Some dilatometric observations on the α-β transformation in zirconium-212 wt. % niobium alloy

JOURNAL

OF THE

LESS-COYMON

SOME DILATOMETRIC IN ZIRCONIUM-@

METALS

OBSERVATIONS

wt. y’ NIOBIUM

ON THE &I

TRANSFOR~~ATION

ALLOY

The effects of repeated heating and cooling on the dilatometric curves in the region of the fi +/3 t ransformation range have been investigated with particular reference to the effect of oxygen contamination at reduced pressures. Data have been obtained on specimen length changes, instantaneous thermal expansion coefficient, dilation change at transformation and transition temperatures. The Goldschmidt expansion on cooling from just below the (m -t /3)//I transition in a vacuum of 2 x 10-6 mm Hg is ten times greater than when the specimen is cooled from just above this transition, whilst the final length change is -I% elongation compared with N ~"/b contraction in the second case. Repeated d: + @ cycling leads to a reduced dilation change and small specimen elongation. The structural implications are discussed. Oxygen contamination at 3 x 10-4 mm Hg virtually eliminates the dilation change through transformation. The mechanism of formation of horizontal portions to the cooling curve is discussed and on cooling is attributed to the interaction of the transfornling outer layers with the still contrasting untransformed p core region. Pressure variations in the range 10-6-10-4 mm Hg have an important influence on transition temperatures.

One of the alloys of potential interest for fuel-element cladding and pressure tubes in water-cooled nuclear reactors is Zr-z+ wt. “/o Nb. Unlike Zircaloy-z and related alloys, Zr--2.& wt.% Nb can be substantially strengthened by heat treatment. This and its excellent corrosion resistance are attractive features which have been reviewed elsewherel-4. The zirconium-niobium constitutional diagram was derived by ROGERS A?;D point at ATKINS~. A monotectoid horizontal exists at -600°C with the monotectoid 17.5% Nb. (The difference between volume oh and weight y. in the Zr-Nb system at 20% Nb is only 0.3%6.) The maximum solubility of niobium in a-zirconium was found J. Less-Commm

Metals, 8 (19G5) x95--208

G. F. SLATTERY

196 to be 6.4 at. y0 Nb5. A monotectoid

loop of j3zr and &,

is present and below the mono-

tectoid horizontal, the phases szr + && are in equilibrium. RICHTER et al.6 considered that the system is effectively a ternary Zr-Nb-0 for technical materials and they set the solubility of Nb in a-Zr at between 1.5 and 2.0 wt. %, the actual value being sensitive to oxygen content. BICHKOV et a1.7, using dilatometric techniques, placed the monotectoid temperature lower at 560°C with a Nb content of 12 %. KNAPTON~, using chiefly metallographic techniques, placed the solubility of Nb at less than I at. o/0at the monotectoid

temperature

of 610’. The high value of solid solubility

of Nb in a-Zr

is associated with the use of “dynamic” techniques such as electrical resistivity and dilatometry which involve the interpretation of curves, whereas the lower values have been obtained

by “static”

methods,

value of 570°C for the monotectoid This

paper

describes

i.e. X-ray

diffraction

temperature

and metallography.

was also obtained

the dilatometric

and structural

The low

dilatometrically.

results

of cycling

the

Zr-24 “/oNb alloy through the a-/3 transformation range. Oxygen (and also nitrogen) are known to raise the transformation temperature in zirconium9 and it is necessary to ascertain the extent of this effect on the transformation of this alloy since it is likely to be used in the heat-treated

condition.

EXPERIMENTAL PROCEDURE Material The Zr-2:

wt.%

Nb alloy specimens

were machined

from a single batch

of

rod, prepared from billet by hot forging between 850” and 950°C followed by hot rolling and annealing at 700°C. The analysis of the important is given in Table I.

elements present in the material

TABLE I CHEMICAL ANALYSISOF ZIRCONIUM-Z;

Wt.

o/o

NIOBIUM

ALLOY

(Oxygen analysis on the rod was 620/660 p.p.m.) Ingot analysis Element Et. Top Middle Bottom

%)

;t.

2.IO

0.05

2.57

0.05 0.05

2.58

%)

&m.)

2P.m.)

40 -

-

30

IO

IO

60

60 65

Ti

AL

(P.B.m.)

(P&m.)

