LJournal of Alloys and Compounds 348 (2003) 119–128www.elsevier.com/ locate/ jallcom

H ydrogen isotherms over a wide temperature range for Pd and for Pd/oxide composites formed by internal oxidation of Pd–Al(Y) alloys

a a a , b*D. Wang , H. Noh , Ted B. Flanagan , R. BalasubramaniamaChemistry Department, University of Vermont, Burlington, VT 05405,USA

bDepartment of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208 016,India

Received 18 April 2002; received in revised form 17 May 2002; accepted 17 May 2002

Abstract

Isotherms have been measured for internally oxidized Pd–M alloys where M5Al or Y. After internal oxidation the alloys becomecomposites of nanosized oxide precipitates within Pd matrices. Following the internal oxidation (1073 K) of the alloys H isotherms are2

closely identical to those of Pd–H, however, differences develop after hydriding/dehydriding (cycling) which increase with %Al (Y). H2

isotherms for the cycled forms of Pd and a Pd/alumina composite have been measured and compared at 323, 473, 513 and 553 K. At thelatter two temperatures the characteristic plateaux appear to disappear for the cycled composite although hysteresis is still presentindicating hydride formation/decomposition. Large differences are also observed between the isotherms for Pd and those for Pdcomposites at lower temperatures especially in the dilute phase and the two-phase region where conversion to the hydride phase is,50%.Similar behavior is found for a Pd/yttria composite obtained from internal oxidation of a Pd Y alloy. The differences between H0.98 0.02 2

isotherms for cycled Pd and internally oxidized (1073 K), cycled Pd–Al(Y) alloys are due to microstructural changes in the Pd matrixresulting from cycling in the presence of the small and closely spaced precipitates. Since internal oxidation at 1273 K results in larger andmore widely-spaced precipitates than internal oxidation at 1073 K, it would be expected that after cycling the former alloys, theirisotherms would be more similar to Pd–H than those internally oxidized at 1073 K. This has been confirmed experimentally. 2002 Published by Elsevier Science B.V.

Keywords: Transition metal alloys; Hydrogen absorbing materials; Gas–solid reaction; X-ray diffraction

1 . Introduction lower one for decomposition,p . A loss of work accom-d

panies hysteresis.Binary Pd alloys containing small amounts of readily When Pd is hydrided and dehydrided, cycled, at moder-

oxidizable solutes such as Al, Mg, and Zr, can be ate temperatures, large dislocation densities form to ac-internally oxidized to form composites consisting of commodate the abrupt volume change between the twoessentially pure Pd matrices containing nano-sized oxide phases [5]. Large dislocation densities are introduced byprecipitates [1–3]. This paper concerns the differences the initial cycle of hydride formation and decompositionbetween H isotherms for Pd and Pd/M O composites [6] but subsequent cycles do not increase the dislocation2 2 3

formed by internal oxidation of Pd–M alloys where M5Al density significantly.or Y. Hydrogen solubilities differ in the dilute phase of Pd and

Pd–H exhibits a two solid phase region, dilute1hydride in the location of thea /(a1b) boundary, i.e. the terminalphases, below 563 K [4] and, according to the phase rule, hydrogen solubility, before and after cycling [7]. Thesuch a two-phase region should have a constantp , the difference is caused by the large dislocation densityH2

introduced by the cycling and the subsequent H-dislocationplateau pressure,p . Experimental plateaux often slope,plat

trapping. There also appears to be some hydride phasehowever, and, due to hysteresis, there are two differentformation before the normal plateau pressure is reachedplateau pressures, one for hydride formation,p , and af

due, perhaps, to macro-stresses from dislocation pile-ups[8]. The H isotherm for cycled Pd shows a gradual2

transition from the dilute to the hydride phase in com-*Corresponding author. Fax:11-802-656-8705.E-mail address: flanagan@emba.uvm.edu(T.B. Flanagan). parison to its annealed form where there is supersaturation

0925-8388/02/$ – see front matter 2002 Published by Elsevier Science B.V.PI I : S0925-8388( 02 )00828-9

120 D. Wang et al. / Journal of Alloys and Compounds 348 (2003) 119–128

followed by an abrupt transition. The plateau pressure for Al content alloys became finely divided due to brittlehydride formation, p , for Pd is slightly greater for the fracture.f

second than for the initial cycle.Nano-sized, closely spaced precipitates such as intro-

duced by internal oxidation cause dispersion hardening [9] 3 . Results and discussionbecause the precipitates are obstacles for dislocationmovement which occurs during deformation. Analogously, 3 .1. Complete isotherms (323 K) for Pd /M O2 3

hydride formation and decomposition causes severe de-composites and single phase Pdformation and should also be affected by small precipi-tates. TEM photomicrographs do not show significant It has been previously observed [3] that initial H2

dislocation densities after internal oxidization of Pd–Al isotherms for internally oxidized Pd/Al alloys are veryalloys up to Pd Al [2,3,10] but after hydriding/ much like those for single phase Pd at the temperatures0.97 0.03

dehydriding, dislocations form with a higher density than where they were compared, e.g.#323 K. If, on the otherin the absence of the precipitates [11]. hand, the internally oxidized Pd–Al alloys are cycled

