Study of metallurgical and electrodeposited Au-Co metastable solid solutions

Journal

of the Legs-~ornrno~

~e~aZs, 96~1984)

249-256

243

STUDY OF METALLURGICAL AND ELECTRODEPOSITED METASTABLE SOLID SOLUTIONS

ZBOU XINMING*, Forschungsinstitut

Au-Co

H. R. KHAN and Ch. J. RAUB fiir Edelmetalle und Metallchemie,

7070 Schwtibisch Gmiind (F.R.G.)

(Received April 10,1983)

Summary Au-Co metastable solid solutions were prepared by spin, water and oil quenching from the melt and by electrodeposition techniques. The solid solubility and structure were investigated using X-ray diffraction, scanning electron microscopy and optical metallography. The electrical resistivity and microhardness of the samples were measured. The relation between the quenching rate and the solid solubility of cobalt in gold is discussed.

1. Introduction Electroplated thin films of gold alloys containing small amounts of nickel or cobalt are used in the electronic industry as contacts. The physical and mechanical properties of these electrodeposited alloys differ substantially from those of their metallurgical counterparts owing to the presence of impurities incorporated in the electrodeposits. In Au-Co these impurities are present in the form of metastable solid solutions [l, 21. The equilibrium solid solubility of cobalt in gold is about 23.5 at.% at 996 “C, corresponding to a lattice parameter of 4.00 A, and is about 0.2 at.% Co at 400 “C [3,4]. The solid solubility of cobalt in gold is increased by fast quenching, and a maximum solid solubility of about 42.00 at.% Co is obtained for cooling rates in excess of 108 K s-l [S]. In these experiments a molten Au-Co alloy was ejected by a helium pulse onto a lightly polished copper strip on the inner periphery of a rotating wheel. The sample thickness was less than 10 pm. The solid solubility of cobalt in gold at room temperature can be substantially increased to lo-12 at.% by electrodeposition [SJ. Gust et al. [7] investigated the precipitation of up to 20 at.% Co in Au-Co solid solutions by water quenching. Some data on the physical properties of Au-

*Permanent

address: Institute of Precious Metals, Kunming, Yunnan, China. 0 Elsevier Sequoia/Printed

in The Netherlands

250

Co solid solutions, such as the resistivity and the magnetic susceptibility, are available. Hurd and McAlister [S] have measured the electrical resistivity, magnetoresistance and Hall effect of both quenched and annealed Au-Co alloys containing between 1 and 15.6 at.% Co at temperatures between 2 and 250 K. The residual resistivity of quenched samples at 4.2 K increases with increasing cobalt content up to 4.5 x lo-’ Q cm for about 16 at.% Co, whereas the residual resistivity of alloys annealed at 300 “C for 4 h remains constant at 0.3 x 10s7 $2 cm. Low temperature (20-300 K) resistivity measurements on electrodeposited Au-O.OBwt.%Co have been reported. The analysis of these data was based on the assumption of electron scattering by impurities [S]. Investigations of metastable Au-Co solid solutions prepared by spin, water and oil quenching from the melt and by electrodeposition are reported in this paper. The solid solubility, structure and microstructure were investigated using X-ray diffraction, scanning electron microscopy (SEM) and optical metallography. The microhardnesses and electrical resistivities of alloy samples prepared using various methods were measured between 65 and 300 K and were compared with each other.

2. Experiments The quenched samples were prepared from gold (purity, 99.95%) and cobalt (purity, 99.99%). Alloys with nominal cobalt contents of 2,9,15,20,27,40 and 50 at.% were melted in a vacuum or in an argon atmosphere using an induction method. The weight losses during melting were negligible. Rapid quenching from the liquid state was carried out using three different techniques: (1) water quenching, (2) oil quenching and (3) spin quenching. Water quenching was performed by melting the sample in an argon-filled quartz tube using an induction method and then forcing the sample under argon pressure through a small orifice into brine. The cooling rate for this technique was about 103-lo4 K s-l. Oil quenching was performed by melting a small piece of alloy in a vacuum of about lo- 5 Torr using a tantalum tube furnace held vertically and then dropping the sample into a Diffelen oil bath (cooling rate, about lo’--lo3 K s- ‘). Spin quenching was performed by using pressurized argon gas to eject a melted sample onto a copper wheel of diameter 20 cm rotating at 4000 rev min-‘. The cobalt content of the Au-Co solid solution was calculated from the lattice parameter which was determined using X-ray diffraction techniques. Temperature-dependent resistance measurements were made between 65 and 293 K using a four-point probe. Microstructural investigations were performed using optical and SEM techniques. The microhardness under a load of 25 gf was measured on the microsections used for the microstructural investigations. Supersaturated Au-Co solid solutions were also prepared by the electrodeposition technique using an electrolyte of the following composition: malic acid, 150-200 g 1-l; KOH, 110 g I-‘; gold (added as KAu(CN),), 8 g 1-l; cobalt

Annealed

Spin quenching

Oil quenching from the melt

Water quenching from the melt

Treatment

_

4.040 4.041* 0.004 4.043 kO.003 4.045 f 0.007

4.056 i 0.005

4.048 +0.004

Au-Co solid solution’ Au-Co solid solutionC

Au-Co solid solution’

4.054 * 0.004 4.047 If:0.005

Au-Co solid solution’ Au-Co solid solution’

8 7.5

9

9

54 7

z-2

(at.%)

(A) 4.068+0.003

Co content of AuCo solid solution

Lattice constant of Au-Cu solid solution

Au-Co solid solution

Phase”

aThe existence of the Au-Co solid solution was confirmed by X-ray diffraction examination. bnisdefinedbyR= R,+AT”. ‘Plus other phases.

