Solid Solubility Metastable Extension ofRare Earth Metals in Gold

Zhou Xinming and Li Qubo

lnstitute ofPrecious Metals, Kunming, Yunnan, 650221, China

Metastable extended solid solubility of rare earth (RE) metals in gold was studied by measuring the latticeparameters of solid solutions of rapidly solidified Au-RE alloys using X-ray diffraction. The cooling rateused for the preparation of the alloys was about 106 K/s. It was found that the metastable solid solubilities ofCe, Sm, Gd, Dy, Er, Yb and Y in gold were 0.3, 0.4, 1.5, 9.0, 13, 13.4 and 10 at% respectively. The solidsolubilities of RE metals in gold increased with decreasing relative difference of atomic radius between theRE metal and gold. The extended solid solubilities of RE metals in Au, Ag, and Cu were compared. Therelatively large solid solubility in gold is explained in terms of electron transfer from the RE atoms to goldatoms. Under the conditions of rapid solidification, the Au-RE alloys tended to form amorphous alloys morereadily than do Ag-RE alloys.

The formation of solid solutions is an important aspectof alloying theory. The Hurne-Rothery, Darken andGurry and other groups have reported significant workon the prediction of solid solubility in alloys. The hasicempirical rules governing the formation of solidsolution alloys are:(1) The 15% size rule: This states that solid solutions

are not to be expected if the atomic size of thesolvent and solute differ by more than 15%.

(2) The electrochemical factor: The electrochemicalnature of the two elements must be similar. Iftheir electrochemical properties are significantlydifferent, compound formation is more likely.Darken and Gurry made use of theelectronegativity to quantify the electrochemicalfactor - if the electronegativity difference betweensolvent and solute is more than +0.4, large solidsolubility is not to be expected.

In the 1960s, Duwez developed the technique ofrapid solidification and found it was associated withthree major effects, ie the extension of solute solubility,the formation of metastable phases, and the formationof metallic glasses (amorphous alloys).

The extension of solute solubility is potentiallyperhaps the most significant of these effects. Not onlydoes its study increase our understanding of theinteractions between alloy constituents but it alsooffers a new approach to the development of alloys

~ GoM Bulletin 1997, 30(2)

with solid solution hardening or age-strengtheningproperties. It is therefore of considerable practicalsignificance.

The authors have previously systematically studiedthe extension of the solid solubility of rare earth (RE)metals in silver, copper and aluminium (l - 3). In thispaper, the metastable extension of solid solubility ofrare earth metals in gold by rapid solidification isdescribed.

EXPERIMENTAL

The alloys used in the present work were preparedfrom high (99.999%)purity gold and pure (99.9%)rare earth metals (RE = Ce, Sm, Gd, Dy, Er, Yb, Y).The alloys were first melted in a boron nitride crucibleunder an argon atmosphere using a high frequencyinduction current. Then a small piece of the alloy wasre-melted and made into thin foils of thickness 0.03­0.05 mm hy an AMHA-2 arc melting hammer-anvilinstallation, made in-house. For the process of re­melting and making the foils, the equipment was firstpumped to vacuum and then filled with high purityargon. The cooling rate for the rapid solidification ofthe Au-RE alloy foils was of the order of 106K/s.

For selected Au-RE foils, chemical analysis wasperformed by inductively coupled plasma atomic

63

14r-----r----,-----"'7'::~---,

LaCc Pc Nd Pm Sm EllGdlb Dy Ho Erlin Vb Lu58 60 62 64 66 68 70

Atomic Number

168 12RE.:u %

4

Figure 2 The maximum solid solubilities C·q(curve 1)and C, (curve 2) ofthe lanthanide metals ingold vs the lanthanide atomic number (thevalue ofCeq is taken from (4))

ec..~ 0.4130 ~--t----=~~~~5;--t---i~a.~ 0.4090 r;;:;~,r-....,...:30.4070 L...-_--I.__-..L...__-L-__...L--.I

1O~---t_---_t_-

~<1

~:3~ 61----+--- -+_-0Vl

2.-----+- --

* Ceq is the maximum solid solubility, slow cooled (equilibrated) alloy; and

C, is the maximum (extended) solid solubility ill rapidly cooled (metastable)

alloy.

