Decomposition of gamma phase in a uranium-9.5 wt % niobium alloy

JOURNAL

OF NUCLEAR

44

MATERIALS

DECOMPOSITION

(1972)

OF GAMMA

207-214.

0 NORTH-HOLLAND

PUBLISHING

PHASE IN A URANIUM-g.5

CO.,

AMSTERDAM

wt O/0 NIOBIUM ALLOY

B. DJURIC “Boris

KidriC”

Institute of Nuclear Sciences, Received

The

isothermal

solution

decomposition

of a U-9.5

of

wt O/bNb

the temperature

interval

diffraction

metallography.

proceeds

and in two

precipitation composed gamma

The

The

first

solid

solution

librium

at

given

~+YZ

modification

phase of the

a

in

During lamellar

constante

Apr&

prolong4

recuit

eutecto’ide l+yz.

de

Der isotherme

this process

zwischen

a

microstructure

Zerfall

450

und

metallographisch sich in zwei

place.

d&omposition

d’un

alliage

suivie

dans

isotherme

U-Nb

l’intervalle

600 “C en utilisant l’examen poursuit tation

de

la diffraction

en deux stades. discontinue

la

solution

d’un

d’une

de Nb

temperatures

m&allographique.

phases compose

1.

de

it 9,576 en poids

La agrbgat

solution

de

des rayons

zweiphasigen

a 6th

und einer

X

k

alpha

ce

se produit lamellaire

hat.

se

eutektoidisch

Nach

Phasen

B deux

eine

et d’une

Schritten.

2.

Der mit

Zerfall

erste

besteht

einem

y-Phase,

in

die

Gleichgewicht

7%. Wiihrend der

dieses

urspriinglich

aus einer

cx-Mischkristall

die eine

Wiirmebehandlung im

und

vollzieht

in einer lamellaren

bei einer bestimmten

liingerer

o( und

U-9,50/, Nb wurde

Der

Ausscheidung

Anordnung

Bnderung

struktur

der y-Phase

600 “C! rijntgenographisch

metastabilen

et

Introduction The decomposition

processus

microst,ructure

untersucht.

Zusammensetzung

est une prbcipi-

lamellaire

solide

solide 450

d&omposition

Le premier

la

par

de phases

longer

into the equi-

diskontinuierlichen La

tempbrature

dans le melange

Durant

modification

& une

elle se d&ompose

initiale.

a metastable gamma has a constant

mixture. original

une

La phase gamma mbtastable

composition

d’hquilibre

and

After

une

un processus

is a discontinuous aggregate

eutectoidally

1972

don&e.

decomposition

temperature.

it decomposes

Yugo&via

Beograd,

phase gamma mhtastable.

solid

two-phase

phase. The metastable

annealing

gamma

was followed

450 to 600 “C, using X-ray

lamellar

a

of alpha

composition

takes

steps.

of

the

alloy

21 March

Vi&a,

konstante

Temperatur zerfiillt

sie

befindlichen

Prozesses

lamellaren

findet Mikro-

statt.

Previous

work

of the U-Nb gamma solid

Gamma solid solution in uranium alloys with

solution in the a+ yz phase region has been investigated by many authors. However, the exact course and mechanism of this process is not elucidated yet. According to the equilibrium diagram, the gamma phase should decompose eutectoidally into the ol+ yz phase mixture. Since previous work has shown that the reaction is not so simple, we have undertaken this study as an attempt to contribute to the better understanding of the decomposition process. We investigated the isothermal transformation of a U-9.5 wt o/o Nb alloy in the temperature interval 450 to 600 “C, using metallography and X-ray diffraction.

niobium contents between 6 and 9 wt o/0 undergoes a transformation to a tetragonal structure on quenching l-3). In alloys with higher niobium content the gamma phase is retained at, room temperature. According to Wilkinson 4), gammaquenched samples of U-10 wt y. Nb alloys, exposed to an anneal below the eutectoid temperature, transform into a metastable phase which later decomposes into the a+ y2 equilibrium structure. Jackson and Miley5) observed, in samples of a U-10 wt o/o Nb alloy annealed 1 h at 500 and 575 “C, gamma phase with lattice parameter somewhere between the parameters characteristic of the starting and equilibrium gamma phases. No details about,

207

208 the nRture aid

B.

the f~rrnat~~~l kinetics

DJURId

of the

quantities

of niobium

in the form of carbides.

