The constitution of alloys of gold and mercury

JOURNAL OF THE LESS-COMMONMETALS

THE

CONSTITUTION

C. ROLFE*

OF

GOLD AND MERCURY

ANDW. HUME-ROTHERY

Department of Metallurgy, (Received

OF ALLOYS

I

University of Oxford, Oxford (Gt. Britain)

October 3rd, 1966)

SUMMARY The complete constitutional diagram of the system Au-Hg has been determined by thermal analysis and X-ray diffraction methods, supplemented by microscopical work on gold-rich alloys. Much of the work was carried out in sealed tubes, and the pressure

was an undetermined

variable.

The gold-rich

alloys contain

two

intermediate phases denoted 01’ and <. The al-phase is formed by a peritectic reaction at 419°C; it has a hexagonal crystal structure with lattice parameters a=8.736 A and c = 9.577 A, and the unit cell contains

36 atoms. The c-phase

tally at 388°C. A phase AuzHg was observed appears to be of fixed composition and c = 17.20 A. In contrast was found for the existence

at temperatures

; it is hexagonal

to the conclusions of any intermediate

is formed peritectibelow IZZ”C, and this

with lattice parameters

a = 13.98 A

of some earlier workers,

no evidence

phase richer in mercury

than AuzHg.

INTRODUCTION Work on the system Au-Hg has been summalised by HANSEN AND ANDERKO~ whose phase diagram is shown in Fig. I. In the supplementary (1965) volume by ELLIOTT~, it was concluded

that Fig.

I

was still the most probable diagram,

that RAYSON AND CALVERT~ had reported as 19.1 and 16.1 at.%

except

the limits of the gold-rich n-solid solution

Hg at 416°C and 225’C,

respectively.

These results were not,

however, conclusive because RAYSON AND CALVERT used other workers’ lattice parameters for two-phase alloys, although they showed this work to be incorrect in the single-phase region. In view of the incomplete nature of Fig. I, the complete diagram

has been redetermined.

EXPERIMENTAL DETAILS The alloys were prepared from spectrographically-pure pure gold, both supplied by Messrs. Johnson, Matthey & Co. The liquidus of gold-rich alloys was determined by alloying being contained in an evacuated silica capsule made * Present address:

Metallurgy

Division,

National

Physical

mercury, and 99.99%Ltd. thermal analysis, the with a small-diameter,

Laboratory,

Teddington,

Middx.

(Gt. Britain)

J. Less-Common

Metals, 13 (1967) I--IO

C. ROLFE, W. HUME-ROTHERY

2

re-entrant thermocouple sheath, similar to that used by MURPHY~ for Ag-Hg alloys. For each experiment 30 g of gold, mixed with the required weight of mercury, were very sIowly heated to slightly above the freezing point of the alloy; by this means explosions were prevented. For subsequent cooling and heating curves, rates of I’---2’C/min were used over the critical ranges. The defects of these methods were that the pressure over the alloy was undetermined and the melt could not be stirred.

‘0 Mercuryfat.%)

Fig. I. The system Au-.Hg; according to

HANSEN.

Slight discrepancies existed between the cooling and heating-curve arrests for peritectic horizontals and, in view of the absence of stirring, the heating-curve results were preferred. For mercury-rich alloys, a small, nichrome-wound resistance furnace was pIaced inside a beaker containing asbestos wool for heat insulation, and the whoie immersed in a vacuum jar containing solid carbon dioxide. This enabled heating and cooling curves to be taken over the range -60°-3oo’C. Thermocouple calibrations were made against the melting points of pure Au, Ag, Zn, Pb and Hg. After the experiments, the thermal-analysis ingots were analysed chemically by Messrs. Johnson, Matthey & Co. Ltd., and the authors must express their thanks to Dr. F. M. LEVER for his care and attention. The synthetic and analysed compositions were in excellent agreement, and no contamination by silicon occurred. The ol-solidus was determined by the microscopical examination of specimens quenched from successive temperatures. The a-solvus was determined microscopically, using polarised light, but distinction between the B’- and r-phases was impossible because of the difficulty of etching gold-rich alloys. 1. Less-Commm

Mel&,

13 (1967)

I--10

CONSTITUTION

OF ALLOYS

3

OF GOLD AND MERCURY

The various gold-rich alloy phases were distinguished by the disappearing-line X-ray method. Some specimens were prepared by the interdiffusion of liquid mercury into gold filings. It was found that ingots prepared by the quenching of liquid alloys exhibited marked porosity, and ingots were therefore prepared by slow cooling followed by homogenisation and strain annealing at the required temperature. Powder taken from these ingots gave results in excellent agreement with those obtained by the interdiffusion method, but the parametric method of obtaining phase boundaries was made difficult or impossible by the overlapping of lines from neighbouring phases when CuKa radiation was used. All of the critical specimens examined by X-ray and microscopical methods were chemically analysed. For mercury-rich alloys, specimens were prepared by filing in liquid nitrogen and were then transferred to their silica capillaries. These specimens were used for exposures in a modified Unicam 19 cm, high-tenlperature, Debye-Scherrer camera. The actual X-ray specimens were sent for analysis but, owing to their small size, the analytical results are accurate to only about + r%,. It was, however, essential to use the analytical values because slight and uncontrollable loss of mercury occurred during the sealing of the capillaries. EXPERIMENTAL