40 40 40

25 25 25

Sfionge analysis Element

Ni

C

P

B

Si

Zn

Cr

Cu

Mn

Cl

Mg

p.p.m.

35

71


0.2

50

<50

107

25

Ig

<50

<50

Dilatometer The thermal dilation of the Zr-2s wt. o/0Nb alloy test specimen was measured in vacua in a Chevenard-type dilatometer where its dilation was compared with a standard of pyros, a non-oxidisable, heat-resistant alloy of known dilation. The J. Less-Common

Metals, 8 (1965) Ig5--208

~~TRANSFO~~~~TION

IN Z-2;

dilation

photographically

was recorded

temperature.

The dilation

temperature

wt.

Xb

was also measured

ALLOY

197

on Kodak

of the pyros

bromide

provided

by a Pt/Pt-Rh

welded bead between the Zr24 Both the pyros standard diameter

1;

foil card as a function

a scale of temperature

(107~) thermocouple

but

of the

lying with its

“/bNb specimen and the pyros. and the test specimen were 50 mm long and 0.36 mm

and they were designed to provide the minimum

of surface contact

with the

silica tubing to avoid sticking. The specimens were inserted in V-shaped grooves in a silica support and enclosed by a transparent outer silica sheath which was attached to the vacuum sensitivity,

chamber.

Silica

non-differential

coefficient

pusher

rods connected

dilatometric

of Zjo mm per IOOO”C and a dilation

The dilatometric

a temperature

amplification

head carried an optical tripod which consisted

fitted on three hardened length. The vertex

actuated by the sliding rods. The specimen and standard a range up to 1100’ & 2T.

triangle

coefficient

of x 300.

of a thin invar plate

remained

stationary,

whilst the other

to the plane of the tripod and were

were heated in an electric resistance

furnace

The heating and cooling rates were maintained

130°C per hour using a high-sensitivity pointer

with a high amplification

steel points and equipped with a concave mirror of long focal

of the right-angled

two points of the tripod moved perpendicularly

indicator

the test specimen

head having

dilation

system

to actuate

with

constant

moving in front of a drum fitted with a profile of the heating

cooling cycle. The specimen temperature,

at

a temperature

as measured by the thermocouple,

and

provided

an accurate check on the furnace temperature. The apparatus was evacuated to 2 x IO-” mm Hg as measured by high-vacuum gauges fitted inside the vacuum chamber and a controlled

leak facility

was provided in the vacuum

chamber.

The temper-

ature points on the dilation curve were taken using a potentiometer attached to the specimen thermocouple. The transition temperatures were read from thr dilation curve with a precision

of &SC.

Metallogvajhic

preparation speThe samples of Zr-z 2r O/ ,0 Nb alloy were sectioned from the dilatometric cimens and prepared by conventional grinding followed by diamond paste polish to $ micron grade. The specimens

were etched in a solution ot 35 ml HN03,45

ml distilled

water and IO ml HF for about 30 sec. Chemical analysis The analysis

for surface

oxygen

together

with bulk oxygen

content

was by

vacuum fusion. Bulk oxygen samples were sawn from the specimen after r/32in. of the oxide surface had been filed off and duplicate tests were made. Surface oxygen samples were taken as sectors of the original specimen to give an oxide surface of approximately 0.25 cm".

The volume of oxygen associated

with

0.25

cm2 of the surface was obtained

as the difference between two measurements which had precisions of &IO%. Unfortunately, the results were not sufficiently precise to be of muchvalue comparison

purposes

because

of the difficulties

inherent

in the technique

for

and it

would be necessary to increase the ratio of surface area to weight to produce more meaningful results. Only a few of the results were statistically significant in so far as a surface oxygen-rich of these significant

