ANELASTIC RELAXATION DUE TO HYDROGEN IN SOLID SOLUTION IN VANADIUM AT LOW TEMPERATURE

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ANELASTIC RELAXATION DUE TO HYDROGEN IN SOLID SOLUTION IN VANADIUM AT LOW

TEMPERATURE

G. Cannelli, R. Cantelli, F. Cordero

To cite this version:

G. Cannelli, R. Cantelli, F. Cordero. ANELASTIC RELAXATION DUE TO HYDROGEN IN SOLID

SOLUTION IN VANADIUM AT LOW TEMPERATURE. Journal de Physique Colloques, 1983, 44

(C9), pp.C9-403-C9-409. �10.1051/jphyscol:1983959�. �jpa-00223409�

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ANELASTIC RELAXATION DUE TO HYDROGEN I N S O L I D SOLUTION I N VANADIUM AT LOW TEMPERATURE

G.

Cannelli,

R.

Cantelli and

F.

Cordero

C.N.R., I s t i t u t o d i A m t s t i c a "O.M. Corbino", V i a Cassia 1216, 00189 Roma, I t a l y

Rbsumb. Nous prksentons un nouveau pic de relaxation anblastique dans le systhme V-H.

Abstract. A new anelastic relaxation effect is reported, due to hydrogen in solid solution in annealed vanadium.

I

-

INTRODUCTION

In the transition metals niobium and tantalum containing hydrogen a relaxation process occurs around 100

K

for vibration frequencies in the kHz range. This effect, when first reported 1 4 was attributed t o the stress- induced reorientation of free hydrogen within the crystal lattice. Later on, the Gorsky effect a1 lowed re1 iable hydrogen diffusion coefficient data to be obtained by means of techniques so different each other a s the internal friction /5-8/ and the elastic after effect /9/. Using those results it appeared clear that a pure H-Snoek peak should have occurred at temperatures remarkably lower than those of the process observed. It was also pointed out that the small local ellipticity introduced by the hydrogen tetrahedral interstitial

/lo/,

and the extreme1 y small amount of H left in sol id solution at the temperatures of the peak, would make a pure H-Snoek process practically not observable.

Subsequent investigations conducted in niobium displayed that the peak at 100 K manifests itself only when hydrogen and oxygen (nitrogen) interstitials are simultaneously present /11/. It is accepted at present that the relaxation effect observed in niobium is due to the stress-induced hopping of single hydrogen atoms around single oxygen (nitrogen) interstitials acting a s trapping centres 1 - 1 4 and such a process is called the O(N)-H peak. On the contrary, the identification of the geometry of the cluster is currently the object of considerable controversy /15-18/.

The phenomenology of the O(N)-H peak presents the following main features. At a fixed vibration frequency, the height of the maximum increases with increase of H concentration and saturates at amounts comparable with the O(N) content, but the peak temperature remains unchanged. For a given H content, the intensity of the peak decreases when the peak is shifted to higher temperature by frequency, because dissociation of the O(N)-H pairs takes place.

The process is thermal 1 y activated and its relaxation time above 1 0 0 K follows the Arrhenius law, while a deviation has been observed at lower temperature /19/. The effect is not described by a single Debye curve, and this is explained by the fact that two types of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983959

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d e f e c t s , t h e 0-H and t h e N-H complexes, t a k e p a r t t o t h e r e l a x a t i o n , and t h a t f o r a same complex s e v e r a l r e l a x a t i o n modes c o n t r i b u t e t o t h e process /13/.

Although t h e O(N)-H and O(N)-D peaks have been s t u d i e d i n d e t a i l i n niobium, o n l y a few papers have appeared concerning t h e corresponding r e l a x a t i o n i n t h e Ta-H, Ta-D / I - 3 / and V-D /20/

a1 l o y s .

I n t h e vanadium-hydrogen system, i n s p i t e o f t h e numerous i n v e s t i g a t i o n s c a r r i e d out a t low temperature, t h i s process has never been r e p o r t e d . I n t h i s paper we present an a n e l a s t i c r e l a x a t i o n e f f e c t o c c u r r i n g around 90 K i n annealed and hydrogen doped vanadium which may be a t t r i b u t e d t o t h e motion o f u n p r e c i p i t a t e d , trapped hydrogen.

VANADIUM

_-- I

H-FREE

I

9'5

1

0.03 at.?&H

.

^{39. kHz}

F i g . 1 I n t e r n a l f r i c t i o n as a f u n c t i o n o f temperature o f H-doped vanadium f o r t h e 1 s t and 2nd v i b r a t i o n modes.

11. EXPERIMENTAL

The specimens were a c i r c u l a r p l a t e (36 mm i n diameter and 1.5 mm t h i c k ) and a r e c t a n g u l a r bar (57x9x0.9 mm3 ) o f 99.9% p u r e p o l y c r y s t a l l i n e vanadium s u p p l i e d by M a t e r i a l s Research Corporation.

