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PREPARATION OF ZIRCONIUM-NIOBIUM ALLOY PRE ^YACARBIDE-OXIDE REACTIONS by R. ViiAatltanjant, S.P. Garg and C.V. Sundaram ;• - Metallurgy Division
BHABHA ATOMIC RESEARCH CENTRE BOMBAY, INDIA 1972 ABSTRACT
This investigation is concerned with thedirect preparation of
Zr-Nb master alloys by reaction between ZrC and Nb2Os. The thermo- dynamics of the reaction are discussed. Starting with 2 different composi- tion* corresponding to 5% and 15$ excess oxygen over the stoichiometric
requirements, and adopting progressive heat-soaking under high dynamic
vacuum, followed by electron beam melting, sirconium-niobium alloys low
in carbon and oxygen have been prepared. The influence of initial oxygen
excess and time of heat soak on the final purity of the alloy has been examined.
The overall material balance for the reaction indicates removal of oxygen both
through carbothermic reduction (as CO) and through sacrificial deoxidation
(as ZrO). PREPARATION OF ZIRCONIUM-NIOBIUM ALLOY BY CARBIDE-OXIDE REACTIONS by R. Venkataramani, S, P. Garg and C.V. Sundaram
the beneficial effects due to the alloying addition of niobium on the mecKanical properties and the corrosion resistance of zirconium have been
extensively discussed in thj literature/1'2'3) A Zr-2. 5% Nb alloy Is re-
yovUd2) to be preferred to zircaloy for the permanent structural compo-
nents - such as coolant tubes - in pressurized water nuclear reactors.
the present investigation has been concerned with the direct pre-
|araltibn of Zx-Nb master alloys by carbo-thermic reduction reacHons. The
fi^exmodynairiics of the reaction of zirconium carbide and zirconium oxide to
form Zirconium metal hav« been discussed by Peterson and Wilhelm(4), who
that with; the techniques presently available, this reaction does not
practicable means for preparing zirconium metal in quantity. On the
other hand, it is well known that the Balke's process*5* based on the high tem-
perature reaction of niobium carbide and niobium pentoxide is one of the
standard practices for producing niobium metal. It has been demonstrated in
the present work that it is possible to produce by a high temperature reaction
between zirconium carbide and niobium pentoxide, Zr-Nb master alloys sub-
ctantially free from carbon and oxygen. ., . «
2. THEORETICAL CONSIDERATIONS The reduction of a metal oxide (M^y) by carbon can take place in
'! the following 2 steps : -2-
—r,x MC + y co , - V) (2)
While the preparation of the reactive metal carbides 1. relatively easy at
.bout 2400«K under a moderate vacuum or in a flow of inert-gas, it Is the .
second step of oxide-carbide reaction which is more significant for the pre- ,
paratior of the metal. The oxide-carbide reactions for the prepa^on of -
zirconium; niobium and Zr-Nb alloy can be written a. follows:
tZ*O2 + ZrC £=±_3/2Zr+CO (3) 7 ,
4 1/5 Nb2O5 + NbC ?± 7/5 Nb + CO r ( )
1/5 Nb2O5 V3*C ^± 7/5(Zr-29 mole pet. Nb alloy) + CO (5)
The standaid'free-energies of formation for the oxides.and carbides
a. available in the literature*6) are presented in Table I for thetemperatUres
1500, 2000 and 2500°C. From these data, standard free energy changes for
the reaction (3) & (4) can be calculated whereas for ..reaction (5), the free
_. iirgrfbrmatton ol Zr-Nb alloy has also to be taken into account; In the
absence of experimental thermodynamic data on the ,Zr,N> system, ideal
solution behaviour hai be*n assumed and for the above particular composition
' of Zr-I^b Alloy the estimated free energy of formation is given by
' •' '1 ^~FT = f-1.20 T Cals/mole. ' «• _ , ,, , ,
Assuirilng unit activities fdr the oxide, carbide and the metal ^s|that the
. equlUbrium constant K as ealc^atedfroto the free energy data can betaken T
at eqoal to the partial pressure of CO - the Equilibrium partfal^essares otfJ
7 CO f« the faction (3). (4) and (s) have been calculated as shoWln Table fl%
The assumptions in laese calculations, however, may not be valid on account
of mutual «oh*bility of (he eaddee, carbides and tke metals. Nevertheless, -3. the Values can be utilised as a rough guide i&t flke comparison of the relative feasibility of the above 3 reaction*.
