An investigation of the zirconium-tantalum system

J. Nuclear Energy, II, 1957, Vol. 5, pp. 247 to 252.

AN INVESTIGATION

Peqsmon

Press Ltd.,

London

OF THE ZIRCONIUM-TANTALUM SYSTEM*

V. S. EMELYANOV, Yu. G. GODIN, and A. I. EVSTWKHIN (Received 17 November 1956) Abstract-Metallography, X-ray diffraction, thermal analysis, electrical resistance and hardness measurements were employed in the study of the zirconium-tantalum system and the determination of its phase diagram. The diagram is a eutectic type and shows limited solubility in the solid state. The maximum solubility of tantalum in a-zirconium at 790°C is less than 0.22 at. per cent while j3-zirconium will dissolve 16 at. per cent of tantalum at 1585°C. Maximum solubility of zirconium in tantalum is. 17 at. per cent at 1585°C. The eutectic occurs at 1585°C and 34 at. per cent tantalum. A eutectoidal transformation of the solid solution based on p-zirconium takes place at 790°C; the eutectoidal composition is 7 at. per cent tantalum. INTRODUCTION

ONLY fragmentary

system is available from published literature. ANDERSONet al.(l) investigated cast alloys of zirconium with tantalum contents of up to 30.3 per cent. The alloys were made in graphite crucibles under vacuum from sponge zirconium and sheet tantalum. A single phase structure was found by them in an alloy containing 5.3 per cent Ta but when the tantalum content was increased to 9.7 per cent the alloys were found to consist, in the solid state, of two phases although at their melting point a solid solution was formed. In alloys containing 14-l per cent Ta approximately 20 per cent of the second phase (eutectic) was found at grain boundaries but in alloys containing 20.8 per cent Ta this phase occupied the whole field. Alloys containing 30.3 per cent Ta consisted mainly of the eutectic together with a bright component of dendritic form which amounted to less than 10 per cent of the whole. PFEIL(~)thought that the second phase in ANDERSON’14.1 S per cent Ta alloys was not likely to be a eutectic because the decrease in hardness found by ANDERSON et al., indicated that the p-phase may have been retained at room temperature in 20.8 per cent Ta alloys. F’FEILcommented that the alloy containing 30.3 per cent Ta might consist of p-solid solution and dendrites of an intermediate phase or of tantalum rich solid solution. LITTON@)found that a cast alloy containing 12.5 at. per cent Ta had a homogeneous structure. KJZELER’*) showed that zirconium alloyed with 2.7 at. per cent Ta had a transformation interval between 807” and 852°C which indicated that addition of tantalum lowers the transformation temperature of zirconium. According to SCHWOPE f5) tantalum when alloyed with zirconium must form a eutectoid, broaden the /?-region and lower the temperature of allotropic transformation in zirconium. From SCHWOPE’Sdata the maximum solubility of tantalum in uzirconium is 5 at. per cent. * Translated by J.

information

ADAM

from

on the zirconium-tantalum

Atomnaya Energiya 2, 42 (1957). 247

248

V. S. EMELYANOV, Yu.

0.

GODM, and A. I. EVSTYUKHIN

The possibility of a eutectoid transformation pointed out in LUSTMANW book. METHOD

OF

PREPARATION

AND

in zirconium-tantalum

EXAMINATION

OF

alloys is also

ALLOYS

Zirconium-tantalum alloys are difficult to prepare because both these metals have high melting points and are very active chemically at elevated temperatures. Melting in an MIFI-SM-3 argon arc furnace equipped with a cooled copper hearth (Fig. 1) TABLE 1 .-ANNEALING TREATMENTS GIVENTO ZIRCONIUM-TANTALUM ALLOYS

-

Temperature*

Duration

(“C)

fir)

155ot 1425t 1425 1200 1200 1000 900 830 800 780 760 740 700 600

2 3 3 80 16 40 70 100 150 200 250 350 600 600

-

Tantalum content of alloys (per cent)

70-99 70-99 0.25-99 0.25-80 025-80 0.25-80 0.25-80 0.25-80 0.25-80 0.25-80 0.25-80 0.25-80 0.25-80 0.25-80

I

_v - lemperature varlauons *5”C.