region was positively

identified

and, where appropriate,

some

results are quoted. .I. Less-Commo~zMetals, 8

(1965) 195-208

G. F. SLATTERY

198 RESULTS

Sequence vacuum

I:Heating

to just below the upper

of 2 x 10-6 mm

transformation

temperature

(a + B)/p in a

Hg Zr-2$

The as-received o/o Nb was heated at the constant rate of r3o”C/h to 865°C and cooled to room temperature at the same uniform rate. The dilation curves for heating and cooling are shown in Fig. I. On heating, the dilation curve showed the normal expansion with temperature corresponding to the (a + BN~) phase region until at 575” (point a on the curve) an increase in slope signified the first formation of ,9zr. The slope increased slightly until 600°C was reached when the curve decreased suddenly in slope (point b) and flattened out to show zero dilation between 600’ and 685°C (point c). Above 685°C the curve showed the normal Goldschmidt contraction to represent the transition of oL-Zr --f p-Zr until heating was stopped at 865°C. The region a + b represents the removal of the ,!?~b constituent while the flat portion b + c represents the resolved component of the competing expansion and contraction coefficients corresponding to the relative proportions of untransformed 01and transforming a-Zr. On cooling from 865°C which is still in the 01 + pzr region, the curve starts to show a specimen expansion immediately due to m formation. It crosses over the heating curve and continues expanding until the transformation is complete at 540°C. The portion represented by b-c on the heating curve is missing from the cooling curve. The transformation contraction on heating produced a dilation change (Al/L) of 1.0% whilst on cooling a much greater dilation expansion (- x 2) was obtained. A permanent elongation of 0.4 mm (0.8%) on the specimen length was produced on cycling back to room temperature. Cycling up to 3 times produced a cumulative extension in the specimen length 1 of 2.7%. On heating, the instantaneous thermal expansion coefficient at 400X, derived from the dilation curve and in the specimen major axis, was 5.08 x 10-6 deg C-i whilst on cooling, it was 6.04 x 10-6 deg C-i for the same temperature. The as-received material had a fine two-phase (x + ,%b microstructure (Fig. 2). After cycling back to room temperature, the microstructure remained similar but was slightly coarser. Data on length change, instantaneous thermal expansion coefficient and transition temperatures are given in Table II. Sequence vacuum

2: Reheating

of 2 x

to above the upper

transformation

temperature

(& + /3)/b in a

IO-~mm Hg

Reheating to 880°C and above (up to ~ooo”C), gave a similar heating dilation curve to the first cycle but in this case, transformation went to completion at 870°C. On cooling, transformation to n-Zr started at 830°C (point d) and the dilation showed an expansion to 740°C when this expansion was followed by a flat portion of zero dilation change. At 715°C the flat portion was changed to a gradually occurring contraction until a change in the curve marked the end of transformation at 57o”C, i.e., O(Z~ + Bzr + B it, --f cczr + /?~b. The dilation curves for heating and cooling are shown in Fig. 3. The extent of the contractual dilation on heating through the transformation remained the same as in the first sequence but the expansion on cooling was reduced J. Less-Common Metals, 8 (1965) Ig5--208

‘99

Fig. I Sequence I. The dilation curves for heating to just below the Temperature in “C. \Tacuum L x 10-6 mm Hg.

(x-1 ,$/,!Itransition and cooling.

TABLE

II

HEATING

COOIJNG

ON

No.5 : Subsequent heat into ,!? phase. Vacuum on heating and cooling 3 x 10-4 mm Hg

10-4

Zero

+o.o3

No.4: Reheated into /I phase region. Vacuum on heating 2 x mm Hg ; vacuum on cooling 3 x mm Hg

10-6

+o.o4

+o.o3

No.3: Heated as above into the B phase region. Vacuum 2 x 10-3 mm Hg

Spec.

-0.58

A

No.2 : Heated above the upper -0.51 transformation temperature ((w+ B)//I.Vacuum 2 x 10-3 mm Hg

Spec.

B

-

6.04

5.08

x

x

Heating

10-6

10-6

x

x

8.19 x

8.36

6.04

Cooling

10-3

10-3

IO-~

THERMAL

0.1

0.7

0.7

11.5

11.3

x

x

x

x

x

Heating

10-3

10-3

10-3

10-3

10-3

x

x

10-3

10-3

10-3

x IO+

x

0.5 x

I.7

2.0

2.5

23.4

COOliPZ~ 10-3

EXPANSION

Dilation change (AZ/Z) at transformation

INSTANTANEOUS

ALLOY

LENGTH, o/o Nb

Instantaneous thermal expansion coefficient (“C-l at 4oo’C)

Wt.

SPECIMEN

OF L?k--2:

THE

Change in length per cycle (mm) Al

TEMPERATURES

AND

+o.44

TRANSITION

No.1 : Heated to just below the +o.38 upper transformation temperature ((w+ fl),/Band cooled to room temperature. Vacuum 2 x 10-6 mm Hg

Cycling sequence

AND

OF SUCCESSIVE

TRANSFORMATION

EFFECT

N.D.

