Short Notes K43

phys. stat . sol . (a) 42, K43 (1978)

Subject classification: 10.1; 12.1; 21

Laboratoire de Mbcanique et Physique des MatBriaux. Equipe de Recherche associBe au C . N . R. S. no 123,

1) Ecole Nationale SuNrieure de MBcanique et d 'ABrotechnique , Poitiers

Evidence of an Internal Friction Phase Trqnsformation Peak

in Ni obi urn-Hydrogen Alloys

BY

G. FERRON and M.QUINTARD

The determination of hydrogen solubility in the group V refractory metals is

of great interest because of the ductile-brittle transition correlated to hydride pre-

cipitation. Several methods have been used in previous investigations dealing with

the determination of the phase diagrams: dilatometric /1/ and resistometric /2, 3/

measurements, appearance of the "precipitation peak", associated with the gene-

ration of dislocations during hydride formation /4/.

The purpose of this work was to precise the properties of a new internal friction

peak, already observed in a previous paper, and then called the p' effect /5/. 2 Hydrogen was introduced by cathodic charging a t room temperature (0.4 A/cm )

in a n electrolytic cell using a solution of 10% H SO as the electrolyte. Internal

friction measurements were carried out in the temperature range between 90 and

350 K at 1 and 15 Hz in an inverted torsion pendulum. The heating and cooling

rates were about 40 K/h.

2 4

The internal friction versus temperature curves obtained during heating,

after some electrolytic charging processes upon specimens priorly annealed 4 h

at 1400 K . a r e shown in Fig. 1. Two main effects a r e observed:

- the precipitation peak. situated near 120 K ,

- the

height and temperature of the maximum increase with increasing hydragen contents.

I effect, observed in the temperature range between 200 and 300 K; the

Furthermore, it. appears that after repeated thermal cyclings between 90 and I effect decreases slightly /6/ 300 K , the precipitation peak develops, whereas the

1) Rue Guillaume VII. 86034-Poitiers C6dex. France.

K44 physica status solidi (a) 46

t .=3

30 a

25

20

15

10

5

G I I I

I

100 200 300 TIKI -

Fig. 1

T/K) --+

Fig. 2

Fig. 1. Damping spectra during heating runs (frequency II 1 Hz); x after annealing for 4 h at 1400 K after hydrogen charging, 0 1 min at 0 . 4 A/cm2 ( ~ 2 0 0 a t p p m ) , 0 4 m i n a t 0 . 4 A / c m ~ ( ~ 8 0 0 a t p p m ) , A 8 m i n a t 0 . 4 A/cm2 ( w 1600 at pprn). + 0 30 min at 0 . 4 A/cm2 ( w 6000 at ppm)

heating run, frequency N 1 H z , 0 heating run, frequency -15 Hz, +

15 min at 0 . 4 A/cm2 (e 3000 at ppm),

2 ' Fig. 2 . Damping spectra after hydrogen charging for 30 min at 0 . 4 A/cm ; 0 x

cooling run, frequency M 1 Hz, cooling run, frequency w 15 Hz

Measurements carried out during cooling runs show an important thermal I hysteresis of the (3 effect, which then exhibits a shift of 50 K towards lower temper-

atures with a hydrogen loading of about 6000 at ppm. Besides, the height of the

$effect is one order of magnitude smaller when the sample i s tested at 15 Hz

(Fig. 2).

Short Notes K4 5

Previous studies have shown that the precipitation peak coincides with the

OL peak, generally observed after deformation; the appearance of the precipitation

peak must be associated with the generation of dislocations during hydride

formation /4 to 6 / . The punching-out of fresh dislocations during the successive

precipitations of the hydride phase can thus account for the increase of this peak

after repeated thermal cyclings. I The p effect exhibits the features of an internal friction peak occurring in the

phase transition temperature region, in agreement with the theoretical predictions

of the phenomenological model of Delorme et al. /7/; in this model, the following

expression i s obtained f o r the internal friction:

where w i s the angular frequency, n i s the relative fraction of new phase, and f(n)

an increasing function of n . I

On the one hand, the fast complete collapse of the (3 effect at the higher fre-

quency of 15 Hz i s consistent with the prediction of a damping in inverse proportion

to the measurement frequency. On the other hand, the variations of the height and

temperature of the peak with hydrogen content and experimental process (i . e.

heating o r cooling run) are in qualitative agreement with the assumption of a damping

varying like the rate of dissolution of the hydride phase during heating (or like the

rate of precipitation during cooling). The slight decrease of the $effect observed

after repeated thermal cyclings might be the result of a decrease of hydrogen

content in the lattice, because of a distribution of hydrogen along the dislocation

loops punched out by the hydride precipitation.

Summarizing, the characterist ics of the p effect a r e indicative of a phase I

transformation peak associated with the a *a+

both experimental and theoretical, should supply valuable information concerning

the hydride precipitation.

transition. Further studies,

Acknowledgement

The authors wish to thank Prof. J. de Fouquet for helpful discussions.

K4 6 physica status solidi (a) 46

References

/1/ B. LONGSON, The Hydrogen Embrittlement of Niobium, TRG Report No. 1035

(c), 1966.

/2/ D . G . WESTLAKE, Trans . MS AIME 239, 1341 (1967).

/3/ D . G . WESTLAKE, Trans . MS AIME 245, 287 (1967).

/4/ 0. BUCK. D . O . THOMSSON, and C . A . WERT. J . Phys . Chem. Solids 32, 2331, (1971).

/5/ G. FERRON, M.QUINTARD. and J . DE FOUQUET. Nuovo Cimento 33, 76

(1976).

/6/ G. FERRON. T h e s i s , PoitIers 1977.

/7/ J .F . DELORME, R. SCHMID, M. ROBIN, and P . GOBIN, J. Physique, Suppl.

- 7 , 101 (1971).

(Received December 5 , 1977)