It caa be seen, from Table II t&at the formation of zirconium metal is
the least favoured reaction amongst the 3 reactions Considered. Etipiilibrlun.
partial pressure values indicatp that £he reduction.vfM1 proceed at a reasonatilfc
rate only at 2000-2500°C,_ under high vacuum. S* t&ls temperature range the
vapour pressure of sirconium metal itself is of fbe o*d«* of 10"4. - 10"2 torr
and that of tbe sub oxide ZrO &bout 10"2 - 1.0 torr as shown in Table III.
This will lead to considerable loss of zirconium as ZrO during the course of
__ ir redUctionand it will be exfcr*mely difficult to balance the carbon and oxygen in
the charge, for their simultaneous removal.
The thermodynamic data are much more favourable for reactions (4)
and (5) and the indications are th&t,both reactions can be easily carried out, ~
at 2t»00°C, under a dynamic vacuum. As already stated, this is one of the
main routes for the production of niobium(5). Reaction (5) was taken up for
investigation in the present work, to examine the feasibility of preparing Zr-Nb
master alloys by direct reaction between ZrC and NbjOs-
* In,oxide-carbide reactions, depending on the operating pressures,
. . there is a certain limit beyond which oxygen removal as CO will not be possi-
ble, because the activity of both oxygen and carbon decrease considerably as
th«i *emett«tt proceed* towards completion. However.-by charging initially
T ,"' sooaef exeast «cyaj«na.s-ZrQ> or Nb2O5, near complete rempval of carbon can " be aftetnptod through CO evolution and Uie removal of residual oxygen then
- • - .- * mM^wmA far BJLCrificJ sacrificial deoxidatton. via volatilisation of ZrO, at more than
kV» etoctrosi b«uri furnace.' r - , 3. EXPERIMENTAL
3.1 Materials
Niobium pentoxide and zirconium oxide used in the present work were
99*67l pure and ground to -325 mesh. Fine carbon black powder of -400 mesh used in the preparation of airconium carbide was pre-treated under vacuum at
1000°C and had an ash content of 0* 001 %. < '
Main impurities in ZrO2 »nd Nb2O5 are given below 5
Impurities in (ppm) •- - _ - Fee M» g Sb Si Sn Hf
ZrO2 530 100 25 600 370
600 70 -440 820 810
3.2 Preparation of Zirconium Carbide
, - jt
Zirconium carbide was prepared by carbothermlc reduction of ZrO2 at 1750°C and at moderate vacuum^10' ll\ A stoichlometric mixture of *ir* conlum oxide and carbon was pelletised at 6-8 tsi pressure using camphor as binder. A hundred gram charge of the pellets was placed in a graphite crucible and heated in a graphite "resistance vacuum furnace. Initially, the tempera- tare was raised slowly upto 500°C to volatiUse the binder. The charge was then gradually Seated further upto 1750°C, so adjusting the rate of heating that the fu?esce pressure wn« malnaunev. i< 10"2 torr. At this temperahire as the reaction proceeded towards completion the vacuum Improved to i0'3 torr. At tia* end of the deslr*d holding time (^1 h>.); the furnace was •witched off the charge allowed to cool., Zirconium carbide thu« prepared corre«po=ded fa&e following analysis,: "' ° ;* * \ - ''* i4«0.|6 w^Tpeibi) Oi76Jwttpet^ V ,- f j>^.. .-i." ,- ," . ,' - '' 3.3 Oxide-Carbide Reactions
The high temperature oxide-carbide reaction! were carried out in a 9 KW electron beam furnace of dictent-gun, self-accelerated-beam design. In this design, the electron gun chamber is mounted on the melting chamber and connected to the same vacuum pumping system. The furnace
chamber of 35 cms dia is served by a 1000 l/«ec capacity diffusion pump,
backed by a 450 1 /min capacity rotary pump.
3.4 Reaction of Zirconium Carbide with Zirconium Oxide A stolchiometric mixture of zirconium carbide and sirconlum-oxlde
/^according to reaction (iil)J7 waa pelletised at 10-11 tsi without any binder.