t Annealed in furnace TVV-2.“’

overcame the preparation difficulties. To remove oxygen from the argon the gases were passed through calcium shavings heated to 800°C and final purification was achieved by the preliminary melting of an iodide zirconium getter in the furnace. Thirty-six ingots were prepared for microscopic examination and thirteen rods, each 60 mm long and 8 to 13 mm in diameter, prepared for electrical resistance measurements from iodide zirconium rods of composition: Zr - 996 per cent, Cl < O+lO25 per cent, W < 0.01 per cent, Ni - 0.001 per cent, Cr < 0.03 per cent, Fe - O-022 per cent, Ca < 0.005 per cent, Si < 0.005 per cent, C - 0.05 per cent, Cd < 3.10e5 per cent, Hf - 0.05 per cent, Mn - 0.002 per cent, N - 0.014 per cent, Ti - 0.0034 per cent, Cu < 0.001 per cent, MO < 0.01 per cent, and tantalum strip of composition: Ta - 99 per cent, Nb - 0.5 per cent, Ti - 0.06 per cent, W - 0.02 per cent, Fe < 0.05 per cent, MO - 0.03 per cent, Si < 0.1 per cent. All alloys were analysed chemically. Homogenization of the alloys was achieved by arc cutting the ingots into pieces and remelting. By repeating the operation three or four times, satisfactory samples were obtained. Their homogeneity was confirmed by metallographic examination of several pieces of each ingot. Hardness measurements made on a control specimen of iodide zirconium, which was melted as one of a set of alloys, served as a test for possible gaseous contamination.

An investigation

of the zirconium-tantalum

system

249

Cast alloys were given a final homogenizing treatment at 1200°C followed by heavy cold rolling, isothermal annealing at various temperatures and quenching. Annealing was done in evacuated silica capsules which were broken under water during quenching, each capsule containing a control sample of iodide zirconium. For annealing at temperatures above 1000°C the capsules were filled with argon and the samples wrapped in molybdenum foil. Samples annealed at 1550°C in a furnace TW-2 were quenched by switching the power “off.” The procedure was considered sufficient because metallographic examination of two samples, one of which was water quenched and the other furnace cooled, showed no difference in microstructure. Annealing treatments given to various alloys are listed in Table 1. In the preparation of metallographic specimens mechanical polishing was followed by polishing on a cloth impregnated with a fine suspension of Cr,O, in water and then etching in a solution of 20 per cent HF and 20 per cent HNOa in water or glycerine.@) The etch, although suitable for most of the alloys, heavily oxidized the alloys near the eutectic composition and consequently such alloys were etched in a solution with an HF content of only 5 per cent. Tantalum rich alloys were successfully etched in a mixture of 90 per cent HF with 10 per cent H,SO,. X-ray examinations were carried out on powders taken from cast and quenched alloys in an RKU-1 camera using copper K-radiation filtered through nickel. Thermal analysis up to 1000°C was carried out in KURNAKOV’S apparatus. In this apparatus the sample and a standard are placed in a special nickel cell fixed inside a calorized steel vessel which is reinforced in its lower part. The upper part of the vessel is water cooled and has a hermetically sealed top through which pass the connexions to two thermocouples. The vessel was heated in a tubular resistance furnace, both the heating and cooling taking place in a vacuum of 10m3mm Hg at a rate of 6°C per minute. DETERMINATION

OF

SOLIDUS

AND

LIQUIDUS

The solidus and liquidus were determined from the appearance of the specimens during heating.cg) Pieces of alloys, each of 3g to 5g weight, with machined faces were placed on a zirconia stand in the TW-2”) furnace and slowly heated to a temperature below the melting point. The specimens were held at this temperature for 15 min and if no signs of melting were seen the temperature was increased by 30-40°C and the process repeated until the first signs of melting were detected. The temperature at which signs of melting appeared was taken as the solidus and the temperature at which the sample assumed a drop-shaped form was taken as the liquidus. The temperature measurements were made with an optical pyrometer and with a platinum/platinum-rhodium thermocouple, sheathed in zirconia, which was placed at the centre of the zirconia base. RESULTS

The presence of a eutectic and regions of considerable solid solubility of tantalum in zirconium and of zirconium in tantalum was established by microscopic examination of cast alloys. Dendrites which appeared in such alloys were identified as tantalum crystals containing zirconium in solid solution. X-ray examination of alloys showed that j3-zirconium cannot be stabilized at room temperature and consequently the only phases which can be detected by X-ray diffraction methods are a-zirconium and y-tantalum rich solid solution. G