metastahle phase were given in the Is& two referencss. The most detailed investigation of the gamma phase de;aamposition in the B + yz field was

Rogers et a1.s) and Ivanov and Terehov 3) have found the r~lationsbip between the niobiu~~~ content and gamma lattice parameter in quenched alloys. Comparing their results wit,h

undertaken by Soviet authors. Ivanov and Virgiliev 61 followed this process by X-ray rli~raGtio~ in uranium alloys cont,aining 23 at 0/O (10.4 w% ye)* 26.3 at % (12.2 wt %f, 30 at 74 (14.3 wt, %f, 40 at % (20.7 wt “/;,f, and 50 at “i;, (27.8 wt %) niobium. In the first alloy, the quenched-in gamma soiid solution undergoes a ho~~oge~~~o~~sde~ompositio~l into the :Xf yz phase mixture on annealing at 500 “C. During this process, the lattice parameter of the gamma component continuously decreases with increasing annealing time. With increased niobium content the decomposition becomes heterogeneous : the starting gamma phase decomposes first into a mixture of alpha ph,ase and an intermediate gamma phase. During this transition the gamma lattice parameter ohanges abruptly. The intermediate gamma decomposes into the equilibrium ct-t-y2 structure, homogeneously in the U-26.3 at1y0 Nb alloy, and heterogeneously in the niobium-richer alloys. Kishinevskii et a1.7) examined the isothermal decomposition of the gamma phase in a U-20 at “4 (8.9 wt %) Nb alloy in the a--cy~ field. They concluded that the decom~ositioi~ of t*he initiaf gamma phase starts by grai~l-boundar.~ precipitations of the alpha phase. The gamma

the lattice parameters given in Soviet papers, the concentration of niobium should amount to 26.5 at ‘lib (12.5 wt y$) in Kishinewkii’s aIIoy and 19.5 and 24.5 at y0 (8.5 and t I .2 wt *&)

phase gradually changes into a metastable intermediate gamma, whose lattice parameter decreases progressively wit,h is~ther~~al treatment. Subsequently the yz phase is heterogeneously nucleated in the presence of the mtermediate phase. After longer annealing the of both gamma phases lattice parameters line intensity of the decrease. The X-ray m~tast‘able phase decreases and that of the y”; phase i~~oreases until equilibrium is attained. When considering the results obtained on different U-Nb alloys one should keep in mind the fact that the exact composition depends on the purity of starting materials, I~a~~~eularly on the aasbon content, which can extra& large

respectively in the nomillal1~ 23 and 26.3 at $$ alloys of Ivanov and Virgiliev. That can probably explain the discrepancy in their findings. 3.

Experimental

For our experiments, a uranium-g.5 wt “/o (2 1.2 at %) niobium alloy was used. The starting materials were uranium of technical purity with about 500 ppm of metallic impurities, and niobium of purity 99.9%. The alloy was produced by argorl-ark melt~ing, followed by vacuum-homogenisation at 950 “C for one week and wat.er-quenching. In order to eliminate the effect of x~iobi~l~l carbide forrn~t,i~~~, the composition was determined not only by chemical analysis but also by electron probe microanalysis and X-ray diffraction. During the experiments, the samples 10 mm in diameter and 3 mm high were reheated for 24 Yt at 9OO “C and then transferred to a tin bath held at the isothermal treating temperature, and finally water-quenched. For longer holding times the samples were treated in a double-stage vacuum furnace, where the upper part was held at 900 “C and bhe lower at the temperature of isothermal treatment. A specimen holder enabled the transfer of a series of samples from the upper to the lower part, of the furnace and successive quenching of the sampfes. After met,allographi~ ~~re~aratioll the q~le~lched under an optical samples were examined microscope and by X-ray diffraction (a diffractometer coupled with a Geiger counter a,nd ohart recorder was used), Attention was mainly directed to the interval between 20=35°-400,

DECQMIPt3SITION

the

where

characteristic

Iines af

alpha

OF

GAMMA

209

PHASE

and

phases appear. The gamma f&&ice parameter was de&rmixled. Using the resr&s of Rogers et aP) we were able to oaTculate the niobium content in the gamma phase on tht3 basis of the Iattfce param&er. gamma

4, 4,1,

Results METALLOGRAPHIC

EXAMINATION

By direct quenching from the gamma region the gamma solid solution is retained at roomtemperature. The isothermal decomposition of the gamma phase in the temperature interval 4.50 to 600 “C begins by nuclea;tion 0-f 8 twophase product at the grain buundaries and inctusions. It spreads gradualtlfy over the whole sample in the form of a Iamell~ aggregate, The lamellae are relatively coarse at higher temperatures (fig. l), their size decreasing with decreasing temperature, At 450 "C the lamellae oan.nat be resolved under the optical microscope,

