Hydride precipitation in vanadium studied by an internal friction technique at high frequency

Scripta METALLURGICA

Vol. 18, pp. 1031-1034, 1984 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

HYDRIDE PRECIPITATION IN V A N A D I U M STUDIED BY AN INTERNAL FRICTION TECHNIQUE AT HIGH FREQUENCY G.Cannelli, R.Cantelli + and F.Cordero Consiglio Nazionale delle Ricerche, [stituto di Acustica "O.M.Corbino" Via Cassia 1216, 00189 Roma, Italy (+) Universitb di Roma "La 5apienza", D i p a r t i m e n t o di Fisica, Piazzale A.Moro 2, 00185 Roma, Italy (Received

February

14,

1984)

(Revised July 22, 1984) Introduction The presence of hydrogen in the metals of group Va, niobium tantalum and vanadium gives rise to an abrupt increase of the curve of elastic energy dissipation vs temperature when the coexistence curve betwe.ep the solid solution(~-phase and the precipitated E-phase is crossed on first cooling. This inflection in the Q- curve is often followed by a maximum which was generically called the "precipitation peak" (1-5). The phenomenology of the peak presents complex features which are not yet satisfactorily understood. In addition, remarkable differences in its behaviour constantly appear whenever results from the three d i f f e r e n t metals or from d i f f e r e n t frequencies are compared. The origin of the precipitation peak is not understood at present and two main mechanisms have been proposed; the interaction between hydrogen (deuterium) and the dislocations generated by the m i s f i t t i n g precipitates (3-5), and the damping connected with the anisotropic growth or dissolution of precipitates (6,7). The present investigation concerning the V-H system aimed at clarifying the influence of vibration frequency on the phenomenology associated with precipitation. A t t e n t i o n has also been devoted to the consequences of mechanical deformation on specimens undergoing precipitation. Experimental The specimen~ were two circular plates (36 and 30 mm in diameter, and 1.5 mm thick) and a rectangular bar (57xgx0.9 m m ' ) of 99.9% pure polycrystaltine vanadium. The i n t e r s t i t i a l gaseous impurities, determined by the heights of the Snack peaks, were; 0.46 at % 0 and 0.16 at %N for the disk of 36 ram; 0.03 at % 0 and 0.10 at %N for the disk of 30 ram. Hydrogen charging was carried out both by electrolysis and thermal treatments at 550°C in 99.999% pure hydrogen atmospheres. The H concentrations were determined by the weight variation of the specimens a f t e r vacuum e x t r a c t i o n . Flexural vibrations were excited and detected by an electrostatic technique in the frequency range 1.4-39 kHz. Results Figure 1 presents the internal f r i c t i o n curves for th~ 1st and 2nd vibration mode exhibited by the 36 mm circular plate charged with 0.0) at%H. The abrupt Q increase at I48K and the associated precipitation peak are caused by hydride formation, and their positions are unaffected by frequency. The maximum below 100K was recently reported by the present authors (8) and identified with the O(NI -H peak extensively investigated in Nb and Ta. The dashed line represents the hydrogen-free dissipation for the 1st vibration mode. The extension of the internal f r i c t i o n measurements down to 1.8 K in the undeformed 30 mm circular plate containing 0.04 at% H (Fig.2), suggests the existence of a new e f f e c t below 2 K. Indeed, a relaxation process has recently been observed at very low temperature ( <:5 K) for comparable frequencies in niobium containing hydrogen trapped by O and N impurities (9). The dislocation peak observed around 20 K for 39 kHz

1031 0036-9748/84 $3.00 + .00 Copyright (c) 1984 Pergamon Press Ltd.

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in the pure and deformed material (2) is absent here, as expected. The peak at 95 K and the abrupt rise at 158 K are caused by the O(N) - H mechanism and by precipitation, respectively. Figure 3 shows the simultaneous measurement of internal friction during the first cooling for the 1st and 5th flexural vibration modes of the vanadium bar containing less than 0.8 at% H. The 1st vibration mode during the subsequent heating is also reported in Fig. 3. The maximum occurring below 100 IK is the O(N)-H peak. The process peaked at 133 IK is connected with hydride precipitation, as indicated by its shift to higher temperature when H concentration increases (see also Fig.4). The precipitation peak for the 5th vibration mode occurs at the same temperature and displays a drastic decrease in its height with respect to the 1st mode. Subsequently, the if-doped sample was 1.5% mechanically deformed and again tested from the conditions of Fig.3. The internal friction curves at 1.4 and 19 kHz are reported in Fig.5 together with the curve for the 1st mode before deformation (Fig.3) for comparison purposes. The concomitant effects of precipitation and deformation give rise to a wide maximum which displays a component whose peak temperature is frequency dependent. The H concentrations are plotted in Fig.6 as a function of the reciprocal temperatures of the internal friction abrupt increase occurring at the onset of hydride precipitation. These data extend the phase diagram of V-H system known to date, in the low concentration region. For comparison purposes, the highest (2) and lowest

(4) solvus line in the literature have also been drawn in Fig. 6.

