INTERNAL FRICTION STUDIES ON HYDROGEN IN SOLID SOLUTION IN V-Nb ALLOYS

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INTERNAL FRICTION STUDIES ON HYDROGEN

IN SOLID SOLUTION IN V-Nb ALLOYS

C. Owen, O. Buck, R. Smith, D. Peterson

To cite this version:

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J O U R N A L D E P H Y S I Q U E

Colloque C10, supplement a u n 0 1 2 , Tome 46, dBcembre 1 9 8 5 page C10-107

INTERNAL FRICTION STUDIES ON HYDROGEN IN SOLID SOLUTION IN V-Nb ALLOYS

C.V.

OWEN,

0. BUCK, R . R . SMITH AND D.T. P E T E R S O N

Ames Laboratory, Iowa State University, Ames, Iowa 50011, U.S.A.

~gsumg

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Nous avons gtudig lleffet de la proportion V:Nb sur le maximum dans la friction intgrieure ?la frGqu$nce de 1 Hz. L1h?uteur slaccro?t avec la i

concentration de H et va aux temperatures plus gleves si on augment la frgquence. Les rgsultats indiqu; que llgnergie dlactivation du maximum dans la friction intgrieure dGpend s$r la proportion V:Nb comme llgnergie d'activation pour la diffusion.

Abstract

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The effects of the V to Nb ratio on the hydrogen internal friction peak have been studied at about 1 Hz. Its height increases with H concentration and shifts to higher temperatures with increasing frequency. The results indicate that the activation energy of the peak depends on the V to Nb ratio in a fashion similar to that found for the activation energy for diffusion.

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INTRODUCTION

The existence of a relaxation peak due to hydrogen in solid solution in b.c.c. alloys has been reported by several investigators 11-51. V-Nb alloys seem to be particularly well suited for such studies in that the hydrogen solubility in these alloys is extremely large /6/ and that hydrides seem not to precipitate in most of these alloys even well below liquid nitrogen temperature. Furthermore, the activation energies and diffusion coefficients for long range diffusion of H have been studied extensively 171, indicating that both quantities depend strongly on the matrix composition. In comparison to earlier anelastic studies of hydrogen diffusion in Nb 181, the peak heights observed in the V-Nb alloys 131 are about a factor of 10 larger and, thus, much easier to evaluate.

The purpose of the present paper is to report initial observations of low frequency internal friction measurements of H-doped V-Nb alloys as a function of H-concentration, frequency, and the matrix composition, to compare the results with those of the long range diffusion measurements, and to draw some preliminary conclusions on the kinetics of the process.

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EXPERIMENTAL METHODS

The internal friction measurements were performed in an inverted pendulum between 0.17 Hz and 1.35 Hz. Q-1 was determined isothermally between 80 (the present low temperature limit of the apparatus) and 300 K on both heating and cooling and at a maximum strain amplitude of about 3 x 10-6. The samples used were wires either 0.76 or 1.5 mm in diameter and 76 mm in length. The high V alloys were recrystallized in vacuum for 1.5 h at 1373 K, all others at 1573 K under the same conditions. H was introduced into the specimens by gas phase charging. Table I shows the alloys employed and the H, 0, and N contents of each.

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JOURNAL

DE

PHYSIQUE

Table I

Chemical Analysis in Atomic percent*

Material 0 N H

"vacuum fusion analysis of charged specimens

111

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EXPERIMENTAL OBSERVATIONS

The background internal friction (Q-l) of all alloys in the uncharged condition was found to be 1 to 2 x 10-4 over the total temperature range studied. Figure 1 shows Q-l versus temperature curves for all six alloys used at the H-concentrations indicated. All alloys (with the exception of V-10 Nb

+

3 at.% H) show a low temperature peak

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or at least the indication of such a peak, as indicated by the increase in Q-1 at the lowest temperatures employed. This low temperature peak seems to be associated with H in solid solution as observed earlier /3/. The results indicate that the temperature position of this peak depends on the matrix composition, being at a maximum temperature in the V-75 Nb alloy. As can be seen in Fig. 1, Y-10 Nb

+

3 at.% H is the only alloy investigated which does not show the low temperature peak but rather an increased Q-1 over a wide temperature range, identified earlier as a hydride precipitation peak /5/. Low temperature microscopy of all alloys was found to be consistent with the observation that this is a precipitation peak since none of the alloys, with the exception of V-10 Nb

+

3 at.% H, showed hydride precipitates as low as 80 K. In the following this precipitation peak will not be discussed any further.

