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Hydrogen-induced segregation in PdPt alloys
Journal of
A~O~' AND COMPOUNDS ELSEVIER
Journal of Alloys and Compounds 231 (1995) 10-14
Hydrogen-induced segregation in Pd-Pt alloys H . N o h a, T e d B. F l a n a g a n a, Y. S a k a m o t o b "Chemistry Department, University of Vermont, Burlington, VT 05405, USA bDepartment of Materials Science and Engineering, Nagasaki University, Nagasaki 852, Japan
Abstract
In this research an initially homogeneous, f.c.c. Pd-Pt alloy, Pdo.gPt0.1, is shown to segregate partially into Pd-rich and Pd-poor regions as a result of being subjected to a hydrogen pressure of 5.5 MPa at 448 K leaving the metastable segregated alloy. It can be returned to its initial homogeneous state by annealing at T/> 673 K. The presence of segregation resulting from the hydrogen treatment is demonstrated from the changes in the dilute phase hydrogen solubility and from Auger depth-profile analysis. Keywords: Hydrogen-induced segregation; Recovery of segregated alloy; P d - P t - H system
1. Introduction P d - R h is known to have a miscibility gap but, in order for its homogeneous alloys to separate according to the phase diagram, prolonged annealing at elevated temperatures is required. For example, Raub et al. [1] found that a Pdo.49Rh0.51 alloy had to be annealed for 200 days at 873 K in order to detect two sets of f.c.c, X-ray reflections; however, two sets of reflections were not found in a Pdo.74Rho.26 alloy after annealing for 1 yea at 873 K. By contrast, after heating for only 4 h in 5.5 MPa of H2(g ) a Pd0.sRh0.2 alloy partially segregated [2]. Segregation was detected in P d - R h alloys from the marked changes in "diagnostic" hydrogen isotherms (323 K) measured before and after the H H T (hydrogen heat treatment). Electron microprobe analysis also revealed that segregation occurred; it was on a fine scale since in the X-ray diffraction patterns two sets of lattice parameters were not found [2]. The segregation in this alloy system undoubtedly corresponds to phase separation because the P d - R h system is known to have a miscibility gap and the composition of the Pd-rich phase which forms is consistent with the binary phase diagram [1]. ]it seems that these intriguing results concerning hydrogen-induced lattice migration (HILM) should be pursued because the phenomenon appears to be of fundamental interest and to have possible technological ramifications, 0925-8388/95/$09.50 © 199:5 Elsevier Science S.A. All rights reserved SSDI 0925-8388(95)01830-1
On the basis of the difference in the melting points of Pd and Pt, Raub [3] suggested some time ago that the P d - P t alloy system may have a miscibility gap and from the difference in melting points he estimated the critical point to be 1043 K. The very hypothetical phase diagram, which is based solely on his estimated critical temperature, is shown in Refs. [3,4]. Antonov et al. [5] have reported that homogeneous, solid solution P d - P t alloys with Xpt/> 0.15 separate into Pdand Pt-rich regions at 623 K under ultra-high H 2 pressures, i.e. 2 GPa and 6.5 GPa. When these ultrahigh pressures were applied to the initially homogeneous alloy, the alloy dissolved a large amount of hydrogen and then separated into Pd- and Pt-rich phases while evolving some hydrogen. The resultant two phases were detected from two sets of f.c.c, lattice parameters. Because of these extreme conditions, i.e. ultra-high hydrogen pressures, high hydrostatic pressures and very high hydrogen contents, these results may reflect a more complex phenomenon than "simple" HILM. This paper is specifically concerned with the occurfence of HILM leading to some metal atom segregation at moderate temperatures in f.c.c. Pd-Pt alloys which are initially homogeneous, solid solutions. The solution of hydrogen in these alloys has been studied earlier and they have been found to behave as "contacted" alloys even though the lattice expands slightly on substitutional alloying [6-8]. The "expanded/con-
11
H. Noh et al. / Journal o f Alloys and Compounds 231 (1995) 10-14
tracted" designation refers to whether the Pd lattice expands or contracts on alloying; this has been employed to divide Pd-rich alloys into two classes of hydrogen solubility behavior, e.g. [9]. The experimental determination of whether segregation has occurred will be made mainly on the basis of "diagnostic" hydrogen isotherms measured at moderate temperatures before and after the H H T . This solubility technique detects sensitively whether any lattice changes have occurred in these Pd-rich alloys. It cannot, however, characterize the nature of these changes except by inference. There are, however, few possibilities for lattice changes in alloys especially if the changes do not occur under comparable conditions of H H T in pure Pd. Although the p h e n o m e n o n is not fully understood, it is, nonetheless, of considerable interest that H I L M occurs at moderate temperatures, Since the earlier research on hydrogen-induced metal migration ( H I L M ) on the P d - R h alloys [2] was carried out, Fukai and O k u m a [10] have carried out similar experiments on the Pd0sRh0.2 alloy but under ultra-high pressures. They find that phase separation takes place within 102S at 5 G P a of H 2 (873K). They also found that very large vacancy concentrations can be produced in Pd and Ni when these pure metals are exposed to ultra-high hydrogen pressures at temperatures greater than or equal to 1073 K. This provides a clue to the mechanism for H I L M because large vacancy concentrations lead to enhanced metal atom diffus ion.
