The Kinetics of Precipitate Coarsening in Zircaloy-2 (Harini Sosiati)

THE KINETICS OFPRECIPITATE COARSENING IN ZIRCALOY-2

Harini Sosiati, SungkonoNuclear Fuel and Recycle Technology Development Centre -BA TAN

Kawasan Puspiptek Serpong Tangerang

ABSTRACTTHE KINETICS OF PRECIPITATE COARSENING IN ZIRCALOY -2. Zircaloy-2 has been used as fuel

element cladding material in both light and heavy water power reactors. The aims of the experiment are to study thekinetics of precipitate coarsening during isothermal a-annealing and to characterize the nucleation and growth ofprecipitate affected by the variations of cold work and heat treatment. The results of the experiment show that allspecimens annealed at 650°C have the average precipitate diameter less than O.Ij.1m whereas those at 750°C have theaverage size larger than 0.1 j.1m except for one annealed for less than 5 hours. The average size of precipitates is 0.07j.1m for specimens deformed before l3-quenching and 0.168 j.1m for specimens deformed after l3-quenching. Thedeformed specimens after l3-quenching have more homogeneous size distribution than that of un deformed specimensand specimens deformed before l3-quenching. The kinetics of precipitate coarsening depends on the changes inannealing temperature and time, and cooling rate from the l3-phase region. The nucleation and growth ofprecipitateshave been influenced by the variations of cold work and heat treatment.

Key words: Zircaloy-2, Kinetics of precipitate coarsening, Fuel element cladding

ABSTRAKKINETIKA PENGASARAN PRESIPIT A T DALAM ZIRCALOY -2. Zircaloy-2 digunakan sebagai materia!

kelongsong elemen bakar reaktor daya berpendingin air ringan atau air berat. Penelitian ini bertujuan untuk mempelajarikinetika pengasaran presipitat selama anil-a isothermal serta karakterisasi pengintian dan pertumbuhan presipitatsebagai fungsi pengerjaan dingin dan perlakuan panas. Hasil penelitian menunjukkan bahwa semua spesimen yangdianil pacta 650°C mempunyai diameter presipitat Terata lebih kecil daTi O,lllm, sedangkan pacta 750°C diameterpresipitat Terata lebih besar daTi 0,1 Ilm kecuali spesimen yang dianil kurang daTi 5 jam. Ukuran Terata presipitatspesimen yang dideformasi sebelum p-quenching adalah 0,07 Ilm, sedangkan untuk spesimen yang dideformasisetelah p-quenching adalah 0,168 Ilm. Spesimen yang dideformasi setelah p-quenching mempunyai distribusi ukuranpresipitat lebih homogen dibandingkan spesimen yang belum dideformasi dan telah dideformasi sebelum p-quenching.Kinetika pengasaran presipitat bergantung pacta perubahan temperatur dan waktu anil, serta laju pendinginan daTidaerah rasa-po Pengintian dan pertumbuhan presipitat dipengaruhi oleh variasi pengerjaan dingin dan perlakuan panas.

Kata kunc;: Zircaloy-2, Kinetika pengasaran presipitat, Kelongsong elemen bakar

INTRODUCTIONIt is well known that zircaloy-2 is mainly used as

cladding and structure materials in the core of both lightand heavy water nuclear reactors. It has an exellent corro-sion resistance in high-water temperature, and also highmechanical strength at elevated temperature, high meltingpoint, high thermal conductivity and low absorption ofther-mal-neutron cross section. Cladding is required to protectthe fuel from direct contact with coolant and to prevent thefission product release from fuel to coolant. The structurematerial is required to support the reactor core and to pre-vent fuel element distortion from various forces or stressimposed as a result of fission process within the fuel.

In zirconium alloy corresponding to zircaloy-2, main

phases occuring succesively on heating are hcp a, ( hcp a+ bcc 13 ) and bcc 13; hcp generally exists between roomtemperature and 865°C, while the coexisting of (hcp a +bcc 13) presents in the range of 865°C and 985°C and bcc 13is in the region above 985°C [I].

Some investigations on microstructure features withrespect to the mechanical and physical properties, and alsothe effects of heat treatments and microchemistry on thecorrosion of zircaloys have been conducted [2,3,4]. Thevariations of cold work and heat treatment, and the changesof their conditions, however, playa significant role on thekinetics of precipitate coarsening that is associated withtheir contributions as a basic information on the design

Jurnal Sains Materi IndonesiaIndonesian Journal of Materials Science

Vol. I No.2, Pebruari 2000, hal: 1-4ISSN: 1411-1098

material of zircaloy. High coarsening rate of precipitatesformed in zircaloy should be prohibited because it can in-crease the corrosion rate and electrical resistivity and alsodecrease the hardness of the material.

