b.c.c. transformation temperature of thorium

JOURNAL OF THE LESS-COMMON METALS

I49

THE EFFECT OF CERTAIN RARE-EARTH SOLUTES UPON THE F.C.C./B.C.C. TRANSFORMATION TEMPERATURE OF THORIUM*

J. C. UY**

AND

A. A. BURR

Rensselaer

Polytechnic

(Received

January z8th, 1966)

Institute,

Troy,

N.Y.

(U.S.A.)

SUMMARY The lutetium thorium

individual

effects

of neodymium,

gadolinium,

holmium,

erbium,

and

solutes upon the f.c.c./b.c.c. allotropic transformation temperature of were determined by an electrical resistance method. At small concen-

trations, all the rare-earth solutes raise the transformation temperature of thorium; the effect reaches a maximum and reverses however, at higher concentrations, thereafter. An attempt at correlating the initial effects at low alloy concentrations with the atomic numbers and atomic sizes of the rare-earth solutes is made in this paper.

INTRODUCTION In attempts to develop theories of alloying and to establish and identify the solid-state interactions which underly the existence of phases in the solid state, it has been shown that certain factors, such as atomic size, valence, and electronegativity, play significant roles. For the evaluation of some of these factors, the rare-earth elements

represent

a unique

series of elemental

are similar and whose electronic

configurations

solutes

whose chemical

vary in a manner

properties

much more regular

than can be found elsewhere. Alloys of the rare earths with a transition element that exhibits an allotropic transformation should present an interesting combination. Although, on the surface, it would appear that atomic size is the only significant factor that would change, the possibility of unusual electronic interactions also exists. Because a reasonable range of solubility is desirable, choice of the solute should include the HUME-ROTHERY rules1 as boundary conditions. For these reasons, a study was undertaken using thorium as the solvent and a selected series of rare earths as solutes. This paper presents the experimental results of how the rare-earth solutes affect the allotropic transformation temperature of thorium as detected by electrical resistivity measurements. * This material was submitted in partial fulfillment of the requirements for the Ph.D. at Rensselaer Polytechnic Institute. N.Y. (U.S.A.). ** Present address: Maggs Research Center, Watervliet Arsenal, Watervliet, J. Less-Common

Metals.

II (1966) 149-156

J. C. UY, A. A. BURR

150 MATERIALS

The chemical analyses of the starting materials are shown in Table I. These data represent the best level of purity presently attainable in these metal systems. The particular rare-earth elements were chosen on the basis of their similarities in hexagonal crystal structure, trivalency, non-radioactivity, and availability. They have a well-shielded inner 4f electron shell which is filling with electrons after the 6s shell has already filled. Thorium is a quadrivalent member of the fourth transition series, and undergoes a f.c.c./b.c.c. transformation at 1405°C. TABLE I ANALYSES 0~ STARTING MATERIALS 02 Thorium Neodymium Gadolinium Holmium Erbium Lutetium * n.d. =

Cn

N2

Ni

cw

Fe

C

MgCrCoAl

72 5 22 n.d.
1600 n.d.* 400 2800

3080

(in p.p.m.)

350 270



5 5

(5

(5 6 <5 500 n.d.

c.5 c.5

c.5 <5 7

Y

Gd

n.d.

2.4 n.d.


Tb

Dy

Ho

Ev

2

n.d. n.d. n.d.

<5oo

100

not detected.

Thorium and rare-earth alloys were prepared by triple arc melting in argon atmospheres. Details of the melting procedure were similar to those previously used with zirconium-rare-earth alloys 2. Weight losses after melting were less than 1%. The ingots were cold rolled and swaged to 0.~77 cm diam., from which lengths of rr cm were cut. DETERMINATION

OF THE ALLOTROPIC TEMPERATURES

The allotropic temperatures were detected by sudden shifts in the electrical resistance-temperature curves. Heating was accomplished in a vacuum chamber flushed several times with high-purity, dry argon. The specimen was clamped in water-cooled copper jaws. To avoid buckling of the specimen due to the large thermal expansion in heating from room temperature to beyond r400°C, one of the jaws was connected electrically through a sliding contact wetted with mercury. A large direct current, sometimes reaching zoo A, was passed through the specimen. For temperature measurement, a fine focusing, disappearing-filament type of infra-red pyrometer was sighted on a small hole drilled into the center of the specimen. This pyrometer setup was calibrated against a Pt-Ptrg%Rh thermocouple up to goo”C, and used the allotropic transformation temperature and melting point of thorium reported in the literature, 1405~ and 1750°C respectively, for additional calibration points. To measure the voltage drop across a 0.6 cm central portion of the specimen, pure thorium wires were percussion welded to the specimen. The current flowing through the specimen was measured by the voltage drop across a shunt in series with the specimen. The electrical circuit was therefore a standard one for four-wire resistance measurement. J. Less-Common

Metals,

II (1966) 149-156

Lu

F.C.C./R.C.C.

