The effect of hydrogen on the tensile properties of palladium

JOURNAL OF THE LESS-COMMON

419

METALS

Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

THE EFFECT PALLADIUM

OF HYDRQ~EN

R. J. SMITH

D. A. OTTERSON

AND

Nutianal Aeronautics and Space ~~inis~rafian,

ON THE TENSILE

Lewis Research

Center,

PROPERTIES

OF

Clevelund, Ohio 44135 (U.S.A.]

(Received March Ist, 1971)

SUMMARY

The tensile properties of both annealed- and work-hardened palladium wire were measured as a function of hydrogen content. Electrolysis at room temperature was the method used for hydrogen absorption. The yield stress, ultimate tensile stress, and elongation at maximum stress were determined and showed abrupt changes with respect to hydrogen content in the neighborhood of the boundaries of the phase diagram. The dependence of the tensile properties on hydrogen content within a single phase region was interpreted in terms of electronic structure and strain hardening. Auxiliary experiments were performed to substantiate that the formation of the P-phase by the electrolytic absorption of hydrogen produced strain hardening.

INTRODUCTION

Physical properties of some transition metals are profoundly influenced by hydrogen in the metal lattice. Because the hydrogen concentration in palladium (Pd) can be readily varied continuously up to a one-to-one atom ratio and because the atom ratio can be accurately determined, extensive studies have been made on this metal. Some of the properties studied in relation to hydrogen concentration in palladium are : electrical resistivity’ y*, magnetic susceptibility3, crystal structure’, electrode potential’, hardness’, Young’s Modulus’, and breaking strainI. Most of these properties show notable changes at hydrogen to palladium atom ratios (sometimes referred to as H/Pd) near 0.02 and/or 0.55-0.6. These two particular regions correspond to phase boundaries: the a-phase with lattice parameter 3.890 to 3.895 A (389.0 to 389.5 picometers) exists for ratios from zero to 0.02; the b-phase with lattice parameter 4.025 A (402.5 pm) exists for ratios 0.6 to 1.0. Between 0.02 and 0.6 atom ratios, the c1-and P-phases coexist 4s . Both phases are f.c.c. with the hydrogen ions occupying the octahedral (interstitial cube edge) sites at room temperature. While publish~ data1*6.7 on the tensile properties of palladia are incomplete, they do suggest that changes could occur near H/Pd values of 0.02 and 0.6. For example, Kruger and Jungnitz6 have shown that Young’s modulus and breaking strain have a broad maximum near H/Pd of 0.08. Also, Takagi and Sugeno7 state that J. Less-Common Met&, 24 (1971) 419-426

420

R. J. SMITH, D. A. OTTERSON

the ultimate tensile stress (UTS) and elongation increase as H/Pd increases from zero to 0.02*, above an H/Pd of about 0.02 these properties decrease. In both of these studies the data points are not sufficiently close to define well the inflection points on the various tensile property curves. Furthermore, some of the data of these investigators appear to be conflicting. The data of Kruger and Jungnitz6 indicate that hydrogen reduced the elongation of annealed palladium while Takagi and Sugeno7 report that hydrogen charging increased the ductility of as-received palladium. Obviously, more definitive work is needed and that is the purpose of the work reported here. The tests were performed on both annealed- and as-received specimens for H/Pd up to 0.9. We determined values for the yield stress (YS), ultimate tensile stress (UTS) and elongation at maximum stress. The results are interpreted in terms of electronic structure and work hardening. EXPERIMENTAL