2. Experimental details P d - P t alloys were prepared by arc-melting the pure components and then annealing the buttons at 1173 K for 3 days. The buttons were then rolled into thin foils and re-annealed for stress relief. Hydrogen isotherms were measured in a Sieverts' type apparatus which was capable of measurements up to about 14 MPa. Several diaphragm gages were available so that the pressure measurements could be made in various ranges, e.g. 0 to 1.4 MPa and 0 to 14 MPa. T E M was carried out on samples after they had undergone H H T , i.e. immediately following the H H T the samples were evacuated above the critical temperature corresponding to the (dilute + hydride) two-phase region. The alloys for T E M were jet electropolished in a solution of perchloric and acetic acids (1:4 by volume). Electron diffraction and electron microscopy were carried out with an Hitachi H-800 electron microscope. Depth-profiling Auger analysis ( J E O L - A M P 10S) was carried out on the foils before and after the H H T . The surface layers were removed by ion etching at
2 nm increments and analysis was done after each incremental removal up to a depth of 200 nm.
3. Results and Discussion It had first to be established that there was no significant dependence of the dilute phase solubilities, plateau pressures and hydrogen capacities of the Pd09Pt0.1 a l l o y - H system on the details of the alloy pretreatment excluding, of course, any changes introduced intentionally by subsequent H H T or cold-work. The solubilities were found to be the same whether the initial alloy had been annealed and then slowly cooled or else quenched from an elevated temperature. It should be noted that the hydrogen solubilities of the P d - R h alloys are affected by the details of the annealing-quenching procedures [11]. It also had to be established that there were no effects on pure Pd under similar conditions of H H T . Fig. 1 shows dilute phase isotherms for annealed Pd at 323 K from this research and from that of Wicke and Nernst [12]; the agreement is seen to be good. Also shown are data for Pd after the introduction of hydrogen at 5.6 MPa above T c (613 K) and then cooling to 448 K where it was subjected to H H T for 3 days. It was then briefly heated to 613 K (5 min), evacuated for 3 min and then quickly cooled to room temperature. This procedure was employed to avoid the introduction of dislocations caused by passing through the miscibility gap. The sample treated in this manner showed no effects of this H H T (Fig. 1) whereas a similar H H T leads to large changes in the Pd0qPt01 alloy as shown below. Fig. 2 shows dilute phase "diagnostic" hydrogen solubilities for the Pd09Pt0 1 alloy (273 K). Solubility data are seen to be identical for the rapidly quenched form, and for the alloy after heating at 448 K for 6 days in vacuo. The hydrogen solubility for the same
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~ , 0.030 t , 0.040 H/ Pd Fig. 1. Dilute phase solubility data for palladium-hydrogen at 323 K: V, annealed sample [12]; ©, annealed sample, these data; A, Pd subjected to HHT at 5.5 MPa (448 K) for 3 days. 0.010
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H. Noh et al. / Journal of Alloys and Compounds 231 (1995) 10-14
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H//N1 Fig. 2. "Diagnostic" isotherms at 273 K for Pd0.gPt0A after HHT: O, annealed alloy; A, 448 K, 6 days in vacuo; V, cold-worked alloy ([(d o - d ) / d o ] = 95%); [3, alloy subjected to HHT, 448 K, 6 days,
12.3MPa (r=0.3). alloy is seen to be greatly affected by H H T at the same temperature, 448 K, and for the same length of time, 6 days, in the presence of 12.3 MPa of H v The enhanced dilute phase solubility arising from H H T can be explained if the alloy has segregated into Pd- and Pt-rich regions because, the solubility will be greater if a Pd-rich phase results from the segregation. The enhanced solubility is greater than can be accounted for by hydrogen segregation to dislocations. The solubility in a heavily cold-worked form of the alloy where the deformation is ((d - do)/do) x 100 = 95% is also shown in Fig. 2 where it can be seen that the solubility in the cold-worked form is significantly greater than in the annealed form but, even at low hydrogen contents, it is smaller than in the alloy which had been subjected to H H T . In a separate experiment the homogeneous Pd09Pt01 alloy was subjected to H H T at 448 K and 12',~3MPa for 3 min. This had no effect on the subsequent isotherms either in the dilute or in the two-phase region, which indicates that the introduction of hydrogen and the brief period of H H T does not either introduce dislocations or cause segregation. The effect of the length of time of the H H T (448 K, 2 . 8 M P a ( r = 0 . 3 ) ) on the subsequent dilute phase solubilities for the Pd0.9Pt0.1 alloy at 273 K are shown in Fig. 3. The HILM, which gives rise to the lattice changes, is a relatiw~.ly slow process under these conditions of temperature and pressure and the effect of H H T is s e e n t o depend on time of exposure. Fig. 4 shows "diagnostic" dilute phase isotherms at