It was reported earlier [3] that, aside from some small through the hydride phase, the H isotherms, especially2

differences in the very dilute region, H isotherms (323 K) those withX $0.015, show significant differences from2 Al

for internally oxidized Pd Al alloys are essentially those of single phase Pd.0.97 0.03

identical to those for single phase Pd. (In order to Complete H isotherms (323 K) for the internally2

distinguish between the Pd matrix in the composites and oxidized (1073 K, 72 h) Pd Al , Pd Al ,0.995 0.005 0.985 0.015

polycrystalline, pure Pd, the latter will be referred to where Pd Al , Pd Al and Pd Al alloys are0.97 0.03 0.955 0.045 0.92 0.08

necessary as single phase Pd and from the context this shown with Pd–H in Figs. 1–4. The initial plateau pressureshould not be confused with H phases within Pd.) In the for the internally oxidized Pd Al alloy is somewhat0.995 0.005

light of the present results this requires some qualification lower than that for cycled Pd but its second cycle is closerbecause significant differencesare observed between to Pd–H (Fig. 1).cycled, internally oxidized (1073 K) Pd alloys and Pd. The The initial isotherms for the internally oxidizedpresent paper is devoted to these differences. Pd Al (Fig. 2) and Pd Al alloys (Fig. 3), are0.985 0.015 0.97 0.03

The effects to be reported are surely related to the very similar to those of single phase Pd in agreement witheffects of cycling on the plateau pressures of Pd alloys [12] our original findings [3]. (Initial isotherms for the internal-because in both situations, ‘impurities’ in the pure Pd ly oxidized Pd Al and Pd Al alloys are not0.955 0.045 0.92 0.08

lattice, i.e. solutes and precipitates, cause changes in the available at 323 K but an isotherm for the latter at 273 Kisotherms. was almost identical to that for single phase Pd.) A small

Since the Pd–H system has been the most well-investi- supersaturation of the dilute phase is observed for an-gated metal–hydrogen system [13], its further characteriza- nealed, single phase Pd (Fig. 2) and also for the internallytion is important and may help in the understanding of oxidized Pd Al and Pd Al alloys (Figs. 10.995 0.005 0.985 0.015

other metal–hydrogen systems. For this reason, it is and 2). Supersaturation disappears after cycling becauseimportant to obtain H isotherms for Pd in as many forms the many dislocations provide sites for nucleation. It is2

as possible, e.g. nanocrystalline [14], thin film [15] and as noteworthy that the alumina precipitates in these compos-the matrix of a metal /oxide composite. Since metal /oxide ites arenot sites for hydride nucleation. The presence ofcomposites are important technologically, any information supersaturation in these internally oxidized Pd–Al alloys isabout them may prove to be useful. consistent with the absence of dislocations found by TEM

in the internally oxidized Pd Al alloy [1–3].0.97 0.03

While the initial, complete H isotherms (323 K) are2

similar for single phase Pd and internally oxidized (10732 . Experimental K) Pd–Al alloys, differences appear after cycling (323 K).

For single phase Pd, thea→b transition becomes roundedPd–M substitutional alloys of compositionsX ,0.10 after cycling and the plateau pressure for hydride forma-M

where M5Al or Y were prepared by arc-melting the pure tion,p , is reproducibly slightly greater, while the de-f

elements, annealing the buttons (1133 K, 72 h) and then composition plateau pressure,p , is nearly unchanged.d

rolling them into foil of dimensions about 2 cm30.3 After cycling, the dilute→hydride transition for the inter-cm3110mm. The Pd–M alloys were internally oxidized in nally oxidized alloys withX .0.005 become significantlyAl

the laboratory atmosphere in a tube furnace generally at more gradual than for cycled single phase Pd. The1073 K for$72 h or at 1273 K for 24 h; both conditions differences between the H isotherms for the cycled forms2

resulted in complete internal oxidation. The foils were of single phase Pd and the internally oxidized alloysquenched to 273 K after oxidation to avoid Pd oxidation. increase with %Al (Figs. 1–4). For example, the differenceThe internally oxidized alloys were generally ductile but between H isotherms for the cycled Pd Al , and2 0.995 0.005

became brittle after cycling such that some of the higher cycled single phase Pd are negligible while the differences