Specimen preparation

The Pd wire was supplied by Materials Research Corporation and was 0.25 mm in diameter. These wires had the manufacturer’s stated impurity level, including interstitials, of less than O.Ol’?. Greatly enlarged shadows of the wires indicated that the wire diameter varied by 5% (10% in cross section area). Some specimens were annealed before charging with hydrogen. The rest were charged in the work-hardened condition, as received from Materials Research. The annealing was accomplished by joule heating to.1 120’K in a vacuum of 5 x lo-’ Torr (7 x 10m6 N/cm’). All specimens were then cut into about 8 cm lengths, rinsed in acetone and then in chloroform. Immediately before the hydrogen absorption, the specimens were soaked for several minutes in concentr.ated hydrochloric acid and were carefully rinsed with distilled water. Hydrogen absorption at room temperature by electrolysis has been discussed elsewhere’ and is briefly described here. The specimen was the cathode of an electrolytic cell. The anode was a spectrographic-grade graphite rod. The electrolyte was 0.1 N sulfuric acid containing formalin and sodium sulfide. These additives allowed rapid charging of the specimens to high hydrogen concentrations. The various concentrations were achieved by varying the charging time. Some specimens were given further treatment to determine if any permanent changes occurred in the palladium lattice which were not dependent on the actual presence of hydrogen. These specimens were first charged either to H/Pd-0.02 or to H/Pd >0.55. The hydrogen was then completely removed (as shown by analysis) by aging overnight at room temperature in a mixture of 2 or 3 cm3 of 30% hydrogen peroxide and 100 cm3 of 0.1 N sulfuric acid’. The effect of handling the annealed specimens prior to tensile testing was evaluated through the use of three sets of specimens. The first set was cold worked more than under normal handling by deliberately bending the specimens. The second set had normal handling. The third set of specimens was annealed (some at 1200°K) and extreme care was used to prevent cold work of the portion of the specimen that was critical for the tensile testing. The value reported in their publication was 0.2. However, an author informed us that this value is a misprint and should be 0.02.

l

J. Less-Common

Metals,

24 (1971) 419426

EFFECT OF

H

ON TENSILE PROPERTIES

OF

421

Pd

Tensile testing An Instron Tensile Testing apparatus was used ; the specimen was held by two

steel grips. One grip was attached to the load cell and the other to the movable crosshead of the tensile machine. The wire was wrapped once around a 6.25 mm diam. post on each grip and was secured to the grip by tightening the end under a bolt. The length of the test section (that is, the length of wire between the post centers) was 25.4 mm. The crosshead speed was 2.54 mm per minute. All tensile tests were done at room temperature and were allowed to proceed until the specimen broke. The data were obtained in the form of load-elongation curves on strip chart recordings. The ultimate tensile stress (UTS), the yield stress (YS), and elongation at maximum stress were obtained from these strip charts. The YS is the stress corresponding to 0.2% permanent extension. The errors associated with the noise in the amplifier and to shifts in the calibration of the Instron are no more than 2%. The maximum error in elongation measurement due to tightening of the wire around the posts is estimated to be a maximum of 20% of the reported values. Determination

of the hydrogen

concentration

The H-Pd atom ratio was determined in the two portions of each specimen immediately adjacent to the point of rupture. The portions used were from 3 to 5 mm long. The method of analysis has been discussed previously’. In brief, the method consisted in desorption of the hydrogen from the specimen at 573°K for $ h in an evacuated quartz container. The gases were expanded into a mass spectrometer which was used to determine the amount of hydrogen. The amount of palladium was determined by weighing the discharged specimen. The error in the H/Pd values is primarily due to uneven charging along the specimen. This error was estimated by comparing the H/Pd obtained for the two segments of the specimen adjacent to the point of fracture. The difference in these values was never greater than 5% of the reported H/Pd for all the specimens where the H/Pd > 0.1 and about 15 % for the specimens where the H/Pd < 0.1. In other words, the error is about 0.003 for the H/Pd near 0.02 and less than 0.03 for the H/Pd near 0.55. These uncertainties were deemed acceptable for this work. RESULTS