250

V. S. EMELYANOV,Yu. G. GODIN, and A. I. EVSTYUKHIN

The position of the eutectic point at a temperature of 1585 -J= 15°C and composition of 34 at. per cent Ta was established by microscopic examination and by determination of the melting point (Fig. 2). In Fig. 3 is shown the eutectic structure of a 34 per cent Ta alloy which was kept at 1570”for 15 min and rapidly cooled in a TVV-2 furnace. The maximum solubility of tantalum in ,Grconium was found to be 16 per cent. The value was found by using I. F.. SHREDER’S equation to extrapolate the data obtained in the temperature range 790°C to 1425°C. Extrapolation was necessary

3000-

4 .”

2800 2600

/

/

/

I I1.

wty

400 0

10 20 30

40

Tantalum

50 content

60

70 80

90 100 at. %

FIG. 2.-Zirconium-tantalum phase diagram. a-solidus and liquidus determination; O-single phase B-alloys; x-two alloys ; + -two phase (or + /?) alloys ; O-two phase (a + 7) alloys; n-single

phase (Jl+ y) phase?-alloys.

because attempts at quenching hypoeutectic alloys from the eutectic temperature were unsuccessful. Figs. 4 and 5 show respectively the microstructure of the transformed p-phase in a 10 per cent Ta alloy and the two phase structure (/3 + y) of a 12.5 per cent Ta alloy. Both alloys were annealed at 1200°C and quenched. The solubility limit of zirconium in tantalum was found by metallographic means to be 17 at. per cent. The microstructure of an alloy containing 83 at. per cent tantalum and quenched from 1550°C is illustrated in Fig. 6. A eutectoidal transformation takes place in alloys containing a few atomic per cent of tantalum. Microstructures of a 2 per cent Ta alloy are illustrated in Figs. 7 and 8. Fig. 7 shows a sample, annealed at 800°C and quenched, which consists of two phases, namely a + metastable @. Fig. 8 was obtained from a sample annealed at 780°C and quenched and shows primary a-grains with eutectoid at the grain boundaries. The position of the eutectoid at 790” -& 10°C and 7 at. per cent Ta was determined by the metallographic examination of a series of alloys quenched from 800°C and

An investigation

of the zirconium-tantalum

system

251

temperatures below this in steps of 20°C. The determination was confirmed by thermal analysis and X-ray studies. The microstructure of an alloy containing 7 per cent Ta quenched from 780°C is shown in Fig. 9. X-ray photographs of alloys containing 1,2,3,4 and 6 at. per cent Ta quenched from 780°C and 800°C show y-phase lines in only the samples quenched from below the eutectoid temperature. The lines in the photographs of samples quenched from temperatures above the eutectoid were very diffuse in the high angle region. The solubility of tantalum in a-zirconium was found to not exceed 0.22 at. per cent. Traces of metastable /?-phase which can be seen in the microstructures of alloys

x

I

90 80 70 60

t

10

Tantalum content

at %

FIG. 1I.-Electrical resistance as a function of composition in Zr-Ta alloys. I-for cast alloys, II-for alloys quenched from 12OO”C,III-for alloys quenched from 770°C.

containing O-25 at. per cent Ta quenched from 800°C (Fig. 10) are absent in the microstructures of 0.20 per cent Ta and O-15 per cent Ta alloys. X-ray photographs taken from all these alloys after quenching from 780°C show no change in the unit-cell size. The final composition diagram of the zirconium-tantalum system is shown in Fig. 2. To obtain further confirmatory evidence electrical resistance measurements were made on a series of alloys in the “as cast” condition and after 40 and 100 hr annealing at 1200°C and 700°C followed by quenching. The electrical resistance measurements were made using a double Thompson bridge. The values of the specific resistance for pure zirconium and pure tantalum samples after annealing for 40 hr at 1200°C were found to be respectively 48.5 x IOV Q cm and 15.6 x 1O-6!Acm. Three resistance vs. composition curves are given in Fig. 11. Curves I and II respectively refer to cast alloys and alloys quenched from 12OO”C,i.e. above the eutectoid temperature. Both the curves have prominent peaks at compositions between 10 per cent and 20 per cent Ta. The peaks can be attributed to the martensitic character of the transformation on quenching and is typical of zirconium alloys. On