D. Wang et al. / Journal of Alloys and Compounds 348 (2003) 119–128 121

Fig. 2. Hydrogen isotherms (323 K) for the Pd Al alloy after0.985 0.015

internal oxidization (1073 K).s, Initial isotherm;n, second cycle. Thecontinuous line is for single phase Pd. The open symbols are forabsorption and the filled ones for desorption.

transition and the fourth is even more gradual. At 273 K,therefore, changes continue after the second cycle. For

Fig. 1. Hydrogen isotherms (323 K) for the Pd Al alloy after0.995 0.005 comparison, the initial and second cycles for single phaseinternal oxidization (1073 K).s, Initial isotherm;n, second cycle. ThePd are shown where the initial one shows supersaturationcontinuous line is for cycled single phase Pd. The open symbols are forand the second one has only a small rounding of theabsorption and the filled ones for desorption.

dilute→hydride transition as compared to the gradualtransition for the second cycle of the internally oxidized

between cycled forms of single phase Pd and internally Pd Al alloy.0.92 0.08

oxidized Pd Al and Pd Al alloys are very Consecutive isotherms (323 K) were measured for an0.955 0.045 0.92 0.08

significant in the dilute→hydride transition region (Fig. 4). internally oxidized (1073 K) Pd Al alloy (323 K).0.97 0.03

An isotherm for the cycled Pd Al alloy internally The second and third cycles were identical indicating a0.97 0.03

oxidized at 1273 K is also shown in Fig. 3 where it can be leveling-off of the cycling effect after the second cycle.seen that the dilute→hydride phase transition is sharper Isotherms (473 K) after each of three consecutive cyclesthan for the alloy internally oxidized at 1073 K. This is were determined for an internally oxidized (1073 K)because larger and more widely-spaced precipitates form Pd Al alloy. This is the only example for internally0.995 0.045

from the higher temperature internal oxidation [9] and oxidized Pd–Al alloys where the initial cycle also ex-these do not affect dislocation movement significantly. hibited a gradual dilute→hydride transition. The ‘plateau’

pressure decreased about 1/3 of the distance between3 .2. Effect of the number of cycles on the isotherms of plateaux for Pd–H (evaluated from the horizontal part ofinternally oxidized Pd alloys the plateau) and the second cycle fell about 2/3 of the

distance. The third cycle duplicated the second one.Fig. 5 shows consecutive isotherms at 273 K for the H isotherms after each of six consecutive cycles for a2

internally oxidized (1073 K) Pd Al alloy in the Pd Al internally oxidized at 1273 K were measured0.92 0.08 0.97 0.03

dilute→hydride transition region. The initial cycle iso- at 473 K. The initial cycle isotherm was very close to thattherm is very similar to that for cycled, single phase Pd. for single phase Pd. A small rounding of theThe second cycle shows a more gradual dilute→hydride dilute→hydride transition was found for the second and

122 D. Wang et al. / Journal of Alloys and Compounds 348 (2003) 119–128

Fig. 4. Hydrogen isotherms (323 K) for the Pd Al and Pd Al0.955 0.045 0.92 0.08

alloys after internal oxidization (1073 K).n, Second cycle forPd Al alloy; h, second cycle for crushed internally oxidized0.955 0.045

Pd Al alloy; ,, second cycle for Pd Al . The continuous lineFig. 3. Hydrogen isotherms (323 K) for the Pd Al alloy after 0.955 0.045 0.92 0.080.97 0.03

is for cycled single phase Pd. The open symbols are for absorption andinternal oxidization (1073 K).s, Initial isotherm;n, second cycle;h,the filled ones for desorption.Pd Al alloy internally oxidized at 1273 K and then cycled. The0.97 0.03

continuous line is for cycled single phase Pd–H. The open symbols arefor absorption and the filled ones for desorption.

at T#323 K, the particle sizes may become large enoughso that their effect on the dislocation movement is in-

the sixth cycle isotherm had a similar gradual transition. Of significant. For very dilute Al alloys, e.g. Pd Al ,0.995 0.005

the temperatures employed for isotherm measurements, it the particle sizes will be small in regions near the surfaceseems that only those measured at 273 K required more but the inter-particle distance will be relatively large whichthan one cycle to reach a limiting effect on the isotherms. explains the observed absence of any cycling effects.

