TECHNICAL INFORMATION

UDC 669.788:699.234:620.172

VARIATION IN THE MECHANICAL PROPERTIES OF HARDENED

PALLADIUM b-HYDRIDE UPON ANNEALING IN HYDROGEN

M. V. Gol’tsova1 and G. I. Zhirov1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 50 – 53, June, 2013.

An experimental technique of obtaining hardened palladium �-hydride consisting in saturation of work-har-

dened palladium with hydrogen under the conditions characteristic of the development of an � � � phase

transformation is developed. Processes of recrystallization and reversal of its mechanical properties are inves-

tigated. The temperature of primary recrystallization of work-hardened palladium �-hydride is established. A

comparative analysis of the plasticity of palladium hydride and of palladium following recrystallization an-

nealing is performed.

Key words: palladium hydride, mechanical properties, recrystallization, annealing in hydrogen.

INTRODUCTION

Palladium and its alloys are widely used in modern engi-

neering. The ranges of application of the substances are

steadily expanding in connection with the development of

hydrogen power engineering and technology. In particular,

such devices as palladium membrane filters are used for

work in media that contain hydrogen or isotopes of hydro-

gen. Such filters are used to achieve deep cleaning and sepa-

ration of hydrogen isotopes [1], separation of hydrogen from

the waste gases of the chemical and petrochemical industry;

sensors to measure leakage of hydrogen from palladium-

based alloys are also used when working with media contain-

ing hydrogen or hydrogen isotopes [2]. The thermodynami-

cally open system Pd – H has been and remains a model sys-

tem for the study of the fundamental laws of the interaction

of hydrogen with materials as a consequence of the relative

simplicity of its equilibrium phase diagram of state. Laws es-

tablished in studies of the Pd – H system have a far broader

scientific and practical value and in many ways are common

to all metal – hydrogen and intermetallic compound – hydro-

gen systems.

Hydrides are usually considered as highly fragile materi-

als. A technique for obtaining the unwork-hardened palla-

dium �-hydride �-PdHx

was first developed in [3]. The tech-

nique consists in saturating annealed palladium with hydro-

gen so as to cause the figurative point in the state diagram of

the Pd – H system to “bypass” the cupola of the two-phase

(� + �) region and keep the material from undergoing a hy-

dride transformation in the course of saturation. It was exper-

imentally established with the use of this technique [4] that

unwork-hardened palladium �-hydride is a very plastic mate-

rial, comparable in terms of properties with annealed palla-

dium.

The objective of the present study is to develop a tech-

nique of obtaining hardened palladium �-hydride and inves-

tigate the reversal of its mechanical properties and recrys-

tallization upon annealing in hydrogen.

METHODS OF STUDY

The mechanical properties of initial work-hardened pal-

ladium (99.98%) in the form of wire measuring 0.5 mm in

the delivered state were first studied. The wire was fabricated

according to a standard technology. Following intermediate

annealing, the palladium was subjected to 95% deformation

on the final stage of drawing. The wire was cut into test sam-

ples measuring 165 mm in length for hydrogen treatment and

mechanical tests as well as into blank samples 40 mm in

length. In the delivered state palladium exhibits the follow-

ing mechanical characteristics: ultimate rupture strength

�ult

= 300 MPa; conditional yield point �0.2

= 240 MPa, and

elongation � = 1%.

Metal Science and Heat Treatment, Vol. 55, Nos. 5 – 6, September, 2013 (Russian Original Nos. 5 – 6, May – June, 2013)

335

0026-0673/13/0506-0335 © 2013 Springer Science + Business Media New York

1Donetsk National Technical University, Donetsk, Ukraine

(e-mail: m goltsova@mail.ru).

Saturation of wire samples with hydrogen and recrys-

tallization annealing in a VVU-3 hydrogen-vacuum plant, a

diagram of which is shown in Fig. 1 [5], were performed.

The plant makes it possible to perform treatment in a vacuum

(about 1 Pa) and in hydrogen (� 4 MPa) at temperatures from

room temperature to 1100°C and to simultaneously measure

the specific electrical resistance of blank samples to monitor

hydrogen absorption of the samples.

The VVU-3 plant consists of a tubular chamber fabri-

cated from stainless steel and hermetically sealed by means

of a flange. A tube through which three types of wire pass

(wires from a thermocouple, wires that remove the voltage

drop from the central part of the wire sample, and wires that

deliver an electric current to the sample) is welded onto the

flange. The pipe with the wires is embedded in thermally sta-

ble epoxide resin and placed in a 1000-W electrical labora-

tory tube furnace. The temperature in the furnace is con-

trolled by means of a VRT-2 temperature regulator and a

UT-2-13 thyristor amplifier.