272 K for the effects of H H T at 448 K from 2 days to 4 days at different hydrogen pressures; only small differe n c e s in the effects of H H T are found between 2 and 4 days (Fig. 3) a n d t h e r e f o r e t h e periods o f t i m e f o r t h e data will be regarded as essentially constant. The results (Fig. 4) will therefore be considered to be entirely due to the changes in hydrogen pressure. For H H T at 1.9 MPa (r = 0.2), there was found to be no effect on the dilute phase solubility; also shown is the expected absence of an effect of heating in vacuo. A small effect of H H T is seen at the same temperature at 2.8 MPa (r = 0.3). A t 5.3 MPa, where r ~ 0.3, there is a significant effect. The effect of H H T can be seen to increase at essentially the same hydrogen content when PH2 increases to 8.0 MPa and then to 12.3 MPa (Fig. 4). Thus it seems that PI%, or, perhaps, the
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H/M Fig. 4. "Diagnostic" isotherms at 273 K for Pd0.gPto.1 which has been subjected to HHT at various pH 2 at 448 K for periods of times from 2 to 4 days: O, annealed alloy: A, in vacuo, 3 days; B, 1.9 MPa, 4 days; O, 2.8 MPa, 2 days; A, 5.3 MPa, 2 days; D, 8.0 MPa, 2 days; v, 12.3 MPa, 2 days.
13
H. Noh et al. / Journal o f Alloys and Compounds 231 (1995) 10-14
l o + I R T l n pile, hydrogen chemical potential p~ = ~/.gH2 is the key variable for H I L M in this system. Fig. 5 shows the "diagnostic" dilute phase solubilities (273 K) for the Pd0qPto. 1 alloy in its annealed condition and after H H T (448 K, 5 days, 12.3 MPa) compared with the annealed, i.e. homogeneous disordered, Pdo.93Pt0.o.o7 and Pdo.q5Pt0.05 alloys. After H H T the hydrogen solubility of the Xpt = 0.10 alloy lies between that for the Xo.05 and Xo.07 alloys; the composition of the Pd-rich phase therefore reflects a considerable amount of segregation. Fig. 6 shows the "diagnostic" dilute phase isotherms (273K) for the Pdo9Ptol alloy after it has been segregated as a result of H H T and then progressively annealed in vacuo at increasing temperatures. The two limiting solubilities shown in Fig. 6 are those for the alloy subjected to H H T at 448 K and 12.3 MPa for 5 days and for the fully annealed alloy. Annealing for periods of 2 days at temperatures up to 573 K had no effect on the diagnostic solubilities; further annealing for 19 h at 623 K and 1 day at 673 K also had no effect on the solubility. Annealing at 723 K for 1 day had a marked effect (Fig. 6) and after annealing for 1 day at 773 K, the alloy returned to the solubility behavior of the well annealed alloy. Annealing of cold-worked Pd and its alloys has been followed similarly [13]. A Pdo.95Ago.05 alloy which had been cold-worked recov-
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Fig. 6. "Diagnostic" isotherms (273 K) for the Pd0qPt0A alloy after it has been subjected to HHT for 5 days, 12.3MPa (r = 0.4) and then annealed in vacuo at progressively higher temperatures for 1 to 2 day periods. The solid line refers to the annealed alloy and the lower dashed line to the alloy subjected to HHT. The following refer to the annealing temperatures and times: ©, 473 K, 2 days; A, 523 K, w days; K], 573 K, 2 days; o, 623 K, 1 day; &, 673 K, 1 day; II,
723K, 1 day; ~, 773K, 1 day. ers almost 50% after annealing at 573 K for 12h. Cold-worked Pd also recovers appreciably after a l day anneal at 573K [14]. In view of the higher temperatures needed for the recovery of the alloy subjected to H H T (Fig. 6) it seems likely that its recovery involves mainly the removal of compositional variations rather than dislocation rearrangement annihilation. Significant effects of H H T were found for Pdo.875Pt0.t25 alloy at moderate temperatures and 12.3MPa. Only small effects from H H T were observed, however, for the 15, 20, 25 and 30at.% Pt alloys. These alloys were subjected to H H T at both moderate, 423 to 523 K, and high temperatures, 700 to 773 K, and no noticeable effects were observed. For example, the Pd0.a5Pto.15 alloy was subjected to H H T at 448 K at 7.0MPa (at r = 0.1) for 5 days; no effect was observed in a subsequent "diagnostic" isotherm (273 K). A 7 day H H T at 423 K at 7.0 MPa (r = 0.2) also did not lead to any significant changes, however, evidence for H I L M was found after H H T at 673 K for 3 days at 11.6 MPa; the subsequent "diagnostic" isotherm exhibited lower equilibrium hydrogen pressures in the low and mid-range of r values than for the initial, homogeneous alloy but there was no significant change in the hydrogen capacity under these c o n d i t i o n s o f HHT, i.e. 423 K, 7.0 MPa, r = 0.2 f o r t h e Pd0.85Pto.15alloy. When Xet is increased the solubility o f hydrogen decreases, and therefore only a small
14
H. Noh et al. / Journal of Alloys and Compounds 231 (1995) 10-14
amount of hydrogen dissolves in, e.g., the Pd0.sPt0.2 alloy at 448 K at pressures up to 12.3 MPa.