610

575

575

575

(4

855 885

870 885

875

870

(d)

Finish

(d)

915 865

855 835

845

830

(4

Start

(al

Start

(“C)

AT

655

600

585

570

540

la)

Finish

CHANGE

Cooling

temperature

DILATION

Heating

Transition

COEFFICIENT,

q!3 TRANSFORMATIOS

IN

Zr-z& wt. yb Nb ALLOY

201

by a factor x IO compared with the cooling change on the first cycle (cf. Figs. I and 3). The expansion on cooling was in the ration -3 : 13 compared with the contraction on heating (Table II). This resulted in an overall length contraction after cycling to room temperature of ---0.55 mm (Table II). The instantaneous thermal expansion coefticient on cooling at 400°C was increased to 8.36 x 10 6 deg C 1 (Table II), and this increase implied a crystallographic reorientation of the transformed 3 Zr to increase the component of its “C” axis into the specimen axis. The microstructure on cycling to room temperature was quite different from the first sequence and now consisted of acicular needles of transformed ,X with a characteristic 90 degree orientation to each other and tending to lie with their major axes in the transverse section of the specimen (Fig. 1). Sequence

3: .Seco& reheat i&o the ,8 bhase region ifa a uacuuna of 2 x IO-~ mm Hg On heating, the curve changes in slope at 575°C to represent the initial formation of ,Bzr; this temperature is in agreement with the previous cycles. The slope of the curve (Fig. 5) decreases continually until at 765”C, a small flat portion of zero dilation occurs before the transformation becomes marked with the characteristic contraction to pz,. Transformation is complete at S75’C. On cooling hysteresis is found and transformation starts at 855°C and finishes at 6oo°C, although a slight flat por-

G. F. SLATTERY

202

d Fig. 3. Sequence 2. The dilation curves for heating to just above the (a + /I)//I transition and cooling. Temperatures in ‘C. Vacuum 2 x 10-6 mm Hg.

tion to the curve was noted in the region of 730% which may represent the first stages of formation of the Nb-rich phase. There was only a slight increase in final macroscopic length from that of sequence 2 (Table II). The a/p transition on heating and cooling gave dilation changes of a similar order, the cooling change being greater (Al/Z of 2 x 10-3 compared with 0.67 x 10-3, see Table II). Sequence 4: Third reheat into the p phase region. Heating in a vacuum of 3 x IO-~ mm Hg. Cooling in a vacuum of 3 x IO-~mm Hg A small pressure of air was admitted through the controlled leak device whilst the specimen was in the b phase so that it cooled in the slightly reduced vacuum of 3 x 10-4 mm Hg. The dilation curve was very similar to sequence 3. The dilation change on heating through the transformation was unchanged but the dilation change on cooling was reduced slightly and this was attributed to the effect of oxygen contamination (Table II). A small macroscopic increase in length was obtained (Table II). The cross-section of the specimen, originally round, was becoming elliptical in shape. Sequence 5: Cycling into the p phase region in a vacuum of 3 x heating and cooling

The lower vacuum

extended

J. Less-CommonMetals,8 (1965) Ig5--208

the transition

temperature

10-4 mm

Hg fey both

range quite markedly

a-/?

TRANSFORMATION IN _&2$

Wt. ‘;; Nb ALLOY

Fig. 4. .-\lpha needles lying in the transverse section of the c,ptcimen.

Fig. 5. Sequence 3. Dilation Vacuum 2 x 10-6 mm Hg.

203

(x

100)

curves for the second reheat into the /l phase. Temperatures

in “C.

whilst reducing the extent of the dilation change on transformation for both heating and cooling (Fig. 6). This effect produced an additional horizontal portion e-d to the upper part of the curve representing

the start of transformation J. Less-Common

during cooling. This Metals,

8 (1965)

1gyzo8

G. F. SLATTERY

204

flat region can extend up to 50°C from 865” to 915” (Table II) and is associated with the oxygen contamination in the surface region due to the increase in pressure to 3 x 10-4 mm Hg. On cycling a few times the dilation change on transformation can be virtually eliminated and a slight change in slope alone signifies the points of transition (Fig. 7). In this specimen, a comparatively high value of surface oxygen was obtained by chemical

analysis,

namely

295 & 125 and 245 + 104 ~1 per 0.25 cm2 sur-

face although the bulk oxygen figure was also increased but to alesser extent. Removing

i 929

tb)

COOLING

d

91b”&

CURVE

Fig. 6. Sequence 5. Dilation curves after continued cycling into the /I phase region. Temperatures in “C. Vacuum 3 x 10-4 mm Hg.