The vacuum chamber is situated inside a laboratory tube

furnace. The temperature in the chamber is monitored by

means of a temperature regulator. The terminals from the

thermocouple are connected to the input of the temperature

regulator. The terminals of the electrical furnace heater are

connected to a thyristor amplifier. The temperature is main-

tained in the furnace to within � 5°C.

An NVR-1-2 guided-vane rotary vacuum pump is used

to produce a vacuum in the chamber. A tank with hydrogen is

connected in order to create a hydrogen atmosphere in the

chamber. A VIT-2P ionization thermocouple vacuum gauge

is used to control the vacuum level.

An IPS-3-1 stabilized constant-current source is con-

nected to conductors that feed an electrical current to the pal-

ladium sample through a milliammeter. The conductors,

which remove the voltage from the central part of the sam-

ple, are connected to the voltage comparator and the output

of the comparator is connected to a KSP-4-01-UKhL-4.2 re-

cording potentiometer.

Following recrystallization annealing, the samples are

extracted from the VVU-3 plant and subjected to tensile test-

ing according to a standard technique on a RMU-0.05-1 ten-

sile-testing machine, which has a maximum force of 500 N.

The working part of the samples is 100 mm in length. Ten-

sion is performed at a constant rate of displacement of the

sliding clamp of the tension-testing machine of 10 mm�min.

The values of �r, �

0.2, and � are determined from the results

of the tests.

The test samples and blank samples are then placed in

the working chamber of the VVU-3 plant for the purpose of

hydrogen saturation. The chamber of the VVU-3 plant is first

pumped out. The samples are then heated to 130°C. Once

this temperature is reached, hydrogen is fed at a rate of

0.1 MPa�min to the working chamber until a pressure of

2.5 MPA is reached. In other words, the samples are sub-

jected to hydrogen absorption in such a way as to induce

them to undergo a direct hydride phase � � � phase trans-

formation. Isothermobaric holding is conducted once a pres-

sure of 2.5 MPa is attained by controlling the variation in the

specific electrical resistance of the blank sample until com-

plete hydrogen saturation of the deformed palladium is

achieved.

It should be noted that palladium �-hydride is a saturated

solid hydrogen solution in palladium, i.e., in terms of nature

it differs from pure palladium, hence to some extent it is not

correct to compare their properties. Thus, the mechanical

properties of hardened and unwork-hardened palladium

�-hydride were compared. According to previous data [4],

unwork-hardened palladium �-hydride has the following

properties: �r= 200 MPa, �

0.2= 31 MPa, and � = 34%.

Thus, unwork-hardened palladium hydride is a highly plastic

and low-strength material, possessing high � with a low level

of �r

and very low �0.2

. On the whole, the mechanical pro-

perties of un-workhardened palladium hydride obtained ac-

cording to the technique of [3] are close to those of pure an-

nealed palladium.

Using the technique of hydrogen saturation of initially

work-hardened palladium [6] that we have developed it is

possible to obtain �-hydride exhibiting a high degree of

hardening. In fact, it appears that palladium �-hydride in the

work-hardened state possesses the following mechanical

characteristics: �r= 280 MPa, �

0.2= 180 MPa, and � = 1%.

This means that the method of producing palladium hydride,

that is, hydrogen saturation of palladium until the compound

�-PdHx

is attained, with and without hydride phase transfor-

mation, is the determining factor of the effect produced in the

mechanical properties of palladium hydride.

336 M. V. Gol’tsova and G. I. Zhirov

1

28

14

13

13

1517

16

18

12

11

10

9

water

54

7

36

Fig. 1. Schematic diagram of VVU-3 hydrogen-vacuum plant:

1 ) thyristor amplifier; 2 ) precision temperature regulator; 3 ) elec-

trical tube furnace; 4 ) samples; 5 ) thermocouple; 6 ) work chamber;

7 ) blank wire sample; 8 ) constant-current milliammeter; 9 ) stabi-

lized constant-current source; 10 ) recording potentiometer; 11 ) con-

stant-current millivoltmeter; 12 ) R3003 comparator amplifier;

13 ) manometer; 14 ) gates; 15 ) cylinder with hydrogen; 16 ) va-

cuum thermocouple sensor; 17 ) vacuum gage; 18 ) vacuum pump.

RESULTS OF EXPERIMENTS AND DISCUSSION

Recrystallization annealing of each lot (three samples per

lot) of hardened palladium �-hydride was performed imme-

diately following hydrogen absorption directly in the VVU-3

plant. The samples were heated at a rate of 4 – 5°C in hydro-

gen H2

( p = 2.5 MPa) until a given temperature in the range

170 – 700°C was attained and were annealed for 90 min fol-

lowing stabilizing holding (5 min). The working chamber

was cooled together with the furnace down to 150°C without

evacuation of the hydrogen, the samples were extracted,

cooled in open air to room temperature, and immediately

tested on a tensile-testing machine.