4. Transmission electron microscopy (TEM) and
Auger results TEM studies were carried out on a Pd0.gPt0.1 alloy which had been subjected to HHT (12.3MPa H 2, 448 K). The electron diffraction patterns show only the fundamental diffraction spots from the disordered f.c.c, lattice after subjecting the alloys to HHT under the above conditions for 3 min, 30 min and 5 days. The bright field images show dislocation densities which increase with the time,; of their exposures to H 2. Since these alloys were in an initially well annealed condition, the dislocation arrays observed by TEM must have arisen from the segregation because they were not subject to hydride formation during the HHT treatment. If the dislocations were caused by the concentration buildup of hydrogen in the outer layers of the sample before penetration very far into the bulk (448 K), the dislocation densities would be expected to be independent of the time of H 2 exposure. This was not the case. The extent of the segregation and the dislocation densities both increase with the time of HHT and it seems clear that they are not independent. The presence of an extensive dislocation array suggests that the segregation into different compositional "phases" is mainly incoherent which is surprising in view of the similar lattice parameters of f.c.c. Pt and Pd and the fine scale of the segregation. Depth-profiling Auger analysis was employed to confirm that segregation results from HHT. A Pd0.9Pt0. ~ alloy was subjected to HHT for 5 days at 448 K and r = 0.4. After this HHT, the sample was evacuated and then examined by Auger spectroscopy; small enrichments anti depletions of Pd and Pt appeared at somewhat irregular intervals, e.g. 3, 5 nm, into the bulk of the alloy. For example, the Pt compositional variations ranged from about 8 to 12 at.%. Before HHT, the composition profiles were found to be very uniform, closely equal to the stated
10 at.% composition. Although the observed composition variations are sraall, They are clearly present after, but not before, the HHT. The fact that the variations are not larger may be attributed partly to the lack of resolution of this analytical technique. 5. Conclusions Metal atom segregation in initially homogeneous f.c.c. Pd-Pt alloys has been shown to result from HHT
at relatively low temperatures compared with those where metal atom diffusion normally occurs; this may be useful for materials applications. This segregation may correspond to phase separation according to a miscibility gap in the binary phase diagram but there has been no experimental evidence for such a gap even though it has been predicted [3,4]. The miscibility gap, if it exists, appears to have a lower critical temperature than that of the P d - R h system [1] because the phase segregation can be annealed out at relatively low temperatures, i.e. at temperatures comparable with those at which HILM occurs for the P d - R h system where the miscibility gap has a critical temperature of approximately 1150 K [1,5]. However, segregation may be a result of the ternary equilibrium and the accompanying ( P d + Pt + H) phase diagram. These possibilities will be discussed elsewhere where more experimental results will be given.
Acknowledgements The authors (TBF and HN) wish to acknowledge financial support from the NSF. D.S. Hazel is thanked for carrying out the Auger-depth profiling analysis.
References [1] E. Raub, H. Beeskow and D. Menzel, Z. Metallkd., 50 (1959) 428. [2] H. Noh, T. Flanagan and M. Ransick, Scr. Metall. Mater., 26 (1992) 353. [3] E. Raub, J. Less-Common Met., 1 (1959) 3.
[4] S. Bharadwaj, A. Kerkar, S. Tripathi and S. Dharwdkar, J. Less-Common Met., 169 (1991) 167, [5] v. Antonov, T. Antonova, I. Belash, E. Ponyatovskii and V. Rashupkin, Phys. Status Solidi A, 78 (1983) 137. [6] A. Maeland and T. Flanagan, J. Phys. Chem., 68 (1965) 1419. [7] J.C. Barton,J.A.S. Green and F.A. Lewis,Trans. Faraday Soc., 62 (1966) 960. [8] B. Baranowski,F. Lewis, S. Majchrzak and R. Wi~niewski,J. Chem. Soc. Faraday Trans. I, 68 (1972) 824.
[9] Y. Sakamoto, U. Miyagawa, E. Hamamoto, F. Chen, T. Flanagan and R.-A. McNicholl,Ber. Bunsenges. Phys. Chem., 94 (1990) 1457.