An interesting effect was found for a partially internallyoxidized (1073 K) Pd Al alloy; partially internally 3 .3. Isotherms for cycled single phase Pd at elevated0.97 0.03

oxidized means that the oxidation zone did not completely temperaturespenetrate into the alloy. The dilute→hydride transition foran alloy internally oxidized to 45% was found to be almost A series of isotherms were measured for single phase Pdas gradual as for the completely internally oxidized at elevated temperatures; these have rather horizontalPd Al alloys. A Pd Al alloy was partially plateaux (Fig. 6). Hysteresis becomes quite small for the0.97 0.03 0.97 0.03

internally oxidized at 983 K to about 15% and, after isotherm at 553 K which is close to the critical tempera-cycling, it also had a very rounded transition similar to ture, 563 K [18]. The horizontal dashed lines (Fig. 6) arecompletely oxidized alloys extending over the whole of the the plateau pressures of isotherms measured at 433 and15%. Therefore the outer regions of the internally oxidized 473 K by Frieske for bulk Pd [19] and it can be seen thatalloys are most affected by the cycling independent of the agreement with the present data is good. The otherwhether or not the remainder has been internally oxidized. temperatures where isotherms were measured (Fig. 6) do

The reason for the relatively large effect of cycling on not coincide with the temperatures of the isothermsthe partially internally oxidized alloys may be due to a measured by Frieske. In the present work the single phaseparticle size increase and accompanying increase in the Pd isotherms were measured using polycrystalline foil,inter-particle spacing with the depth of penetration of the which was not annealed between isotherm measurementsinterface [9,16,17]. This effect can be significant and at at the different temperatures, i.e. the isotherms in Fig. 6 aredepths corresponding to.50% conversion to the hydride all for cycled Pd.

D. Wang et al. / Journal of Alloys and Compounds 348 (2003) 119–128 123

Fig. 5. Hydrogen isotherms (273 K) at low H contents for single phase Pd and the Pd Al alloy after internal oxidization (1073 K).s, Annealed Pd0.92 0.08

showing supersaturation;d, cycled Pd;n, initial cycle for internally oxidized Pd Al ;m, second cycle for internally oxidized Pd Al ;,, third0.92 0.08 0.92 0.08

cycle for internally oxidized Pd Al .0.92 0.08

3 .4. Isotherms at elevated temperatures for cycled elsewhere [20] that neither plateau corresponds to equilib-internally oxidized Pd–Al alloys rium.

Cycled, internally oxidized Pd–Al alloys are quite brittleFig. 7 shows H isotherms at 473, 513 and 553 K for a and can be crushed into powder; this presumably takes2

composite prepared by internal oxidation (1073 K, 72 h) of place by fracture at grain boundaries. Examination of thea Pd Al , alloy and cycled at a lower temperature crushed powder by SEM reveals that some of the particles0.955 0.045

with the single phase Pd–H isotherms shown for com- are still polygranular with cracking visible along grainparison. (Although the isotherms for the composite are far boundaries; the smaller particles observed by SEM may befrom horizontal, the expression ‘plateau’ will, nonetheless, individual grains. The isotherm of a cycled, crushedbe employed because two phases co-exist and because the internally oxidized Pd Al alloy was measured at 3230.97 0.03

corresponding regions for Pd–H are nearly horizontal (Fig. K (Fig. 3) and found to be similar to that of the uncrushed7).) The differences between the isotherms are greatest in internally oxidized alloy. Since there appear to be somethe region of H contents where less than 50% hydride individual grains in the crushed alloy, whatever causes thephase has formed. The H capacity does not seem to be difference between isotherms of Pd and the internallyaffected. The isotherm at 553 K is particularly interesting oxidized alloys, is intergranular.because there no longer appears to be a plateau although Isotherms (473 K) for the cycled Pd Al and0.94 0.06

there is a small hysteresis indicating that two phases form Pd Al alloys which had been internally oxidized at0.92 0.08

but no longer at a constant, plateau pressure. While it is 1073 K are shown in Fig. 8. Although the size of thetrue that for the 323 and 473 K isotherms the precipitates increases with concentration of solute [21], thedilute→hydride transition, as reflected by the ‘plateaus’ for inter-particle distance decreases, which may account forhydride formation in the low H content region, show the the fact that differences between isotherms for cycledmost difference from single phase Pd, this is not the case at internally oxidized alloys and Pd increase with %Al. It isthe higher temperatures where both plateaux differ by also clear that the alumina precipitates for the Pd Al0.94 0.06

about the same extent from those of Pd–H (Fig. 7). The and Pd Al alloys are still small enough to interact0.92 0.08

effect is not simply a reduction of hysteresis as for the strongly with dislocations.cycling effect [12] but a lowering of both plateaupressures. These results therefore do not prove or disprove3 .5. Isotherms for Pd /yttria compositesthe proposition that the desorption plateau corresponds toequilibrium rather than the absorption plateau [18] because Some other properties of cycled internally oxidized Pdboth plateau branches differ significantly from those of alloys can be illustrated with Pd–Y alloys which can besingle phase Pd–H at 513 and 553 K. It has been argued internally oxidized to give small yttria precipitates within a