Figure 1 Lattice parameters ofAu-rich solid solution vsRE concentration in rapidly solidifiedAu-Jill(RE=Gd, Dy, Er, Yb, Y) alloys

The atomic radius and electronegativity differencesbetween rare earth metals and gold are quite large; therelative atomic radius difference is about 20-30%, andthe e1eetronegativity difference is about 0.7 unit. Thesedifferences are unfavourable for the formation of solidsolutions between rare earth metals and gold accordingto Hume-Rothery or Darken and Gurry criteria. But,in fact, the equilibrium and metastable extended solidsolubilities (Ceq and Cs*) of some rare earth metals ingold are quite large. In Figure 2 the values for Ceq and

RESULTS AND DISCUSSION

The lattice parameters of solid solutions change almostlinearly with solute concentration. When the soluteconcentration goes beyond the solid solubility limit, asecond phase will appear in the alloy but the latticeparameters of the solid solution remain constant. Fromthe relationship between the lattice parameters and thealloy compositions, and the accompanying TEMobservations, the metastable extended solid solubilitiesof RE in gold in alloys obtained using rapidsolidification can be determined.

The lattice parameters of gold-light RE solidsolutions had no obvious increase with the increasingRE concentration in alloys under rapid solidification.The results indicated that the extension of solidsolubility of light rare earths (RE = Ce, Sm) in gold byrapid solidification was negligible. Rider et al (4)studied the solid solubilities of the rare earths in gold.They attempted to estimate the solid solubility limits oflight rare earths in gold from the slopes of the latticeparameter vs composition curves in the single-phaseregion for gold-heavy RE alloys plotted against the radiiof RE metals. Using the same method, we determinedthe metastable extended solid solubilities for Ce and Smin gold, which were found to be 0.3 and 0.4 at%respectively. The dependence of the lattice parametersof gold-rich solid solutions on RE concentration inrapidly solidified Au-RE (RE=Gd, Dy, Er, Yb, Y) alloysis shown in Figure 1. The extended solid solubilities ofGd, Dy, Er, Yb and Y in gold were found to be 1.5, 9.0,13, 13.4 and 10 at% respectively.

ernrssron spectroscopy. Analytical results indicated thatthe actual composition was quite close to the nominalcomposition because loss of RE by vaporization had beentaken into consideration. The listed compositions havebeen corrected in accordance with the analytical results.

The metastable extension of the solid solubilitylimit of the rare earth metal in gold was determined bymeasuring the lattice parameters of gold-rich solidsolutions of rapidly solidified alloy, and verified bytransmission electron microscopy (TEM), if necessary.The lattice parameters of gold-rich solid solutions weremeasured by X-ray diffraction using Cu Kcx 1 radiationwith a D/max-12K diffractometer. The positions of thediffraction peaks were corrected using 99.999% highpurity gold. The lattice parameters were calculated on acomputer using the least-squares method. Thesemeasurements indicated that the method used had highprecision. The TEM used was the JEM-2000EX model.

64 ($9' GoldBulle/in 1997, 30(2)

Table 1 Apparent atomic radii and solid solubilitiesofRE metals in gold

App rent atomicRE metal dll

ra, r nm

Yb 0.1619

Er 0.1656

Dy 0.1712

Gd 0.1772

ra.a.r- rAux 100 Ext nd old

rAu olubilityc; t"

12.3 13."

14.8 13

17.1 9

22.8 1.5

Figure 3 The maximum solidsolubilities Ceq (curve 1 - seereference 4), c:;. (curve 2 )oflanthanide metalsingold vs their relative atomic radius diffirences

value. So the electron transfer that would favour theformation of solid solution occurred in gold-RE alloysonly if the relative difference of atomic radius wassmaller than the critical value. This opinion is inaccord with the experimental values for the Ceq and C,but its physical nature needs further research.

Ce, Sm and Cd have large atomic radii, and theirextended solid solubilities in gold under rapidsolidification are small. But we found that if theirconcentration exceeded 15 at%, these alloys showed atendency to form amorphous material under meltquenching. The X-ray diffraction patterns of meltquenched Au-RE (RE = Ce, Sm, Cd) alloy foilsshowed that the diffraction peaks of the second phasecompound disappeared, and were characteristic oftypical amorphous alloy when RE concentrations were

C, of the lanthanide metals in gold are plotted againstatomic numbers. The Ceq and C, values for the rareearths studied are plotted versus their relative radiusdifferences in hgure 3. It can be seen that both Ceqand C, increase with the increasing lanthanide atomicnumber or the contracting lanthanide atomic radius.The increases were very limited before Cd, but thenincreased approximately linearly with the decreasing ofthe relative atomic radius difference.