[10] Y. Fukai and N. Okuma, Jpn. J. Appl. Phys., 32 (1993) L1256. [11] H. Noh, W. Luo and T. Flanagan, J. Alloys Comp., 196 (1993)
7. [12] E. Wicke and G. Nernst, Ber. Bunsenges. Phys. Chem., 68 (1964) 224. [13] S. Kishimoto, N. Yoshida, T. Yao, T. Itani and T.B. Flanagan, Ber. Bunsenges. Phys. Chem., 96 (1992) 1477. [14] J.F. Lynch, PhD Thesis, University of Vermont, 1975. [15] J. Shield and R. Williams, Scr. Metall., 21 (1987) 1475.
A~O~' AND COMPOUNDS ELSEVIER
Journal of Alloys and Compounds 231 (1995) 10-14
Hydrogen-induced segregation in Pd-Pt alloys H . N o h a, T e d B. F l a n a g a n a, Y. S a k a m o t o b "Chemistry Department, University of Vermont, Burlington, VT 05405, USA bDepartment of Materials Science and Engineering, Nagasaki University, Nagasaki 852, Japan
Abstract
In this research an initially homogeneous, f.c.c. Pd-Pt alloy, Pdo.gPt0.1, is shown to segregate partially into Pd-rich and Pd-poor regions as a result of being subjected to a hydrogen pressure of 5.5 MPa at 448 K leaving the metastable segregated alloy. It can be returned to its initial homogeneous state by annealing at T/> 673 K. The presence of segregation resulting from the hydrogen treatment is demonstrated from the changes in the dilute phase hydrogen solubility and from Auger depth-profile analysis. Keywords: Hydrogen-induced segregation; Recovery of segregated alloy; P d - P t - H system
1. Introduction P d - R h is known to have a miscibility gap but, in order for its homogeneous alloys to separate according to the phase diagram, prolonged annealing at elevated temperatures is required. For example, Raub et al. [1] found that a Pdo.49Rh0.51 alloy had to be annealed for 200 days at 873 K in order to detect two sets of f.c.c, X-ray reflections; however, two sets of reflections were not found in a Pdo.74Rho.26 alloy after annealing for 1 yea at 873 K. By contrast, after heating for only 4 h in 5.5 MPa of H2(g ) a Pd0.sRh0.2 alloy partially segregated [2]. Segregation was detected in P d - R h alloys from the marked changes in "diagnostic" hydrogen isotherms (323 K) measured before and after the H H T (hydrogen heat treatment). Electron microprobe analysis also revealed that segregation occurred; it was on a fine scale since in the X-ray diffraction patterns two sets of lattice parameters were not found [2]. The segregation in this alloy system undoubtedly corresponds to phase separation because the P d - R h system is known to have a miscibility gap and the composition of the Pd-rich phase which forms is consistent with the binary phase diagram [1]. ]it seems that these intriguing results concerning hydrogen-induced lattice migration (HILM) should be pursued because the phenomenon appears to be of fundamental interest and to have possible technological ramifications, 0925-8388/95/$09.50 © 199:5 Elsevier Science S.A. All rights reserved SSDI 0925-8388(95)01830-1
On the basis of the difference in the melting points of Pd and Pt, Raub [3] suggested some time ago that the P d - P t alloy system may have a miscibility gap and from the difference in melting points he estimated the critical point to be 1043 K. The very hypothetical phase diagram, which is based solely on his estimated critical temperature, is shown in Refs. [3,4]. Antonov et al. [5] have reported that homogeneous, solid solution P d - P t alloys with Xpt/> 0.15 separate into Pdand Pt-rich regions at 623 K under ultra-high H 2 pressures, i.e. 2 GPa and 6.5 GPa. When these ultrahigh pressures were applied to the initially homogeneous alloy, the alloy dissolved a large amount of hydrogen and then separated into Pd- and Pt-rich phases while evolving some hydrogen. The resultant two phases were detected from two sets of f.c.c, lattice parameters. Because of these extreme conditions, i.e. ultra-high hydrogen pressures, high hydrostatic pressures and very high hydrogen contents, these results may reflect a more complex phenomenon than "simple" HILM. This paper is specifically concerned with the occurfence of HILM leading to some metal atom segregation at moderate temperatures in f.c.c. Pd-Pt alloys which are initially homogeneous, solid solutions. The solution of hydrogen in these alloys has been studied earlier and they have been found to behave as "contacted" alloys even though the lattice expands slightly on substitutional alloying [6-8]. The "expanded/con-
11
H. Noh et al. / Journal o f Alloys and Compounds 231 (1995) 10-14
tracted" designation refers to whether the Pd lattice expands or contracts on alloying; this has been employed to divide Pd-rich alloys into two classes of hydrogen solubility behavior, e.g. [9]. The experimental determination of whether segregation has occurred will be made mainly on the basis of "diagnostic" hydrogen isotherms measured at moderate temperatures before and after the H H T . This solubility technique detects sensitively whether any lattice changes have occurred in these Pd-rich alloys. It cannot, however, characterize the nature of these changes except by inference. There are, however, few possibilities for lattice changes in alloys especially if the changes do not occur under comparable conditions of H H T in pure Pd. Although the p h e n o m e n o n is not fully understood, it is, nonetheless, of considerable interest that H I L M occurs at moderate temperatures, Since the earlier research on hydrogen-induced metal migration ( H I L M ) on the P d - R h alloys [2] was carried out, Fukai and O k u m a [10] have carried out similar experiments on the Pd0sRh0.2 alloy but under ultra-high pressures. They find that phase separation takes place within 102S at 5 G P a of H 2 (873K). They also found that very large vacancy concentrations can be produced in Pd and Ni when these pure metals are exposed to ultra-high hydrogen pressures at temperatures greater than or equal to 1073 K. This provides a clue to the mechanism for H I L M because large vacancy concentrations lead to enhanced metal atom diffus ion.