Figure 2 shows (as an example) the peak shift with frequency for the V-75 Nb

+

1.4 at.% H alloy. Figure 3 indicates an almost linear increase in peak,height with increasing H-concentration for the V-85 Nb alloy. Note the peak shift to lower temperatures as the H-concentration is increased.

Test Temperature, O K

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Test Temperature , O K

Test Temperoture, O K

Fig. 2. Q-l of H-charged V-75 Nb +1.4 Fig. 3. Q-1 of V-85 Nb ag a func- at.% H as a function of frequency. tion of H-concentration. IV

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DISCUSSION

Based on the results obtained on V-75 Nb (Fig. 2) and on V-85 Nb, (not shown) the width of the low temperature peak is roughly a factor of 3 to 3.5 broader than would be expected from a single relaxation process. Ignoring this complication, estimates of the activation energies of the process were obtained for some of the alloys using the high temperature flank of the peak and, whenever possible, the shift of the peak temperature versus frequency. The values obtained are somewhat below those obtained for long range diffusion extrapolated to zero H-concentration /7/, as shown in Fig.

4. It is interesting to note that a shift of the peak to lower temperatures with increasing H concentration, as shown in Fig. 3, was also observed in Nb-Ti-H alloys and attributed to the formation of Ti-H, complexes /4/. In the latter case /4/ it was found that the activation energy decreases with increasing H-concentration. Assuming that a similar effect occurs in V-Nb-8 it thus seems possible that the difference between the activation energy for diffusion and that observed by the

I

0 20 40 60 80 100

Alloy Composition, Atomic Percent Nb

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'210-110 J O U R N A L D E PHYSIQUE

internal friction measurements is a H concentration effect. Further experiments to test this possibility will have to be performed. However, both types of measurements yield a maximum activation energy for the V-75 Nb alloy, a trend which is also reflected in the shift of the peak position as shown in Fig. 1.

Relaxation times T~ were also estimated, with the V-75 Nb alloy yielding a minimum value of ro 2 x 10-12 sec. On the other hand, Herro /7/ observed a slight maximum in Do for the same alloy. Assuming that Do a ~ ~ - 1 , the results obtained on both types of measurements are in qualitative agreement. Indications are, however, that the changes in ro with composition as obtained from ~ - 1 measurements, assuming a single relaxation process, are larger than those found in Do 171. Therefore, it is felt that the mechanism giving rise to the internal friction peak is closely related to the diffusion of H in these alloys.

Presently it is thought that H occupies tetrahedral sites /9/ in the alloys giving rise to a Snoek-type relaxation process. Using slow neutrons, quasielastic scattering measurements on pure Nb indicate 191 that the H does not simply jump between nearest neighbor sites. In the alloys this situation is further complicated by a random distribution of host atoms so that, as the H jumps, it could find a multitude of host environments. It thus is not surprising to find a peak which is much broader than a Debye peak. Future efforts will concentrate on determining the continuous relaxation spectrum both in activation energy and relaxation time, as well as on the effects of interstitials (particularly 0) on the observed peak 181. ACKNOWLEDGMENT

This work was performed for the U.S. Department of Energy, Office of Basic Energy Sciences under contract No. W-7405-Eng-82.

REFERENCES

/1/ Cannelli, G., and Cantelli, R., Proc. 6th Int. Conf. on Internal Friction and Ultrasonic Attenuation in Solids, Tokyo (1977) p. 491.

/2/ Tanaka, S., and Koiwa, M., Scripta Met.

15

(1981) 403.

131 Owen, C. V., Buck, O., and Scott, T. E., Scripta Met. _15 (1981) 1097. /4/ Cannelli, G., Cantelli, R., and Koiwa, M., Phil. Mag.

A46

(1982) 483. /5/ Owen, C. V., and Buck, O., Scripta Met.

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(1983) 649.

/6/ Miller, J. F., and Westlake, D. G., in Hydrogen in Metals, Proceedings of the Second JIM International Symposium, Suppl. to Trans. JIM

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(1980) p. 153. /7/ Herro, H. M., Ph. Dissertation, Iowa State University (1982).

/8/ Baker, C., and Birnbaum, H. K., Acta Met.

2

(1973) 865.