RESULTS

It is to be emphasised that the present results involve an undetermined

b Heating

cut’ve

v Cooling curve 0 Solid phase

900

0 Liquid and solid phases

3

a_600

s

Is

500

I!

~I~-: 419*c

8 400

300

.

200

1

122Y

100 0

-100

.

0

>

1C

20

30

40

50

1

I

I

1

I

60

70

80

90

100

.Mercury(at.%)

Fig. 2. The system

Au-Hg;

according

to present

work

J. Less-Common

Metals,

13

(1967)

I-IO

C. ROLFE, W. HUME-ROTHERY

4

pressure variable, and that the constitutional diagram presented is not necessarily that of true equilibrium at any one pressure. This uncertainty of the pressure effect may be responsible for some of the discrepancies between results of previous investigators. The complete diagram obtained in the present work is shown in Fig. 2, whilst Fig. 3 shows the results for the gold-rich phases, and Fig. 4 those for alloys 200 ,

,

-10011’ taf

20 Mercury

22 (at.%)

24

26

-100

60

(b)

’

30

,

”

’

,

( ‘

,

1

’

’

’

I

40 50 Mercury (at%)

70

80 Mercury

I 1

,

’

’

60

’

70

90 (at %)

Fig. 3. The system Au-Hg; gold-rich alloys. 0 Liquid plus solid; o single-phased, X-ray points full and microscopical points open.

q

two-phased;

Fig. 4. The system Au-Hg; mercury-rich alloys. V Cooling curve arrest; A Heating curve arrest; R two-phase solid (X-ray points full, microscopical points open); 0 liquid plus solid (X-ray points full, microscopical points open).

of higher mercury content. The liquidus points (see also Table I) are accurate to kz”C, and show the addition of mercury necessary to cause a fall in the liquidus and solidus curves from the melting point of pure gold to a value of 419’fO.5”C, at which temperature the solubility of mercury in gold is 19.8 at.% (RAYSON AND CALVERT 19.1 at.%). Previous workers have given widely differing results for the solubility of mercury in gold, and the present results are in good agreement with those of OWEN AND ROBERT+. The present peritectic temperature is in good agreement with that given by HANSEN, but the second peritectic horizontal (a’+ Liq 1: c) at 388°C is 14°C lower than that given by HANSEN, although it is higher than the value (380°C) given by PARRAVAN05. The compound AueHg is formed by a peritectic reaction at IZZ~C, and no evidence was found for the horizontal at 310°C claimed by some previous workers. The present diagram is remarkable for the relatively-sudden, steep fall in the liquidus which occurs between 90 and 92 at.% Hg.

CONSTITUTION TABLE THERMAL

OF ALLOYS

5

I ANALYSIS

composition (aLo/, Hg) -.