2. Experimental details P d - P t alloys were prepared by arc-melting the pure components and then annealing the buttons at 1173 K for 3 days. The buttons were then rolled into thin foils and re-annealed for stress relief. Hydrogen isotherms were measured in a Sieverts' type apparatus which was capable of measurements up to about 14 MPa. Several diaphragm gages were available so that the pressure measurements could be made in various ranges, e.g. 0 to 1.4 MPa and 0 to 14 MPa. T E M was carried out on samples after they had undergone H H T , i.e. immediately following the H H T the samples were evacuated above the critical temperature corresponding to the (dilute + hydride) two-phase region. The alloys for T E M were jet electropolished in a solution of perchloric and acetic acids (1:4 by volume). Electron diffraction and electron microscopy were carried out with an Hitachi H-800 electron microscope. Depth-profiling Auger analysis ( J E O L - A M P 10S) was carried out on the foils before and after the H H T . The surface layers were removed by ion etching at
2 nm increments and analysis was done after each incremental removal up to a depth of 200 nm.
3. Results and Discussion It had first to be established that there was no significant dependence of the dilute phase solubilities, plateau pressures and hydrogen capacities of the Pd09Pt0.1 a l l o y - H system on the details of the alloy pretreatment excluding, of course, any changes introduced intentionally by subsequent H H T or cold-work. The solubilities were found to be the same whether the initial alloy had been annealed and then slowly cooled or else quenched from an elevated temperature. It should be noted that the hydrogen solubilities of the P d - R h alloys are affected by the details of the annealing-quenching procedures [11]. It also had to be established that there were no effects on pure Pd under similar conditions of H H T . Fig. 1 shows dilute phase isotherms for annealed Pd at 323 K from this research and from that of Wicke and Nernst [12]; the agreement is seen to be good. Also shown are data for Pd after the introduction of hydrogen at 5.6 MPa above T c (613 K) and then cooling to 448 K where it was subjected to H H T for 3 days. It was then briefly heated to 613 K (5 min), evacuated for 3 min and then quickly cooled to room temperature. This procedure was employed to avoid the introduction of dislocations caused by passing through the miscibility gap. The sample treated in this manner showed no effects of this H H T (Fig. 1) whereas a similar H H T leads to large changes in the Pd0qPt01 alloy as shown below. Fig. 2 shows dilute phase "diagnostic" hydrogen solubilities for the Pd09Pt0 1 alloy (273 K). Solubility data are seen to be identical for the rapidly quenched form, and for the alloy after heating at 448 K for 6 days in vacuo. The hydrogen solubility for the same
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~ , 0.030 t , 0.040 H/ Pd Fig. 1. Dilute phase solubility data for palladium-hydrogen at 323 K: V, annealed sample [12]; ©, annealed sample, these data; A, Pd subjected to HHT at 5.5 MPa (448 K) for 3 days. 0.010
,
0.020
H. Noh et al. / Journal of Alloys and Compounds 231 (1995) 10-14
12
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O. 8
H//N1 Fig. 2. "Diagnostic" isotherms at 273 K for Pd0.gPt0A after HHT: O, annealed alloy; A, 448 K, 6 days in vacuo; V, cold-worked alloy ([(d o - d ) / d o ] = 95%); [3, alloy subjected to HHT, 448 K, 6 days,
12.3MPa (r=0.3). alloy is seen to be greatly affected by H H T at the same temperature, 448 K, and for the same length of time, 6 days, in the presence of 12.3 MPa of H v The enhanced dilute phase solubility arising from H H T can be explained if the alloy has segregated into Pd- and Pt-rich regions because, the solubility will be greater if a Pd-rich phase results from the segregation. The enhanced solubility is greater than can be accounted for by hydrogen segregation to dislocations. The solubility in a heavily cold-worked form of the alloy where the deformation is ((d - do)/do) x 100 = 95% is also shown in Fig. 2 where it can be seen that the solubility in the cold-worked form is significantly greater than in the annealed form but, even at low hydrogen contents, it is smaller than in the alloy which had been subjected to H H T . In a separate experiment the homogeneous Pd09Pt01 alloy was subjected to H H T at 448 K and 12',~3MPa for 3 min. This had no effect on the subsequent isotherms either in the dilute or in the two-phase region, which indicates that the introduction of hydrogen and the brief period of H H T does not either introduce dislocations or cause segregation. The effect of the length of time of the H H T (448 K, 2 . 8 M P a ( r = 0 . 3 ) ) on the subsequent dilute phase solubilities for the Pd0.9Pt0.1 alloy at 273 K are shown in Fig. 3. The HILM, which gives rise to the lattice changes, is a relatiw~.ly slow process under these conditions of temperature and pressure and the effect of H H T is s e e n t o depend on time of exposure. Fig. 4 shows "diagnostic" dilute phase isotherms at