Rider et al (4) considered that upon alloying goldwith rare earths, electron transfer from rare earthmetals to gold occurred in the alloys, and that thiswould decrease the size of RE atom, and wouldincrease its clectronegativity. Conversely, it wouldincrease the size of the gold atom and decrease itselectronegativity. These changes in their electronicstructures made the formation of solid solutions morefavourable. Rider also considered that when the sizedifference between the constituent metal atoms wastoo large (>25%), such electron transfer would notoccur. The authors consider that this explanation isreasonable. The apparent atomic radii (ra.a. f ) of the rareearth metals in gold were derived by extrapolation ofthe graph of lattice constant of gold-rich solid solutionvs composition to 100% rare earth metals. Theapparent atomic radii of the rare earth metals in gold­rich solid solutions were smaller than the acceptedatomic radii in the pure state, see Table 1. The relativedifference between the ra.a.r of some heavy rare earthmetals and rAu was already less than 15%. homhgures 2 and 3, it can be seen that the behaviour isdistinctly different after the relative size differenceachieved a certain critical value (about 25%). Both theequilibrium. solid solubility Ceq and the metastableextended solid solubility C, increased fast with thedecreasing of the relative difference of the atomicradius. There was no obvious relationship between thesolid solubility and atomic size difference if the relativedifference of atomic radius exceeded some critical

2t--- f--....

0.20 0.25r - r ,.,.

r ""

La

0.30

§9' GoldBulletin 1997, 30(2) 65

more than 15at%. Figure 4 is the X-ray diffractionpattern of pure gold, and low and high concenttationAu-Sm alloys. In the Differential ScanningCalorimetry measurements on these alloy foils therewas an obvious exothermic peak during theisochronous heating process. But the results of TEMobservation and electron diffraction tests indicatedthat these rapidly solidified foils were mainlycharacteristic of crystalline alloy. These melt quenchedalloys may be in a mixed state of crystal and noncrystal. Johnson (5) had pointed out that Au-La alloysin certain concentration ranges could becomeamorphous alloy through melt quenching. Comparedwith Ag-RE alloys the Au-RE alloys thus showed astronger tendency to become amorphous.

The authors have studied the metastable extensionof solid solubility of rare earths in Ag, Cu and Al (l ­3). Results from this previous work and the presentresearch are given in Table 2. The atomic radii of Au,Ag, AI and Cu are 0.1442, 0.1445, 0.1432 and 0.1278nm, respectively. The atomic radii of Au, Ag, and AIare nearly the same, and the electronegativitydifferences between gold and the rare earths are largerthan those between the rare earths and Ag, AI. But theextended solid solubilities of the rare earths, especiallythe heavy rare earths, in gold, are larger than those inAl and Ag. From the relationship of the extended solidsolubilities of the rare earths and their atomicnumbers, the solid solubilities of Eu and Yb in Ag and

Table 2 Metastable extension (at%) of the solidsolubilities ofRE metals in Au, Ag,Al and Cu

Au

Figure 4 X-ray diffraction patterns ofrapidly solidifiedpuregold, and lowand high concentration Au­Sm alloys

100.00 120.00

Au -II .S I m

AI are obviously smaller than might be expected. Thisis because Eu and Yb showed divalent behaviour whenthey alloyed with Ag and AI, and the difference ofatomic radius was very large. But the solid solubilities(both Ceq and C, ) of Eu and Yb in gold were normal,the solid solubilities of RE in gold changed regularlywith the decreasing of the difference of atomic radius,see Figure 3, if the atomic radius of Yb is taken as themean value of the lutetium and thulium atom radii. Euand Yb, in common with the other rare earth elements,showed trivalent behaviour in their alloys with gold.From the comparison of solid solubilities of the rareearths in Au, Ag, AI, and the fact that Eu and Yb aretrivalent in gold alloys, we considered that the electrontransfer from RE atoms to gold atoms would bestronger than to Ag or Al atoms. This leads to largersolid solubility in gold, and the atomic size is the mainfactor controlling solid solubility when rare earths alloywith Au, Ag, or AI. The atomic radius differencesbetween RE and copper were in the range 35.8-54.7%.This was too large, and was larger than the critical value(25%) needed for electron transfer to occur. So when rareearths alloy with copper, the metastable extended solid