F&a 3. 24 h at 500°C.

x600

4.2. X-IthY EXAMINATION

Prolonged isothermal. annealing of the trsnsformed samples brings about a change in the microstructure. A new two-phase aggregate is nucleated at various points of the original lamellnr structure, forming a network throughout t.hc sample in the initial stage (fig. 2). The rcesotion proceeds by thickening of the netwurk (fig. 3) nntiH *‘he whole snmplc is transformed to the new structure.

The lattice parameter of the gamma phase retained at room-temperature by quenching corresponds to ~2content uf 9.5 wt Q/0niobium. The cuursc of the isothermal decurn~os~~~o~~ at 450, !%I@, and 550 “C is similar and wilX be described in more detail by taking the ox~mple of samptes treated at 450 “C. After 20 min at 450 “C the X-ray diffraction pattern shows only sharp peaks of the starting yl phase with lattice parameter o+== 3,448 A, After 45 und 90 min the pattern is the same, except that! in the latter, traces of alpha lines are visible. In the sample held 3 h at, 450 “C, beside the ye and a-lines, traces of two other cubic phases are present, with fat&ice param&ers N 3-41 and 3% w. The first is design&d y’

. -t--

Fig. 4.

t

I

I

Diffraction

I

I

I

,

,

,

t

t

t

t

t

pattern of samples held various times at 450 “C. (a) 45 min; (b) 10 h; (c) 20 h; (d) 24 h; (e) 72 h;

(f) 144 h.

DECOMPOSITION

and the second t#he equilibrium 50 wt %

Nb.

OF

GAMMA

211

PHASE:

yz, because it corresponds to y2 phase containing about In

the

further

course

of

TABLE

the

reaction t,he intensity of the yl lines decreases, and that of the y’ lines increases. The yz lines remain weak, while the intensity of the alpha lines continuously increases with time. After 15 h the yi lines completely disappear, and the y’ lines are very strong, They correspond to a lattice parameter ay’=3.407 A. In the to the equinext, stage, a gradual t ran&ion

1

Lat.tice ~aramwters of the gamma phases (A) and the intensity

I 450

lines

of X-ray

Y’

Yl

72

CK

-

-

“C

20 min

3.448 (s)

-

45 min

3.447 (s)

-

90 min

3.448 (9)

’

-

/_

3h 3.446 {s) I 3.41 librium nr+ya structure takes place. The first 6h : 3.447 (8) / 3.41 step of this process is aceompa~lied by 10 h 3.446 (m) 3.403 broadening of the y’ lines. For example, after 15h / / 3.407 20 h the (110) line of the y’ phase becomes j diffuse ‘20h / i 3.407 ’ broad enough to cover the (110) line of the 3~2 24h _ I 3.405 4s h phase. The intensity maximum lies between 72h 3.404 the positions of the y’ and ys lines. This 144 h 3.404 broadening happens in a relatively short time 500 “C , interval. Soon afterwards y’ and progressively 20 min / 3.448 (8) more intense ys lines become visible. The 30 min / 3.446 (6) 3.41 transition proceeds in such a way that the 60 min / 3.417 intensity of y’ lines decreases and ys lines 120min j 3.414 become more intense and sharp. The lattice 340 min 3.413 ! diffuse parameters of the y’ and yz phases do not change. 7h ! diffuse 24h j The amount of alpha phase steadily increases. 48 h 3.415 In all samples the lattice parameters of the 550 “C : alpha phase correspond to those of almost pure 15 min / 3.449 (9) _ uranium. 40 min 3.446 (R) 3.42 In fig. 4 a-f some characteristic diffractograms lh ~3.445 (s) ~ 3.424 are shown. Table 1 gives details of the course 3h _ / 3.426 of reaction. 6h ! 3.425 At 600 and 550 “C the reaction path is almost diffuse 10 h diffuse 24 h identical, only the rate of the decomposition 48h 3.42 : process is changed. The value of the y’ lattice ! paramet.er changes with temperature, but 600 “C remains constant during the course of the lh 3.444 (9) j 3h 3.444 (6) I isothermal reaction. Details can be seen from / 3.445 (s) 9h table 1. 3.447 (6) ’ 24 h At 600 “C the process is different. We did 40 h diffuse not observe lines corresponding to the y’ phase. 137 h After a few hours beside the yl lines broad diffuse peaks appear, which resemble the lines (w) - weak intensity preceding the y’ ++yz transition at lower (m) - medium intensity (sf - strong intensity temperatures. Later, the y2 lines appear and after about 140 h the reaction is accomplished. Using the relationship between lattice para-