TRANSFORMATION

TEMPERATURE

After setting up the specimen high-purity,

dry argon, a typical

OF THORIUM

151

and flushing the vacuum

chamber

run would start with an adjustment

thrice with

of the circuit

current to attain about IOOO’C on the specimen. A few minutes were allowed for the temperature, voltage, and current indications to stabilize, then simultaneous readings of these three variables were taken. The current was then increased in small steps to raise the temperature correspondingly. At each step, the steady-state values were recorded. A typical run from IOOO’C to over 16oo’C back down to IOOO’C, for both heating and cooling data, took over an hour. A few specimens were lost due to melting when they were heated beyond

the 01+ fl//3 point.

RESULTSAND DISCUSSION Considering

the small specimen size, the high temperatures

involved,

and the

slow, stepped heating and cooling rates, practical equilibrium conditions were believed to have been achieved. However the results showed that a slight hysteresis, varying from o to 3o”C, and sometimes even slightly negative, occurred at the transformation points. This could be accounted for by the tendency of the measuring system to drift when a specimen

was held at the high fl temperatures

for prolonged

periods of

time. Also, repeat runs on the same alloy under identical experimental conditions showed that the on-heating data were more nearly reproducible than the on-cooling data. Thus, only the on-heating results are reported here.

9M

,000

,100

,200

1300

1400

1500

1600

1700

T, *C Fig.

1. Resistance-temperature

curve

of thorium

on heating.

The resistance-temperature data for thorium on heating are plotted in Fig. I. Ideally, the curve should show a discontinuity at the phase-transformation temperature; however, thorium is sensitive to the impurity contents shown in Table I J. Less-Common

Metals,

II (1966) 149~156

J. C. UY, A. A. BURR

152

fh

I 1100

1.2 ,000

-

I

2.5

Nd

I

1200

,300

I

I

I

1400

1600

1600

1700

T, ‘C Fig. 2. Typical resistance-temperature

curve of thorium-rare

isoo,

I

earth alloy.

IS00

p 1700

1

P

1600-

P 2

1500

2

”

5 f ? -&

@

1400

1300-

12000 L.

---e----k

I5

ATOMIC

PERCENT

Nd

IN

20

Th

Fig. 3. Allotropic

phase boundaries of thorium-neodymium

Fig. 4. Allotropic

phase boundaries of thorium-gadolinium

J. Less-Common Metals,

II (1966)

149-156

5 ATOMIC

system. system.

IO PERCENT

I5 Gd

IN Th

20

F.C.C./B.C.C.

TRANSFORMATION

and consequently

TEMPERATURE

the transformation

I53

OF THORIUM

took place over a small range of temperatures.

Figure z shows a typical run for a thorium-rare-earth to the b.c.c. curve was more gradual, indicating

alloy. The shift from the f.c.c.

a larger range of temperatures

over

which the f.c.c.-b.c.c. transition took place. The oc/oc+ p and the LXf/?/p points were determined by the intersections of the extrapolations of the o( and /I lines with a straight line through the inflection point having the slope at the inflection. Lsoo-

P 1600.-

CY 1300-

1200

0

5

IO

ATOMIC

Fig. 5. Allotropic

phase

boundaries

of thorium-holmium

Fig. 6. Allotropic

phase

boundaries

of thorium-erbium

PERCENT

15 Er

IN

20

Th

system. system.

By a series of such resistance-temperature

curves,

the phase boundaries

of

Figs. 3-7 have been constructed. It can be seen that, in agreement with the prediction of DWIGHT~, all the five rare-earth solutes initially raise the allotropic transformation temperature of thorium. However, a maximum effect is reached at some alloy concentration beyond which the transformation temperatures are lowered by further alloying additions. It can be expected that this lowering of the allotropic temperatures continues to the eutectoid temperature. The cause of the change in slopes of the phase boundaries is not clear. It is doubtful that this is a scavenging effect, however, because the maxima occur at rather high alloy contents of about 5 at.%. The ~/LX+ /I and 01+ /I/o boundaries for thorium-holmium and thorium-erbium systems seem anomalous beyond the initially rising portions. However, redeterminations of the allotropic temperatures of Th-roO/ Ho, Th-5% Er, and Th-6.8% Er alloys proved the results to be reproducible. Although it was felt that at the high temperatures and slow heating rates used, equilibrium conditions were being obtained, the phase boundaries do not have a common tangent at their maxima and it might be argued that this is in violation J. Less-Common

Metals,

II (1966)

149-156

J. C. UY,

154

A. A. BURR

the rules of thermodynamics. It was, however, realized early in the experiments that the allotropic temperatures were very sensitive to small amounts of impurities, such that the alloys used would not show the properties of true binary systems. On the other hand, the transformation points could be reproduced satisfactorily by repeat runs on the same alloy. Therefore, while it is not the intention to present the of

1600

t

5

IO

ATOMIC

Fig. 7. Allotropic

PERCENT

/ 20

I5 Lu

phase boundaries

IN Th

of thorium-lutetium

system.