OF GOLD AND MERCURY

RESXJLTS FOR

Themal

.-

_

.Xu-Hg

ALLOYS

arrests

(“C) ______ (C = COOLIA~G, H = HE.4TI.VG.) c H C C

C

H

H ._ _- --.-

H

.___._

1051

4.1 5.2 8.1 9.8 II.1

1030 998 984 978 958 940 893 861 768

13.0 14.8 18.3 IO,9 25.2

4T9 417

419 4’9

379

388

4’8 408

479 419

384 377

388 388

408 407

4’9 4’9

376 376 369

388 388

122

28.1

680

30.2

62Q

35.1 37.9 .pJ. 2

567 514 469 418 375 351 328

45.0 50.0 55. ’ 59.9 (‘4.9 70.0 75.1 79.7 85. I

4’9

92.5

._

122

-39

-39

I20

180

I”1

146 148 191

-38 -39 --39 -39 -39 -39 -39 -39 ~- 40 -- 39 -~ 39 --39 - 39 -39 -39

- 37 -37 -- 39 -38 -37 -38 -38 -39 -39

122 122 100

123

103

123

102 99 98

202

172 129

122

109

321 308 303 292 290

go.0

94.Q 96.7 97.8 99.1

C

122

130 I22

99 IO0

-. .-. _._

..__ ,.-. _-. ._

~_

-_

Earlier investigators have not distinguished between the CC’-and <-phases, and this fact is responsible for some of the discrepancies between thediagramssuggested previously. The X-ray diffraction film showed a large number of lines, some of which were overlapping. Thirty-six lines could be separated and measured, and the O-values could be indexed on the basis of a hexagonal cell with ra=8.736 A and c=9.577 A. The density was 18.8 g/cm3 and showed that the unit cell contained 36 atoms. Figure 5 shows the atomic volumes of the a-(f.c.c.), LY’-,and <- (c.p.h.) phases as a function of composition, and it will be seen that the points for all three phases lie very nearly on a straight line. From this it may be concluded that the s’-phase is close-packed. The atomic diameters of gold and mercury are 2.88 and 2.99 A, respectively, so that the mean atomic diameters of an alloy containing zo at.% Hg is approximately 2.9 A. A normal c.p.h. cell would, thus, have lattice spacings a=~.9 A, c=z.9 x 1.63=4.73 A, and the a’ cell is therefore three times as wide and twice as high as this. The X-ray atomic scattering factors of gold and mer-

C. ROLFE, W. HUME-ROTHERY

6

cury are too nearly equal for the relative positions of the two kinds of atom to be determined. The [-phase

At 15o”C, the composition limits of this phase lie between 21.3 and 25.8 at.% Hg, and therefore include the whole number ratio AuaHg. The c.p.h. structure found by MAssALsK17 was confirmed, and Fig. 6 shows the present results for the a and c parameters compared with those of MASSALSKI;the present results indicate a moremarked increase in both parameters with increasing mercury content. The 5 alloys were gold in colour whilst those of the AuzHg phase were silver.

2.91sI2.9lEl-

17.7-

2.9li

x a - phase 0 a’- phase

2.9lEi-

3 2.91:j-

+ g - phase

0

I%617.5n^ ” E 1?42 9 .v 17.3E 0 2 17.2-

2.9lLLI2.91: >_ 2.91:

- 4.808

- 4.805 l

Present

0 Massalski

17.1-

-4.804 _ 4.803 - 4.602

1.65( 2 u

work

1.645 1.641 ,,,6

Mercury(at%)

Mercury

(at%)

Fig. 5. Atomic volumes of gold-rich alloys. Fig. 6. Lattice parameters in the c-phase of Au-Hg.

The AuzHg-phase

The present results showed the composition of this phase to include the whole number ratio AuzHg, and any variation in composition was less than -t_I at.% from the ideal ratio. The X-ray data are given in Table II, and the lines can be indexed to agree with a hexagonal cell with a = IX.98 A and c = 17.20 A. The d-values did not agree with those given by the A.S.T.M. card index file, or with the values of WINTERHAGER AND SCHLOSSER~. DISCUSSION

The system Au-Hg follows the general alloying trends shown by the systems J. Less-Common

Metals, 13 (1967) I--IO

CONSTITUTION OF ALLOYS OF GOLD AND MERCURY TABLE INDEXING

7

II OF THE

PHASE

AuaHg

The X-ray film used to measure the z&values was taken at room temperature with filtered CuKa radiation and the specimen contained 45 at.% Hg. The 20 calculated values were computed assuming a hexagonal cell with a = 13.98 and c = 17.20 .& Lint

Intensity

Index

2tL1r.

006 206 5oo 324 330 502

31.197 34.036 37.136 38.639

so. w

w S

w

w VW

w

w

207 503 334 227 4’6 432 514 520

d

31.199 34.620

2.8669

37.128

2.5897 2.4209

38.587

1.3301

38.641 38.628 2.2777 2.2260

39.566

39.583

40.524 44.248

40.436 44.210

45.073

45.103

2.0113

46.799

46.757 46.850

1.9411

2.0469

46.815 46.861

w w

522 440

48.080

48.110

I.8923

II

52.357

52.353

I.7474

12

w

2,0,10

55.580

55.541

I.6535

IO

13 14 15 16

533

55.492

703

55.492

w w

727

60.909

60.862

1.5rog

632

61.836

61.765

1.5002

w

634

64.869

1.4373

550

66.950

64.857 66.929

m

I7 18 I9 20 21

w w w

m w

22

23 24 25 26

27

4,2,10

66.920

68.155

68.210

1.3578

68.166

813 806

70.312

70.341

I.3388

3,1,12

71.518

71.575

I.3192

72.845

I.2975 I.2822

3.3>11 539

72.901 72.916

73.903

709

73.903

904

73.803

727

74.969

74.804

735

74.990

911

75.042

I,O,14 3,0,14

VW

751

\v

422x13 I,O,II 2,O,I5

1.2668

74.937

650

W

w

I.3976

66.893

724 812

78.142 81.847 83.636 84.588

78.202

1.2231

81.976

1.1768

83.560

1.1562

84.606

1.1456

84.494

86.301

1.1272

86.352 86.258

657 10,1>3 925 917

86.342 86.308 86.258

J. Less-Common

Metals,

13 (1967)

I--IO

8 TABLE

C. ROLFE, W. HUME-ROTHERY II (Continued) Intensity

Index

28

w

1o,1,4 739 932

88.027

87.955 88.051 97.981

I. 1095

29

w

7n2.11 760 927

91.759

91.685 91.706 91.836

1 .o739

30

w

5,o,I5

96.186

96.122

1.0358

Line

NO.