272 K for the effects of H H T at 448 K from 2 days to 4 days at different hydrogen pressures; only small differe n c e s in the effects of H H T are found between 2 and 4 days (Fig. 3) a n d t h e r e f o r e t h e periods o f t i m e f o r t h e data will be regarded as essentially constant. The results (Fig. 4) will therefore be considered to be entirely due to the changes in hydrogen pressure. For H H T at 1.9 MPa (r = 0.2), there was found to be no effect on the dilute phase solubility; also shown is the expected absence of an effect of heating in vacuo. A small effect of H H T is seen at the same temperature at 2.8 MPa (r = 0.3). A t 5.3 MPa, where r ~ 0.3, there is a significant effect. The effect of H H T can be seen to increase at essentially the same hydrogen content when PH2 increases to 8.0 MPa and then to 12.3 MPa (Fig. 4). Thus it seems that PI%, or, perhaps, the
15c o
~-c~ aoc ~£ 5o
o
, 0
l 0.02
,
I 0.04
,
i 0.06
,
t 0.08
H/M Fig. 4. "Diagnostic" isotherms at 273 K for Pd0.gPto.1 which has been subjected to HHT at various pH 2 at 448 K for periods of times from 2 to 4 days: O, annealed alloy: A, in vacuo, 3 days; B, 1.9 MPa, 4 days; O, 2.8 MPa, 2 days; A, 5.3 MPa, 2 days; D, 8.0 MPa, 2 days; v, 12.3 MPa, 2 days.
13
H. Noh et al. / Journal o f Alloys and Compounds 231 (1995) 10-14
l o + I R T l n pile, hydrogen chemical potential p~ = ~/.gH2 is the key variable for H I L M in this system. Fig. 5 shows the "diagnostic" dilute phase solubilities (273 K) for the Pd0qPto. 1 alloy in its annealed condition and after H H T (448 K, 5 days, 12.3 MPa) compared with the annealed, i.e. homogeneous disordered, Pdo.93Pt0.o.o7 and Pdo.q5Pt0.05 alloys. After H H T the hydrogen solubility of the Xpt = 0.10 alloy lies between that for the Xo.05 and Xo.07 alloys; the composition of the Pd-rich phase therefore reflects a considerable amount of segregation. Fig. 6 shows the "diagnostic" dilute phase isotherms (273K) for the Pdo9Ptol alloy after it has been segregated as a result of H H T and then progressively annealed in vacuo at increasing temperatures. The two limiting solubilities shown in Fig. 6 are those for the alloy subjected to H H T at 448 K and 12.3 MPa for 5 days and for the fully annealed alloy. Annealing for periods of 2 days at temperatures up to 573 K had no effect on the diagnostic solubilities; further annealing for 19 h at 623 K and 1 day at 673 K also had no effect on the solubility. Annealing at 723 K for 1 day had a marked effect (Fig. 6) and after annealing for 1 day at 773 K, the alloy returned to the solubility behavior of the well annealed alloy. Annealing of cold-worked Pd and its alloys has been followed similarly [13]. A Pdo.95Ago.05 alloy which had been cold-worked recov-
25o
2oo
_7~ 15c _~ ~" ~ loz
5o
o o
i oo2
I os~4
i oo6
t 0.08
H/M Fig. 5. "Diagnostic" isotherms (273 K) for the PdogPtoA alloy before and after it has been subjected to HHT for 5 days, r = 0.4 (12.3 MPa) and isotherms for annealed (homogeneous) Pd0.95Ptoo5 and Pd093Ptoo7 alloys: ©, annealed (homogeneous) Pd0.gPt0A alloy; [2], PdogPto. ~ alloy subjected to HHT at 448 K, 12.3MPA for 5 days; A, annealed Pdo925Pto075 alloy; V, annealed Pdo.95Pto.05 alloy,
aoc
l~o -~ ~ -qa.~
o o . .......... . ........... /,////(////// .~A/,/. / ,~. , ..m~.m~. ......................... ...... ///~f,,/~'//,, ~
loo
5o /.,6"//,, //,:' , o
. . oo2
. . . ao4 o.~ H/ M
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Fig. 6. "Diagnostic" isotherms (273 K) for the Pd0qPt0A alloy after it has been subjected to HHT for 5 days, 12.3MPa (r = 0.4) and then annealed in vacuo at progressively higher temperatures for 1 to 2 day periods. The solid line refers to the annealed alloy and the lower dashed line to the alloy subjected to HHT. The following refer to the annealing temperatures and times: ©, 473 K, 2 days; A, 523 K, w days; K], 573 K, 2 days; o, 623 K, 1 day; &, 673 K, 1 day; II,