The samples were annealed f o r 15 min a t 1500.C i n a vacuum b e t t e r than 1(:)-' Torr. The i n t e r s t i t i a l oxygen and n i t r o g e n c o n t e n t s subsequently determined i n t h e c i r c u l a r p l a t e by t h e h e i g h t s o f t h e Snoek peaks, were 0.46 and 0.16 at.%, r e s p e c t i v e l y . The carbon c o n t e n t o f t h e as r e c e i v e d m a t e r i a l was 0.024 a t . %

.

Hydrogen doping was c a r r i e d o u t b o t h by e l e c t r o l y s i s and thermal t r e a t m e n t s a t 550 " C i n 99.999% p u r e hydrogen atmospheres. The H c o n c e n t r a t i o n s were determined by t h e weight v a r i a t i o n o f t h e specimens a f t e r vacuum e x t r a c t i o n . F l e x u r a l v i b r a t i o n s o f t h e specimens were e x c i t e d and detected by an e l e c t r o s t a t i c technique /21/ i n t h e frequency range 1.5-40 kHz.

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O.OSat.%H

F i g . 2 I n t e r n a l f r i c t i o n and v i b r a t i o n frequency as a f u n c t i o n o f temperature o f H-doped vanadium.

111. RESULTS

The i n t e r n a l f r i c t i o n

6 '

and t h e concomitant frequency f measurements were c a r r i e d o u t as a f u n c t i o n o f temperature a t a c o o l i n g r a t e o f about 0.3 K/min.

I n t h e annealed and hydrogen f r e e c i r c u l a r specimen no r e l a x a t i o n was observed between room temperature and 2 K. The background damping (dashed l i n e s i n F i g s . 1 and 2) m o n o t o n i c a l l y d e c l i n e s as temperature decreases. Above room temperature, t h e ~ ~ c i w v e (Fig. 3 ) d i s p l a y s t h e oxygen and n i t r o g e n Snoek peaks, from whose i n t e n s i t i e s t h e corresponding c o n c e n t r a t i o n s were evaluated.

A f t e r doping t h e specimen w i t h hydrogen t h e i n t e r n a l f r i c t i o n measurement c a r r i e d o u t over t h e temperature range 2-300 K, d i s p l a y s two peaks l o c a t e d between SO and 150 K. The f i r s t peak ( l a b e l l e d as process P i ) i s c e n t r e d around 90 K , and i t i s seen t h a t , a t a f i x e d c o n c e n t r a t i o n (Fig. 1)

,

i t s h i f t s t o h i g h e r temperature w i t h i n c r e a s e o f frequency w h i l e i t s h e i g h t decreases. The peak l o c a t e d a t h i g h e r temperature ( l a b e l l e d as process P2) d i s p l a y s a sharp i n c r e a s e a t t h e same temperature (145 K ) f o r both frequencies, w h i l e t h e h e i g h t f o r t h e 2nd mode i s lower. Above 150 K , t h e h i g h e r values o f t h e i n t e r n a l f r i c t i o n a t t h e lower frequency a r e due t o t h e s t r o n g e r c o n t r i b u t i o n o f t h e t h e r m o e l a s t i c e f f e c t /22/.

F i g u r e 2 shows t h e i n t e r n a l f r i c t i o n curve of t h e same c i r c u l a r specimen a t a h i g h e r hydrogen c o n c e n t r a t i o n ( 0 . 0 5 a t . % H ) . The i n t e n s i t y of t h e process P1 i s h i g h e r than t h a t o f t h e corresponding curves drawn i n Fig.1, b u t t h e maximum temperature i s unchanged. The peak P2 i s more pronounced, too, and t h e temperature a t which t h e sharp i n c r e a s e occurs i s s h i f t e d t o h i g h e r values. The abrupt v a r i a t i o n o f d 3 s accompanied by a d i s c o n t i n u i t y i n t h e slope of t h e frequency curve, as shown i n t h e same Fig.2. Again,the temperature

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of the 03inf lection for the 2nd mode coincides with that for the 1st one (result not shown in this figure)

.

The internal friction measurements conducted in the rectangular bar down to 1400 Hz confirmed that the position of the process Pi on the temperature scale depends on the vibration frequency, while that of the process P 2 depends on the hydrogen content.

Figure 4 shows the behaviour of the peak frequency of P1 as a function of the maximum reciprocal temperature. The Arrhenius law is followed in the frequency range investigated; the derived activation energy and the pre-ex onential factor of the relaxation time are Es=0.16 eV and V, =2xiO s, respectively. 43

Fig.3 Oxygen and nitrogen Snoek peaks in the annealed vanadium specimen.