0.015 in. off the specimen surface layer was sufficient to change the dilation curve towards a more normal appearance showing an expansion (although small) on cooling through the transformation (Fig. 8). There was evidence that the orientation of the OLneedles in the rim region of the specimen, though remaining characteristically orthogonal to each other, altered to lie at 45 degrees to the major axis in the longitudinal section of the specimen (Fig. 9). Some needles ofm showed a “veined” structure associated with polygonisation. A similar phenomenon of polygenisation has been found in transformation in titaninm alloys and correlated

with the presence

J.

8 (1965)

Less-Common

Metals,

Ig5--208

of oxygenlo.

2%

TRUE

Dl‘A7ilON

(nmf s

SCALE

i

D,Lari‘T’*@vnf i49

Fig. 7. Dilation curves on continued cycling into the /?phase region. (+ o c~clcs.) Vacuum 3 x xwJ mm Hg. Temperatures in “C.

HEAT

CURVE

FOR

2nd HEAT

Ist.

CURYE

WiiH

OXYGEN

o-015”

SURF*CE OF

5”RFACE

tONTIWNC.flON ------REMOVSD

-

3XlCrmmMg 3xdmm~

Fig. 8. Effect of surface oxygen contamination on dilation cur\.cs. ‘fcmpcraturcs in “C.

The contraction observed on heating through the transformation range is due to a. decrease in “atomic size” on going to a structure of lower co-ordination number. The interatomic distance in /? zirconium is about 2.6 ‘I&smaller than in hexagonal LX at 862°C~~. The interesting feature of stopping a cycle just short of the upper transition temperature is that on cooling the corresponding expansion is twice as great producing an overall extension at room temperature. This extension can be reversed simply by raising the upper temperature of the cycle by -15~ so as to exceed the transition temperature. The considerable reduction of x IO in the corresponding cooling expansion (compare Figs. I and 3) through transformation results in an even greater contraction of the specimen on reaching room temperature. These dilation changes are measured along the specimen length. Some explanation for these effects lies in the microstructural changes (compare Figs. 2 and 4). The alignment of the transformed a needles to lie in the transverse section will result in a dilation reduction along the specimen length. Once the needles have oriented in this manner, their effect on subsequent cycling behavi(~ur is not very marked until surface oxygen contamination begins to become significant. This causes some re-orientation in the rim regions and

206

Fig. 9. Oriented

G. F. SLATTERY

alpha needles

in the longitudinal

section

of the specimen.

(X

1000)

an angular spread in this re-orientation is thought to be the cause of the developing ellipticity in cross-section. Oxygen raises the transition temperatures of Zr-24 7’ Nb and it would appear that there is enough residual oxygen to contaminate the specimen surface layers, even at the reduced pressures of IO- PIO-5 mm Hg. The effect of this on the dilation curve is to introduce a horizontal portion at the start of transformations on cooling. The explanation is that the surface region transforms first on cooling before the relatively uncontaminated

bulk and the expansion

of the transformed

outer layers is counter-

balanced by the contraction of the untransformed /3 core region. The subsequent formation of a needles will be oriented to relieve the transformation strains. The net result is to reduce the dilation change through transformation. The other effect of the oxygen is to extend the temperature range over which transformation proceeds. A similar effect has been observed in titanium12 where MCQUILLAN found that the presence of very small amounts of oxygen increased the transformation temperature up to 955°C. An unfortunate effect of flattening the dilation curve is to give difficulty in assessing the actual values of the transition temperatures thereby complicating the interpretation of the dilatometric curves, This is bound to be the case where several phases of different thermal expansion coefficients are changing their proportions J. Less-Common

Metals,

8 (1965)

