decomposition on the mechanical properties of palladium

Scripta Materialia, Vol. 35, No. 8, pp. 1013-1018, 1996 Elsevier Science Ltd Copyright 0 1996 Acta Metallurgica Inc. Printed in the USA. All rights reserved 1359-6462196 $12.00 + .OO

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INFLUENCE OF PARTIAL HYDRIDE FORMATION/DECOMPOSITION ON THE MECHANICAL PROPERTIES OF PALLADIUM Christian Anderton, Natasha &other, John Pote, R. Foley’, K. Rebei2, S. Nesbi? and A. Craft Department of Chemistry, Lafayette College, Easton, PA 18042 ’ Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401

* Department of Civil Engineering, Lafayette College, Easton, PA 18042 3 Department of Mechanical Engineering, Lafayette College, Easton, PA 18042 (Received January 30,1996) (Accepted June 12, 1996) Introduction The ability of absorbed hydrogen to alter the mechanical properties of metals and alloys has been recognized since the earliest investigations on metal-hydrogen systems, where embrittlement accompanying hydrog,en absorption was commonly encountered [l]. Since that time much effort has been directed at characterizing and understanding the ways by which absorbed hydrogen modifies the mechanical properties of metals. Many of the early studies involved M-H systems with absorbed hydrogen present during mechanical characterization. It was not until more recently that attention was focused on systems for which absorbed hydrogen had been removed from the metal matrix prior to mechanical testing [2-51. These investigations into the residual effects of hydrogen absorption/desorption on mechanical properties have focused on thermal cycling [2,3] or isothermal pressure cycling [4,5] of the metal-hydrogen system. The latter process usually involves subjecting the system to a phase transformation via passing through a two-phase region consisting of a dilute solid solution a-phase and a p (a’) hydride phase. The changes in mechanical properties brought about by this treatment are attributed mainly to the dislocations generated during the a ---> p and p ---> a transitions (due to the abrupt volume change of the matrix that accompanies hydride formation and decomposition) [4-61. Thermal cycling and pressure cycling studies have reached a point such that some have suggested that hydrogen may be used to alter the mechanical properties of certain materials in a controlled fashion, though much work remains to be done before this is realized. Most pressure cycling studies have involved a complete traversing of the two-phase coexistence region during hydrogen exposure treatment. A very important factor which has been ignored to a large degree is the extent to which the two-phase region is crossed during cycling (i.e. the fraction of the system converted to hydride). This paper reports the results of a study on the palladium-hydrogen system during which the miscibility gap that separates the a and p phases was traversed to varying 1013

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breadth. The results indicate that changes in the tensile properties of Pd, due to the absorption/desorption of hydrogen, depend markedly on the degree to which the miscibility gap is crossed. Experimental Palladium (99.9%) foil of thickness 0.25 mm was used in this study. This foil was cut into specimens of length 38.1 mm and width 6.4 mm. A reduced section of width 3.2 mm was machined into each specimen that was used for tensile testing. All specimens were then stress-relieved in vacua at 723 K for 24 h followed by a 2 h annealing in vacua at 823 K. Hydrogen absorption/desorption by the annealed specimens was carried out at 323 K in an all-metal system of calibrated volumes. Hydrogen pressures were measured with MKS diaphragm gauges and the hydrogen content of samples was determined via the gas law from changes in the hydrogen pressure. In this study, respective samples were dosed to H/Pd atom ratios of 0.02,0.20, 0.35, 0.49, 0.66, and 0.72. Upon reaching the desired H/Pd ratio, samples were evacuated for 24 h in order to remove the absorbed hydrogen. Tensile tests were carried out on an Instron Series IX Automated Materials Testing System using a constant elongation rate of 1.27 mm/min. Knoop microhardness tests were performed on a Kentron Microhardness Tester using a load of 200 g. Grain structures were investigated by metallography. Results Figure 1 shows typical engineering stress-strain curves for palladium samples that were exposed to the hydrogen absorption/desorption treatments described above. [note: to minimize clutter, the curves for specimens cycled to 0.02 and 0.72 are not shown, the former is virtually identical to that of the vacuum annealed specimen and the latter is virtually identical to that of the specimen cycled to 0.661. As this figure illustrates, in many instances the tensile properties of Pd are quite sensitive to the final hydrogen

Figure 1. Engineering stress-strain curves for palladium samples that were subjected to hydrogen absorption, to various final hydrogen contents, followed by complete hydrogen desorption; (A) vacuum annealed, (B) cycled to H/Pd = 0.20, (C) cycled to HPd = 0.35, (D) cycled S UP = 0.49, (E) cycled to HIPd = 0.66.

