I ATOMIC BNBStaV v: •M i rr ' UP", +**z B.A.R.C.-612 GOVfiRNMENf OF INDIA ATOMIC ENERGY COMMISSION 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.