INTERNAL FRICTION STUDY OF MULTIPLE RELAXATION PHENOMENA IN N b-O ALLOYS WITH HIGH OXYGEN CONTENT

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INTERNAL FRICTION STUDY OF MULTIPLE

RELAXATION PHENOMENA IN N b-O ALLOYS

WITH HIGH OXYGEN CONTENT

B. Heulin

To cite this version:

JOURNAL DE PHYSIQUE

Colloque

C10,

supplbment

au

n012, Tome 46, dbcembre

1985

page

(210-51

INTERNAL FRICTION STUDY OF MULTXPLE RELAXATION PHENOMENA IN N b-0 ALLOYS WITH HIGH OXYGEN

CONTENT

8 . HEULIN

D e p a r t e n t e n t

Sciences

d e s M a t e r i a u x - E . N . S . C . S . , 67008 S t r a s b o u r g C e d e x , F r a n c e

Resume

-

La mise en evidence de paires et de triplets d'atomes interstitiels en solution solide dans les metaux cubiques centres par mesure du frottement intbrieur a 6t6 tres controversee. Des mesures effectubes 1 Hz sur des alliages Nb-0 contenant jusqu'A 4

% at. d'oxyghne permettent d'observer un elargissement asymetrique du pic qui peut Btre decompose en'trois pics simples attribues

A

la relaxation d'atomes isoles, de paires et de triplets d'oxyghne.

Abstract

-

The existence, based on internal friction measurements, of pairs and triplets of interstitial atoms in solid solution in bcc metals seems to be questionable. Measurements performed at 1 Hz on Nb-0 alloys with oxygen content up to 4% at. show an asymmetric broadening of the peak that can be resolved into three elementary peaks due to the relaxation of singlets, pairs and triplets of oxygen atoms.

I- INTRODUCTION

The Snoek peak associated with oxygen atoms in Ta-0 and Nb-0 solid solutions has been studied by several authors. Powers /I/, Powers and Doyle /2,3/, Gibala and Wert /4/, Ahmad and Szkopiak /5/ resolved the internal friction peak due to oxygen in more than one peak. According to these authors, the additional peaks were attributed to the relaxation of oxygen atom either from oxygen atom

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pairs, or from clusters of three and even four atoms. More recently Weller and al. /6/ suggested that the broadening and skewing of the measured internal friction peak on the high temperature side, as observed by increasing the oxygen content (up to 0.6 % at.), was not asymmetrical, but symmetrical ; furthermore these authors refute the existence of an additional peak and assume that the modification has to be related to a change in the activation enthalpy and/or in the frequency factor leading to a statistical distribution of relaxation times.

We have performed internal friction measurements on Nb-0 alloys with higher oxygen content (up to 4 % at.) in order to obtain new information about the existence or the non-existence of additional peaks and also on the solubility of oxygen in niobium

I1 MTERIAL AND EXPERIHENTAL PROCEDURE

The specimens were prepared from high purity niobium (about 99.95 at %

purity) ; 2 mm diameter wires cold-swa ed to 0.5 mm diameter and annealed in a dynamic vacuum of about lo-' Pa for 15 mn at a temperature of 1065°C. Nb

-

0 alloys were prepared by direct electrical heating of the wires (60 cm length) in an atmosphere of pure oxygen (lo-' Pa) at a

C10-52 JOURNAL DE PHYSIQUE

temperature of 975'~. Samples were prepared with different oxygen concentrations by annealing for times varying between 30s and 2 hours-Cons cutive annealing at llOOOc for 30 mn in a dynamic vacuum of

t

about 10- Pa was performed in order to homogenize the oxygen in interstitial solution; finally the samples were quenched with helium immediately after switching off the current. This treatment leads to an average grain diameter of 50 pm. The oxygen concentrations, determined by a method of extraction by melting the samples under vacuum /7/, ranged from 0.1 to 4 % at. Interstitial nitrogen was also present as an impurity at about 100 at. ppm.

Internal friction measurements were carried out in an inverted vacuum torsion pendulum at a frequency of about 1 Hz. The specimen was heated from room temperature up to 600'~ at a constant rate of 1 K/mn. The maximum shear strain was less than 5.10-~.