--

1w

(w)

3.35

(w)

(w)

(w) (m) fs)

3.347 (w) 3.348 (w)

(m) (m)

_

I

i:i

lines (9)

3.353 (w)

(8)

(m)

j 3.352 (m)

(S)

(m)

3.353 (m)

(w)

3.353 (s)

I 1 (w)

-

(8)

~ b) -

3.35

(w)

(w)

, 3.35

(w)

(m)

(s)

3.35

(w)

(s)

: 3.35

(w)

(m) / (b) ’ fs)

(s)

lines lines

(s)

(m)

3.349 (m)

/ (8)

(m) (s) (s) (m) lines

3.352 (w) 3.353 (w)

i 3.353 (w) ’

lines (w)

’ 3.346 (m)

i

3.349

(w)

/ 3.348 (w) ! 3.350 (w) diffuse lines lines 3.348 (a)

(w) (m) (m)

j j (9) (4 fs) (s)

i (w) / (w) (w)

i (m) / (4 (8)

212

B.

meter and niobium content solutions) we calculated the tration in the y’ phase. The into the phase diagram, fig. 5.

DJURIC:

in gamma solid niobium concenresults are drawn 5.

Discussion

According to the X-ray examination it is possible to conclude that during the transformation of the original yi phase into the equilibrium LX+ ys phase mixture an intermediate y’ phase appears, having a nearly constant composition at a given temperature. The transitions yi+‘x+y’ and y’+~~+ys are continuous in the sense that the amount of the starting phase gradually diminishes and amount of products increases. However, neither is there a continuous decrease of the yi lattice parameter, nor do the lattice parameters of the y’ and ys phases change with time. We suppose that in this respect our results obtained on a diffractometer are more precise than results obtained earlier by film-recording 637). The position of the points which denote the compositions of the y’ phase in the phase diagram (fig. 5) gave us the idea for a possible explanation of the yi + y’ +n transition and

Fig. 5.

Part of the U-Nb

phase diagram

the nature of the y’ phase. The curve which connects these points (the dashed curve in the diagram) may be the continuation of the (01-tyr)/yi phase boundary. According to this view, y’ is metastable yi of composition appropriate to the temperature of anneal, whereas yi in the quenched alloy has the same composition as the alloy itself (vertical line in fig. 5). In this case, the free-energy principles which govern the shape of phase-diagrams can be applied to this boundary of metastable equilibrium. We may suppose that the freeenergy curves for the U-Nb system at the eutectoid temperature, 647 “C, should approximately have the shape given in fig. 6a. (Quite arbitrarily, the common tangent to the OL:,yr and ys phase free energy curves at 647 “C is taken to be horizontal). By lowering the temperature, the free energy curves of the phases which are nearer to the equiatomic composition (yi and ys) rise quicker than the alpha-phase curve, so that at lower temperatures we should have free energy diagrams as given in fig. 6b-d. In these figures the common tangent to the 01 and yi phase curves is also given (dashed line). We suppose that during

with the compositions

of the y’ phase.

DECOMPOSITION

OF

GAMMA

213

PHASE

of the y’ phase during

the decomposition

at

600 “C: assuming that fig. 6b corresponds to 600 “C, the tangent touches the yl free energy curve near the composition of the examined alloy. After the first stage of the reaction, when practically only alpha and y’ phases are present in the sample, further lowering of the free energy

L 5ooc

F

I

0 Fig. 6.

10

20

30

Hypothetical

LO

50

60 wt%Nb

free energy versus composition

curves for the U-Nb

system.