results found in this investigation as the equilibrium phase boundaries for the true binary systems of thorium and the rare-earth solutes, it is felt that these results serve a useful engineering purpose in showing the effects of the rare-earth solutes upon the allotropic temperature of thorium. CORRELATION

OF THE

INITIAL

PHASE

BOUNDARIES

WITH

ATOMIC

NUMBER

AND

ATOMIC

SIZE DWIGHT~ has predicted the effects of dilute rare-earth solutes upon the allotropic transformation temperature of thorium. Making use of a thermodynamic parameter, which is the difference in the differential heats of mixing of a solute in the polymorphic phases, he was able to show that the allotropic phase boundaries of dilute binary solutions follow a periodicity similar to that of atomic size. The thermodynamic parameter (QB) which DWIGHT used can be related to the form of the

allotropic

J. Less-Common

phase

Metals,

boundaries

by

the

II (1966) 149-156

equation

F.C.C./B.C.C. TRANSFORMATION

where X,

TEMPERATURE

and Xb are the atomic

OF THORIUM

concentrations

155

of the solute in the 01 and in the /3

phases, respectively, in equilibrium at any given temperature. It can be seen from this equation that a negative QB indicates the low-temperature

a-phase

(i.e., X, > X,),

and a positive

stabilization

of

QB, B-phase stabilization

(X,< X0) ; furthermore, the magnitude of this QB indicates the relative slopes of the two phase boundaries. The sensitivity of the thorium-rare-earth allotropic boundaries to impurities, as shown by the existence of a two-phase region at zero solute concentration, has made the calculation of QB values difficult. What follows is based on rough estimates of QB from the partial

phase diagrams

obtained

in this investigation.

Figure 8 shows QB values based on the 1480°C isotherm

vs. the atomic number

of the rare-earth solute. Qualitatively, it can be seen that, in agreement with DWIGHT’S predictions, the f.c.c. structure is stabilized by dilute rare-earth solutes. However,

Fig. 8. Variation of QB with solute atomic number. Fig. 9. Variation of QB with relative atomic radii.

the trend of QB seems to be towards larger negative values with increasing atomic number, in contrast to the negative slope predicted by DWIGHT on the same coordinates. It has been shown that the PAULING valence and the atomic size of the solute are significant factors in allotropic phase stabilizationd. It has further been shown that, in the case of zirconium-based binaries, the allotropic phase boundaries depend primarily upon the metallic valence and to a less extent upon the atomic sizes. The evaluation of the size effect can be made only in a series of binaries wherein the valence effect is constant. Figure 9 is an attempt to evaluate the size effect in these thorium-rare-earth systems. Due to a constant valence difference of one between the solutes and the solvent, the curve is not expected to pass through the zero point, which seems to be the case. Furthermore, the trend seems to be towards smaller negative values of QB with increasing algebraic values of the difference in atomic size. The derivation of the QB equation as employed here was based on the following assumptions : (I)

(2)

QR is independent QB is independent

of concentration of temperature J.

Less-Comnzon

Metals,

II (1966)

149-156

J. C. UY, A. A. BURR

156 (3) ideal conditions exist throughout the reactions. Perhaps better correlations can be obtained if these assumptions correspond to the actual, non-ideal conditions.

were modified

to

CONCLUSIONS

(I) A technique for determining the allotropic transformation temperatures of thorium-based alloys by electrical resistance measurements has been devised. (2) Small concentrations of neodymium, gadolinium, holmium, erbium, and lutetium raise the allotropic transformation temperature of thorium. However, at higher concentrations, the effect reaches a maximum and reverses thereafter. ACKNOWLEDGEMENTS

The authors would like to acknowledge the support of the U.S. Atomic Energy Commission in carrying out this work. Also, the assistance of Mr. DONALD B. JUGLE in design and construction of the apparatus, and the helpful advice of Dr. WILLIAM R. CLOUGH

are gratefullyacknowledged.

REFERENCES AND G. V. RAYNOR, The Structure of Metals and Alloys, The Institute of 1962, pp. 97-11~. zz T. UY. D. LAM AND A. A. BURR, The effect of rare-earth elements on the allotropic transformation of zirconium, Final Report, Contract No. AT (30-1-2159). Rensselaer Pol$technic Institute, Troy, N.Y., April, 1961. 3 A. E. DWIGHT, Allotropic transformations in titanium, zirconium, and uranium alloys, AECD3673, Argonne National Laboratory, Argonne, Ill., Sept. 1953. 4 J. UY, Ph. D. thesis, Rensselaer Polytechnic Institute, Troy, N.Y., 1963. 5 J. UY AND A. A. BURR, The effects of valence and size upon the allotropic phase boundaries of zirconium-based binary systems, accepted for publication in Tra?as. A IME.

I W.

HUMSROTHERY

Metals, London,

J. Less-Common

Metals, II (1966) 14g-156