The results of the experiments are presented in Table 1. It

is clear that annealing at temperatures up to 225°C does not

practically alter the mechanical characteristics of hardened

palladium �-hydride. Reversal of the mechanical characteris-

tics is observed following annealing at 250°C: �r

decreases

by 5%, �0.2

by 0.5%, while � grows to 5%. Annealing in the

temperature interval 275 – 350°C leads to primary recrys-

tallization and practically complete reversal of the mechani-

cal properties (cf. Table 1). With a further increase in temper-

ature the mechanical properties of palladium hydride un-

dergo only small variations with �0.2

falling to 35 MPa fol-

lowing annealing at 700°C. The reversal of the mechanical

properties of initially hardened palladium hydride is illus-

trated particularly graphically in Fig. 2.

It is very interesting that following recrystallation an-

nealing, palladium hydride possesses higher plasticity

(� = 42%) than does annealed palladium (� = 37%). It is this

which we believe is the principal manifestation of the partic-

ular nature of palladium hydride, as a concentrated solid so-

lution of hydrogen in a transition metal.

In fact, when tensile tests of annealed samples of palla-

dium �-hydride are performed, a so-called “traveling neck,”

in the form of a successive formation of thin parts (“necks”)

along the length of the sample, is observed. The appearance

and mechanism of this effect consists in the following pro-

cess. The first neck to appear in the sample at first, as as-

sumed, successively develops until some limit is reached. A

new neck is then formed, and the process is repeated a multi-

ple number of times. From three to five necks are formed im-

mediately before a rupturing in different samples. We believe

this phenomenon is of the same type as what is known as the

TRIP effect. The essential nature of the TRIP effect in

metastable steels is known to consist in the fact that a phase

transformation that hardens the metal locally develops upon

the formation of a neck in a sample (i.e., in the region of

maximum plastic deformations).

In our case, a local reverse � � �-hydride phase trans-

formation develops upon the formation of the first neck as a

consequence of the resulting stresses and possible local re-

moval of hydrogen from the Pd – H alloy. Local hydro-

gen-phase work-hardening of the material also occurs. As a

result, the development of the first neck is halted. A second

neck next forms in another part of the sample. This process

repeats and, on the whole, elevated elongation of recrys-

tallized palladium hydride subjected to tensile deformation is

realized.

CONCLUSIONS

1. A technique of obtaining hardened palladium �-hy-

dride by means of hydrogen saturation of initially work-

hardened palladium at temperatures much below the critical

temperature of the system Pd – H (Tcr

= 292°C) is developed.

The hardened palladium hydride has the following mechani-

cal characteristics: �r= 280 MPa; �

0.2= 180 MPA, � = 1%.

2. When hardened palladium �-hydride is annealed in

hydrogen, its primary recrystallization occurs in the tempera-

ture interval 275 – 350°C.

3. In the course of tensile tests of palladium �-hydride

annealed at temperatures above the temperature interval of

recrystallization, multiple necks form in the samples as a

consequence of the development of a hydride TRIP effect.

Variation in the Mechanical Properties of Hardened Palladium b-Hydride upon Annealing in Hydrogen 337

300

250

200

150

100

50

0

100 200 300 400 500 600 700 tan

, °C

� � �r 0.2; , ÌPà

�r

�0.2

� 40

30

20

10

Fig. 2. Mechanical characteristics of hardened palladium �-hydride

as a function of annealing temperature in hydrogen.

TABLE 1. Mechanical Properties of Palladium �-Hydride Follow-

ing Annealing in a Hydrogen Medium at Different Temperatures

tan

, °C �r, MPa �

0.2, MPa �, %

170 276 183 1.13

200 285 192 1.5

225 280 186 1.8

250 224 175 5.0

275 193 59 39.0

300 193 53 41.0

350 191 40 46.0

400 190 40 47.0

500 181 39 43.0

600 171 36 43.0

700 168 35 42.0

4. Following recrystallization annealing, palladium hy-

dride possesses higher plasticity (� = 42%) than does an-

nealed palladium (� = 37%). This feature emphasizes the

special nature of palladium hydride as a concentrated solid

hydrogen solution in a transition metal.

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terials 2002 – 2003, Catalogue Alfa Aesar�

, Johnson Matthey,

London (2001), p. 1387.

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posited palladium mesowire arrays,” Science, 293, 2227 – 2231

(2001).

3. M. V. Goltsova, Ya. A. Artemenko, and V. I. Zaitsev, “Kinetics of

reverse � � �-hydride transformations in thermodynamically

open palladium-hydrogen system,” J. Alloys & Compounds,

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338 M. V. Gol’tsova and G. I. Zhirov