124 D. Wang et al. / Journal of Alloys and Compounds 348 (2003) 119–128

Fig. 7. H isotherms for the internally oxidized Pd Al alloy2 0.955 0.045Fig. 6. H isotherms for single phase Pd (cycled) at elevated tempera-2 (cycled) at elevated temperatures. Isotherms for single phase Pd aretures. The dashed horizontal lines are isotherm plateaux from Frieske

shown by the continuous lines without points. The open symbols are for[19]. The open symbols are for absorption and the filled ones for

absorption and the filled ones for desorption.desorption.

for single phase Pd (Fig. 9). In both cases, the desorptionPd matrix, i.e. Pd/yttria composites. It should be noted that isotherms are similar to single phase Pd (473 K).the more stable the oxide, the smaller are the precipitateswhich form from internal oxidation [9]; since the stability 3 .6. Hysteresisof the two oxides are similar, the sizes of the oxides shouldbe similar for Pd–Al and Pd–Y. An isotherm was mea- An isotherm for a cycled, internally oxidizedsured at 323 K for the internally oxidized (1073 K, 72 h) Pd Al alloy was measured at 473 K and has been0.985 0.015

Pd Y alloy and the initial isotherm was very similar plotted as 1/2 ln (p /Pa) againstr (Fig. 10). The area of0.98 0.02

to that for single phase Pd and the second cycle was alsothe hysteresis cycle shown in this figure, 150 J/mol Pd, issimilar because its plateaup was greater than for the the work lost due to hysteresis per mol Pd. For singleH2

initial cycle. Thus the Pd Y alloy differs from the phase Pd, 1/2RT e ln ( p /Pa) dr 5 265 J/mol Pd at 4730.98 0.02

cycled internally oxidized Pd Al and Pd Al K, or when evaluated as 1/2RT e ln ( p /p ) dr wherep0.985 0.015 0.97 0.03 f d f

alloys which exhibit gradual dilute→hydride phase transi- andp are the formation and decomposition plateaud

tions (Figs. 2 and 3). pressures, it is about 660 J/mol H and the correspondingA Pd Y alloy which was internally oxidized at value for the Pd/alumina composite shown in Fig. 10 is0.98 0.02

1073 for 72 h and then cycled (323 K), was heated to 473 about 370 J/mol H. Because of the sloping of theK where a H isotherm was measured (Fig. 9); significant composite’s isotherms, the hysteresis value from the areas2

deviations from the single phase Pd isotherm are apparent. are more accurate than that using onlyp and p whichf d

When a Pd Y alloy was internally oxidized at 1273 varies with % conversion to the hydride phase (Fig. 10). It0.98 0.02

K, however, the isotherm at 473 K is very similar to that is clear that hysteresis is appreciably reduced for the Pd

D. Wang et al. / Journal of Alloys and Compounds 348 (2003) 119–128 125

Fig. 9. H isotherms (473 K) for cycled Pd Y alloys after internal2 0.98 0.02

oxidations at 1273 and 1073 K. The isotherm for single phase Pd isFig. 8. H isotherms for internally oxidized Pd–Al alloys (cycled) at 4732 shown by the continuous line.s, Cycled Pd Y alloy internally0.98 0.02K. The isotherm for single phase Pd is shown by the continuous line. oxidized at 1273 K;n, cycled Pd Y alloy internally oxidized at0.98 0.02Dashed line without points, internally oxidized Pd Al alloy0.955 0.045 1073 K. The open symbols are for absorption and the filled ones for(cycled);s, internally oxidized Pd Al alloy (cycled);n, internally0.94 0.06 desorption.oxidized Pd Al alloy (cycled). The open symbols are for absorption0.92 0.08

and the filled ones for desorption.

ible) and (≠m /≠r) (rev) is obtained from the slope of theH T

matrix in the cycled composite compared to single phase small desorption excursions; this equation has been derivedPd at this temperature, 473 K. elsewhere [22]. If the small reversible desorption excur-

sions coincide with the absorption data, then the absorptiondata must also be reversible indicating that only single3 .7. Irreversibility for dilute phase H solubilities in2 phase H solution takes place.2cycled and uncycled single phase Pd and Pd /Al O2 3 The H solubility is known to be completely reversible2composites at 323 Kfor single phase Pd in the dilute phase and, after cycling,its H solubility is reversible to aboutr #0.02 (Fig. 11).2Dilute phase solubilities (323 K) for annealed Pd andFor the desorption excursion starting fromr50.023, therethe internally oxidized Pd Al alloy in their uncycled0.97 0.03 can be seen to be significant irreversibility and thereforeand cycled forms were measured during absorption andsome hydride formation has taken place up to this contentthen interrupted at various H contents for the measurementas previously observed [23]. The internally oxidized alloyof small desorption excursions (Fig. 11). Irreversibility inin Fig. 11 is a Pd Al alloy which does not have as0.97 0.03the absorption/desorption data indicates that some hydridelarge differences from Pd in this region as the higher Alformation/decomposition has occurred because irrever-content alloys, however, its trends should be similar tosibility should be absent in single phase regions. Thethose with greater solubilities. There is seen to be irrever-fraction of reversibility, f , can be calculated fromrev sibility for the uncycled internally oxidized Pd Al0.97 0.03

alloy near the phase boundary atr¯0.03 but, otherwise, it(≠m /≠r) (exp)H T]]]]]f 5 512 f (1)rev a→b is reversible. After cycling, some irreversibility is seen at a(≠m /≠r) (rev)H T

low H content,r¯0.012, but, surprisingly, its extent doeswhere f is the fraction of hydride formation (irrevers- not increase significantly withr despite the large increasea→b