the decomposition of the yl solid solution below the eutectoid temperature, the 01+ yl phase mixture, corresponding to this tangent, is still formed. Of course, being metastable, in the further course of the reaction it transforms to the equilibrium 8 + ys. On decreasing the temperature, the freeenergy curve of the alpha phase sinks more and more with respect to the gamma curves. This explains the experimentally established variation of the composition of the y’ phase with temperature. Assuming this explanation, we can simply describe the yl + OL + y’ reaction as a discontinuous precipitation. The hypothetical free-energy diagrams in fig. 6 are drawn so as to explain the absence

demands the decomposition of the y’ ol+yz phase. phase into the equilibrium According to the free energy diagrams, this should be a typical eutectoidal transformation, y’ +01+ ys. By means of the X-ray diagrams one can follow the gradual decrease of the intensity of y’ lines and increase of the intensity of ys lines without change in lattice parameters. There is an interesting point in this process: this is the transition period preceding the intensive y’ + (x+ y2 transformation. At all temperatures, in a relatively short time interval, broad diffraction lines appear, covering the whole region between the positions of the yl and the yz lines. It is difficult to determine the exact position of these diffuse peaks, but they approximately correspond to a concentration of 30 to 40 wt o/o niobium. A possible explanation for this phenomenon could rely upon Cahn’s 9) theoretical argument that during eutectoid decomposition the equilibrium composition of the product phases can be achieved only in the ideal case when the reaction rate approaches zero. The deviation from the equilibrium composition depends on the position of the free energy curves of the present phases. For example, in our case the composition of the ys phase before reaching the equilibrium niobium content can have all the values which lie to the right of the intersection of the n/y1 tangent with the ys free energy curve in fig. 6. Another possibility could be derived from the work of Strelova et al.ls), who found a pronounced tendency to short-range ordering in the equiatomic U-Nb alloy. This tendency could lower the free energy of the system and allow another, intermediate, state during the decomposition of the gamma phase. Elucidation of these phenomena lies outside the scope of

214

B.

DJURIC:

the present work because it demands other techniques to be applied to alloys of near equiatomic composition. On the basis of the X-ray results it is possible to interpret the metallographic findings. The formation of the primary lamellar structure corresponds to the discontinuous precipitation of this of a+y’ phases. The modification structure is a result of the eutectoid y’ + o(+ ya A similar structural change was reaction. metallographically observed in copper 1~12) and uranium la) alloys: there also a two-phase structure is nucleated in the original pearlite and grows at its expense. According to Spencer and Mack 14) this process seems to be possible in two-phase systems where at least one of the supersaturated. The phases is appreciably secondary reaction then leads to the equilibrium structure. Having in mind our X-ray results, this applies to our case very well. 6.

Conclusions

Our conclusion is that the yl+ #x+ ya transition takes place in two steps. The first is a discontinuous precipitation of a two-phase structure consisting of alpha solid solution and a metastable gamma phase. The second step is a eutectoidal decomposition of the metastable phase into the 0~+ ys phase mixture. During this latter step the equilibrium is not obtained immediately. The appearance of the metastable gamma phase and its composition are determined by the free energy curve of the yl solid solution. Acknowledgements The author is grateful to Prof. A. Mihajlovic

for valuable discussion and comments. Thanks are also due to S. MalEi for help in X-ray work, and P. Saponjic for help in metallography.

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ANL-5581

(1957)

K. Tangri and D. K. Chaudhuri, J. Nacl. Mater.

‘) 15 (1965) 278 3) 0. S. Ivanov and G. I. Terehov, splavov (Alloy

nekotoryh

sistem

Const,itution

of

Uranium and Thorium)

in Strcenie s uranom i toriem

Some

Systems

(Moscow,

with

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1961) p. 20

“1 W.

D.

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Uranium

science

Publishers

(New

Met,allurgy,

York,

1962)

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p. 1231

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“1 0.

S.

Mater.

7) V.

336 Ivanov

ant1 Yu.

6 (1962)

B.

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S.

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and 0.

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splavov

i tugoplavkikh

ru-anom

(Physical

Tretyakov,

L.

I.

in Fiziko-khimiya

soedinenii

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of

s toriem Alloys

i

and

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Nauka,

1968) p. 42

*) B. A. Rogers, D. F. Atkins, E. J. Manthos and M. E. Kirkpatrick,

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387

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Cahn, Acta

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212

18

S. V. Strelova, Ya. S. Umansky and 0. S. Ivanov, J. Nucl.

“1

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160

R. H. Fillnow and D. J. Mack, Trans. Met. Sot. AIME

188 (1950)

1229

12) C. VI’. Spencer and D. J. Mack, J. Inst. Metals 82 (1953354)

81

13) G. H. May, J. Nucl. Mater. 7 (1962) 72 ‘4) C. W. Spencer and D. J. Mack, in Decomposition of Austenit,e by Diffusional Processes, Interscience Publishers

(New York,

1962) p. 549