A 10' gm pellet was kept over the water cooled copper hearth in the melting
chamber of the electron beam furnace. It was observed that high beam power
could not be imposed directly over the pellet on account of rapid evolution
of CO gas which in turn caused tripping of the electron beam. The beam
power was therefore gradually increased upto 0.8KW (100 mA and 8 it?)
maintaining the furnace vacuum at 10"4 torr. As the reaction proceeded
towards completion in a period of about 20 hour* the vacuum improved to
1*5 x-10-"5 fcwfsf. Melting of the charge started at a stage when the beam
Increased to 6 KW (550MA and 11 KW). After observing a clean
charge was allowed^to cool. The resulting zirconium metal was
analysed for residual oxygen and carbon,
3.5 Reaction between zirconium carbide and niobium oentoxide
' -. This reductions** attempted in 2 different ways:
charge AnJfre=eie_ctron beam furnace,'' -6-
(il) by prior vacuum sintering of the charge, followed by electron beam melting. (i) Direct treatment of charge In electron beam furnace
i1 _ * A charge'of zirconium, carVlue and niobium pentoxide containing
• >\ 3% excess oxygen over the s?tolt:hiometric requirement (according to reaction
j' l • (v)) was pelletised at 10-11 tsi without any binder. A 15 gm pellet was directly
heat toaked in the electron beam furnace. Initially a low beam power of 0. 6
K.W (11 KV & 55 mA) was imposed over the charge for a period of .about 7 hours
for controlled evolution of CO, maintaining the furnace pressure at 1-' : 1C~*
torr. The power was then gradually increased to 3. 6 KW (9 KV & 400 mA)
again maintaining the same low pressure. As the reaction proceeded towards
completion, the pellet melted in the form of a button and the vacuum improved
1-3 x 10" torr. At this stage a portion of the button was cut out for analysis.
The rest of the button was further melted for 90 minutes at 4 KW (10 KV, 400 mAj
maintaining a pressure of 1 x 10~5 to if*. A plot of beam power and progres-
sive weight loss against time for the above run is shown in Fig. 1..
(ii) Vacuum sintering followed by electron beam melting of the charge
A. 15.gm pellet of zirconium carbide and niobium pentoxide con-
taming about 15 wt. pet. excess oxygen over stoichiometric requirement was
placed in an alumina- crucible and heated in a tantalum resistance vacuum, r
furnace at 1750°C and 5 x 10*4 torr pressure for about 3 hours. The sintered
pelletwas then charged In the elect*** beam furnace for further treatment
• In computing this charge, the c&rboa sjmitent of eiiconium carbide was taken *?&&**&_ the equivalent oxfgen for fofmattcm ofnCtf was calcu- l»ted, an^ a.5% exce«* prodded to a» charge. Th* main source of (WtTf en was the Nb2O5 addition; but A« residual oxygen ^reHeiit in Zrd . was also taken into account in comptill^ the total oxygen. , J -7- at higher temperature!. In this case a relatively high beam power of
1. 6 KW (8 KV Ic 200 mA) could.be. imposed even at the start maintaining the pressure at 10"4 torr. Under these condition., after about 3 hours, the vacuum improved to 10'5 torr and the charge started melting. A clear melt in the form of a button was observed when the beam power was increased to
2.7 KW (90 KV and 300 mA). At thi, stage a portion of the button was cut for analysis and rest of the button was further melted at *. beam power of
4 KW (10 KV and 400 mA) for about 90 minutes.
3.6 Analysis
The analysis of alloy sample* at the intermediate and final stages of the runs are 8hown in Table IV. Carbon in the alloy sample was determined by the low-pressure method<12) and oxygen both by vacuum fuuion(13) and by D-C arc gas chromatography(14). XlrcOnium and niobium contents of the alloy samples were chemically analysed employing Schoeller's gravimetric method^15}.