126 D. Wang et al. / Journal of Alloys and Compounds 348 (2003) 119–128

about r50.03 in the cycled, internally oxidizedPd Al alloy is almost entirely due to solution in the0.97 0.03

dilute phase; contributions from hydride formation becomeimportant forr.0.03 especially where the curvature in thesolubility plot is significant (Fig. 11). Atr 5 0.05, f ¯a→b

0.75.

3 .8. Possible sources for the differences of isotherms forcycled Pd in a composite and cycled single phase Pd

Single phase Pd isotherms reported in the literatureshow reproducible differences in dilute phase H solu-2

bilities for annealed, cycled, and cold-worked forms [24].The latter two are characterized by gradual transitions tothe hydride phase rather than an abrupt one with supersatu-ration as found for annealed Pd. The plateaup for cycledH2

single phase Pd is reached at about H/Pd5r50.032instead of 0.018 for the annealed form (323 K), however,since the total width of the plateau is aboutr50.6, this isonly about 2% of the total H capacity. By contrast, forcycled internally oxidized Pd–Al alloys up to 35% of theplateau can be affected (323 K) (Fig. 4). Pd/Al O2 3

composites resulting from internal oxidation of Pd–AlFig. 10. H isotherms (473 K) for a Pd Al alloy after internal2 0.985 0.015 alloys are composed of oxide precipitates within pure Pd1 / 2oxidation at 1273 K and cycled plotted as ln (p /Pa) againstr. TheH2 matrices. Deviations of the isotherms for the cycledisotherms for single phase Pd are shown by the continuous line. The areas

composites from those of single phase Pd cannot beof the complete hysteresis cycles give the hysteresis per mol metal foreach. The open symbols are for absorption and the filled ones for attributed to impurities but can only be due to changes ofdesorption. the microstructure of the Pd in the matrix, i.e. dislocation

arrays in the Pd.in solubility after cycling. Whenr.0.03 the isotherm The differences in isotherms for cycled forms of Pd instarts to curve sharply towards the plateau where hydride the composites appear to increase with temperature (Figs.formation becomes a significant factor. From these results, 2 and 7) but, when evaluated as free energy changes, theit is concluded that the large dilute phase solubility to effect is larger at 323 K than 473 K, e.g. values of 1/2RT

Fig. 11. Dilute phase H absorption and desorption solubilities (323 K) for single phase Pd and an internally oxidized Pd Al alloy before and after2 0.97 0.03

cycling (323 K). The absorption isotherms have been interrupted to measure desorption data starting with the lowest H contents and then the absorption iscontinued and interrupted at progressively higher H contents. The desorption data are all initiated from the nearest absorption data point lying at aslightlyhigher H content than the beginning desorption point. Dashed line, Pd;h, cycled Pd;n, internally oxidized Pd Al alloy;s, cycled internally0.97 0.03

oxidized Pd Al alloy. The open symbols are for absorption and the filled ones for desorption.0.97 0.03

D. Wang et al. / Journal of Alloys and Compounds 348 (2003) 119–128 127

e ln p /p dr evaluated in the early part of the requirements for reversing plastic deformation because of(compos.) (s.p.)

plateau regions, where the maximum effect occurs at each the build-up of long-range stresses in dislocation arraystemperature, are greater at 323 K where compos. and s.p. during the forward deformation [28]. It is much morerefer to Pd in the composite and single phase Pd, respec- pronounced in composites than in single-phase materialstively. This is expected if there is a redistribution of H in [30]. Long-range residual stresses which develop in com-the stressed and unstressed interstices, i.e. there will be posites from dislocation arrays introduced by cyclic de-fewer occupying the tensile stressed sites at elevated formation are greater than those from uniaxial deforma-temperatures. At the elevated temperatures, however, the tions [31]. In view of this, it seems that cyclic hydridingwhole isotherm is affected rather than just the first 35% as and dehydriding of Pd/Al O composites will be a very2 3

at 323 K (Fig. 4). favorable framework for the development of internal long-When well-annealed, single phase Pd absorbs H initial- range stresses. Back stresses and long range residual2

ly, there is a barrier for nucleation as shown by the stresses assist the hydriding phase change by effectivelysupersaturation of the dilute phase [25] and only whenp lowering p as observed. It does not appear to have asH f2

exceeds a certain value, does it fall to the plateau pressure. much effect on the lowering ofp . Although it is reason-d

There is no supersaturation for single phase Pd which has ably certain that the cause of the effect lies in thebeen either cold worked or cycled and therefore nucleation dislocation microstructures created by cycling in thewill not be a problem for Pd in a cycled composite and presence of small precipitates, a quantitative connectioncannot be the source of the large differences of isotherms. with the isotherm differences would be difficult.

