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ZENER AND SNOEK-KOESTER EFFECTS IN THE Pd/Pt/H SYSTEM
F. Mazzolai, F. Lewis, P. Marzola
To cite this version:
F. Mazzolai, F. Lewis, P. Marzola. ZENER AND SNOEK-KOESTER EFFECTS IN THE Pd/Pt/H SYSTEM. Journal de Physique Colloques, 1987, 48 (C8), pp.C8-269-C2-274.
�10.1051/jphyscol:1987838�. �jpa-00227142�
ZENER AND SNOEK-KOESTER EFFECTS IN THE Pd/Pt/H SYSTEM
F.M. MAZZOLAI, F.A. LEWIS* and P. MARZOLA
Physics Department, U n i v e r s i t y o f P e r u g i a , I - 0 6 1 0 0 P e r u g i a , I t a l y
" ~ u e e n s U n i v e r s i t y o f B e l f a s t , G B - B e l f a s t BTG 5AG N o r t h e r n I r e l a n d , G r e a t - B r i t a i n
Abstract-
Results are reported of elastic energy dissipation associated w i t h hydrogen/dislocation and Zener relaxation processes arising from the introduction of hydrogen into a series of palladium/platinum alloys.Comparisons are made w i t h analogous effects observed i n the case of the pal ladium/si lver/hydrogen system.
1. introduction
A substantial literature concerning anelastic relaxation effects associated w i t h hydrogen in metals has by now been developed and compiled [I]. In the particular case of palladium the introduction of hydrogen has been found t o lead t o the appearance of elastic energy dissipation (internal friction) effects, which have been associated either w i t h stress-induced changes in the short-range order of hydrogen interstitials and vacant -octahedral interstitial sites i n the non stoichiometric J3 hydride phase (Zener effects),or w i t h hydrogen/dislocation interact ion processes (Snoek-Koester effects). In the case of hydrogen/dislocation interaction effects the dislocations have been introduced both by mechanical deformation and also as a consequence of the a
-
13 phase transitions [21.Seemingly quite analogous elastic energy dissipation peaks have also been observed i n the case of Pdt-x/Agx/Hn alloys w i t h ~50.23. At higher silver contents the Zener effect has been found t o split into t w o distinct peaks [31. Such peak separation i s believed t o reflect differences i n the short-range order of the binary metal Pd/Ag alloys, which differences induce wide modulations i n the local hydrogen concentration.
Both Pd/Pt/H and Pd/Ag/H are examples of hydrogen systems f o r which
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987838
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the most comprehensive and unambigous information i s so f a r avaiable, concerning hydrogen chemical potential or pressure composition temperature (p-c-T) relationships. Additionally they are also systems for which changes of lattice parameter and other physical properties as a function of the alloy composition and hydrogen contents have been extensively studied [5,61. It has thus appeared of interest t o further investigate the features of the Zener effect i n the Pd/Pt system. The results presented here are an extension of measurements earlier reported [41 t o alloys w i t h higher platinum contents.
2. Ex~erimental
As w i t h other compositions [41 the palladium/platinum alloys had been prepared by argon-arc melting. They had been machined into bars of rectangular cross section w i t h overall dimensions of 4 0 x 6 ~ 2 mm and then annealed i n vacuo f o r 2 h a t 700 'C. Hydrogen was introduced into and removed from specimens by electrolysis i n I M H2S04 After each alteration of the hydrogen content the specimens were kept a t room temperature for 24 h before each measurement run.
Resonant vibration frequencies , f, were i n the range 4.6
-
4.9 kHz.These were measured together w i t h the elastic energy dissipation
coefficient, Q-l, during cooling at a cooling rate of 1 K min-l over the temperature range 300-80 K. The hydrogen content of the specimens was determined by weight differences and i s expressed here as the atomic ratio n= H/(Pd+Pt). Composition changes over the course of the experimental runs were < 1%.
3. Results
Comparisons of the relationships between Q - I and temperature f o r six Pd I -x/Pt, compositions a t similar values of hydrogen contents are shown i n Fig.1. These comparisons indicate that f o r each of the alloy compositions there i s a dominant peak i n Q-l a t temperatures included between 120 and 130 K. Using a previously employed convention [21 this peak has been denoted as H(
1.
For the alloys of lower platinum content (x< 0.17) it may also be seen that a less precisely defined peak i s found over the approximate higher temperature range of 150-190 K. This peak has been denoted as
H(2).