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content reached during the absorption segment of the cycling treatment. In most cases, as the final hydrogen content reached during absorption increases, the strength of palladium increases and the ductility decreases. This sensitivity of tensile properties to the final hydrogen content reached during absorption is made more apparent in Figures 2 and 3. Figure 2 shows the yield strength and ultimate tensile strength of palladium as a function of the final H/I’d ratio reached during the absorption segment of the cycling treatment. Figure 3 shows the total elongation (i.e. elongation at failure) of palladium as a function of final H/Pd ratio reached during the absorption segment of the cycling treatment. In these figures, results corresponding to I-I/Pd = 0 refer to palladium that was annealed in vucuo as described above and not subsequently exposed to hydrogen. Knoop microhardness as a function of the final H/Pd ratio reached during the absorption segment of the cycling treatment is shown in Figure 4. As can be seen, in most cases the hardness of palladium increases as the maximum I-I/Pd achieved during cycling increases. The hardness value corresponding to H/Pd = 0 again refers to palladium that was annealed in vacua as described above and not subsequently exposed to hydrogen. When considering Figures 1 - 4 it is important to remember that, although hydrogen content is reported as a variable, all palladium samples were hydrogen-free during testing. The H/Pd values refer to the hydrogen content reached during the absorption segment of the absorption/desorption cycling treatment; all albsorbed hydrogen was removed from each specimen prior to testing. Discussion At the temperature of the hydrogen absorption/desorption cycling of the present study (323 K), the miscibility gap of the Pd-H system exists between I-I/Pd ratios of 0.03 and 0.62 [7].

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E/M Figure 2. Yield sbongth (0) and ultimate tensile strength (A) of palladium as tkction of the final H/Pd reached during the absorption segment of a cycling treatment. [The dashed line represents the values of yield strength predicted by a weighted average of the yield strengths of Pd that was cycled to the onset H/Pd of the miscibility gap and Pd that was cycled to the terminal H/Pd of the miscibility gap. The solid line representsvalues of ultimate strengthpredictedby a weighted average of the ultimate strengths‘ofPd that was cycled to the onset H/Pd of the miscibility gap and Pd that was cycled to the terminal H/Pd of the miscibility gap ]

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Palladium

Figure 3. Total elongation of palladium as a function of the final HIPd reached during the absorption segment of a cycling treatment. [The dashed line represents the values of total elongation predicted by a weighted average of the total elongations of Pd that was cycled to the onset H/Pd of the miscibility gap and Pd that was cycled to the terminal HiPd of the miscibility gap.]

Thus two sets of samples investigated in this work (the vacuum annealed samples and those cycled to I-I&d = 0.02) did not enter the miscibility gap. Comparison of the tensile properties and hardness characteristics of these two samples (Figures 2-4) shows them to be similar. This infers that formation followed by decomposition of the single phase dilute solid solution of hydrogen in palladium has only a slight effect on the observed mechanical properties of the metal matrix. Conversely, two sets of samples (those cycled to H/Pd = 0.66 and 0.72) completely traversed the miscibility gap. During hydrogen absorption, both of these samples underwent complete formation of the p hydride phase and essentially differ in the amount of hydrogen ultimately accommodated in the hydride. Comparison of the tensile and hardness traits of these two samples (Figures 2-4) are also quite similar to one another. This finding similarly infers that the presence of varying amounts of hydrogen in a single phase configuration of the Pd-H system has only a minor effect on the mechanical properties of the Pd matrix, once that hydrogen is removed from the matrix. The remaining three sets of samples partially traversed the miscibility gap. The present results clearly show that the observed strengthening of the Pd matrix occurs almost exclusively in the course of this progressive traversing of the miscibility gap. Goods and Guthrie showed that subjecting palladium to a hydriding/dehydriding cycle, such that the system completely traverses the Pd-H miscibility gap, results in a marked increase in the strength of the palladium while significantly reducing the ductility of the matrix [4]. The results of the present study confirm this finding and additionally show that the tensile and hardness characteristics of palladium are greatly affected by the extent to which the miscibility gap is traversed. In particular, progressive partial traversing of the Pd-H miscibility gap (i.e. a larger and larger fraction of the matrix experiences hydride formation and decomposition) is accompanied by a steady increase in yield and ultimate tensile strength and microhardness of the palladium matrix while the ductility of Pd steadily decreases. With regard to the samples that partially traversed the miscibility gap, the lever rule dictates that as the H/Pd ratio increases the fraction of the palladium matrix experiencing the a --A p transition (during absorption) and the p --A a transition (during desorption) increases in a linear fashion. It is of interest to note if the measured tensile and hardness characteristics of the Pd matrix also follow the lever rule. In other words are the observed mechanical properties simply a weighted average, in accord

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H:M Figure 4. Knoop microhardnessof palladium as a function of the final H/Pd reached duringthe absorptionsegment of a cycling treatment.[The dashed line representsthe values of Knoop microhardnesspredictedby a weighted average of the Knoop microhardnessvalues of Pd that was cycled to the onset It’d of the miscibility gap and Pd that was cycled to the terminal H/F’dof the miscibility gap.]