The objectives of this work are to evaluate the ki-netics of precipitate coarsening during isothermal a-an-nealing and to characterize the nucleation and growth ofprecipitates with respect to their size distribution affectedby variations of cold work and heat treatment.

MATERIALS AND METHODSThe material used in this experiment was as-forged

zircaloy-2 with the composition ofZr-I.6 wt. % Sn-O.2 wt. %Fe-O.1 wt. % Cr-O.O5 wt. % Ni, supplied by Kobe Steel Co.,Japan. Specimens were distinguished by three steps.

In the first step, specimens were j3-annealed thenwater quenched (wq) at about 400°C/s. It was used as aninitial state. The j3-quenched specimens were subsequentlyannealed in the a-phase for various durations followed bywater quenching as tabulated in Table I. In the second andthird steps, specimens were deformed (rolled) before andafter j3-quenching, and then both were annealed at 650°Cand 750°C for 5 hours, as shown schematically in Figure 1and Figure 2, respectively.

Figure 1. Scheme of second step heat treatment

Table 1. Heat treatment history (first step)

Thin foils for TEM were prepared by a twin-jetelectropolishing technique in the polishing solution of 10vol.% perchloric acid (60%) and 90 vol. % methanol (99.8%).The solution temperature was maintained between -45°Cand -35°C with the voltage varied from 9 to 20 volt duringthinning. TEM examinations was conducted with a JEM-2000 EXII.

Figure 1. Scheme of third step heat treatment

of the precipitates becomes more profound than by increas-ing the annealing time. This finding agrees with that ofMaussner et.al [5] who conducted study on zircaloy-4.

When a precipitate nucleates, its growth is as fast

RESULTS AND DISCUSSIONCoarsening of precipitates

The size of precipitates tends to become coarserwith increasing annealing time and temperature. As theprecipitates become coarser, however, their number becomeslower. By increasing the annealing temperature; the growth

,

001

0008

1

0008

".\, 0004

0002

0 ~-L 40 60 60 100 120 140

o.-anncllling lime (hour)

Figure 4. Dependency of volume precipitate on time and

temperature.

0

10

The Kinetics of Precipitate Coarsening in Zirca/oy-2 (Harini Sosia

as the diffusion of solute from the surrounding matrix. Dur-ing this period, the volume of the precipitates increasesand the solute content of the matrix decreases. After thesolute concentration of the matrix has decreased to a near-equilibrium level, the microstructures become static. Onthis basis, solute content in equilibrium with smaller pre-cipitates will be greater than that with larger precipitates. Itcauses flux of solute atoms from smaller to larger precipi-tates. This flux results in the shrinkage of the small precipi-tates and the growth of the larger ones [6].

The kinetics of precipitate coarsening during iso-thermal a-annealing is illustrated in Figure 3. It shows thatall specimens annealed at 650°C have the average precipi-tate diameter less than O. I ~m whereas those at 750°C havethe average size larger than 0.1 ~m except for one annealedfor < 5 hours. Precipitates formed in the specimens weremostly distributed at the grain boundaries.

025

j 02~.'0..015

3a.~ 01b.cfi 0 05~

40 60 60 tOO

a-annealing limc (hQur)

120 1.020

Effects of cold work and heat treatment variationsTaking into account the observation results in the

first step, the average size of precipitates less than O.ll.1mwas formed in specimens annealed either at 650°C for 5hours or at 750°C for less than 5 hours. The specimensmight have been indicated as relatively good materials withrespect to low corrosion rate tendency.

Based on the above conditions, the effects of de-formation (cold work) before and afterf3-quenching wereconducted on the nucleation and growth of precipitates.The difference of precipitate coarsening formed in the speci-mens in the second step and that in the un deformed speci-mens annealed at 750°C can be clearly observed from theelectron micrographs as shown in Figure 5. The averagesize of precipitate is 0.071.1m for specimens deformed be-fore f3-quenching and 0.168 I.1m for specimens deformedafter ~-quenching. It can be said that the coarsening rate

Figure 3. Coarsening of isothermal a-annealing precipitate

The coarsening of precipitates has been consider-ably extended by a diffusion rate that follows the simplifiedmodel of the Oswald ripening. The equations (1) as follows

dJ -doJ = k' .0 .t .exp (-Q/R. T) ( I )

If do is assumed to be small compared with d, then

dJ=k'.D.t.exp(-Q/R.T) (2)

whered = mean diameter of precipitates, m

k'= constant, m0= diffusion constant, m2/s.t = time, s

T= absolute temperature, KQ = activation energy = J/moleR = gas constant = 8.3143 J/mole.K

The variation of the cube of precipitate diameterwith time and temperature is given in Figure 4, Q and k'.Oare 192 kJ/mole and 1.19x 1 0-7 m3 Is, respectively.