Figure 1 shows that w i t h increasing the platinum content of the alloys this HQ) peak gradually decreases and becomes hardly evident for alloys w i t h contents of Pt
,
17%. At the highest hydrogen levels, the peak H(2) i s seen to disappear for a1 l the Pd/Pt alloys examined.Plots in Fig.2 illustrate the dependence on hydrogen content of the helght, 0-',, of the H( I) peak ( i n the upper parts of the diagrams and of the temperatures, TM, corresponding t o this peak maximum ( i n the lower
passes through a maximum f o r a hydrogen content n approximately equal to 0.5, while TM shows relatively small changes w i t h changing both the hydrogen and the platinum contents.
4.Discussion
The form of the relationships between 0 - I and temperature i n Fig.l for the alloys of lower platinum contents i s quite closely analogous t o the form of relationships f o r the Pd/H system i n which case the H(,) peak has been shown to be a Zener effect associated w i t h stress-induced changes i n the short-range order of hydrogen interstitials. For a random distribution of hydrogen atoms Q - ' ~ i s expected t o depend on n according LO the simple relationship Q - ' ~ 0~ n * ( ~ - n ) ~ , which has a maximum f o r n=0.5. Results i n Fig.2 are i n agreement w i t h this expectation, thus showing that these Pd/Pt/H alloys are substantially random.
Also i n the case of the Pd/H and Pd/Ag/H systems the H(?) peak has been associated w i t h hydrogen/dislocation interaction processes, where dislocations have been introduced i n the course of a-I3 phase hydride transitions taking place during successive hydrogen loadings. The absence of an H(2) peak i n the case of the alloys w i t h x > 0.17 seems good evidence that very few dislocations have been introduced i n these alloys either i n the course of the successive hydrogen loadings or in the course of the cooling and heating cycles of measurements, which had taken place between each additional loading without any intermediate higher temperature anneal.
The relatively smaller density of dislocations created by hydrogenation i n these higher platinum content alloys i s consistent w i t h the reduction of the critical temperature, Tc, w i t h increasing platinum content, as indicated by lattice parameters measurernets [71, which show that f o r x l 0.15, T, i s smaller than 293 K.
The sequence of changes of the 0-I against T relationships w i t h increasing Pt and H contents i n Fig.2 i s somewhat different from the sequence of changes w i t h increasing Ag and H contents i n the case of the Pd/Ag/H system.
In the case of the Pdl-,/AgX/Hn alloys a substantial diminution i n the height of the H(l) peak i s observed w i t h increasing Ag content at comparable values of n. However, for x>0.23 this diminution of the H(
peak i s accompanied f o r any value of n by the development of a peak i n the same temperature range as the H(2) peak i n Fig. 1.
In w e l l annealed specimens w i t h overall silver contents, x, higher than 0.23 such a peak has been found not t o be a Snoek-Koester peak but rather
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a further Zener process occurring within microdomains of low hydrogen concentration. Such microdomains are believed t o be regions of high local silver content, XL, (probably x ~ = 0.75) as discussed i n [31, where results other than internal friction have been examined, which suggest an incomplete randomness of Pd/Ag alloys.
The fact that a second higher temperature Zener effect i s not seen t o develop, as a consequence of hydrogen loading, i n any of the specimens examined here seems t o suggest that for x i 0.27, ideal solid solutions are formed by palladium and platinum. However, this i s not the only explanation that can be given of the absence of a second Zener effect, namely as it can be seen from diagrams i n the lower part of figure 2 the temperature T,., does not strongly change either w i t h changing x or n. Thus the eventual local inhomogeneities i n the composition of the binary metal alloy do not give rise t o separate internal friction peaks, differently from what happens in the Pd/Ag alloys, where TM i s found t o be strongly affected by changes i n x or n. As a matter of fact a separation into Pt-rich and Pt- poor phases of an i n i t i a l l y homogeneous alloy has been observed t o occur a t high hydrogen pressures (p = 2 GPa) and temperatures (T= 350
"C) f o r x? 0.15 by means of x-ray diffraction
techniques [81. Thus i n principle the existence of microdomains of Pt-rich and Pt-poor compositions could not be excluded f o r some of our alloys. However, also taking into account the results on Q - ' ~ vs n dependence, the assumption of randomness of such alloys seems t o be the more appropriate.
Acknowledgements
The provision of the Pd/Pt alloys by Johnson Matthey PLC and support under the terms of NATO Research Grant No 0349/87 are gratefully acknowledged.
References
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Fig.1 Q-I vs T r e l a t i o n s h i p f o r PdImx/PtX/Hn.
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Fig.2 Dependence o f Q M -I and TM of H peak on n.
(1)