Most of the data are presented as graphs in which the ultimate tensile StreSS (UTS), the yield stress (YES),and the elongation at maximum stress is plotted against the H-Pd atom ratio (H/Pd) (Figs. 1,2, and 3). The UTS and YS are expressed in ksi (thousands of pounds per square inch) and newtons per square meter. The elongation is expressed as percent elongation. The YS curves for annealed and work-hardened palladium are shown in Fig. 1. The curve for annealed palladium shows a sharp rise as hydrogen increases up to H/Pd near 0.02. Between H/Pd values of 0.02 and about 0.55, the slope is not as steep. Above H/Pd of about 0.55, the change in hydrogen concentration does not appear to influence the YS. The .data for work-hardened palladium show that the YS is much greater at all values of H/Pd than for annealed palladium. The two curves show opposite effects in the region of H/Pd above 0.02. The YS of annealed palladium shows an increase before becoming constant. The YS J. Less-Common

Metals,

24 (1971) 419426

422

R. J. SMITH, D. A. OTTERSON

(a)

“r

(b)

6OI 0

0.2

H-Pd atom ratio Fig. 1. (a) Effect of H/Pd on yield stress for annealed annealed palladium.

0

0.2

0.4 0.6 H-Pd atom ratio

0.8

palladium.

0.4 0.6 H-Pd atom ratio

0.8

1.0

(b) Effect of HjPd on yield stress for un-

1.0

Fig. 2. (a) Effect of H/Pd on ultimate tensile stress for annealed tensile stress for unannealed palladium.

H-Pd atom ratio

palladium.

(b) Effect of H/W

on ultimate

of work-hardened palladium shows a gradual decrease as H/Pd goes from about 0.02 to 0.9. Figure 2 illustrates the effect of hydrogen on the UTS. The UTS for workhardened palladium was much greater than for annealed palladium over the entire hydrogen concentration range. However, both curves were very similar in appearance, that is, there is a sharp increase in I-ITS as H/Pd increases from 0 to about 0.02. The UTS then decreases as H/Pd increases to about 0.55. Above this value the UTS appears to be constant with increasing hydrogen concentration. Figure 3 illustrates the effect of hydrogen on the elongation at fracture. The curves for work-hardened- and for annealed palladium appear to coincide with each other and have the same general shape as the UTS curves. It should be noted that work-hardened Pd and /I-Pd had essentially no ductility. The elongation shown occurs almost totally in the elastic region. Table I presents tensile data for hydrogen-free (H/Pd < 0.0004) palladium that had been subjected to various treatments. The treatments for annealed palladium include normal handling, special handling to minimize cold work, special handling to increase cold work, and complete hydrogen removal at room temperature from J. Less-Common

Metals, 24 (1971) 419426

EFFECT OF

423

H ON TENSILE PROPERTIES OF Pd (b)

0.2

0.4 0.6 H-Pd atom ratio

0.6

1. 0

II

0.2

0.4 0.6 0.8 H-Pd atomratio

LO

Fig. 3. (a) Effect of H/W on elongation at maximum stress fox annealed palladium. (b) Effect of HjPd on elongation at maximum stress for unannealed palladium.

TABLE I TENSILE

PROPERTIES

OF PALLADIUM

Specimens containing no hydrogen (H/Pd ‘Z0.0004) Specimen history ajter annealing

YS (ksi)*

UTS (ksif*

Elongation

10-15

24-26

6-18

(%)

Uncharged Regular handhng Uncharged Minimum cold work Uncharged Enhanced cold work Discharged from a-phase Discharged from p-phase

6-13

18-23

7-14

14-15 15-16 33-54

2421 22-23 34-54

6-12 6-10 2-4

As-Received palladium

YS

UTS

Elongation

(ksi)

(ksi)

(%)

71-80 91-92

75-80 92-97

3 4-5

Uncharged Regular handling Discharged from P-phase * ksi is converted

to N/m2 by multiplication

by 6.895( 10)6.