Discussion The process occurring below 100 K in V-H was recently reported by the present authors (8) and identified with the O(N)-H peak. Its features are similar to those of the corresponding effect in Nb and Ta. The peak is a thermally activated process caused by the stress induced reorientation of hydrogen around oxygen (nitrogen); the values of the pre-~j~ponential factor and of the activation energy determined by the peak shift with frequency, are ~ =2xl0- s and E =0.16 eV, respectively. o s From the data on vanadium presently reported it is clearly seen that the peak connected with precipitation displays characteristics markedly different from those of the corresponding processes in Nb and Ta in the same frequency range. In those two metals the peak introduced by precipitation is thermally activated and its maximum temperature T is independent of the precipitation temperature T ; it is triggered during the 1st coohng at T t and manifests itself as a sharp increase in dissipation. A~so the vibration frequency undergoes an abrupt change at the same temperature. The peak, once generated, persists during the subsequent coolings and heatings, even at temperatures higher than the precipitation temperature, where precipitates are dissolved. Because the peak in Nb and Ta at high frequency is of different nature from the one in vanadium, it needs a different classification, and w i l l be called the "post precipitation peak" (PPP). On cooling, the peak in vanadium always manifests itself at the onset of hydride precipitation with a sharp increase in the internal friction (Fig.4). On heating, the respective cooling curve is retraced (with a small temperature hysteresis) whatever the transition temperature is. In other words, the peak is not thermally activated, but is always associated with the phase transformation and shifts with H concentration (Figs 1 and 3). It has been proved that the peak in Nb and Ta is due to the dislocations generated during precipitation (3,5). By contrast, the dissipation associated with precipitation in vanadium is strictly connected with the a ~ t r a n s i t i o n itself. In addition, the peak does not seem to be due to dislocations, since mechanical deformation introduces a thermally activated relaxation process (Fig.D) distinct from the precipitation peak, the position of which, on the contrary, is frequency independent. Deformation of H-free vanadium produces •

.

m

a peak at about ZO K, whereas small additions of hydrogen cause the suppression of that peak and the appearance of a process at about 150 K (2,10), which is the observed peak. This is interpreted

in terms of interactions between hydrogen and dislocations. The precipitation peak observed in vanadium at frequencies in the kHz "range, being due to the presence of hydrides, w i l l be labelled as the "hydride peak" (liP). The need of a different classification derives from the fact that the definition of "precipitation peak" has been used up to now to denote any abrupt rise associated with precipitation. The HP curves although risinR on cooling, at different temperatures accordin~ to their H concentrations,

are coincident

below 120 IK, where a very small amount of hydrogen is available in g-phase (<50 at ppm as evaluated from the extrapolation of the solvus of Fig. 6). This behaviour suggests that the dissipation mechanism giving rise to the HP requires the presence of both the ~ and ~ phases. It is not well understood at present why at high

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frequency the HP is not observed in Nb and Ta and, conversely9 why precipitation does not produce in V the PPP observed in Nb and Ta. The decrease of the height of the hydride peak in V with increase of frequency (Fig.3) recalls the phenomenology of Nb, Ta, and V at low frequency (~1 Hz) found by Yoshinari and Koiwa (697). However, a quantitative comparison between the two sets of experiments is not possible at present due to the lack of data. The present results have also been used to determine the coexistence line between the a and fl phase (Fig.6), in the very low H concentration region where no data were available. In conclusion, the process due to the relaxation of the O(N)-H cluster in V presents features very similar to those of the O(N)-H peak in Nb and Ta. On the contrary, the peak associated with precipitation in V is of different nature from that in Nb and Ta. Mechanical deformation in V introduces an additional internal friction maximum, but the difference between the nature of the "post precipitation peak" and the H-cold work peak needs still to be clarified.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

References G.Cannelli and F.M.Mazzolai, Nuovo Cimento 64B, 171 (1969). G.Cannelli and F.M.Mazzolai, 3.Phys. Chem. Solids 3__1,1913 (1970). O.Buck, D.O.Thompson and C.A. Weft, J.Phys. Chem. Sol. 3--2, 2331 (1971). H.Y. Chang and C.A. Weft, Acta Metalh21__z 1233 (1973). G.Cannelli and R.Cantelli, Apph Phys. 3, 325 (1974). O.Yoshinari and M.Koiwa, Acta Metalh 3--0, 1979 (1982). O.Yoshinari and M.Koiwa, Acta Metalh 3-0, 1991 (1982). G.Cannelli, R.Cantelli and F.Cordero, J.de Physique 44_. 2 C9 403 (1983). O.Cannelli and R.Cantelli, Sol. Stat. Comm. 43, 567 (1982). H.Mizubayashi, S.Okuda and M. Daikubara, Scripta Met. 1-3, 1131 (1979).

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