723K, 1 day; ~, 773K, 1 day. ers almost 50% after annealing at 573 K for 12h. Cold-worked Pd also recovers appreciably after a l day anneal at 573K [14]. In view of the higher temperatures needed for the recovery of the alloy subjected to H H T (Fig. 6) it seems likely that its recovery involves mainly the removal of compositional variations rather than dislocation rearrangement annihilation. Significant effects of H H T were found for Pdo.875Pt0.t25 alloy at moderate temperatures and 12.3MPa. Only small effects from H H T were observed, however, for the 15, 20, 25 and 30at.% Pt alloys. These alloys were subjected to H H T at both moderate, 423 to 523 K, and high temperatures, 700 to 773 K, and no noticeable effects were observed. For example, the Pd0.a5Pto.15 alloy was subjected to H H T at 448 K at 7.0MPa (at r = 0.1) for 5 days; no effect was observed in a subsequent "diagnostic" isotherm (273 K). A 7 day H H T at 423 K at 7.0 MPa (r = 0.2) also did not lead to any significant changes, however, evidence for H I L M was found after H H T at 673 K for 3 days at 11.6 MPa; the subsequent "diagnostic" isotherm exhibited lower equilibrium hydrogen pressures in the low and mid-range of r values than for the initial, homogeneous alloy but there was no significant change in the hydrogen capacity under these c o n d i t i o n s o f HHT, i.e. 423 K, 7.0 MPa, r = 0.2 f o r t h e Pd0.85Pto.15alloy. When Xet is increased the solubility o f hydrogen decreases, and therefore only a small
14
H. Noh et al. / Journal of Alloys and Compounds 231 (1995) 10-14
amount of hydrogen dissolves in, e.g., the Pd0.sPt0.2 alloy at 448 K at pressures up to 12.3 MPa.
4. Transmission electron microscopy (TEM) and
Auger results TEM studies were carried out on a Pd0.gPt0.1 alloy which had been subjected to HHT (12.3MPa H 2, 448 K). The electron diffraction patterns show only the fundamental diffraction spots from the disordered f.c.c, lattice after subjecting the alloys to HHT under the above conditions for 3 min, 30 min and 5 days. The bright field images show dislocation densities which increase with the time,; of their exposures to H 2. Since these alloys were in an initially well annealed condition, the dislocation arrays observed by TEM must have arisen from the segregation because they were not subject to hydride formation during the HHT treatment. If the dislocations were caused by the concentration buildup of hydrogen in the outer layers of the sample before penetration very far into the bulk (448 K), the dislocation densities would be expected to be independent of the time of H 2 exposure. This was not the case. The extent of the segregation and the dislocation densities both increase with the time of HHT and it seems clear that they are not independent. The presence of an extensive dislocation array suggests that the segregation into different compositional "phases" is mainly incoherent which is surprising in view of the similar lattice parameters of f.c.c. Pt and Pd and the fine scale of the segregation. Depth-profiling Auger analysis was employed to confirm that segregation results from HHT. A Pd0.9Pt0. ~ alloy was subjected to HHT for 5 days at 448 K and r = 0.4. After this HHT, the sample was evacuated and then examined by Auger spectroscopy; small enrichments anti depletions of Pd and Pt appeared at somewhat irregular intervals, e.g. 3, 5 nm, into the bulk of the alloy. For example, the Pt compositional variations ranged from about 8 to 12 at.%. Before HHT, the composition profiles were found to be very uniform, closely equal to the stated
10 at.% composition. Although the observed composition variations are sraall, They are clearly present after, but not before, the HHT. The fact that the variations are not larger may be attributed partly to the lack of resolution of this analytical technique. 5. Conclusions Metal atom segregation in initially homogeneous f.c.c. Pd-Pt alloys has been shown to result from HHT
at relatively low temperatures compared with those where metal atom diffusion normally occurs; this may be useful for materials applications. This segregation may correspond to phase separation according to a miscibility gap in the binary phase diagram but there has been no experimental evidence for such a gap even though it has been predicted [3,4]. The miscibility gap, if it exists, appears to have a lower critical temperature than that of the P d - R h system [1] because the phase segregation can be annealed out at relatively low temperatures, i.e. at temperatures comparable with those at which HILM occurs for the P d - R h system where the miscibility gap has a critical temperature of approximately 1150 K [1,5]. However, segregation may be a result of the ternary equilibrium and the accompanying ( P d + Pt + H) phase diagram. These possibilities will be discussed elsewhere where more experimental results will be given.
Acknowledgements The authors (TBF and HN) wish to acknowledge financial support from the NSF. D.S. Hazel is thanked for carrying out the Auger-depth profiling analysis.
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