Au-Ga, Au-In and Au-Sn, in all of which the composition range up to 20-30 at.% of solute includes a series of phases with close-packed structures. Beyond this concentration of solute, intermediate phases exist which are of fixed, or very nearly fixed, composition, and often correspond with simple atomic ratios e.g. AuGa, AuGaz, AuIn, AuIne, AuSn4 and AuzHg. The first five of these give rise to maxima on the liquidus curves, whereas AuzHg decomposes peritectically. This difference may be related to the electrochemical factor, since the electronegativity difference in Au-Hg (Ax =0.5) is less than those in the other systems-Au-Ga (Ax =0.8), Au-In (Ax=o.7) and Au-Sn (Ax=o.6). Silver and gold have almost the same atomic diameters, and are in the same group of the Periodic Table. It might, therefore, be expected that the phase diagrams of the systems Ag-Hg and Au-Hg would show similarities as do the diagrams for the systems Ag-Cd and Au-Cd. Actually, as can be seen from Fig. 7 this is not the case. The a-f.c.c. solid solution in the system Ag-Hg extends to 35 at.% Hg, i.e. further than the whole sequence of a-, LX’-and [-close-packed phases in the system Au-Hg. There seems no doubt, therefore, that the tendency to form a sequence of close-packed phases just outside the limits of the a-phase is a characteristic of the gold atom*, as distinct from those of silver and copper. It seems improbable that this is related to the electronegative nature of gold, since theocl phases in the systems Au-Ga and Au-In have random arrangements of the atoms, whereas ordering would be expected if the electrochemical factor was responsible for the formation of these phases. It is possible that the greater polarisability of the gold ion is of significance. It is also of interest that neither b.c.c. ,!?nor ordered /3’ equiatomic phases are found in the systems Au-Hg and Ag-Hg, although they are prominent in Au-Cd and Ag-Cd. It seems that, although there is a typical y-brass phase, AgjHg8, the mercury atom is unable to form a characteristic + b.c.c. electron phase, and this may be related to the abnormal valency characteristics of mercury, as shown in the mercurous compounds. * The system Cu-Hg is of unfavourable size-factor and its equilibrium diagram cannot be compared simply with those of the systems Ag-Hg and Au-Hg. However, in the systems Cu-Zn and Cu-Ga where the size-factors are favourable, there are no close-packed phases immediately outside the a-solid solutions. J. Less-Common Metals, 13 (1967) I-IO

CONSTITUTION

OF ALLOYS

c

OF GOLD AND MERCURY

.-__ 900

BOO700\ 600-

\

500Y 5 400‘;;

a

$300!

2001000

0 t

-100 1 0

30

50

(at.%)

Mercury

60

70

80

90

1

-1001

0

)

’ 10

’ ’ ’ ’ 20 30 40 50 Cadmium (at.%)

’ 10

’ ’ ’ ’ 20 30 40 50 Cadmrum (at %I

’ 60

’ 70

1 80

’ 90

boo-

”

L)

al

5 500E t E 4oo f 300200looO-

0

10

20 30 40 50 Mercury (at.%/.)

Fig. 7. The systems

Ag-Hg,

60

70

Ag-Cd,

80

Au-Hg

90

I

-1001

0

’ 60

’ 70

’ 80

’ 90

10

and Au-Cd

ACKNOWLEDGEMENTS

The authors must express their thanks to the Council of the Royal Society for financial assistance in connection with the present work. The Science Research Council are also thanked for the award of a Research Studentship to C.R. J.

Less-Common

Metals,

13 (1967)

I-IO

C. ROLFE, W. HUME-ROTHERY

IO REFERENCES I 2 3 4 5 6 7 8

M. R. H. A. N. E. T. H.

HANSEN AND K. ANDERKO, Constitution of Binary Alloys, McGraw-Hill, P. ELLIOTT, Constitution of Binary AZZoys, First Supplement, McGraw-Hill, W. RAYSON AND L. D. CALVERT, J. Inst. Metals, 87 (1958) 88. J. MURPHY, J. Inst. Metals, 46 (1931) 507. PARRAVANO, Gazz. chim. ital.. 48(ii) (1918) 123. A. OWEN AND E. A. O’D. ROBERTS, J. Imt. Metals, 71 (1945) 213. B. MASSALSKI, Acta. Met., 5 (1957) 541. WINTERHAGER AND W. SCHLOSSER, Metall., 14 (1960) I.

J. Less-Common Metals, 13 (1967) I--IO

New York, New York,

1958. 1965.