with the fraction of the matrix converted to the hydride phase, of the mechanical properties of Pd that was cycled to the onset H/Pd of the miscibility gap (i.e. no hydride formation/decomposition involved) and Pd that was cycled to the terminal H/Pd of the miscibility gap (i.e. complete formation/decomposition of the hydride). Our results indicate that the mechanical properties of Pd specimens cycled to I-I/Pd = 0.02 are representative of Pdl cycled to the onset H/Pd of the miscibility gap while the mechanical properties of specimens cycled to H/Pd = 0.66 are representative of Pd cycled to the terminal H/Pd the miscibility gap. Using the:se measured values of mechanical properties and the accepted onset H/Pd (0.03) and terminal H/Pd (0.62) of the Pd-H miscibility gap at 323 K, we have calculated the expected values, using the lever rule and a simple weighted average, of each mechanical property for specimens cycled to H/Pd ratios within the miscibility gap. The predicted values of the mechanical properties are represented by da&d or solid lines in Figures 2-4. It is very interesting that ultimate strength and microhardness seem to exhibit behavior that is in accord with the lever rule but yield strength and ductility show considerable deviations from behavior predicted by the lever rule. Metallographic analysis of the various specimens (including as-annealed specimens) revealed no perceptible differences in microstructure. This finding indicates that hydrogen absorption/desorption cycling, including situations where hydride formation and decomposition is involved, has little effect on the microstructure of palladium. In all likelihood then, the observed hydrogen-induced changes in the mechanical characteristics of palladium are not influenced to a significant extent by microstructure factors. It is well established that the formation or decomposition of a hydride phase via a two-phase region results in the introduction of dislocations into the metal matrix owing to the abrupt volume change that accompanies such hydride formation and decomposition [6]. For those Pd samples that partially traversed the miscibility gap, only a fraction (dictated by the lever rule) of the matrix experiences the abrupt volume change between dilute solid solution and hydride phase. Thus the matrix is expected to have a smaller number of dislocations introduced into it, as compared to a fully cycled sample. Be-

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cause of the smaller dislocation density in a partially cycled matrix, the increase in strength (relative to the vacuum annealed matrix) should not be as pronounced as in a fully cycled matrix. Also, the greater the fraction of matrix that experiences hydride formation/decomposition and the accompanying abrupt volume change, the greater will be the number of dislocations introduced into the matrix. Thus, as the palladium matrix is cycled such that higher hydrogen contents are achieved during absorption, more dislocations are generated within the matrix. This finding is consistent with the trends shown with respect to strength, hardness and ductility. In all likelihood, it is this increase in dislocation density that plays a major role in the observed increases in strength and hardness and decreases in ductility that arise in palladium as the matrix progressively traverses the Pd-H miscibility gap during hydrogen absorption/desorption cycling. However, the observed deviations of yield strength and ductility from behavior predicted by the lever rule indicate that the situation may be more involved than a simple enhancement of dislocation density as the miscibility gap is progressively traversed. More work is certainly needed to fully understand the observed results. Conclusions We have found that isothermal hydrogen absorption/desorption cycling of palladium can have a significant effect on the strength, hardness, and ductility of the Pd matrix. The greater the extent to which the miscibility gap of the Pd-H system is traversed during cycling, the greater are the observed increases in strength and hardness as well as the observed decreases in ductility. However, addition and removal of hydrogen such that hydride formation and decomposition is not involved has only a small effect on these properties. These changes in mechanical properties of the Pd matrix point to a correlation between the number of dislocations introduced into the Pd matrix and the extent to which the PdH miscibility gap is traversed during isothermal pressure cycling. The present results however are conflicting with regard to a linear relationship between the extent to which the miscibility gap is traversed and the degree to which the mechanical properties are altered. If hydrogen absorption and desorption is to be used as a tool to manipulate the mechanical characteristics of metals and alloys then the present results indicate that one important and rather easy to control factor may be the extent to which two-phase coexistence regions are traversed during materials preparation. For by controlling this factor, the strengthening that results from the build-up of dislocations (due to the phase transition occurring through the coexistence region) may also be controlled. Acknowledgements This work was principally supported by a Cottrell College Science Award of Research Corporation (grant C-2990R). C.A. acknowledges the support of the Chemistry Scholarship Program sponsored by Dow Chemical Company. References 1. W.M. Mueller, J.P. Blackledge, and G.G. Libowitz (eds.): Metal Hydrides, Academic Press, New York, N.Y., 1968. V.A. Goltsov, Muter. Sci. Eng., &) (1981) 109. V.A. Goltsov, V.M. Dekanenko and N.N. Vlasenko, Mater. Sci. Eng., u (1990) 239. S.H. Goods and SE. Guthrie, Scrip& Me&N. Muter., 26 (1992) 561. J. Shenk, A. Moss, D. Jonsen, S. Nesbit, K. Rebeiz and A. Craft, J. Muter. Sci. I&t., JJ(l994) 496. T. Flanagan and C.N. Park: in Hydrogen Storage Materials, Materials Science Forum, R. Barnes, ed., 1988), chap. 12, p. 291. 7. F.A. Lewis: The Palladium Hydrogen System, Academic Press, New York, 1961.

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