195-208

a-p TRANSFORMATION

IN

Zr-24 wt. 1’; Nb ALLOS

207

with respect to each other so that the curve is in essence the resolved component of competing factors. The upper transition temperature was raised from 830” to 915’ on repeated cycling. This latter figure agrees with a value of 920' reported for the @/‘(a + @) transition by BELL AND EVASS 13.It is important to realise that in any practical heattreatment schedule of the Zr--24 wt. y/ONb alloy to improve its mechanical strength and involving a quench from the beta phase, oxygen contamination at reduced preswill be sures will extend the (a + /I) range and the value of the transition temperature related to the degree of vacuum obtained in the heat-treatment furnace. The lower transition temperature was found to vary probably for similar reasons. The initial value of 570°C was close to the monotectoid temperature of 560°C obtained by BICHKOV et al.7 using dilatometric techniques. The present work has shown that this value can be raised to 610°C by repeating the runs in a very good vacuum. Even so, this is sufficient to risk a slight oxygen contamination. This higher value obtained dilatometrically is now brought into agreement with the values obtained from static methodsj.R and the difference in results between the so called “dynamic” and “static” methods may be due to small variations in pressure of about 10-5 mm Hg in the experimental equipment. CONCLUSION

(I) The dilation changes experienced by the Zr-z + wt. yONb alloy on going through the X/P transformation range are complex. The expansion dilation on cooling from just below the (a + /I)/p transition temperature is ten times greater than when the specimen is cooled from just above the transition temperature. In the first case, this results whereas in the in an overall expansion of N 1”/0 on cycling back to room temperature second case a contraction of a similar order takes place. This variation in transformation dilation is associated with an orientation of the transformed a-zirconium needles to lie with their major axes in the specimen cross-section. (2) Continued cycling into the b-phase range produces a reproducible effect of reduced dilation changes through the transformation range leading to a small cumulative elongation of the specimen at room temperature. (3) The effect of oxygen contamination even in a reduced pressure of 3 x IO-~ mm Hg is sufficient to reduce the dilation change on transformation virtually to zero. As an intermediate stage, horizontal portions in the transformation curve are developed leading to a flattening of the dilation curve. These horizontal portions are attributed to interaction between the contaminated rim and untransformed bulk core regions. The control of vacuum is very important in assessing transformation temperatures and the values obtained vary with the pressure in the range 10-6-10-4 mm Hg. ACKNOWLEDGEMEKT

This paper is published by permission of the Managing Director of the Reactor Group of the United Kingdom Atomic ‘Energy Authority. The author wishes to acknowledge the experimental assistance of Messrs. I. L. MCDOUGALL and R. HORSFIELD and also of Chemical and Metallurgical Services, Springfields Works, for carrying out the oxygen analyses. J. Less-Common

Metals,

8 (196.5) 195-208

G. F. SLATTERY

208 REFERENCES

I R. S. AWBARTSUMYAN et al., Proc. Second Intern. Conf. Peaceful Uses of Atomic Energy, Geneva, 1958, Vol. 5. p. 12. 2 0. S. IVANOV AND U. K. GRIGOROVICH, Proc. Second Intevn. Conf. Peaceful Uses of Atomic Energy, Geneva, 1958, Vol. 5, p. 34. 3 C. R. CUPP, J. Nucl. &fat., 6 (1962) 241-255. 4 J. H. FOLEY, Heavy Water Reactov International Newsletter, No. 6, Sept. 1963, Chalk River, Ontario. 5 B. A. ROGERS AND D. F. ATKINS, Trans. A.I.M.E., 203 (1955) 1034. 6 H. RICHTER, P. WINCIERZ, K. ANDERKO AND U. ZWICKER, J. Less-Common Metals, 4 (1962)

252-265. 7 Y. F. RICHKOV, A. N. ROZANOV AND D. M. SKOROV, At. Energ., 2 (1957) 146, translated in J. Nucl. Energy, 5 (1957) 402-407. 8 A. G. KNAPTON, J. Less-Common Metals, 2 (1960) 113~124. g J, H. DE BOER AND J. D. FAST, Rec. Tram. Chim., 55 (1936) 459. IO D. J. DELAZERO AND W. ROSTOKER, Acta Met., I (1953) 674. I I A. R. KAUFMANN AND J. T. MAGEL, in B. LUSTMAN AND F. KERZE, (eds.), Metallurgy ofZirconium. McGraw-Hill, New York, 1955, p. 378. I2 A. D. MCQUILLAN, J. Inst. Metals, 78 (1950) 249-257. 13 L. G. RELL AND W. EVANS, Atomic Energy of Capbada Rept. AECL-1395

J. Less-Common

Metals, 8 (1965) 195-208

(1961).