0.95

0.5

1.2

0.75

1.0

0.9

1.1

n Cu

0."0.15

0.21

0.21

0.30

0.5

0.1

0.6

0.6

0.7

0.75

0.2

InAI

2

1.5

2.5

3.0

3.5

0.3

5.0

5.0

5.4

6.0

0.8

InAg

10

15

0."

0.3

9

13

13.4

In u

Pr

Y

La

Er

Yb

Dy

Cc

Nd

Gd

Tb

Sm

Eu

REMetals

66 Cfi9' GoldBulletin 1997, 30(2)

solubilities are small, and there is no obvious relationshipwith the RE atomic size - in this case, the size is not themain factor influencing solid solubility. Other factors suchas the characteristics of the copper-rich intermetalliccompound which is formed and the eutectictemperatures, etc, became more important.

CONCLUSIONS

Under rapid solidification conditions, with a coolingrate of 106K/s, the metastable extended solid solubilitiesof Ce, Sm, Gd, Dy, Er, Yb and Y in gold were 0.3, 0.4,1.5, 9.0, 13, 13.4 and 10 at% respectively. Themetastable extended solid solubilities of rare earths ingold were related to the relative atomic radius differencebetween the rare earth element and gold, and as a result,increased approximately linearly with the decrease ofrelative difference when the relative difference was lessthan 25%. But there was no obvious relationshipbetween the two if the relative difference was greaterthan the critical value (about 25%).

The electron transfer from the rare earth metal tothe gold was thought to occur when the RE alloyedwith gold. The electron transfer was favourable forsolid solution formation. But this transfer would notoccur if the relative atomic radius difference was largerthan the critical value. The extended solid solubilitiesof the rare earth in gold were larger than that in silveror aluminium. It was considered that the electrontransfer from the rare earth to gold was stronger thanthat from the rare earth to silver or aluminium.

Eu and Yb showed divalent behaviour when theyalloyed with Ag and Al. The solid solubilities of Eu, Ybin Ag and Al were distinctly smaller than those of thenearby rare earth elements. But as in the case of theother rare earths, Eu and Yb showed trivalentbehaviour when they alloyed with Au. The solidsolubilities of Eu and Yb in gold changed regularlywith the relative atomic radius difference.

When rare earths alloy with Au, Ag or Al and formsolid solutions, their size is the main factor controllingtheir solid solubility. But when rare earths alloy with

(fi9' GoldBulletin 1997, 30(2)

Cu ;l'nd form a solid solution, the importance of thesize factor decreases, and other factors became moreimportant.

Under rapid solidification conditions, the Au-REalloys showed a stronger tendency to becomeamorphous than was the case for Ag-RE alloys.

ACKNOWLEDGEMENTS

The authors wish to express their appreciation tosenior engineers Wang Zhongming and Li Shilin fortheir assistance in the X-ray diffraction and TEManalysis.

ABOUT THE AUTHORS

Professor Zhou Xinming is currently a Research Fellowin the Institute of Precious Metals, Kunming, Yunnan,China. He has made many original contributions in thefield of precious metals alloy materials, including highpurity gold alloys for the electronics industry. He hasalso carried out substantial research on metastable alloys.

Li Qubo is a member of Professor Zhou's researchgroup. His recent research has been concerned withgold and gold alloys, including their use in themicroelectronics industry.

REFERENCES

N. Yuantao, Z. Xinming and D. Hong, Chin. JMet. Sci. Technol., 1991,7,391

2 Z. Xinming, N. Yuantao, and L. Qubo, J Alloysand Compounds, 1992, 183, 168

3 N. Yuantao and Z. Xinming, Acta Metall Sin.,1992, 28, B95 (in Chinese)

4 P.E. Rider, K.A. Gschneider and O.D.McMasters, Trans. Met. Soc. A/ME, 1965, 233,1488

5 w.L. Johnson, S.]. Poon and P. Duwez, Phys.Rev., B, 1975, (11), 150

67