252

V. S. EMELYANOV, Yu. G. GODIN, and A. I. EVSTYUKHIN

the right hand side of the figure the curves show a sharp drop which is attributed to the. formation of solid solution. The solid solubility region determined from the electrical resistance measurements (curve II) agrees well with the region obtained by microscopic examination of alloys annealed at 1200°C. It is seen that the shapes of curves I and II differ only in the size of the peaks, from which it is inferred that the quenching temperature of the cast alloys is higher than the eutectoid. Curve III refers to samples quenched from 770°C and is typical of alloys in the equilibrium state. It gives satisfactory confirmation of the metallographic results, in particular, confirming the low solubility of tantalum in cr-zirconium.

0

K) 20

30

40

Tantalum

50

60

content

70

80

90

loo at. %

FIG, 12.-Hardness as a function of composition in Zr-Ta alloys. I-for

alloys

quenched from lZOO’C,II-for alloys quenched from 77O’C.

To complete the study of the zirconium-tantalum system some hardness measurements were made on alloys annealed at 1200°C and 770°C and quenched. Two hardness vs. composition curves are shown in Fig. 12. Both curves show that the addition of tantalum to zirconium results in an increase of hardness. The peak in curve I is attributed to martensitic transformation in alloys quenched from 1220°C. REFERENCES 1. ANDERSON C. T., HAYESE. T., ROBERTKIN A. H., and KROLLW. J. U.S. Bureau qf Mines report No. 4658 (1950). 2. PFEILP. C. L. A.E.R.E. report No. MT/N 11, Harwell(l952). 3. LITTONF. B. Iron Age 167,95, 112 (1951). 4. KEELERJ. H. G. E. Res. Lab. Report No. RL 640, New York (1952). 5. SCHWOPEA. D. in Metallurgy of Rarer Metals-Zirconium Butterworths Scientific Publications, London (1954). B. in The Metallurgy of Zirconium p. 442 McGraw-Hill, U.S.A. (1955). 6. LUSTMAN Hatinoqribor leaflet (1951). 7. High temperature vacuum electric muffle furnace type TW-2. 8. D~MAGALAR. F., MAcPH~R~~ND. J., and HANSENM. in Metallurgy of Rarer Metals-Zirconium Butterworths Scientific Publications, London (1954). Yu. F. Private communication (1956). 9. BYCHKOV

FIG. l.-Arc

furnace MIFI-SM-3.

p. 250

FIG. 3.-Alloy containing 34 at. per cent Ta, annealed at 1570°C for 15 min and etched in 20 per cent HNO, + 5 per cent HF in glycerine. Eutectic /%Zr + y. Magnification X 500.

FIG. 5.-Alloy containing 12.5 at. per cent Ta, quenched from 1200°C and etched in 20 per cent HNOa + 20 per cent HF in water. Decomposed /?-solid solution, segregated y-phase. Magnification X 500.

FIG. 4.-Alloy containing 10 at. per cent Ta, quenched from 1200°C and etched in 20 per cent HNOs + 5 per cent HF in water. Needle-shaped grains of transformed B-phase. Magnification x 100.

FIG. 6.-Alloy containing 83 at. per cent Ta, quenched from 1550°C and etched in 90 per cent HF + 10 per cent H,SO,*y-solid solution. Magnification x 200.

FIG. 7.-Alloy containing 2 at. Ta, quenched from 800°C and 20 per cent HNO, + 20 per in water. a and metastable Magnification x 200.

per cent etched in cent HF B-phases.

FIG. 8.-Alloy containing 2 at. per cent Ta, quenched from 780°C and etched in 20 per cent HNO, + 20 per cent HF in water. Primary u-grains and eutectoid. Magnification X 500.

FIG. 9.-Alloy containing 7 at. per cent Ta, quenched from 780°C and etched in 20 per cent HNO, + 5 per cent HF in glycerine. Eutectoid c( + y. Magnification x 500.

FIG. lO_Alloy containing 0.25 at. per cent Ta, quenched from 800°C and etched in 20 per cent HNO, + 20 per cent HF in water. c( and metastable P-phases. Magnification X 200.