4. RESULTS AND DISCUSSION
4;1 Zirconium oxide - zirconium carbide reaction
The rate of this reaction was observed to be very slow AS antici- pated from.therznodynamic considerations. Even after 20 hours of conti- nuous heating the metal obtained was found to be highly inhomogeneous. The microhardne.se of the metal varied between 300-1350 DPH and while the oxygen content was only 600 ppm* carbon content was very high (>I %). As the arun was carried out at a relatively high temperature (Z000°-250Q°C)
that oxygen would have been lost also a B volatile \ -8-
4.2 Niobium oxide - girconium carbide reaction
In the direct treatment of this charge in the electron beam furnace it can be seen from Fig. 1 that in the initial stages, only about 0.5 KW of bz~ai power could be imposed over the charge. In all about 10-11 hours progressive heat soak was necessary te obtain the final alloy. In the second
test with pre-sintered charge, 1.6 KW of beam power could be imposed
straightaway and within 4 hours the final alloy was obtained. During the pre-
sintering step (at 1750°C, 5x10** torr for 3 hours), about 40% reduction
was achieved as assessed from the weight loss of the pellet and good conso-
lidation of the pellet during sintering avoided any spurting in the subsequent
heating in the electron beam furnace.
The intermediate analysis for oxygen in the alloy in both the runs
/"l(a) and 2(a£7 gave high values (1.4 wt. pet. and 0. 90 wt. pet.) as shown in
Table IV. However,, subsequent remelting of the alloy for 90 minutes reduced
the oxygen contents to 1600 and 405 ppni respectively. Though the oxygen
removal in the alloy in this step was accompanied by CO evolution also as
noted from decrease in carbon content during melting, most.of the excess
oxygen -was lost as volatile ZrO. This was confirmed from material balance
of zirconium and niobium in the charge. The loss of niobium was found to
be negligible while zirconium loss corresponded to the removal of excess .
oxygen as ZrO. This is also reflected in the low zirconium content of the V alloy obtained from Znli run as the excess oxygen charged in this run was more than in the first run. - . "
. Oxygen excess charged was found to have a marked.influence over
>\th« catrWn^sonUfnrorthe^fxDal ttfloy^Wlfch an increase of initial oxygen 4 excess.-iii.:the charge from 5% to !£%» the residual carbon content ia the final
alloy decreased from 0. 34 wt.pct. to 600 ppm, though the corresponding
residual oxygen content of the alloy increased from 400 to 1600 ppm, It can
be noted however that the oxygen content can be. further decreased by vacuum
melting of the alloy via volatilisation of ZrO.
The optimum condition for the excess oxygen to be charged depends
upon the impurity level that can be tolerated in the final alloy, For the pre-
paration of Zr-Nb alloy of low oxygen and carbon contents, oxygen excess in
the charge should be high and subsequently the melting time for the removal
of excess oxygen should also be increased. The Zr-Nb master alloy can be
conveniently used for the preparation of dilute Zr-Nb alloys such as Zr-2. 5%
Nb for application in nuclear reactors.
5. ACKNOWLEDGEMENT
The authors are thankful to Dr. V. K. Mcorthy, Head, Metallurgy
Divisions Bhabha Atomic Research Centre, for his kind interest in the pre-
sent investigation. Thanks are also due to Dr. T. S. Krishnan, (Nuclear
Fuel Complex, Hyderabad), Dr. K. K. Majumdar (Metallurgy Division, ..
B.A.R. C), Dr. N.A. Narasimhan (Head, Spectroscopy Division, B.A.R.C.)
and Dr. M. Shankar Das (Head, Analytical Division, B.A.R. C*) for arrang-
ing fo* the analysis of the various samples. The authors are grateful to A . - ' • • ^ •' I5r. J. Shankar (Head, Chemistry Division), and Shri R. K. Garg (Chemical Engineering Division) for the supply of niobium and jslreonium.oxides fc? the -J • investigations. ., ' • ••. .10-
REFERENCES 1. D.L. Dougla**,- 'The Metallurgy of Zirconium', Atomic Energy . Review, Supplement 1971 (International Atomic Energy Agency Review Publication, Vienna, 1971). 2. C, P. William*, •Development Potential of Zirconium Alloys for High Temperature Application*', Reactor Technology, Vol. 13, No. 2, 1970.