specimens after charging to either the a-phase (~/Pd~O.O2) or the @-phase (Ii/Pd 7 0.55). The general agreement in the data for uncharged palladia with varying amounts of cold work indicated that normal handling procedures did not adversely affect the specimens. The desorption experiments for a-Pd yielded data which were identical with that for the uncharged specimens. The UTS and YS for palladium desorbed from the &phase were significantly higher than for annealed palladium, and the elongation was less. Table I also lists data for work-hardened specimens. The treatments for these work-hardened specimens were normal handling and complete hydrogen removal at room temperature after charging to the B-phase (H/Pd 20.55). Higher YS and UTS were also noted in the desorption experiments for work-hardened specimens. J. Less-Common

Metals, 24 (1971) 419-426

424

R. J. SMITH, D. A. OTTERSON

DIfXXJSSION

From the data, it is obvious that the tensile properties are highly dependent on the phase involved. In the a-phase region, an increase in hydrogen concentration is accompanied by an increase in the value of the tensile properties studied. In general, the curves indicate that the tensile properties are not affected by changes in the hydrogen concentration within the P-phase region. [The YS of work-hardened palladium appears to continue to decrease with increasing H/Pd in the P-phase (Fig. 1 (b)). However, the scatter in the data probably obscured the small change in slope to horizontal that is needed to support the other observations concerning the a-phase.] Because the effect of hydrogen on the tensile properties depends on the phase involved, we shall discuss the properties of the a-phase and the P-phase independently. We assume the mixture rule can be applied when both phases are present, i.e., for 0.025 H/Pd 5 0.55. The a-phase

The increases for YS and UTS observed in the a-phase are typical of what is termed solution strengthening. Two important considerations for solution strengthening are changes in lattice parameter and electron/atom ratios. Recent work shows a linear increase in YS with increasing electron-lattice site atom ratio*. Because the hydrogen atom enters into the palladium lattice interstitially, the presence of the hydrogen electron increases the effective electron to lattice site atom ratio. In keeping with this, we observe a linear increase in YS with this increase in electron-lattice site atom ratio. Also, in a-Pd, there is a small and linear increase in lattice parameter with increasing H/Pd in agreement with the recent ideas’. The increasing elongation with increasing H/Pd for cr-Pd agrees with the work of Takagi and Sugeno’ for as-received palladium. We show somewhat similar data for annealed palladium. However, Kruger and Jungnitz, who did work with annealed palladium report no data in this region (between H/Pd=O and 0.08). Thus far, we have been unable to determine the cause for the increased elongation. The fact that both YS and elongation increase with increasing H/Pd leads to a similar dependence of UTS upon H/Pd. The j-phase

There are two aspects of the P-phase to be considered. The first is the effect on tensile properties due to the addition of hydrogen within the P-phase. The second is the relative behavior of tensile properties in the /?-phase with respect to the behavior of these properties in a-phase. In general, the Figures show that a change of hydrogen concentration within the P-phase region has little or no effect on the tensile properties. In the b-phase region, we have an NaCl type structure. An increase in the hydrogen content decreases the vacancy concentration of the hydrogen lattice. Because the hydrogen now occupies a lattice site, the addition of hydrogen is no longer equivalent to an increase in the electron-lattice site atom ratio (as it was for the a-phase). Furthermore, the lattice parameter in /3-Pd (4.025 A; 402.5 picometers) is relatively constant. For these two reasons we are not surprised to find the absence of solid solution strengthening. Before comparing the results for cr-Pd with those for P-Pd, we would refer to J. Less-Common