Au

Au 0 73C00.27 Au 0.&o,.,,

20

Auo.&o,.,, km%

Au o.sscoo.o2

(at.%)

Alloy composition

Properties of Au-Co alloys prepared by various methods

TABLE 1

0.15

0.84 0.81 0.71

0.72

0.73

0.85

Resistance ratio R(65 K)/R(293 K)

1.6 (65-95 K) 1.1(95293 K)

0.6 (65-293 K) 0.9 (65-293 K) 1.1(6!%120 K) 0.8 (12&293 K)

1.0 (65-110 K) 0.70 (llG-293 K)

0.80 (65130 K) 0.50 (13C293 K)

0.60 (65-293 K)

nb

252

(added as CoSO,.7H,O), l-10 g I- ‘; pH 4.2. The variation in the cobalt content in the bath affects the cobalt content in the electrodeposited Au-Co samples. The qualitative variation in internal stress in the electrodeposited samples was checked by the X-ray diffraction linewidth because the width of the X-ray diffraction peaks is related to the internal stress. 3. Results and discussion The starting compositions of the quenched samples are shown in Table 1. After quenching in brine the Au,,,,Co,,,, alloy is a single-phase solid solution, as determined by X-ray diffraction examination, Other samples quenched from the melt, which contain 9, 15, 20, 27 and 50 at.% Co, are multiphase, e.g. Au~.~~Co~.~~ shows a multiphase structure (Fig. l), with a solid solution phase constituent containing about 6 at.% Co as determined from lattice parameter measurements. Samples of various compositions quenched from the melt in Diffelen oil also show a solid solution phase containing 5-6 at.% Co as shown in Table 1. Alloys of various compositions quenched from the melt in brine show a solid solution phase containing 7-8 at.% Co. However, when the samples were spin quenched (cooling rate, about lo’--lo6 K s-l), the solid solubility of cobalt in gold was about 9 at.%. Two samples with compositions Au,~s,Co,,,, and Au 0.73C00.27 showed two solid solutions containing about 9 at.% Co and about 20 at.% Co when quenched at a cooling rate of about lo6 K s-l or less. Klement [5] has reported that the solid solubility of cobalt in gold can be increased to 42 at.% by using a special spin quenching technique where the cooling rate exceeds lo8 K s- l. These quenched samples are small flakes of thickness less than 10 pm. A comparison of our values with the data of Klement [5] shows that the solubility of cobalt in gold in the solid state cannot be increased to more than 8 at.% by quenching at a rate of about lo5 K s-l. Our alloys, which were quenched at a cooling rate of lo6 K s- 1 or less, contain an Au-Co solid solution with a maximum cobalt concentration of about 20 at.%, whereas Klement’s Au-Co solid solution which was quenched at a cooling rate in excess of 10’ K s- ’ contained 42 at.% Co. Moreover a comparision of the samples quenched using various techniques (water, oil and spin quenching) at various cooling rates shows that the solid solutions contain similar cobalt contents (about 6-9 at.%). However, when the cooling rate exceeds 105 K s- ’ a solid solubility of about 20 at.% Co can be obtained. A further increase in the cooling rate to about lo8 K s- ’ results in a solid solution containing about 42 at.% Co. This indicates that there is a critical cooling rate above which there is a sharp increase in the solubility of cobalt in gold. The same phenomenon appears to exist in Ag-Cu alloys. The lattice parameters of the Au-Co solid solutions for various samples are also shown in Table 1. The microhardness data for the Au-Co alloys quenched in brine are shown as a function of cobalt concentration in Fig. 2. The microhardness increases with increasing cobalt content up to about 15 at.% and then remains almost constant.

253

Fig. 1. Microstructure

of the Au~.~~Co~,~~alloy.

Fig. 2. Microhardness

of Au-Co alloys quenched in brine.