3. D.A. ProkoshklnandE.V. Vasileva, 'Alloy* of Niobium1, Ed. A. M. Samarin, Israel programme for Scientific Transactions, Jerusalem, 1965 6. Gerasimov, Kretovnikov and Shakov, 'Chemical Thermodynamics in Non-ferrous Metallurgy1, Vol. HI, Israel Programme for Scientific Translation*, Jerusalem (1965). 7. Molly Gleiser, Trans., AIME, Vol. 221, 1961 (pp. 300-304), S. H.R. Smith, 'Vacuum Metallurgy1, Ed. R. F. Bunshah, N.Y. 1958 (pp.227-232). 9. R. F. Bunshah and Michael A. Coccia, 'Technique* of Metal* Research1, Ed. R. F. Bunahah, Vol.I, Part 2, Interscience Publisher s, N.Y. 1968. 10. T.Y. Kosolapova, 'Carbiden1, N. B. Vaghaun Ed., Plenum Press, London (1971), pp 111-117. 11* K. Venkafiaramani, Ph. D. Thesis, Dept. of Metallurgical Engineering, 1.1.T., Bombay (1969). . - 12. VtT. Ainlavale et al, Anal. Chim. Acta, 35, (1966), pp. 247-253. _,. \ ^ 1 ' ______-_.-_ " - -. _ j = 13. T.S; Krishnamurthi, C.V. Krishn&n and Ch. Venkateswar&lu, Report B. A. R. C, -277, BARC, Bombay, 1967. 14. A.P, D'Silva. P.K. WahlandS.S. Biswas, Report B. A. R. C. -336, BARC, Bombay, 1968. 15. W»R. Schoeller and A.R. Powell, 'The Analysis of Minerals and Ores of the Rarer Compounds*/ Charles Griffin k Company Ltd. (Publication*), = = London W.C. (2), (1955), pp.137, 220-222. -L - , • .A.' Table I STANDARD FREE ENERGIES QF, FORMATION OF CARBIDES AND O&DES >••••••- °C ,1500°C 2000°C 2500°C -37.5 -35.5 . -33.3 -31.8 -31.8 -31.2 -183.0 -160.0 -139.0 -275.0 -236.0 -182.7 -64.0 -74.0 -83.5 Table II THERMODYNAMIC DATA FOR THE OXIDE-CARBIDE REACTIONS Reaction (iii) Reaction (iv) Reaction (v) I ZrO2 + ZrC^ 1/5 Nb2O5 + NbC^* 1/5 Nb2O5+ZrC^ Tempera- 3/2 Zr + CO 7/5Nb+CO 7/5(Z*-Nballoy)+CO ture C __ PCO (K Call) (torr) (K Call) (torr) (K Cals) (torr) 1500 + 65.0 7.6xl0"6 +23.0 1.1 +25.7 0.5 2000 + 41.5 7.7xlO"2 +5.0 2;5xlO2 +4.9 Z. 6xlO2 2500 V 19.0 23.0 -15.7 l.OxlO4 -18.3 2.1xlO4 Table THERMODYNAMIC DATA FOR REACTION i ZrO2(s) ^S ZrO(g) Tem]>erature °C 2000°C 2500°C ^F°R K Cal«/mole + 55.0 + 35.0 " PZrO(i) (t*»«J 4.0 x 10-2 1.30 / - ' l / ' ji i: // --„ - — * - <. 'il ',i h i 1 1 I i t 1? 1 i ,' i } _ / 1 '/ . 4 • 3 Table IV ANALYSIS AND HARDNESS VALUES FOR THE Zr-Nb ALLOY OBTAINED AT DIFFERENT STAGES OF HEAT TREATMENT Excess Analysis in. wt. pet oxygen Run No. in the Thermal History of the Charge Zx Nb 0 charge in % 00 Direct treatment of the charge. in 70.10 28.20 -0.74 0.90 electron beam furnace for about 9 hours- (b) Same as l(a) followed by vacuum melt- 69.90 29.40 0.34 0.04 ing La electron beam furnace at 10 KV and 400 tnA for 90 minutes, 2 (a) Pre-Bintered at 1750°C under a vacuum 62.75 35.87 0.12 1.40 of 5x10"4 torr for 3 hours followed by treatment in electron beam furnace for 2 hours. Same as 2(a) followed by vacuum 61.25 38. 75 0.06 0.16 melting in electron beam furnace at 10 KV and 400 mA for 90 minutes. TTTIE IKOKCl BEAM POWER AND WEIOKT LO8« AOAINST TIME. F!O. No. I ft: in . o 10 II TIME ( hi; ) MHARKS SCALE. CHKV HCTALLURGY OIVtSION