Metals, 24 (1971) 419-426

EFFECT OF TABLE

H

ON TENSILE PROPERTIES OF

Pd

425

II

Metal

Ni CU

85 81

355 338

Pd Ag

110 68

460 284

Pt Au

127 92

531 384

Table II. In this Table, we show a comparison of the cohesive energies of the transition metals, Ni, Pd, and Pt with their respective, nei~boring noble metals, Cu, Ag, and Au9. In each case, the noble metal has exactly one more electron than its corresponding transition metal. The Table shows that each transition metal has a higher cohesive energy than its respective noble metal. Because the d-band of j?-Pd is assumed to be lilled’*3*4,/I-Pd is electronically similar to the noble metal, Ag a-Pd is still a transition metal. Thus we speculate that the filled d-band of fi-Pd tends to decrease its cohesive energy with respect to ol-Pd. With this decrease in cohesive energy, the decrease in YS observed in Fig. 1 (b) is expected. In Fig. 1 (a), however, we find that for annealed palladium, the YS of the J?phase exceeds the YS of the E-phase. This must be due to some phenomenon which opposes the expected decrease in YS caused by filling the d-band. We believe this phenomenon to be strain hardening caused by deformation of the lattice when hydrogen enters and converts E-phase to P-phase. This deformation is due to the large increase in lattice parameter in going from a-Pd (389.5 pm) to ,&Pd (402.5 pm). At least some of this strain is permanent and remains in the lattice after hydrogen desorption. This is somewhat supported by the data in Table I. As seen in this Table, palladium, obtained by desorbing hydrogen from the P-phase, exhibits increases in YS and UTS. The values of YS and UTS, obtained by desorbing hydrogen from initially annealed Pd, approach the values of the work-hardens Pd. We attribute these higher values of YS and UTS to strain hardening. Work-hardened palladium also shows this strain hardening effect after hydrogen is desorbed from the b-phase but to a lesser degree than the annealed palladium. This is because the work-hardened palladium already has a large amount of strain in its lattice prior to charging. We further note that when annealed palladium is desorbed from the cl-phase, the work-hardening effect is absent. Thus, on re-examining Fig. 1, the relative values of the YS for the Mand p-phases must be considered to be the combined effect of work hardening and the aforementioned electronic structure. In addition to this data, earlier works”*” also are interpreted in terms of strain occurring because of the p-phase. These works include: X-ray diffraction, electrical resistivity, hardness, thermoelectric power, and internal friction. The brittleness found in the elongation curves for @-Pd is associated with the severe strain hardening caused by the formation of the P-phase. One effect of this brittleness is that the YS and UTS coincide for H/Pd N 0.55. Thus, for work-hardened palladium, both YS (Fig. 1 (b)) and UTS (Fig. 2(b)) decrease with hydrogen content J. Less-Common Metals, 24 (1971) 419-426

426

R. J. SMITH,

D. A. OTTERSON

in the two-phase region. In contrast, for annealed palladium, YS increases while UTS decreases with hydrogen content in the two-phase region.

REFERENCES 1 F. A. LEWIS, The Palladium-Hydrogen System, Academic Press, New York, 1967. 2 D. A. OTTER~~NAND R. J. SMITH, NASA Tech. Note D-5441, 1969. 3 N. F. MOTT AND H. JONES,The Theory of the Properties of Metals and Alloys, Dover Press, London, 1936. 4 ‘G. BAMBAKIDIS,R. J. SMITHAND D. A. OTTERSON,Phys. Rev., 177 (1969) 1044. 5 R. J. SMITH AND D. A. OTTERSON,in Proc. Conf Aerospace Structural Mater., NASA, SP-227, 1969, pp. 269-278. 6 F. KRUGER AND H. JUNQNITZ,Z. Tech. Physik, 17 (1936) 302. I S. TAKAGI AND T. SUGENO,Japan. J. Appl. Phys., 4 (1965) 772. 8 B. CHALMERS,Physical Metallurgy, Wiley,.New York, 1959, p. 169, 171. 9 F. SEITZ, Modern Theory of Soli&, McGraw-Hill, New York, 1940, p. 427. 10 R. J. SMITH, A. I. SCHINDLERAND E. W. KAMMER, Ph_vs. Rev., 127 (1962) 179-183. 11 F. A. LEWIS, G. E. ROBERTSAND A. R. UBBELOHDE,Proc. Roy. Sot. (London), Z2OA (1953) 279-289. J. Less-Common Metals, 24 (1971) 419426