Another investigation [5] shows that zircaloy-2rolled sheet isothermally annealed at 750°C and 800°C after Figure Sa. BFI of undeformed specimens annealed at 750 "C

3

'Ii)

j3-quenching with cooling rate of 800°Cfs gave Q = 250 kJfmole andk'.D = 0.9xI0-3m3fs. According to the discrepancy

in the value of the activation energy between these twoinvestigations, there could be an effect of cooling rate fromthe j3-phase region on the coarsening rate of the precipi-tates, that has been controlled by the diffusion of the al-loying elements through the matrix into the precipitates.

Jurnal Sains Materi IndonesiaIndonesian Journal of Materials Science

Vol. 1 No.2, Pebruari 2000, hal.. 1-4ISSN.. 141/-1098

Figure 5b. BFI of defamed specimens annealed at 750 .C

of precipitate in the specimen deformed after ~-quenchingtends to be higher than that of the undeformed specimensand specimens deformed before ~-quenching. Accordingto the size distribution of the precipitates, however, thedeformed specimens have more homogeneous size distri-bution.

On the other hand, specimens prepared in the third

step have relatively low in average size of precipitate. It is

0.03 I.Lm for specimens deformed after p-quenching and 0.05

I.Lm for specimens deformed for both before and after p-

quenching. The comparison of their precipitate sizes is

depicted in Figure 6.Normally the specimens def0rmed after p-quench-

Figure 6a. BFI of undeformed specimens annealed at 650 "C

ing fonD a zone with quite high dislocation density whichcreates high concentration of vacancy. These play morerole in the precipitation ( nucleation and growth) which iscontrolled by migration of atoms in the alloy.

CONCLUSIONAccording to the evaluation and observation re-

sults of precipitate coarsening, it can be pointed out thatthe kinetics of precipitate coarsening depends on thechanges in the annealing temperature and time, and thecooling rate from the ~-phase region that has been con-trolled by the diffusion of alloying elements through thematrix into the precipitates. The variations of cold workand heat treatment have also strongly influenced the nucle-ation and growth of precipitates which have been con-trolled by migration of atoms.

Among all specimens examined in this work, thespecimens annealed at 7S0°C for more than 5 hours andspecimens defonned after ~-quenching then annealed at7S0°C cannot be classified as good materials owing to theirhigh rate of precipitate coarsening.

ACKNOWLEDGEMENTSThe authors would like to thank Kobe Steel Co.,

Japan for supplying zircaloy-2 material and for making th iswork possible. Our grateful is also given to all my colleaguesat the Centre of Nuclear Fuel and Recycle Technology De-velopment, National Nuclear Energy Agency for their sup-ports and assists that have made this work succesful.

REFERENCES[I]. ARIAS, D. and GUERRA, R.C., J: Nucl. Mater., 144

(1987), 196.[2]. HARINI S.," Hardness and Electrical Resistivity of

Heat Treated Zircaloy-2 at 650 and 750 'C ", Proceed-ing of Nuclear Fuel Cycle, Jakarta, March 1996.

[3]. KRUGER, R.M., ADAMSON, R.B., AND BRENNER,S.S., J: Nucl. Mater., 189 ( 1992 ), 193.

[4]. INAGA, M., AKAHORI, K., AND MAKI, H., Effectsof Chemical Composition and Precipitation on Corro-sion Resistance of Zircaloy, Research Thesis, 32,5

(1990).[5]. MAUSSNER, G., ORTLIEB, E. AND TENKHOFF, E.,

Nucleation and Growth of Intennetallic Precipitates inZircaloy-2 and Zircaloy-4 and Correlation to NodularCorrosion Behaviour in Zirconium in the Nuclear In-dustry, 7th. Symp., ASTM STP 939, ASTM,Philadhelphia, 1987, pp. 307 -320.

[6]. SMALLMAN, R.E., Modem Physical Metallurgy, 4 ed.,Butterworths, London, 1985, pp. 81.

Figure 66. BFI of deformed specimens annealed at 6S0 "C

4