IV. DISCUSSION

The peak P Z presents features which allow the mechanism from which it is originated to be ideqtified. The process is characterized by a sharp increase in the 13 and f curves occurring at a temperature Ttwhich increases with

H

concentration ut which

-9

is independent of vibration frequency. In vanadium, the Q abrupt rise was systematically observed in concomitance with the hydride (deuteride) precipitation /25, 23,24/ and the process was called

"precipitation peak". Discontinuity in the temperature derivative of the frequency curve also takes place in correlation with the H (D) precipitation, a s it was first reported for the tantalum- deuterium system / 2 6 / . Finally, the temperatures Tt and the corresponding H concentrations obtained in the present investigation fit the extrapolated solvus 1 ine previous1 y reported for the V-H system at higher H concentrations /23,27 /. All the features described here clearly characterize the process P 2 as the precipitation peak. It should be emphasized that this peak has been

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F i g . 4 V i b r a t i o n frequency as a f u n c t i o n o f r e c i p r o c a l peak temperature o f process P I i n t h e H-doped vanadium specimens.

When deformation i s present a d d i t i o n a l peaks appear, and t h e p i c t u r e o f t h e behaviour o f t h e r e l a x a t i o n spectrum f o r vanadium can be completed r e c a l l i n g t h e f 01 l o w i n g f eatut-es. I f p l a s t i c deformation i s i n t r o d u c e d i n t h e hydrogen f r e e sample, a t h e r m a l l y a c t i v a t e d peak occurs around 20 K /23/ which i s considered t o be caused by an i n t r i n s i c d i s l o c a t i o n r e l a x a t i o n / 2 8 / . A d d i t i o n o f v e r y s m a l l amounts o f hydrogen t o t h e s t r a i n e d m a t e r i a l r e s u l t s i n t h e gradual suppression o f t h e i n t r i n s i c d i s l o c a t i o n peak and t h e appearance o f a r e l a x a t i o n e f f e c t around 200 K , t h e so c a l l e d * peak, generated by t h e i n t e r a c t i o n o f d i s l o c a t i o n s w i t h hydrogen.

The process P 1 p r e s e n t s p e c u l i a r i t i e s which do n o t make i t a s s i m i l a b l e w i t h any o t h e r o f t h e e f f e c t s j u s t described. The peak occurs o n l y i n t h e presence o f hydrogen, whose c o n c e n t r a t i o n a f f e c t s i t s h e i g h t , and i t s s h i f t w i t h frequency i n d i c a t e s t h a t i t i s a t h e r m a l l y a c t i v a t e d process. The values o f t h e pre-exponential f a c t o r

ee

and o f t h e a c t i v a t i o n energy o f t h e e f f e c t a r e c l o s e l y comparable w i t h those obtained f o r t h e r e l a x a t i o n s o f O(N)-H i n Nb and Ta, and o f O(N)-D i n V. F i g u r e 5 shows t h e normalized i n t e r n a l S r i c t i o n values f o r peak P 1 o f a l l t h r e e s e t s o f d a t a r e p o r t e d i n F i g s . 1 and 2, r e f e r r i n g t o d i f f e r e n t v i b r a t i o n f r e q u e n c i e s and H contents. I t can be seen t h a t a l l experimental p o i n t s f i t q u i t e we1 1 t h e same t h e o r e t i c a l Debye c u r v e c a l c u l a t e d f o r an a c t i v a t i o n energy E g 0 - ( 3 7 0 eV. T h i s v a l u e i s remarkably d i f f e r e n t from t h e a c t i v a t i o n energy E S d e r i v e d from t h e peak s h i f t w i t h frequency, denoting t h a t t h e process F l i s n o t c h a r a c t e r i z e d by a s i n g l e r e l a x a t i o n time. T h i s i s again a p e c u l i a r i t y common w i t h t h e O ( N ) - H

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r e l a x a t i o n . Considering t h a t oxygen and n i t r o g e n i m p u r i t i e s a r e p r e s e n t i n t h e t e s t e d specimens, i t may be i n f e r r e d t h a t t h e process P I i s due t o t h e stress-induced r e o r i e n t a t i o n o f hydrogen atoms around 0 or N t r a p p i n g centres.

An e x p l a n a t i o n why t h e e f f e c t l a b e l l e d h e r e as process P1 was n o t r e p o r t e d up t o now may be s u p p l i e d c o n s i d e r i n g t h a t t h e s o l u b i l i t y l i m i t o f H i n V i s small. As a consequence o f t h i s , i n t e n s e p r e c i p i t a t i o n peaks a r e t r i g g e r e d below room temperature even a t H c o n c e n t r a t i o n s lower than 1 at.%, which can t h e r e f o r e aask t h e process PI.

,

VANADIUM - Debye c u m

Fig.5 Normalized i n t e r n a l f r i c t i o n values o f t h e peak P1 a s a -i -4

f u n c t i o n o f tEw/k)s (T,-T ) a t d i f f e r e n t v i b r a t i o n frequencies and H c o n c e n t r a t i onsg k i s t h e Bol tzmann constant and T, t h e ma:.:imum temperature. The continctous l i n e r e - p r e s e n t s t h e t h e o r e t i c a l Debye curve.

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