i a s s t r e s s values a t
various temperatures /7/. Typical data i s shown i n Figs. 1 t o 4. A l l
changes d e t e c t upon f i r s t c o o l i n g a low observed solvus temperature o f 250°K
f8r
200 ppm H specimens and 270°K f o r 300 ppm H specimens r e s p e c t i v e l y . These temperature correspond t o t h e non-zero s e l f s t r e s s temperatures (TSO) discussed by Grossbeck and Birnbaum / 3 / . A f t e r one o r more thermal cycles, however, even though t h e a t t e n u a t i o n and v e l o c i t y appears t o have f u l l y recovered a f t e r hydride r e d i ssol u t i o n a t elevated temperature, t h i s technique d e t e c t s a higher sol vus temperature. The higher solvus temperature i s almost up t o phase diagram solvus temperatures presented by Wenzl and Welter /8/, i.e. about 284OK f o r 200 ppm H specimen's and 310°K f o r 300 ppm H specimens. A l l o f these temperatures are p r e d i c t e d t o s h i f t upwards by only 0.2"K f o r a b i a s s t r e s s o f 21 MPa 131. A d e t a i l e d analysis o f a l l o f the data shows t h a t a homogeneous t e n s i l e b i a s s t r e s s up t o-
21 MPa i n Nb-3 w i t h 300 ppm H causes no detectable r i s e i n solvus temperature f o r t h i s closed system i n accordance w i t h expectations o f theory5 A l l o f our b i a s s t r e s s data reveals our modulus d e f e c t (AMIM) (when corrected f o r d e n s i t y and Poisson c o n t r a c t i o n e f f e c t s under bias s t r e s s ) t o be negative and t o become more negative w i t h i n c r e a s i n g bias stress.
I V DISCUSSION AND SUMMARY
JOURNAL DE PHYSIQUE
FIG. 4 (2Av/v
versus T("K) f o r sp?cimen Nb-3 w i t h and w i t h o u t H
doping and a t 0 and
f i n i t e b i a s s t r e s s o.
(a) t h e non-zero s e l f - s t r e s s solvus temperature (TsO) observed i n t h e f i r s t thermal c y c l e /5/ r i s e s upon subsequent thermal c y c l e s and approaches t h e e q u i l i brium solvus temperature p r e d i c t e d by phase diagram a n a l y s i s /8/
(b) t h e solvus temperature c l o s e t o t h a t p r e d i c t e d by phase diagram a n a l y s i s i s n o t observed t o be a f f e c t e d by a homogeneous t e n s i l e b i a s up t o = 20 MPa f o r t h i s closed system as expected by t h e o r y /5/
and ( c ) b i a s stresses up t o 213 o f t h e y i e l d p o i n t o f Nb c o n t a i n i n g 300 ppm H by weight produce a modulus decrease as t h e b i a s s t r e s s increases i n t h e temperature range 250 t o 310°K.
P o i n t (a) can be explained as follows. Upon formation of 0 h y d r i d e p r e c i p i t a t e upon 1 s t c o o l i n g , t h e 12% volume expansion /3/ i s accompanied by a l a t t i c e e l a s t i c s t r a i n and l o c a l d i s l o c a t i o n generation and re1 axation. Subsequent c y c l i c c o o l i n g nucleates t h e 6 h y d r i d e a t these d i s l o c a t e d c e n t e r s and hence a t regions o f
i nhomogeneous stress, which promotes a h i g h e r sol vus temperature. The r e s i d u a l d i s l o c a t i o n s are so few i n number, they h a r d l y a l t e r t h e h i g h temperature a t t e n u a t i o n o r v e l o c i t y values, t h u s suggesting r e d i s s o l u t i o n o f h y d r i d e i s complete. Point (b) r e q u i r e s no e x p l a n a t i o n and p o i n t ( c ) suggests t h e b i a s s t r e s s i s probably a c t i v a t i n g more donble k i n k s on non-screw d i s l o c a t i o n s and hence c o n t r i b u t i n g t o t h e lower modulus.
Acknowled ments
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One o f t h e authors (O.F.) acknowledges a Master's Fellowship Grant fr& C w q and another (J.M.R.) i s g r a t e f u l f o r support as a v i s i t i n g professor a t DEMS-UFSCar from CNPq, Brasi 1.
REFERENCES
/I/ Birnbaum, H. K., Grossbeck, M.L., and Amano, M., J. Less-Common Metals,
49
(1976) 357.
/2/ Gahr, S., Grossbeck, M.L. and Birnbaum, H.K., Acta Met., 25 (1977) 125.
/3/ Grossbeck, M.L. and Birnbaum, H.K., Acta Met., 25 (1977) T55.
/4/ Gahr, S. and Birnbaum, H.K., Acta Met.,
26
( 1 9 m 1781./5/ Flanagan, T. B., Mason, N.B. and Birnbaum, H.K., S c r i p t a Met.,
&
(1981) 109./6/ Papadakis, E.P., J. Acoustic Soc. Am. 42 (1967) 1045.
/7/ ~ l o r $ n c i o ~ O., Masters Thesis i n ~ p n e d Physics and Science o f M a t e r i a l s , U n i v e r s i t y o f Sao Paulo, I n s t i t u t e o f Physics and Chemistry o f