According given by PT =

PO +

to Matthiessen’s

rule, the resistivity

of a non-magnetic

metal is

PitTI

where p0 is the residual resistivity, which depends on the impurities and the lattice defects, and p,(T) is the temperature-dependent contribution and is equivalent to the ideal resistivity. It is caused by the interaction of conduction electrons, lattice vibrations etc. Figure 3 shows plots of the resistance ratio R( ~)/~(273) as a function of temperature between 65 and 293 K for water- and spin-quenched Au-Co alloys. There is a marked change in the temperature dependence of the resistance ratio of pure gold on addition of cobalt. The temperature dependence of the resistance is determined using the equation R = R, + AT" from the slope of logarithmic plots of R - R. uer.sus T. The value of n for pure gold is 1.6 between 65 and 95 K and it changes to 1.1 between 95 and 293 K. The value of n for the solid solution obtained by water quenching is 0.6. The values of n for Au,.,&o,.,,

TEMPERATURE

Fig. 3. Resistance Au 0.98coo.W; AU o.,$om;

0,

ratio of Au-Co Au,.,&o,.m;

0, hm,~o,.,d.

0,

alloys

between

Au,.,&o,.m)

65 and 293 K after

IK!

(a) water quenching (El,

and (b) spin quenching

(I,

Au,.s&o,

20; 0.

254

other alloys containing Au-Co solid solutions together with other phases are listed in Table 1. They vary from 0.50 to 1.1 in the different temperature ranges. Owing to the multiphase nature ofAu-Co alloys containing more than 2 at.% Co, the exact mechanism of the resistivity behaviour cannot be analysed. Table 2 shows the lattice parameters of electrodeposited Au-Co solid solutions. The maximum solid solubility of cobalt in gold from this method is about 13 at.%. The variation in the lattice parameter (solid solution) and the concentration of cobalt in solid solutions (determined via the lattice parameter) is shown as a function of the cobalt concentration in the electrolyte in Fig. 4. The width of the X-ray diffraction peaks of the Au-Co solid solutions increases steadily up to 10 at.% Co and is constant at higher cobalt concentrations. This indicates that stress in the lattice of electrodeposited Au-Co solid solutions increases with the concentration of cobalt in the electrolyte. The average widths of the X-ray diffraction lines are shown in Fig. 5 and are also listed in Table 2.

L

I

I ‘

2 COBALT

6

CONCENTRATION

Fig.4. The lattice solutions. Fig. 5. The average solutions.

IN BATH

parameter

8

IO

and the cobalt

5 &lLT

GRAMSILITAEi

concentration

width of the X-ray diffraction

6

8

9

IN BATH

CdCENTRATId

(t/L)

in electrodeposited

Au-Co

solid

lines of the electrodeposited

Au-Co

solid

TABLE 2 Influence of the cobalt concentration in the electrolyte on the composition, the width of the X-ray diffraction line of electrodeposited Au-Co alloys Sample

Co content in bath k-‘)

Lattice constant of Au-Co solid solution

Co content of Au-Co solid solution

(4

(at.%)

the lattice constant and

Average width of Xray diffraction lines at half-maximum” (-1

1

1

2 3 4 5

2 4 6 10

’ 1 cm is equivalent to 1”

4.062 k 0.002 4.041* 0.001 4.035 *0.004 4.029 * 0.006 4.024 &-0.003

2.5 8.5 10 11 13

8 12.5 17 18.5 19

255

(b) Sample 1

Sample 2

Fig. 6. (a) The surface microstructure alloys (Au Ka radiation).

Sample 4

Sample 3 and (b) the gold distribution

for electrodeposited

Au-Co

TABLE 3 Sample Temperature ofelectrolytic bath (“C) Co concentration in electrolyte (g l- ‘) Co concentration in solid solution (at.%)

1 35 2 8

2 55 4 10

ENERGY Fig. 7. Cobalt distribution

in electrodeposited

Au-Co alloys (Co Ka radiation).

3 55 6 11

4 55 10 13

256

The surface microstructures and various compositions electrodeposited Fig. 6 and Table 3. As shown by the X-ray these alloys is homogeneous within qualitative analysis of cobalt in Au-Co X-ray analysis is shown in Fig. 7.

the distribution of gold in alloys of at various temperatures are shown in photographs, the distribution of gold in the sensitivity of the method. The alloys determined by energy-dispersive

Acknowledgment The financial support of the Ministerium fiir Wirtschaft, Mittelstand und Verkehr Baden-Wiirttemberg to one of the authors (Z.X.) during the period of his stay is gratefully acknowledged. References E. Raub, Plating Surf. Finish., 63 (1976) 29. E. Raub, Plating Surf. Finish., 63(1976) 30. E. Raub and P. Walter, 2. Metallkd., 41(1950) 234. M. Hansen and K. Anderko, Constitution of Binary AZZoys, McGraw-Hill, New York, 1958, p. 195. W. Klement, Trans. MetaZZ. Sot. AIME, 227(1963) 965. J. Socha, E. Raub and A. KnBdler, Metalloberfliiche Angew. Elektrochem., 27(1973) 1. W. Gust, R. Wurster-Scheiffele and B. Predel, J. Less-Common Met., 79(1981) Pll. C. M. Hurd and S. P. McAlister, Philos. Mug. B, 42(1980) 221. H. R. Khan, T. Muramaki and Ch. J. Raub, MetaZZoberfldche, 33 (1979) 102.