The r value where the isotherms rise steeply at the end It seems that the present observations may be related toof the plateau is determined by ther-dependence ofm the ‘cycling’ effect observed in Pd-rich alloys [12] forH

[26] which is the same for Pd in the composites or single which no explanation has been provided except that it mustphase Pd, i.e. the closely-spaced nanometre-sized precipi- also be connected with the dislocation microstructuretates do not affect ther-dependence ofm . This is an which develops during cycling. Changes in the isothermsH

important observation because for some other forms of Pd take place in Pd–M alloys with cycling at moderatesuch as thin films or nanocrystalline, smaller capacities are temperatures; the extent of these changes increase withfound [14,17]. In the case of thin films the decrease of the %M up to a value dependent upon the nature of M. Theplateau width and capacity is attributed to the effect of changes which occur differ for the cycling of these solid‘clamping’ to the substrate [27]. It is clear that such solution alloys from the cycling of composites because theinfluences are absent for the composites either before or former tends to uniformly decreasep and, to a lesserf

after cycling. extent, increasep , thereby reducing the hysteresis gapd

The difference in the isotherms after internal oxidation whereas cycling the composites causes mainly changes inof the Pd Al alloy at 1073 K and 1273 K (Fig. 9) the first half of the absorption plateau at least at lower0.98 0.02

shows that small oxide precipitates such as formed by temperatures. For the ‘cycling’ effect in alloy solid solu-internal oxidation at 1073 K are necessary for there to be a tions, hardening occurs because the solute atoms play thelarge difference between the isotherms of the internally role of the oxide precipitates in the present investigation.oxidized alloys and those for single phase Pd. Small, An important difference is that the solid solution hardeningclosely spaced precipitates are also needed for dispersion- will be uniform throughout the samples whereas hardeninghardening [9] which is due to the precipitates acting as from the internally oxidized alloys will be a function of theobstacles for dislocation motion. Plastic deformation of varying precipitate size with penetration distance.composites like the present ones by external stressingresults in a dislocation microstructure, e.g. Orowan loops,about the precipitates [28]. In an analogous way, cycling 4 . Conclusionsinternally oxidized (1073 K) Pd–Al alloys causes plasticdeformation because of the11% expansion and contrac- Although the initial H isotherms for Pd/Al O compos-2 2 3

tion accompanying hydride phase formation and decompo- ites are almost identical to single phase Pd, the subsequentsition (298 K). In the presence of small, closely spaced ones differ markedly for alloys withX $0.015. At higherAl

precipitates resulting from internal oxidation at 1073 K temperatures, e.g.$513 K, the nearly horizontal plateauxdislocations are stabilized [9,29] and therefore cycling characteristic of single phase Pd disappear. The similaritiessuch a Pd/composite will result in a different dislocation and differences of isotherms for cycled single phase Pdmicrostructure than cycling in their absence. The different and Pd/Al O composites prepared by internal oxidation2 3

microstructures must lead to the observed differences of Pd–Al(Y) alloys can be summarized as follows.between H isotherms of the cycled Pd composites and2

cycled single phase Pd. It should also be noted that after (a) For cycled Pd/Al O composites withX $0.015 the2 3 Al

cycling, the internally oxidized alloys readily undergo transition from the dilute to hydride phase is gradual atbrittle fracture indicating their increased hardness. all temperatures examined. The plateau pressure for

The Bauschinger effect [28] refers to the smaller stress hydride formation, which should be invariant, is lower

128 D. Wang et al. / Journal of Alloys and Compounds 348 (2003) 119–128

[2] J. Eastman, M. Ruhle, Ceram. Eng. Sci. Proc. 10 (1989) 1531.for ,50% conversion to the hydride phase than for[3] H. Noh, T. Flanagan, R. Balasubramaniam, J. Eastman, Scriptahigher fractions of conversion to the hydride.