I11 RESULTS AND THEIR INTERPRETATION

Internal friction measurements have been perfomed on Nb - 0 alloys for 15 different oxygen concentrations. The eight of the experimental peak increases linearly to a Q-lmaX of ll.10-4 for an oxygen content of 2.55

% at. and then slightly deereases to 9-lmax = 10-I for higher concentration. Figure 1 shows a normalized representation of the peak heights of some experimental spectra versus 1000/T1. T1 is the temperature of the experimental points corrected according to

l/T1 =1/T +k/H. In (f/fl) (fl being the frequency of 1Hz and f the experimental frequency for each point,

H

the activation enthalpy obtained by Ueller /8/ for Nb

-

0 is 1.15 eV). The temperature Tm of the maximum shifts from 156'~ to 183'~ for the highest oxygen concentration. The peaks appear asymmetric on the high temperature side for the lowest concentrations and on the low temperature side for the highest concentrations.

Fig.1 Normalized representation of experimental spectra.

The peak at 280°c corresponding to the Nb-N Snoek peak remains constant

for all the samples (Q-~,,,

=

1,5.10-~). The variations of the height of

the four other peaks versus oxygen content are represented on figure 2. The peak located at 156Oc is the Snoek peak Nb-0. At first it increases linearly with oxygen content, then stabilizes itself as soon as other peaks appear at higher temperature.

Fig.2 Variation of the decomposed peaks versus oxygen content.

The peaks at 170'~ and 193'~ are attributed respectively to the

relaxation of oxygen atoms in pairs and triplets according to the

results of. Ahmad and Szkopiak / 5 / . The variation of these two peaks is

represented on a logarithmic scale on figure

3.

We can observe within

the linear parts of these curves a slope of 2 and 3 which supports

strongly arguments attributing relaxation phenomena to oxygen pairs and triplets.

(210-54 JOURNAL DE PHYSIQUE

Thus the process of occupation of the interstitial positions could be :

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up to 0.3 % at. 0, occupation by isolated oxygen atoms ;

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beyond 0.3 % at. 0, creation of pair8 ;

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beyond 0.9 % at. 0, creation of triplets, the number of isolated atoms and pairs remaining constant ;

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beyond 2.5 % at. 0, the solubility limit of oxygen at 1 1 0 O 0 ~ is reached and oxygen precipitates during quenching.

The decreasing of the peak due to triplets indicates that the precipitation occurs in the areas where there are clusters (0

-

0

-

0 ) of oxygen. An additional study performed on N b

-

0 alloys quenched at 700'~ and X ray parameter measurements confirm these results.

The peak at .220°c is the peak attributed by Powers and Doyle / 2 / and Ahrnad and Szkopiak /5/ to 0

-

N interactions. It' increases with oxygen content up to the solubility limit of oxygen. The decreasing of this peak indicates a precipitation of oxygen in areas where nitrogen is present.

V CONCLUSION

The large concentration range studied by us lead us to conclude that for a high oxygen content, more than 1 % at., it is difficult to explain the asymmetry of the peak only through variation of frequency, presence of nitrogen as impurity or variation in the relaxation time. On the other hand the existence of multiple relaxation phenomena seems to be in good accordance with the experimental results. The asymmetry of the peak on the high temperature side for low oxygen contents corresponds to the existence of few clusters of oxygen. Then when the oxygen content increases the number of clusters increases too and the main peaks located at higher temperature correspond to an interaction of oxygen atoms in pairs and triplets. Thus the Snoek peak is stabilized and gives an asymmetric shape to the experimental curves. Nevertheless the theory of Ueller could be accepted at the same time because we can imagine that interactions between oxygen atoms at high oxygen contents could lead to a variation in the migration activation enthalpy and consequently to a statistical distribution of relaxation times for each relaxation phenomena: singlets, pairs and triplets.

REFERENCES

/1/ Powers, R.U., Acta M e t . 2 (1955) 135

/2/ Powers, R.W. and Doyle H.V., Appl. Phys.

2

(1959) 514 /3/ Powers, R.U. and Doyle M.V. Trans. AIME,

215

(1959) 655

/4/ Gibala, R. and Wert, C., Acta Met.,

14

(1966) 1095 / 5 / Ahmad, U.S. and Szkopiak, Z.C., J. Phys. Chem. Solids

31

(1970) 1799

/6/ Ueller, M. and al., Acta Met.

29

(1981) 1055 /7/ Heulin, B., Thesis, University of Strasbourg (1981)