Metall. Mater. 34 (1996) 863.(b) The H capacities wherep rises markedly in theH [4] E. Wicke, J. Blaurock, Ber. Bunsenges Phys. Chem. 85 (1981) 1091.2

single hydride phase are the same for the composites [5] M. Wise, J. Farr, I. Harris, J. Hirst, in: L’Hydrogene dans lesand single phasep . Metaux, Vol. 1, Pergamon Press, London, 1972, p. 1.d

[6] T. Flanagan, B. Bowerman, G. Biehl, Scripta Metall. 14 (1980) 443.(c) There is little difference in the isotherms near the[7] J. Lynch, J. Clewley, T. Curran, T. Flanagan, J. Less-Commonupper, (a1b) /b, phase boundary whereas there is a

Metals 55 (1977) 367.marked difference in the region of the lower,a /(a1 [8] T. Flanagan, T. Kuji, J. Less-Common Metals 99 (1984) L5.b), phase boundary. [9] J. Meijering, in: H. Herman (Ed.), Advances in Materials Research,

(d) Hysteresis, which is proportional to the area of the Vol. 5, Wiley, New York, 1971, p. 1.¨[10] X. Huang, PhD Thesis, Universitat Stuttgart, 1989.loop in a 1/2 lnp versusr plot, is greater for singleH2 [11] R. Balasubramaniam, H. Noh, T. Flanagan, Y. Sakamoto, Actaphase Pd than for the cycled composites.

Metall. Mater. 45 (1997) 1725.(e) Both the absorption and desorption branches of the [12] T. Flanagan, D. Wang, H. Noh, J. Alloys Comp. 253–254 (1997)

hysteresis loop are lower than the corresponding 216.[13] F. Lewis, The Palladium/Hydrogen System, Academic Press, Newbranches for single phase Pd–H at 513 and 553 K in

York, 1967.the region below about 50% conversion to the hydride[14] T. Mtitschele, R. Kirchheim, Scripta Metall. 21 (1987) 135;phase.

T. Mtitschele, R. Kirchheim, Scripta Metall. 21 (1987) 1101.(f) Cycled Pd–Al or Pd–Y alloys, which have been [15] E. Salomons, R. Feenstra, D. deGroot, J. Rector, R. Griessen, J.

internally oxidized at 1273 K rather than at 1073 K, do Less-Common Metals 130 (1987) 415;E. Salomons, R. Feenstra, D. deGroot, J. Rector, R. Griessen, J.not show such large differences in isotherms fromLess-Common Metals 172–174 (1991) 42.those of cycled single phase Pd.

[16] P. Bolsaitis, M. Kahlweit, Acta Metall. 15 (1967) 772.[17] L. Zhou, X. Wei, Scripta Mater. 40 (1999) 365.

It is known that small precipitates in metals give rise to [18] E. Wicke, J. Blaurock, J. Less-Common Metals 130 (1987) 351.dispersion hardening. The strength of the hardening is ¨[19] H. Frieske, Ph.D. Thesis, University of Munster, Germany, 1972.

[20] T. Flanagan, D. Wang, Phys. Chem. Chem. Phys., in press.derived from the interaction of the moving dislocations[21] N. Birks, G. Meier, Introduction to High Temperature Oxidation ofwith the precipitates which act as obstacles to be sur-

Metals, Edward Arnold, London, 1983.mounted [28]. It is suggested that the dislocation micro-[22] B. Bowerman, C. Wulff, G. Biehl, T. Flanagan, J. Less-Common

structure and resulting long range residual stresses re- Metals 73 (1980) 1.sulting from plastic deformation in the presence of obsta- [23] T. Flanagan, S. Kishimoto, B. Biehl, in: N. Gokcen (Ed.), Chemical

Metallurgy—a Tribute to Carl O. Wagner, The Metal Society AIME,cles are the cause of the phenomenon observed here. This1981, p. 471.is one of the few examples where the dislocation micro-

[24] S. Luo, T. Flanagan, J. Alloys Comp. 330–332 (2002) 29.structure clearly influences a chemical reaction; the absorp-[25] E. Wicke, G. Nernst, Ber. Bunsenges Phys. Chem. 68 (1964) 224.

tion/desorption of H is a chemical reaction because2 [26] T. Flanagan, W. Oates, Annu. Rev. Mater. Sci. 21 (1991) 269.strong H–H bonds are broken when H dissolves into [27] R. Feenstra, G. de Bruin-Hordijk, H. Bakker, R. Griessen, D. de

Groot, J. Phys. F Metal Phys. 13 (1983) L13.interstices in Pd.[28] D. Hull, D. Bacon, Introduction to dislocations, in: International

Series on Materials Science and Technology, Vol. 37, Oxford, 1984.[29] R. Smallman, Modern Physical Metallurgy, Butterworths, London,

A cknowledgements 1970.[30] L. Zhonghua, S. Schmauder, A. Wanner, Metall. Mater. Trans. A 3A

(2000) 2943.The authors acknowledge Westinghouse Savannah River[31] H. Mughrabi, Acta Metall. 31 (1983) 1367.Corporation for partial financial support of this research.

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